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Gareth Magennis Gareth Magennis is offline
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Default Diodes on Cathode?

Hi,

what is the purpose of the reverse biased diodes on the OPT/Cathodes as in
this Fender circuit?

http://www.schematicheaven.com/fende...er_60_rack.pdf

I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can I
make up the 3kV diode required using standard 1Kv rectifier types?


Thanks,


Gareth.


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John Byrns John Byrns is offline
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Default Diodes on Cathode?

In article ,
"Gareth Magennis" wrote:

Hi,

what is the purpose of the reverse biased diodes on the OPT/Cathodes as in
this Fender circuit?

http://www.schematicheaven.com/fende...er_60_rack.pdf

I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can I
make up the 3kV diode required using standard 1Kv rectifier types?


I don't see any reversed biased diodes on the OPT/Cathodes, do you mean
the reversed biased diodes on the OPT/Anodes?

I believe the purpose of these diodes is to clamp the peak anode voltage
at twice the B+ voltage, limiting the magnitude of the high voltage
spikes that can be produced when the amp is overdriven into an inductive
load. These voltage spikes can destroy tubes and transformers if not
controlled.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Phil Allison Phil Allison is offline
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Default Diodes on Cathode?


"Gareth Magennis"
Hi,

what is the purpose of the reverse biased diodes on the OPT/Cathodes as in
this Fender circuit?



** They are not simple diodes, nor are they on the cathodes.

They ARE diode strings wired from each plate to ground.


http://www.schematicheaven.com/fende...er_60_rack.pdf



** The two " R3000 " prevent the plate voltage from swinging negative by
more than 2 volts or so - something all tube amps do when overdriven into a
reactive ( ie speaker) load.

Peak negative plate voltage can exceed 2000 volts and damage the insulation
of the tube, the socket or the OT.


I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can
I make up the 3kV diode required using standard 1Kv rectifier types?



** The later is generally OK.

Use " fast " diodes, if possible



........ Phil






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Sander deWaal Sander deWaal is offline
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Default Diodes on Cathode?

"Gareth Magennis" said:


what is the purpose of the reverse biased diodes on the OPT/Cathodes as in
this Fender circuit?


http://www.schematicheaven.com/fende...er_60_rack.pdf


I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can I
make up the 3kV diode required using standard 1Kv rectifier types?



I use UF4007 or UF5408 diodes for that purpose, 3 in series.
Don't use 1N4007 or 1N5408, they're too slow for this application.

The reason why they are there is already explained by Phil Allison.

--

- Maggies are an addiction for life. -
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Patrick Turner Patrick Turner is offline
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Default Diodes on Cathode?



Gareth Magennis wrote:

Hi,

what is the purpose of the reverse biased diodes on the OPT/Cathodes as in
this Fender circuit?

http://www.schematicheaven.com/fende...er_60_rack.pdf

I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can I
make up the 3kV diode required using standard 1Kv rectifier types?


The diodes prevent the anodes ever swinging below 0V when no load ins
connected and the
volume is turnd up.
Without the diodes, the OPT primary peak signal voltage can be up to
say +2,000V on one side, and -1,000V on the other OPT side.
This can cause arcing in the OPT and the windings to fuse.

If you wish to use alternative diodes, i suggest 3 seriesed IN5408 which
are each PIV rated for 1,000V,
have 3amp rating, and you should have 1M resistances across each diode.
Then the voltage max at one of the OPT can only be +1,000V and -2.1V at
the other,
and arcing won't happen.

Patrick Turner.





Thanks,

Gareth.



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Patrick Turner Patrick Turner is offline
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Default Diodes on Cathode?



John Byrns wrote:

In article ,
"Gareth Magennis" wrote:

Hi,

what is the purpose of the reverse biased diodes on the OPT/Cathodes as in
this Fender circuit?

http://www.schematicheaven.com/fende...er_60_rack.pdf

I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can I
make up the 3kV diode required using standard 1Kv rectifier types?


