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Edward Morris Edward Morris is offline
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Default B+ rectifier question

Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the + and -
leads together in the middle of the rectifiers and the other lead attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a common tie
with a number of resistors that tie into a number of capacitors. Does
anyone have a ballpark guess what value of resistors I need for the voltage
drop.

Thanks in advance,
Eddie

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Restless Fingers Syndrome Restless Fingers Syndrome is offline
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Default B+ rectifier question

Edward Morris wrote:
Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+


http://www.pmillett.com/file_downloa...sh/C22_sch.pdf

Why do you need a voltage drop ? and by how much ?
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:-:Ghost Chip:-: :-:Ghost Chip:-: is offline
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Default B+ rectifier question


"Edward Morris" wrote in message
news
Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a common
tie
with a number of resistors that tie into a number of capacitors. Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.

Thanks in advance,
Eddie

Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.


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Peter Wieck Peter Wieck is offline
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Default B+ rectifier question

On Dec 12, 4:04*pm, ":-:Ghost Chip:-:" wrote:
"Edward Morris" wrote in message

news


Hello,
* *I have a McIntosh C22 preamp. *I'd like to replace the two diode B+
selenium rectifiers. *One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead attaches
to a 40mfd capacitor. *One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a common
tie
with a number of resistors that tie into a number of capacitors. *Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.


Thanks in advance,
Eddie


Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text -

- Show quoted text -


Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.

Others may have differing opinions.

Peter Wieck
Melrose Park, PA
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Default B+ rectifier question - C22 schematic

C22 schematic is at alt.binaries.schematics.electronic




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Default B+ rectifier question


"Peter Wieck" wrote in message
...
On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote:
"Edward Morris" wrote in message

news


Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead
attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a common
tie
with a number of resistors that tie into a number of capacitors. Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.


Thanks in advance,
Eddie


Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted
text -

- Show quoted text -


Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.

Others may have differing opinions.

Peter Wieck
Melrose Park, PA

Yes. That's a good idea.


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bigwig bigwig is offline
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Default B+ rectifier question

On 12 Dec, 22:00, Peter Wieck wrote:
On Dec 12, 4:04*pm, ":-:Ghost Chip:-:" wrote:





"Edward Morris" wrote in message


news


Hello,
* *I have a McIntosh C22 preamp. *I'd like to replace the two diode B+
selenium rectifiers. *One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead attaches
to a 40mfd capacitor. *One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a common
tie
with a number of resistors that tie into a number of capacitors. *Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.


Thanks in advance,
Eddie


Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text -


- Show quoted text -


Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. *Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.

Others may have differing opinions.

Peter Wieck
Melrose Park, PA- Hide quoted text -

- Show quoted text -


Agreed,
I would measure the HT with the selenium rectifiers in place (If
they still work). Then measure the curent in the red/yellow wire (be
carefull if you do this). Use a bit of good ole Ohms law to determine
what the equivalent load resistance on the supply is. Disconnect the
HT from the rest of the circuit. bodge up a resistor combination of
appropriate power rating and connect this to the now disconnected HT
(the letters HT should be enough warning) leave the heaters intact for
now as if you disconnect them the HT will rise. Now replace the
rectifiers with whatever you choose, as long as the voltage/current
ratings are ok. Measure the voltage, back to Ohms law again to work
out your series resistance needed.
Thats the long winded way. I think Peters 10-20R ballpark figure
will be fine. There is another way to work it from the AC output from
the transformer taking into account diode drop, ripple current etc.
This is even more long winded and only really if you are designing
from scratch.
I just looked at the diagram. Cant believe I just typed all that,
10R will be fine with IN4007s or anything else for that matter. The
only thing to watch is the heaters if you change to silicon.
Especially as you guys have a higher than designed for mains. In the
UK they were suposed to go down from 240V to 230V to get in with the
rest of Europe. My mains is 254V in the evening dropping to 250 or so
at "put the kettle on watch a crap soap time". I must admit that the
distribution transformer is at the end of my garden so thats the
reason. I work as a sparky by trade and I have seen as low as 200V in
some rural locations. Even seen one house with glowing line taps.
Matt.
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Edward Morris Edward Morris is offline
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Posts: 47
Default B+ rectifier question

Hello all,
Thanks for all your help. What a great bunch of people here! Would you
recommend using a resistor between the red/yellow wire and the capacitor?

Edward

"bigwig" wrote in message
...
On 12 Dec, 22:00, Peter Wieck wrote:
On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote:





"Edward Morris" wrote in message


news


Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead
attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a
common
tie
with a number of resistors that tie into a number of capacitors. Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.


Thanks in advance,
Eddie


Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted
text -


- Show quoted text -


Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.

Others may have differing opinions.

Peter Wieck
Melrose Park, PA- Hide quoted text -

- Show quoted text -


Agreed,
I would measure the HT with the selenium rectifiers in place (If
they still work). Then measure the curent in the red/yellow wire (be
carefull if you do this). Use a bit of good ole Ohms law to determine
what the equivalent load resistance on the supply is. Disconnect the
HT from the rest of the circuit. bodge up a resistor combination of
appropriate power rating and connect this to the now disconnected HT
(the letters HT should be enough warning) leave the heaters intact for
now as if you disconnect them the HT will rise. Now replace the
rectifiers with whatever you choose, as long as the voltage/current
ratings are ok. Measure the voltage, back to Ohms law again to work
out your series resistance needed.
Thats the long winded way. I think Peters 10-20R ballpark figure
will be fine. There is another way to work it from the AC output from
the transformer taking into account diode drop, ripple current etc.
This is even more long winded and only really if you are designing
from scratch.
I just looked at the diagram. Cant believe I just typed all that,
10R will be fine with IN4007s or anything else for that matter. The
only thing to watch is the heaters if you change to silicon.
Especially as you guys have a higher than designed for mains. In the
UK they were suposed to go down from 240V to 230V to get in with the
rest of Europe. My mains is 254V in the evening dropping to 250 or so
at "put the kettle on watch a crap soap time". I must admit that the
distribution transformer is at the end of my garden so thats the
reason. I work as a sparky by trade and I have seen as low as 200V in
some rural locations. Even seen one house with glowing line taps.
Matt.

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bigwig bigwig is offline
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Default B+ rectifier question

On 13 Dec, 07:52, "Edward Morris" wrote:
Hello all,
* * Thanks for all your help. *What a great bunch of people here! *Would you
recommend using a resistor between the red/yellow wire and the capacitor?

Edward

"bigwig" wrote in message

...
On 12 Dec, 22:00, Peter Wieck wrote:





On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote:


"Edward Morris" wrote in message


news


Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead
attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a
common
tie
with a number of resistors that tie into a number of capacitors. Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.


Thanks in advance,
Eddie


Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted
text -


- Show quoted text -


Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.


Others may have differing opinions.


Peter Wieck
Melrose Park, PA- Hide quoted text -


- Show quoted text -


Agreed,
* I would measure the HT with the selenium rectifiers in place (If
they still work). Then measure the curent in the red/yellow wire (be
carefull if you do this). Use a bit of good ole Ohms law to determine
what the equivalent load resistance on the supply is. Disconnect the
HT from the rest of the circuit. bodge up a resistor combination of
appropriate power rating and connect this to the now disconnected HT
(the letters HT should be enough warning) leave the heaters intact for
now as if you disconnect them the HT will rise. Now replace the
rectifiers with whatever you choose, as long as the voltage/current
ratings are ok. Measure the voltage, back to Ohms law again to work
out your series resistance needed.
* Thats the long winded way. I think Peters 10-20R ballpark figure
will be fine. There is another way to work it from the AC output from
the transformer taking into account diode drop, ripple current etc.
This is even more long winded and only really if you are designing
from scratch.
* I just looked at the diagram. Cant believe I just typed all that,
10R will be fine with IN4007s or anything else for that matter. The
only thing to watch is the heaters if you change to silicon.
Especially as you guys have a higher than designed for mains. In the
UK they were suposed to go down from 240V to 230V to get in with the
rest of Europe. My mains is 254V in the evening dropping to 250 or so
at "put the kettle on watch a crap soap time". I must admit that the
distribution transformer is at the end of my garden so thats the
reason. I work as a sparky by trade and I have seen as low as 200V in
some rural locations. Even seen one house with glowing line taps.
* *Matt.- Hide quoted text -

- Show quoted text -


I would stick it in series with SR1. SR2 doesnt really need the series
R.
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John Byrns John Byrns is offline
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Posts: 1,441
Default B+ rectifier question

In article ,
bigwig wrote:

On 13 Dec, 07:52, "Edward Morris" wrote:
Hello all,
* * Thanks for all your help. *What a great bunch of people here! *Would you
recommend using a resistor between the red/yellow wire and the capacitor?

Edward

"bigwig" wrote in message

...
On 12 Dec, 22:00, Peter Wieck wrote:





On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote:


"Edward Morris" wrote in message


news


Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead
attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a
common
tie
with a number of resistors that tie into a number of capacitors. Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.


Thanks in advance,
Eddie


Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted
text -


- Show quoted text -


Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.


Others may have differing opinions.


Peter Wieck
Melrose Park, PA- Hide quoted text -


- Show quoted text -


Agreed,
* I would measure the HT with the selenium rectifiers in place (If
they still work). Then measure the curent in the red/yellow wire (be
carefull if you do this). Use a bit of good ole Ohms law to determine
what the equivalent load resistance on the supply is. Disconnect the
HT from the rest of the circuit. bodge up a resistor combination of
appropriate power rating and connect this to the now disconnected HT
(the letters HT should be enough warning) leave the heaters intact for
now as if you disconnect them the HT will rise. Now replace the
rectifiers with whatever you choose, as long as the voltage/current
ratings are ok. Measure the voltage, back to Ohms law again to work
out your series resistance needed.
* Thats the long winded way. I think Peters 10-20R ballpark figure
will be fine. There is another way to work it from the AC output from
the transformer taking into account diode drop, ripple current etc.
This is even more long winded and only really if you are designing
from scratch.
* I just looked at the diagram. Cant believe I just typed all that,
10R will be fine with IN4007s or anything else for that matter. The
only thing to watch is the heaters if you change to silicon.
Especially as you guys have a higher than designed for mains. In the
UK they were suposed to go down from 240V to 230V to get in with the
rest of Europe. My mains is 254V in the evening dropping to 250 or so
at "put the kettle on watch a crap soap time". I must admit that the
distribution transformer is at the end of my garden so thats the
reason. I work as a sparky by trade and I have seen as low as 200V in
some rural locations. Even seen one house with glowing line taps.
* *Matt.- Hide quoted text -

- Show quoted text -


I would stick it in series with SR1. SR2 doesnt really need the series
R.


Why would SR1 need a series R any more, or any less, than SR2? The need, or
lack thereof is equal for both diodes. If I were of a mind to use a series R I
would connect it in series with the "RED" wire from the PT where it connects to
the junction of SR1 & SR2.

There does seem to be a minor drawing error in the schematic where a short piece
of wire is missing.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/


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bigwig bigwig is offline
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Posts: 118
Default B+ rectifier question

On 13 Dec, 18:49, John Byrns wrote:
In article ,





*bigwig wrote:
On 13 Dec, 07:52, "Edward Morris" wrote:
Hello all,
* * Thanks for all your help. *What a great bunch of people here! *Would you
recommend using a resistor between the red/yellow wire and the capacitor?


Edward


"bigwig" wrote in message


....
On 12 Dec, 22:00, Peter Wieck wrote:


On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote:


"Edward Morris" wrote in message


news


Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead
attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a
common
tie
with a number of resistors that tie into a number of capacitors.. Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.


Thanks in advance,
Eddie


Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted
text -


- Show quoted text -


Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.


Others may have differing opinions.