I don't see any reversed biased diodes on the OPT/Cathodes, do you mean
the reversed biased diodes on the OPT/Anodes?

I believe the purpose of these diodes is to clamp the peak anode voltage
at twice the B+ voltage, limiting the magnitude of the high voltage
spikes that can be produced when the amp is overdriven into an inductive
load. These voltage spikes can destroy tubes and transformers if not
controlled.

Regards,

John Byrns


Your'e right.

Patrick Turner.

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

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Iain Churches Iain Churches is offline
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Default Diodes on Cathode?


"Phil Allison" wrote in message
...

"Gareth Magennis"
Hi,

what is the purpose of the reverse biased diodes on the OPT/Cathodes as
in this Fender circuit?



** They are not simple diodes, nor are they on the cathodes.

They ARE diode strings wired from each plate to ground.


http://www.schematicheaven.com/fende...er_60_rack.pdf



** The two " R3000 " prevent the plate voltage from swinging negative by
more than 2 volts or so - something all tube amps do when overdriven into
a reactive ( ie speaker) load.

Peak negative plate voltage can exceed 2000 volts and damage the
insulation of the tube, the socket or the OT.


I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can
I make up the 3kV diode required using standard 1Kv rectifier types?



** The later is generally OK.

Use " fast " diodes, if possible


Guitar amp technicians seem to refer to them as "flyback" diodes.
This may or may not be a general term.

Iain





....... Phil








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Chris Hornbeck Chris Hornbeck is offline
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Default Diodes on Cathode?

On Mon, 11 Jun 2007 22:26:51 +0300, "Iain Churches"
wrote:

Guitar amp technicians seem to refer to them as "flyback" diodes.
This may or may not be a general term.


That's an excellent choice of a name. It comes from old TV
receiver practice, where the horizontal sweep output transformer
was combined with a high-ratio step-up "secondary" and rectified
to make the multi-KiloJolts needed for CRT anodes.

The transformer itself (later with internal high voltage
rectifiers) was called the "flyback" transformer, because
its primary was shunted with a "reverse" biased diode,
whose purpose was to fly the flying spot back from right
to left.

The vacuum valve diodes used for this gig in their day are
very popular now as slow-heating rectifiers with amazingly
large heater-cathode voltage ratings (unidirectional! arf.).
They're commonly called damper diodes.

Much thanks, as always,

Chris Hornbeck
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Gareth Magennis Gareth Magennis is offline
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Default Diodes on Cathode?


"Gareth Magennis" wrote in message
...
Hi,

what is the purpose of the reverse biased diodes on the OPT/Cathodes as in
this Fender circuit?

http://www.schematicheaven.com/fende...er_60_rack.pdf

I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can
I make up the 3kV diode required using standard 1Kv rectifier types?


Thanks,


Gareth.



Thanks for all the information and added confidence.


(Incidentally, I had measured some good R3000's from another amp which
showed a higher than single PN junction Vf - Phil has given the reason why).




Cheers,


Gareth.


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Iain Churches Iain Churches is offline
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Default Diodes on Cathode?


"Chris Hornbeck" wrote in message
...
On Mon, 11 Jun 2007 22:26:51 +0300, "Iain Churches"
wrote:

Guitar amp technicians seem to refer to them as "flyback" diodes.
This may or may not be a general term.


That's an excellent choice of a name. It comes from old TV
receiver practice, where the horizontal sweep output transformer
was combined with a high-ratio step-up "secondary" and rectified
to make the multi-KiloJolts needed for CRT anodes.

The transformer itself (later with internal high voltage
rectifiers) was called the "flyback" transformer, because
its primary was shunted with a "reverse" biased diode,
whose purpose was to fly the flying spot back from right
to left.

The vacuum valve diodes used for this gig in their day are
very popular now as slow-heating rectifiers with amazingly
large heater-cathode voltage ratings (unidirectional! arf.).
They're commonly called damper diodes.