Peter Wieck
Melrose Park, PA- Hide quoted text -


- Show quoted text -


Agreed,
* I would measure the HT with the selenium rectifiers in place (If
they still work). Then measure the curent in the red/yellow wire (be
carefull if you do this). Use a bit of good ole Ohms law to determine
what the equivalent load resistance on the supply is. Disconnect the
HT from the rest of the circuit. bodge up a resistor combination of
appropriate power rating and connect this to the now disconnected HT
(the letters HT should be enough warning) leave the heaters intact for
now as if you disconnect them the HT will rise. Now replace the
rectifiers with whatever you choose, as long as the voltage/current
ratings are ok. Measure the voltage, back to Ohms law again to work
out your series resistance needed.
* Thats the long winded way. I think Peters 10-20R ballpark figure
will be fine. There is another way to work it from the AC output from
the transformer taking into account diode drop, ripple current etc.
This is even more long winded and only really if you are designing
from scratch.
* I just looked at the diagram. Cant believe I just typed all that,
10R will be fine with IN4007s or anything else for that matter. The
only thing to watch is the heaters if you change to silicon.
Especially as you guys have a higher than designed for mains. In the
UK they were suposed to go down from 240V to 230V to get in with the
rest of Europe. My mains is 254V in the evening dropping to 250 or so
at "put the kettle on watch a crap soap time". I must admit that the
distribution transformer is at the end of my garden so thats the
reason. I work as a sparky by trade and I have seen as low as 200V in
some rural locations. Even seen one house with glowing line taps.
* *Matt.- Hide quoted text -


- Show quoted text -


I would stick it in series with SR1. SR2 doesnt really need the series
R.


Why would SR1 need a series R any more, or any less, than SR2? *The need, or
lack thereof is equal for both diodes. *If I were of a mind to use a series R I
would connect it in series with the "RED" wire from the PT where it connects to
the junction of SR1 & SR2.

There does seem to be a minor drawing error in the schematic where a short piece
of wire is missing.

--
Regards,

John Byrns

Surf my web pages at, *http://fmamradios.com/- Hide quoted text -

- Show quoted text -


oops yes both diodes. You could just put one in at the junction of the
two.
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:-:Ghost Chip:-: :-:Ghost Chip:-: is offline
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Posts: 11
Default B+ rectifier question


"John Byrns" wrote in message
...
In article
,
bigwig wrote:

On 13 Dec, 07:52, "Edward Morris" wrote:
Hello all,
Thanks for all your help. What a great bunch of people here! Would you
recommend using a resistor between the red/yellow wire and the
capacitor?

Edward

"bigwig" wrote in message

...
On 12 Dec, 22:00, Peter Wieck wrote:





On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote:

"Edward Morris" wrote in message

news
Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode
B+
selenium rectifiers. One lead from the transformer attaches to
the +
and -
leads together in the middle of the rectifiers and the other lead
attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the
bridge
rectifier that powers the filaments and the other leg ties into a
common
tie
with a number of resistors that tie into a number of capacitors.
Does
anyone have a ballpark guess what value of resistors I need for
the
voltage
drop.

Thanks in advance,
Eddie

Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide
quoted
text -

- Show quoted text -

Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US
in
recent years from a typical ~115V to as much as 125V as a
steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.

Others may have differing opinions.

Peter Wieck
Melrose Park, PA- Hide quoted text -

- Show quoted text -

Agreed,
I would measure the HT with the selenium rectifiers in place (If
they still work). Then measure the curent in the red/yellow wire (be
carefull if you do this). Use a bit of good ole Ohms law to determine
what the equivalent load resistance on the supply is. Disconnect the
HT from the rest of the circuit. bodge up a resistor combination of
appropriate power rating and connect this to the now disconnected HT
(the letters HT should be enough warning) leave the heaters intact for
now as if you disconnect them the HT will rise. Now replace the
rectifiers with whatever you choose, as long as the voltage/current
ratings are ok. Measure the voltage, back to Ohms law again to work
out your series resistance needed.
Thats the long winded way. I think Peters 10-20R ballpark figure
will be fine. There is another way to work it from the AC output from
the transformer taking into account diode drop, ripple current etc.
This is even more long winded and only really if you are designing
from scratch.
I just looked at the diagram. Cant believe I just typed all that,
10R will be fine with IN4007s or anything else for that matter. The
only thing to watch is the heaters if you change to silicon.
Especially as you guys have a higher than designed for mains. In the
UK they were suposed to go down from 240V to 230V to get in with the
rest of Europe. My mains is 254V in the evening dropping to 250 or so
at "put the kettle on watch a crap soap time". I must admit that the
distribution transformer is at the end of my garden so thats the
reason. I work as a sparky by trade and I have seen as low as 200V in
some rural locations. Even seen one house with glowing line taps.
Matt.- Hide quoted text -

- Show quoted text -


I would stick it in series with SR1. SR2 doesnt really need the series
R.


Why would SR1 need a series R any more, or any less, than SR2? The need,
or
lack thereof is equal for both diodes. If I were of a mind to use a
series R I
would connect it in series with the "RED" wire from the PT where it
connects to
the junction of SR1 & SR2.

There does seem to be a minor drawing error in the schematic where a short
piece
of wire is missing.

--
Regards,

John Byrns


Wow. What good eyes you have. I had to zoom to 400 to see the little "poor
solder joint."
Good idea; 1 resistor in the red wire will do nicely. This circuit is a
voltage doubler riding on top of the rectified 27.6 volts for the filaments.
There's more than one way to get HV.
Ghost


  #13   Report Post  
Posted to rec.audio.tubes
Ian Iveson Ian Iveson is offline
external usenet poster
 
Posts: 960
Default B+ rectifier question

Edward Morris wondered:

I have a McIntosh C22 preamp. I'd like to replace the two
diode B+
selenium rectifiers. One lead from the transformer
attaches to the + and -
leads together in the middle of the rectifiers and the
other lead attaches
to a 40mfd capacitor. One outside rectifier leg attaches
to the bridge
rectifier that powers the filaments and the other leg ties
into a common tie
with a number of resistors that tie into a number of
capacitors. Does
anyone have a ballpark guess what value of resistors I
need for the voltage
drop


Just been trying to find out about selenium rectifiers. It
seems they can have several cells in series, and each cell
can vary in several ways. The voltage drop can be measured,
or could have been estimated from an appropriate data sheet
which is unlikely to be available now. If you can measure
the drop across and current through each one, you can get an
idea of what resistor you would need in series with each,
using ohm's law. The waveforms aren't sinusoidal, though, so
you would need to measure the two final voltages and adjust
to suit, depending on how exact they need to be. You would
also need to be careful with power ratings, for the same
reason.

A much easier way is to measure the resistance and
open-circuit voltage of the transformer secondaries, and
then simulate the circuit with SPICE. Especially if you
don't happen to have a stock of real resistors to try.

If you can measure but are unable to simulate, someone here
might do that for you.

Or perhaps someone has already made a near-enough guess?

There is a question of strategy. Would it be better to add
resistance to the heater circuit after the feed to the HT,
and to add no resistance to that feed, thus raising the
floor of the HT circuit. Then add resistance to the HT after
the doubler?

It would be a bit interesting to find the optimum
arrangement.

I wonder how you should dispose of the old rectifiers?
Authorities here are, quite rightly I suppose, getting very
touchy about what goes in the bin. I noticed, passing a so
called "health food" shop the other day, that selenium is
essential for fitness, so perhaps you could grind them up
and eat them?

Ian


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Peter Wieck Peter Wieck is offline
external usenet poster
 
Posts: 2,418
Default B+ rectifier question

On Dec 15, 9:17*am, "Ian Iveson"
wrote:

Just been trying to find out about selenium rectifiers. It
seems they can have several cells in series, and each cell
can vary in several ways. The voltage drop can be measured,
or could have been estimated from an appropriate data sheet
which is unlikely to be available now. If you can measure
the drop across and current through each one, you can get an
idea of what resistor you would need in series with each,
using ohm's law. The waveforms aren't sinusoidal, though, so
you would need to measure the two final voltages and adjust
to suit, depending on how exact they need to be. You would
also need to be careful with power ratings, for the same
reason.

A much easier way is to measure the resistance and
open-circuit voltage of the transformer secondaries, and
then simulate the circuit with SPICE. Especially if you
don't happen to have a stock of real resistors to try.

If you can measure but are unable to simulate, someone here
might do that for you.

Or perhaps someone has already made a near-enough guess?

There is a question of strategy. Would it be better to add
resistance to the heater circuit after the feed to the HT,
and to add no resistance to that feed, thus raising the
floor of the HT circuit. Then add resistance to the HT after
the doubler?

It would be a bit interesting to find the optimum
arrangement.

I wonder how you should dispose of the old rectifiers?
Authorities here are, quite rightly I suppose, getting very
touchy about what goes in the bin. I noticed, passing a so
called "health food" shop the other day, that selenium is
essential for fitness, so perhaps you could grind them up
and eat them?


WOW ------ WAY, way to complicated.

Selenium rectifiers have a much higher voltage drop than silicon
diodes. In *some* (but not all) circuits where the down-line voltage
is critical, that drop must be replaced if the selenium diodes are
replaced with silicon.

So far, so good.

Not all selenium rectifiers are created equal. Some are single-
element, some are stacks of 2 - 20-or-more depending on the current
they are to carry. The specific SIZE of the diode does not matter, the
NUMBER OF ELEMENTS (stack) does matter relative to this question.

Rule-of-thumb, all other things being equal (Ed's ballpark) is 10 ohms
per element. After which additional resistance *may* be desired to
further reduce B+ (or C- if a bias circuit) to make up for higher line
(wallplate) voltage common in some areas these days. Naturally, this
is to be verified in the field by actual measurement.

Ed was requesting ball-park numbers for a single-element system. 10 -
20 ohms covers the from home plate to far-away center field. Close
enough.

Sometimes a small brass hammer in the hands of a Vermont Blacksmith is
all that is necessary.

In our township, one may drop "hazardous waste" at the township
transfer station for separate disposal without additional charge. In
general, I collect this sort of stuff in an empty (clean) paint can -
when it gets nearly full (perhaps once per year), I drop it off at the
transfer station about 2 miles away. From there it is taken to a
certified hazardous-waste landfill for proper disposal.

Peter Wieck
Melrose Park, PA
  #15   Report Post  
Posted to rec.audio.tubes
Ian Iveson Ian Iveson is offline
external usenet poster
 
Posts: 960
Default B+ rectifier question

Peter Wieck wrote:

Just been trying to find out about selenium rectifiers. It
seems they can have several cells in series, and each cell
can vary in several ways. The voltage drop can be
measured,
or could have been estimated from an appropriate data
sheet
which is unlikely to be available now. If you can measure
the drop across and current through each one, you can get
an
idea of what resistor you would need in series with each,
using ohm's law. The waveforms aren't sinusoidal, though,
so
you would need to measure the two final voltages and
adjust
to suit, depending on how exact they need to be. You would
also need to be careful with power ratings, for the same
reason.

A much easier way is to measure the resistance and
open-circuit voltage of the transformer secondaries, and
then simulate the circuit with SPICE. Especially if you
don't happen to have a stock of real resistors to try.

If you can measure but are unable to simulate, someone
here
might do that for you.

Or perhaps someone has already made a near-enough guess?

There is a question of strategy. Would it be better to add
resistance to the heater circuit after the feed to the HT,
and to add no resistance to that feed, thus raising the
floor of the HT circuit. Then add resistance to the HT
after
the doubler?

It would be a bit interesting to find the optimum
arrangement.


***
WOW ------ WAY, way to complicated.

Selenium rectifiers have a much higher voltage drop than
silicon
diodes. In *some* (but not all) circuits where the down-line
voltage
is critical, that drop must be replaced if the selenium
diodes are
replaced with silicon.

So far, so good.

Not all selenium rectifiers are created equal. Some are
single-
element, some are stacks of 2 - 20-or-more depending on the
current
they are to carry. The specific SIZE of the diode does not
matter, the
NUMBER OF ELEMENTS (stack) does matter relative to this
question.

Rule-of-thumb, all other things being equal (Ed's ballpark)
is 10 ohms
per element. After which additional resistance *may* be
desired to
further reduce B+ (or C- if a bias circuit) to make up for
higher line
(wallplate) voltage common in some areas these days.
Naturally, this
is to be verified in the field by actual measurement.