Chris. Interesting to know from where the term has originated.
I was not sure if "flyback" was a general term,. as here in Scandinavia
Swedish, Finnish and also Danish technicians seem to intersperse
their vocabulary with English language expressions that may or may
not mean the same thing in the US. Sometimes ago there was some
confusion over the term "common cathode", for example.

Best regards
Iain





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Ian Iveson Ian Iveson is offline
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Default Diodes on Cathode?

Iain wrote

Interesting to know from where the term has originated.
I was not sure if "flyback" was a general term,. as here in
Scandinavia
Swedish, Finnish and also Danish technicians seem to intersperse
their vocabulary with English language expressions that may or may
not mean the same thing in the US. Sometimes ago there was some
confusion over the term "common cathode", for example.


Maybe in Europe other names were used for the TV circuit. See

http://www.repairfaq.org/sam/flytest.htm#flyfly

It now has common currency as a form of switch-mode power supply,
which uses a circuit similar to the TV application.

Possibly the origin outside the US and the UK seems mysterious.

Ian


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robert casey robert casey is offline
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Default Diodes on Cathode?



I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can I
make up the 3kV diode required using standard 1Kv rectifier types?



Looks like the circuit is expecting the diode to start conducting when
the voltage gets higher than the 3KV, so a generic replacement might not
cut it.

I've seen (in solid state "hot chassis" radios, yes, those were made
back in the early 70's) neon bulbs (with a resistor about 5K in series)
placed across the primary of the audio output transformer. Presumably
to limit voltage excursions, to avoid blowing the output transistor up.
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John Byrns John Byrns is offline
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Default Diodes on Cathode?

In article ,
robert casey wrote:


I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can I
make up the 3kV diode required using standard 1Kv rectifier types?



Looks like the circuit is expecting the diode to start conducting when
the voltage gets higher than the 3KV, so a generic replacement might not
cut it.

I've seen (in solid state "hot chassis" radios, yes, those were made
back in the early 70's) neon bulbs (with a resistor about 5K in series)
placed across the primary of the audio output transformer.


These were not limited to the "early 70's", they were made at least as
far back as the mid 1960s.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Phil Allison Phil Allison is offline
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Default Diodes on Cathode?


"robert casey"

Looks like the circuit is expecting the diode to start conducting when the
voltage gets higher than the 3KV,



** If the R3000 diode (string) ever conducts in the reverse direction -
that is the end of the diode.

It is NOT a damn zener - you clot - the voltage limiting effect works by
transformer action.


BTW:

There are in fact several ways in that Fender amp the R3000s CAN cop more
than 3 kV in reverse, both at the same time.

Then it is kaput for one of them.

See if you can figure even one of them out.



......... Phil


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John Byrns John Byrns is offline
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Default Diodes on Cathode?

In article ,
"Phil Allison" wrote:

"robert casey"

Looks like the circuit is expecting the diode to start conducting when the
voltage gets higher than the 3KV,



** If the R3000 diode (string) ever conducts in the reverse direction -
that is the end of the diode.

It is NOT a damn zener - you clot - the voltage limiting effect works by
transformer action.


BTW:

There are in fact several ways in that Fender amp the R3000s CAN cop more
than 3 kV in reverse, both at the same time.

Then it is kaput for one of them.

See if you can figure even one of them out.


Leakage inductance between the two primary halves is one of them, I am
curious what the others are?


Regards,

John Byrns

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


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Chris Hornbeck Chris Hornbeck is offline
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Default Diodes on Cathode?

On Wed, 13 Jun 2007 02:44:09 GMT, John Byrns
wrote:

Leakage inductance between the two primary halves is one of them, I am
curious what the others are?


dI/dT can make some serious voltage spikes, but might require
either major overdrive or oscillation.

In a gitfiddle amplifier? Nah... Never happen.

Much thanks, as always,

Chris Hornbeck
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robert casey robert casey is offline
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Default Diodes on Cathode?



** If the R3000 diode (string) ever conducts in the reverse direction -
that is the end of the diode.

It is NOT a damn zener - you clot - the voltage limiting effect works by
transformer action.