Ed was requesting ball-park numbers for a single-element
system. 10 -
20 ohms covers the from home plate to far-away center field.
Close
enough.

Sometimes a small brass hammer in the hands of a Vermont
Blacksmith is
all that is necessary.

***

The situation is no less complicated than I wrote. You may
believe that the complications don't matter, and Edward may
not care.

We don't have ball-parks here in England, but anyway, if
Edward asks what's the right ball-park voltage for his
heaters, would 35V be in the right field? How close is close
enough for Edward? We don't even know the voltage ratings of
his capacitors.

If the voltage drop across a selenium diode is truly "much"
greater than that of a silicon diode, and particularly if it
is a true diode drop rather than the result of inherent
series resistance, then the resistance necessary to restore
it will depend on the current through the diode. So if, say,
the current is 1A, then 10 ohms will result in a 10V
reduction, whereas if the current is 10mA, then 10 ohms will
result in a drop of only 100mV, each assuming DC or
sinusoidal current.

Now, the circuit in question includes a relatively
high-current heater supply, and a low-current HT supply, and
an even-lower-current link between the two. Further, the
heater supply uses a bridge rectifier, whereas the other two
are half-wave. Further still, the high-current supply is
relatively low voltage, so the greater voltage difference
will be a much more significant proportion of the supply
voltage. What is more, we don't know the specifications of
Edwards selenium diodes, although the knowledgeable may be
able to divine them from the voltages.

If 10-20 ohms is a good guess for one, it is unlikely to be
a good guess for the others, surely?

Included in your answer is the assumption that Edward
doesn't care too much about the consequent voltages. Maybe
you're right, and maybe they will be close enough for
reasonable operation.

In which case, considering 20 ohms will have so little
effect in the two lower-current situations, he could leave
them out and concentrate on getting the heater voltage
right.

What power rating should the added resistor(s) be, there,
and exactly where in the circuit should he put it, or them,
do you think?

A simulation would not be complicated at all, BTW, unless an
exact mean power dissipation is required because that's
awkward to do.

Perhaps Edward will report the results of his experiments.
I'm quite interested.


I wonder how you should dispose of the old rectifiers?
Authorities here are, quite rightly I suppose, getting
very
touchy about what goes in the bin. I noticed, passing a so
called "health food" shop the other day, that selenium is
essential for fitness, so perhaps you could grind them up
and eat them?


***
In our township, one may drop "hazardous waste" at the
township
transfer station for separate disposal without additional
charge. In
general, I collect this sort of stuff in an empty (clean)
paint can -
when it gets nearly full (perhaps once per year), I drop it
off at the
transfer station about 2 miles away. From there it is taken
to a
certified hazardous-waste landfill for proper disposal.

***

Probably the same here. I haven't managed to fill my jar
yet...just one dead valve and one extremely dead tantalum
capacitor. If there were a damaged selenium diode in there I
would want to get rid of it sooner.

Ian





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Peter Wieck Peter Wieck is offline
external usenet poster
 
Posts: 2,418
Default B+ rectifier question

On Dec 15, 12:10 pm, "Ian Iveson"
wrote:

The situation is no less complicated than I wrote. You may
believe that the complications don't matter, and Edward may
not care.


Um... um...

But, most of the parameters are already given by the nature of the
beast in question - single-element selenium diode(s), in a McIntosh
C22 pre-amp.

So, the application is low-current, single-element diodes. Wattage on
the resistor will be 2W or less, thereabouts. I would counsel a 5-
watter as they are certainly cheap enough for the insurance. And 10
ohms/element by l-o-n-g experience in low-current applications of this
nature (lots-O-vintage radios have selenium diodes in various
applications as well as a fair amount of vintage audio stuff) if
straight-up voltage-drop replacement is required. Beyond that, one
must experiment (or, Heaven forfend!! *Calculate*) the additional or
lower/none resistance wished.

Were this a piece of unknown antecedents, brand or application, I
would not have been so glib.

Sometimes, a Cigar is just a Cigar.

Peter Wieck
Melrose Park, PA

  #17   Report Post  
Posted to rec.audio.tubes
Edward Morris Edward Morris is offline
external usenet poster
 
Posts: 47
Default B+ rectifier question

Hey guys,
I appreciate all the input. I replaced all the bumble bee capacitors.
I replaced all the electrolytic capacitors (cans if possible). I couldn't
find the can for the filament circuit so I had to go with 3 separate
capacitors for that circuit. I went with keeping the capacitor voltages the
same as the originals or at least close. I replaced the resistors in the
power supply as well as the filament bridge which was originally 4 separate
diodes mounted on a strip and I replaced the original 2 diodes for the B+.
I have to run the preamp on a variac at 110v because the filament voltage
tested at each tube is right at 12 volts. My goal is to get the resistance
added necessary to be able to plug the preamp into the wall outlet about
123V but I also don't want to add too much more heat to the innards. The
filament circuit pulls a lot of current. I've got a 6 amp bridge for the
filament circuit and it runs warm. The resistors (2-7.5 ohm 5 watt) run so
hot you can even touch them. But they were that way before the new parts.
The preamp at this point sounds great. I've used it running from the variac
for a number of months now with no problems and the PT runs just a little
warm. I have a number of resistors to be able to experiment. It's just
getting the thing unhooked and out of my cabinet. I know I sound lazy. I
guess I just don't like the idea of getting it out again. It was a lot of
work at time to (rebuild it). You guys know what I mean? I have the
schematic so I know the recommended voltages. I know just about enough
about electronics to be dangerous. If you go to www.nosvalves.com/C22.htm
you can see the 2 grey B+ diodes at the left of the pictures. I still have
them but they don't have stacks per se like I've seen some other selenium
rectifiers have. Do you recommend for the B+ using resistance before the
diodes or after? Also the 2 HT wires Red and Red/Yellow; do I need to
install a resistor between the capacitor and the Red/Yellow wire; the
voltage is running a little high, or will reducing the B+ voltage take care
of that?