Duh! Of course! It's the diode on the other side of the transformer
that does the clamping by going forward conduction. :-)
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Phil Allison Phil Allison is offline
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Default Diodes on Cathode?


"robert casey"


** If the R3000 diode (string) ever conducts in the reverse direction -
that is the end of the diode.

It is NOT a damn zener - you clot - the voltage limiting effect works
by transformer action.



Duh! Of course! It's the diode on the other side of the transformer that
does the clamping by going forward conduction. :-)



** I can see that light bulb on top of your head is now brightly lit !!

Or is it maybe in your mouth, like " Uncle Fester ".



........ Phil


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Patrick Turner Patrick Turner is offline
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Default Diodes on Cathode?



John Byrns wrote:

In article ,
"Phil Allison" wrote:

"robert casey"

Looks like the circuit is expecting the diode to start conducting when the
voltage gets higher than the 3KV,



** If the R3000 diode (string) ever conducts in the reverse direction -
that is the end of the diode.

It is NOT a damn zener - you clot - the voltage limiting effect works by
transformer action.


BTW:

There are in fact several ways in that Fender amp the R3000s CAN cop more
than 3 kV in reverse, both at the same time.

Then it is kaput for one of them.

See if you can figure even one of them out.


Leakage inductance between the two primary halves is one of them, I am
curious what the others are?


Speakers are inductive loads at HF, especially in guitar amps where
there is never any attempt made to place an impedance equalization Zobel
across the speaker.
A pair of Zobel networks are never placed network where they are
usually most effective, across each 1/2 of the primary on the OPT.

So a stray bit of noise with F content up to 20kHz can produce
high voltages at the anodes of a guitar amp in pentode mode because the
load
at the sec becomes high, as well as being decoupled from the primary
because of the
leakage L.
However, the high Ra of the pentoads and the shunt C of the OPT DOES
tend to shunt the
extreme HF gain of the pentoad.

I like to see a pair of Zobels beginning to load the output tubes at
20kHz at least.

I have often included the anode to 0V diodes to limit transformer
primary voltage excursions
with no load and have had no trouble with 3 seriesed diodes with PIV =
1,000V each.

The highest voltages geberated in an unloaded OPT seem to be with
pentoad amps,
and about the highest voltages I have seen are up to about 3 times the
maximum
Va-a with rated load at clipping.

So with an amp making 100 watts into 5k, this is 707Vrms, and with no
load
I have seen maybe 2,200Vrms a-a.

UL will also produce high no load voltages, and triodes not so much.

The amount of loading needed to prevent the strange non-commonsense
phenomena observed
when an amp continues to produce a really high output voltage with no
load
isn't anything like the rated load; it appears the load to prevent
more than 10% above high back-emf no load voltages is an RL of many more
ohm
than the rated normal load, but just what the general rule for such a
load is I don't know.
I've never made the trials to find out.

Some older amp schematics show what look to be a silly amount of R
across the output,
say 100 ohms, for where the load might normally be 16 ohms.
There is never a reason given for the 100 ohms, but perhaps its is to
guard against the
back emf voltage production with no load caused by the cut off of
current in one pentode
slowly turning off
while the other one which conducts until the anode Va goes down to 0V
and then it too cuts off sharply
because its anode becomes negative.

A loading of 100 ohms would not cause a huge power loss. 100W into 16
ohms = 40Vrms,
and this voltage across 100 ohms = 16 watts.
Perhaps 220 ohms would be OK, and the extra reliablity would be much
worth
the few watts wasted in the permamnent high ohm load across the output.
A better place to put such a load of say 33k would be across each 1/2
primary of the OPT.

People are welcome to investigate further, **cautiously!** and perhaps
not have
use any diodes.

But because the diodes don't waste any power, they are in favour.

Patrick Turner.



Regards,

John Byrns

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

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robert casey robert casey is offline
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Default Diodes on Cathode?


Or is it maybe in your mouth, like " Uncle Fester ".


:-) Good one!
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