Thanks again,
Edward Morris

"Ian Iveson" wrote in message
...
Peter Wieck wrote:

Just been trying to find out about selenium rectifiers. It
seems they can have several cells in series, and each cell
can vary in several ways. The voltage drop can be
measured,
or could have been estimated from an appropriate data
sheet
which is unlikely to be available now. If you can measure
the drop across and current through each one, you can get
an
idea of what resistor you would need in series with each,
using ohm's law. The waveforms aren't sinusoidal, though,
so
you would need to measure the two final voltages and
adjust
to suit, depending on how exact they need to be. You would
also need to be careful with power ratings, for the same
reason.

A much easier way is to measure the resistance and
open-circuit voltage of the transformer secondaries, and
then simulate the circuit with SPICE. Especially if you
don't happen to have a stock of real resistors to try.

If you can measure but are unable to simulate, someone
here
might do that for you.

Or perhaps someone has already made a near-enough guess?

There is a question of strategy. Would it be better to add
resistance to the heater circuit after the feed to the HT,
and to add no resistance to that feed, thus raising the
floor of the HT circuit. Then add resistance to the HT
after
the doubler?

It would be a bit interesting to find the optimum
arrangement.


***
WOW ------ WAY, way to complicated.

Selenium rectifiers have a much higher voltage drop than
silicon
diodes. In *some* (but not all) circuits where the down-line
voltage
is critical, that drop must be replaced if the selenium
diodes are
replaced with silicon.

So far, so good.

Not all selenium rectifiers are created equal. Some are
single-
element, some are stacks of 2 - 20-or-more depending on the
current
they are to carry. The specific SIZE of the diode does not
matter, the
NUMBER OF ELEMENTS (stack) does matter relative to this
question.

Rule-of-thumb, all other things being equal (Ed's ballpark)
is 10 ohms
per element. After which additional resistance *may* be
desired to
further reduce B+ (or C- if a bias circuit) to make up for
higher line
(wallplate) voltage common in some areas these days.
Naturally, this
is to be verified in the field by actual measurement.

Ed was requesting ball-park numbers for a single-element
system. 10 -
20 ohms covers the from home plate to far-away center field.
Close
enough.

Sometimes a small brass hammer in the hands of a Vermont
Blacksmith is
all that is necessary.

***

The situation is no less complicated than I wrote. You may
believe that the complications don't matter, and Edward may
not care.

We don't have ball-parks here in England, but anyway, if
Edward asks what's the right ball-park voltage for his
heaters, would 35V be in the right field? How close is close
enough for Edward? We don't even know the voltage ratings of
his capacitors.

If the voltage drop across a selenium diode is truly "much"
greater than that of a silicon diode, and particularly if it
is a true diode drop rather than the result of inherent
series resistance, then the resistance necessary to restore
it will depend on the current through the diode. So if, say,
the current is 1A, then 10 ohms will result in a 10V
reduction, whereas if the current is 10mA, then 10 ohms will
result in a drop of only 100mV, each assuming DC or
sinusoidal current.

Now, the circuit in question includes a relatively
high-current heater supply, and a low-current HT supply, and
an even-lower-current link between the two. Further, the
heater supply uses a bridge rectifier, whereas the other two
are half-wave. Further still, the high-current supply is
relatively low voltage, so the greater voltage difference
will be a much more significant proportion of the supply
voltage. What is more, we don't know the specifications of
Edwards selenium diodes, although the knowledgeable may be
able to divine them from the voltages.

If 10-20 ohms is a good guess for one, it is unlikely to be
a good guess for the others, surely?

Included in your answer is the assumption that Edward
doesn't care too much about the consequent voltages. Maybe
you're right, and maybe they will be close enough for
reasonable operation.

In which case, considering 20 ohms will have so little
effect in the two lower-current situations, he could leave
them out and concentrate on getting the heater voltage
right.

What power rating should the added resistor(s) be, there,
and exactly where in the circuit should he put it, or them,
do you think?

A simulation would not be complicated at all, BTW, unless an
exact mean power dissipation is required because that's
awkward to do.

Perhaps Edward will report the results of his experiments.
I'm quite interested.


I wonder how you should dispose of the old rectifiers?
Authorities here are, quite rightly I suppose, getting
very
touchy about what goes in the bin. I noticed, passing a so
called "health food" shop the other day, that selenium is
essential for fitness, so perhaps you could grind them up
and eat them?


***
In our township, one may drop "hazardous waste" at the
township
transfer station for separate disposal without additional
charge. In
general, I collect this sort of stuff in an empty (clean)
paint can -
when it gets nearly full (perhaps once per year), I drop it
off at the
transfer station about 2 miles away. From there it is taken
to a
certified hazardous-waste landfill for proper disposal.

***

Probably the same here. I haven't managed to fill my jar
yet...just one dead valve and one extremely dead tantalum
capacitor. If there were a damaged selenium diode in there I
would want to get rid of it sooner.

Ian



  #18   Report Post  
Posted to rec.audio.tubes
Peter Wieck Peter Wieck is offline
external usenet poster
 
Posts: 2,418
Default B+ rectifier question

Ed:

Please note the interpolations:

On Dec 15, 2:26*pm, "Edward Morris" wrote:
Hey guys,
* * I appreciate all the input. *I replaced all the bumble bee capacitors.
I replaced all the electrolytic capacitors (cans if possible). *I couldn't
find the can for the filament circuit so I had to go with 3 separate
capacitors for that circuit. *I went with keeping the capacitor voltages the
same as the originals or at least close. *I replaced the resistors in the
power supply as well as the filament bridge which was originally 4 separate
diodes mounted on a strip and I replaced the original 2 diodes for the B+..
I have to run the preamp on a variac at 110v because the filament voltage
tested at each tube is right at 12 volts.


Actually, the filaments for a 12AX7 are 12.6V nominal. Running at 12V
will help tube life somewhat, and not materially affect peformance.

*My goal is to get the resistance
added necessary to be able to plug the preamp into the wall outlet about
123V but I also don't want to add too much more heat to the innards. *The
filament circuit pulls a lot of current. *I've got a 6 amp bridge for the
filament circuit and it runs warm. *The resistors (2-7.5 ohm 5 watt) run so
hot you can even touch them. *But they were that way before the new parts.


Too hot! Nothing should get that hot inside that amp. I know that in
many cases OEM equipment does have dropping resistors that do run
seriously hot - my preference would be to replace them with a higher
wattage resistors - but in this case, pay _special_ attention to
fusing. Heat and equipment are enemies. Heat and phenolic circuit
boards are enemies.

So, if you have put in silicon diodes and are running at 110V to get
12v at the filaments with 7.5 ohm resistors, I would *suggest* you go
with 20 ohms (additional 12.5 ohms) at 5-watts and see what you get at
124V. Again, you are working with single-element (originally) diodes,
so a massive amount of additional resistance over the OEM 7.5ohms will
not be needed.

The preamp at this point sounds great. *I've used it running from the variac
for a number of months now with no problems and the PT runs just a little
warm.


What is "just a little"? Egg-frying hot after a few hours/minutes/
ever?
Or warm-to-the-touch-but-no-more forever? T

The latter is what you want.

*I have a number of resistors to be able to experiment. *It's just
getting the thing unhooked and out of my cabinet. *I know I sound lazy. *I
guess I just don't like the idea of getting it out again. *It was a lot of
work at time to (rebuild it). *You guys know what I mean? *I have the
schematic so I know the recommended voltages. *I know just about enough
about electronics to be dangerous. *If you go towww.nosvalves.com/C22.htm
you can see the 2 grey B+ diodes at the left of the pictures. *I still have
them but they don't have stacks per se like I've seen some other selenium
rectifiers have. *Do you recommend for the B+ using resistance before the
diodes or after?


Always _after_ the diodes.

*Also the 2 HT wires Red and Red/Yellow; do I need to
install a resistor between the capacitor and the Red/Yellow wire; the
voltage is running a little high, or will reducing the B+ voltage take care
of that?


It should. But when you finally walk away from the amp, the voltage on
the caps should be at/slightly below the listed voltage(s), and B+
should be at or slightly below the listed voltage(s). But you would
like to work to as few points as possble, not have new resistors
spread all over the place. Further, be sure you document what you have
done so you or your successors do not have to re-invent the wheel each
time you service the amp in the future.

Peter Wieck
Melrose Park, PA
  #19   Report Post  
Posted to rec.audio.tubes
Edward Morris Edward Morris is offline
external usenet poster
 
Posts: 47
Default B+ rectifier question

Hi Peter,
I can leave the preamp on for hours on end and the PT only gets slightly
warm. You can hold your hand over it and it's not uncomfortable at all.
I'll do what you suggest and give you an update. I used an volt meter and
measured voltage from the chassis to ground. It's pretty low (around 8
volts) and just a little difference when changing the position of the plug
in the outlet. The preamp had 6 Telefunken 12AX7's and I replaced them with
Sovtek 12AX7LPS. Do you think the Sovtek's require more filament current
than the Telefunken? I guess now it a matter of getting the voltages lower
so I can safely use it without a variac.

Thank to all,
Edward Morris

"Peter Wieck" wrote in message
...
Ed:

Please note the interpolations:

On Dec 15, 2:26 pm, "Edward Morris" wrote:
Hey guys,
I appreciate all the input. I replaced all the bumble bee capacitors.
I replaced all the electrolytic capacitors (cans if possible). I couldn't
find the can for the filament circuit so I had to go with 3 separate
capacitors for that circuit. I went with keeping the capacitor voltages
the
same as the originals or at least close. I replaced the resistors in the
power supply as well as the filament bridge which was originally 4
separate
diodes mounted on a strip and I replaced the original 2 diodes for the B+.
I have to run the preamp on a variac at 110v because the filament voltage
tested at each tube is right at 12 volts.


Actually, the filaments for a 12AX7 are 12.6V nominal. Running at 12V
will help tube life somewhat, and not materially affect peformance.

My goal is to get the resistance
added necessary to be able to plug the preamp into the wall outlet about
123V but I also don't want to add too much more heat to the innards. The
filament circuit pulls a lot of current. I've got a 6 amp bridge for the
filament circuit and it runs warm. The resistors (2-7.5 ohm 5 watt) run so
hot you can even touch them. But they were that way before the new parts.


Too hot! Nothing should get that hot inside that amp. I know that in
many cases OEM equipment does have dropping resistors that do run
seriously hot - my preference would be to replace them with a higher
wattage resistors - but in this case, pay _special_ attention to
fusing. Heat and equipment are enemies. Heat and phenolic circuit
boards are enemies.

So, if you have put in silicon diodes and are running at 110V to get
12v at the filaments with 7.5 ohm resistors, I would *suggest* you go
with 20 ohms (additional 12.5 ohms) at 5-watts and see what you get at
124V. Again, you are working with single-element (originally) diodes,
so a massive amount of additional resistance over the OEM 7.5ohms will
not be needed.

The preamp at this point sounds great. I've used it running from the
variac
for a number of months now with no problems and the PT runs just a little
warm.


What is "just a little"? Egg-frying hot after a few hours/minutes/
ever?
Or warm-to-the-touch-but-no-more forever? T

The latter is what you want.

I have a number of resistors to be able to experiment. It's just
getting the thing unhooked and out of my cabinet. I know I sound lazy. I
guess I just don't like the idea of getting it out again. It was a lot of
work at time to (rebuild it). You guys know what I mean? I have the
schematic so I know the recommended voltages. I know just about enough
about electronics to be dangerous. If you go towww.nosvalves.com/C22.htm
you can see the 2 grey B+ diodes at the left of the pictures. I still have
them but they don't have stacks per se like I've seen some other selenium
rectifiers have. Do you recommend for the B+ using resistance before the
diodes or after?


Always _after_ the diodes.

Also the 2 HT wires Red and Red/Yellow; do I need to
install a resistor between the capacitor and the Red/Yellow wire; the
voltage is running a little high, or will reducing the B+ voltage take
care
of that?


It should. But when you finally walk away from the amp, the voltage on
the caps should be at/slightly below the listed voltage(s), and B+
should be at or slightly below the listed voltage(s). But you would
like to work to as few points as possble, not have new resistors
spread all over the place. Further, be sure you document what you have
done so you or your successors do not have to re-invent the wheel each
time you service the amp in the future.

Peter Wieck
Melrose Park, PA

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Edward Morris Edward Morris is offline
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Posts: 47
Default B+ rectifier question

Hello,
I used 330 ohm resistors for each of the replacement B+ diodes and the
highest B+ voltage is just under 4 volts from the schematic and I replaced a
7.5 ohm 5 watt with a 14 ohm 5 watt resistor in the filament supply and the
filaments are running 12 volts.

Thanks everyone for all your help,
Edward

"flipper" wrote in message
...
On Mon, 15 Dec 2008 17:10:28 -0000, "Ian Iveson"
wrote:

Peter Wieck wrote:

Just been trying to find out about selenium rectifiers. It
seems they can have several cells in series, and each cell
can vary in several ways. The voltage drop can be
measured,
or could have been estimated from an appropriate data
sheet
which is unlikely to be available now. If you can measure
the drop across and current through each one, you can get
an
idea of what resistor you would need in series with each,
using ohm's law. The waveforms aren't sinusoidal, though,
so
you would need to measure the two final voltages and
adjust
to suit, depending on how exact they need to be. You would
also need to be careful with power ratings, for the same
reason.

A much easier way is to measure the resistance and
open-circuit voltage of the transformer secondaries, and
then simulate the circuit with SPICE. Especially if you
don't happen to have a stock of real resistors to try.

If you can measure but are unable to simulate, someone
here
might do that for you.

Or perhaps someone has already made a near-enough guess?

There is a question of strategy. Would it be better to add
resistance to the heater circuit after the feed to the HT,
and to add no resistance to that feed, thus raising the
floor of the HT circuit. Then add resistance to the HT
after
the doubler?

It would be a bit interesting to find the optimum
arrangement.


***
WOW ------ WAY, way to complicated.

Selenium rectifiers have a much higher voltage drop than
silicon
diodes. In *some* (but not all) circuits where the down-line
voltage
is critical, that drop must be replaced if the selenium
diodes are
replaced with silicon.

So far, so good.

Not all selenium rectifiers are created equal. Some are
single-
element, some are stacks of 2 - 20-or-more depending on the
current
they are to carry. The specific SIZE of the diode does not
matter, the
NUMBER OF ELEMENTS (stack) does matter relative to this
question.

Rule-of-thumb, all other things being equal (Ed's ballpark)
is 10 ohms
per element. After which additional resistance *may* be
desired to
further reduce B+ (or C- if a bias circuit) to make up for
higher line
(wallplate) voltage common in some areas these days.
Naturally, this
is to be verified in the field by actual measurement.

Ed was requesting ball-park numbers for a single-element
system. 10 -
20 ohms covers the from home plate to far-away center field.
Close
enough.

Sometimes a small brass hammer in the hands of a Vermont
Blacksmith is
all that is necessary.

***

The situation is no less complicated than I wrote. You may
believe that the complications don't matter, and Edward may
not care.

We don't have ball-parks here in England, but anyway, if
Edward asks what's the right ball-park voltage for his
heaters, would 35V be in the right field? How close is close
enough for Edward? We don't even know the voltage ratings of
his capacitors.

If the voltage drop across a selenium diode is truly "much"
greater than that of a silicon diode, and particularly if it
is a true diode drop rather than the result of inherent
series resistance, then the resistance necessary to restore
it will depend on the current through the diode. So if, say,
the current is 1A, then 10 ohms will result in a 10V
reduction, whereas if the current is 10mA, then 10 ohms will
result in a drop of only 100mV, each assuming DC or
sinusoidal current.

Now, the circuit in question includes a relatively
high-current heater supply, and a low-current HT supply, and
an even-lower-current link between the two. Further, the
heater supply uses a bridge rectifier, whereas the other two
are half-wave. Further still, the high-current supply is
relatively low voltage, so the greater voltage difference
will be a much more significant proportion of the supply
voltage. What is more, we don't know the specifications of
Edwards selenium diodes, although the knowledgeable may be
able to divine them from the voltages.

If 10-20 ohms is a good guess for one, it is unlikely to be
a good guess for the others, surely?

Included in your answer is the assumption that Edward
doesn't care too much about the consequent voltages. Maybe
you're right, and maybe they will be close enough for
reasonable operation.

In which case, considering 20 ohms will have so little
effect in the two lower-current situations, he could leave
them out and concentrate on getting the heater voltage
right.

What power rating should the added resistor(s) be, there,
and exactly where in the circuit should he put it, or them,
do you think?

A simulation would not be complicated at all, BTW, unless an
exact mean power dissipation is required because that's
awkward to do.

Perhaps Edward will report the results of his experiments.
I'm quite interested.


Sounds to me like there are a number of assumptions but, first...

Individual selenium cells are only good for, depending on the
particular unit, between 20V and 45V reverse voltage and that's the
reason for the stacking. I think 20V was the most common value back
then.

The metal plate inherent to the construction acts as a heatsink and
the 'rule of thumb' I heard was about 500mA per sq inch.

The break over knee actually looks a bit lower per cell (about .45V)
than your typical silicon diode but, then, you seldom have one cell in
HV rectifiers because of the low reverse breakdown voltage.

Impedance in the 500mA area looks to be between .67 and 1 ohm per
cell.

Also, by silicon standards they're quite leaky, passing as much as 6mA
in reverse at the rated voltage.

I wonder how you should dispose of the old rectifiers?
Authorities here are, quite rightly I suppose, getting
very
touchy about what goes in the bin. I noticed, passing a so
called "health food" shop the other day, that selenium is
essential for fitness, so perhaps you could grind them up
and eat them?


***
In our township, one may drop "hazardous waste" at the
township
transfer station for separate disposal without additional
charge. In
general, I collect this sort of stuff in an empty (clean)
paint can -
when it gets nearly full (perhaps once per year), I drop it
off at the
transfer station about 2 miles away. From there it is taken
to a
certified hazardous-waste landfill for proper disposal.

***

Probably the same here. I haven't managed to fill my jar
yet...just one dead valve and one extremely dead tantalum
capacitor. If there were a damaged selenium diode in there I
would want to get rid of it sooner.

Ian



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