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  #1   Report Post  
Bryan McGivney
 
Posts: n/a
Default Old amp reconditioning and mods questions

I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube
amp in it and I wanted to pirate the amp for a fun project. Well I pulled it
out of the console and fired it up and it hummed badly. It has two multi
part can style caps on the chassis. I found caps at the local supply house
and wired them in below the chassis to the terminals on the original caps.
The amp sounds fine now but I figured I'd ask if there are any issues with
not having removed the original caps from the circuit? I hooked the caps up
with what I think is about 18ga speaker wire, with the copper conductor on
one side and silver conductor on the other. This shouldn't cause any issues
should it?

How hot might one expect the power transformer to get? I can touch it but
it's hot enough you don't want to hold you finger on it long. My guess is
around 110 - 120 degrees.

I have seen reference to adding a modern power socket like a computer would
use and using the 3 prong grounded cord, grounding the chassis to help
reduce hum or noise. Is this usually OK or might I somehow get myself into
trouble doing this?

I want to make a wooden enclosure for this with a metal screen top or
something like that. It seems that 17" is about the width of most
components?

Reading the comments along with new tubes it looks like JJ/tesla might be
the winner of not being the cheapest or the most expensive, just inexpensive
yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well
or might I consider others?

The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There
are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8
ohm? I think the volume goes up a bit in the 1 positions.

Are there voltages I should be checking or stuff like that?

There is a little pot on the back marked HUM what should I do with this
adjustment?

Well it's late and I am sure there may be more questions but that's all I
can think of for now. More than enough for one post anyways.

TIA,
Bryan


  #2   Report Post  
Mikkel C. Simonsen
 
Posts: n/a
Default

Bryan McGivney wrote:

I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube
amp in it and I wanted to pirate the amp for a fun project. Well I pulled it
out of the console and fired it up and it hummed badly. It has two multi
part can style caps on the chassis. I found caps at the local supply house
and wired them in below the chassis to the terminals on the original caps.
The amp sounds fine now but I figured I'd ask if there are any issues with
not having removed the original caps from the circuit? I hooked the caps up
with what I think is about 18ga speaker wire, with the copper conductor on
one side and silver conductor on the other. This shouldn't cause any issues
should it?


If the original capacitor decides to short, you may have a problem. I
would disconnect the original capacitor.

How hot might one expect the power transformer to get? I can touch it but
it's hot enough you don't want to hold you finger on it long. My guess is
around 110 - 120 degrees.


Hot is normal. What type of degrees are you talking about?

I have seen reference to adding a modern power socket like a computer would
use and using the 3 prong grounded cord, grounding the chassis to help
reduce hum or noise. Is this usually OK or might I somehow get myself into
trouble doing this?


I don't think it will reduce noise or hum, but it will increase
safety...

I want to make a wooden enclosure for this with a metal screen top or
something like that. It seems that 17" is about the width of most
components?


Most equipment is in the 42-43.5cm range.

Reading the comments along with new tubes it looks like JJ/tesla might be
the winner of not being the cheapest or the most expensive, just inexpensive
yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well
or might I consider others?


Any new tubes should work fine. But the 12AX7 tubes will most likely
work fine for the next 50 years, so I would just leave them. The 6BQ5
may be worn out - just buy a new pair and see if you sound gets better
or worse.

The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There
are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8
ohm? I think the volume goes up a bit in the 1 positions.

Are there voltages I should be checking or stuff like that?


Check the voltage on grid 1 of the 6BQ5 tubes. It should be either 0V or
negative. If it's positive you have bad coupling caps that should be
replaced. Do you have a schematic?

There is a little pot on the back marked HUM what should I do with this
adjustment?


Turn up the volume with no signal and adjust for minimum hum.

Best regards,

Mikkel C. Simonsen
  #3   Report Post  
Nothing40
 
Posts: n/a
Default

"Bryan McGivney" wrote in message ...
I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube
amp in it and I wanted to pirate the amp for a fun project. Well I pulled it
out of the console and fired it up and it hummed badly. It has two multi
part can style caps on the chassis. I found caps at the local supply house
and wired them in below the chassis to the terminals on the original caps.
The amp sounds fine now but I figured I'd ask if there are any issues with
not having removed the original caps from the circuit? I hooked the caps up
with what I think is about 18ga speaker wire, with the copper conductor on
one side and silver conductor on the other. This shouldn't cause any issues
should it?

How hot might one expect the power transformer to get? I can touch it but
it's hot enough you don't want to hold you finger on it long. My guess is
around 110 - 120 degrees.

I have seen reference to adding a modern power socket like a computer would
use and using the 3 prong grounded cord, grounding the chassis to help
reduce hum or noise. Is this usually OK or might I somehow get myself into
trouble doing this?

I want to make a wooden enclosure for this with a metal screen top or
something like that. It seems that 17" is about the width of most
components?

Reading the comments along with new tubes it looks like JJ/tesla might be
the winner of not being the cheapest or the most expensive, just inexpensive
yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well
or might I consider others?

The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There
are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8
ohm? I think the volume goes up a bit in the 1 positions.

Are there voltages I should be checking or stuff like that?

There is a little pot on the back marked HUM what should I do with this
adjustment?

Well it's late and I am sure there may be more questions but that's all I
can think of for now. More than enough for one post anyways.

TIA,
Bryan




I would remove the original caps,and maybe mount the new ones in
thier place,under the chassis is fine also.The old caps may be leaky
(electricaly) and drawing excess current from the power tranny.(making
it run warmer.) Make sure your new caps are close to the same
values,and rated for the voltage,or higher.
You might want to use wire rated at 600V..speaker wire might not cut
it at B+ voltages.
Those JJ tubes should work fine for ya. I think i have a pair in some
SE 6BQ5 amps myself. ;-)
The HUM control is just that.:-) Adjust it for the least amount of
hum. (I believe you'll want to unplug the input first.)

Good luck!
  #4   Report Post  
Bryan McGivney
 
Posts: n/a
Default

As for the tubes in it now, they sound pretty good but are mismatched, two
different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I
figured I might as well replace them while I was at it since they don't cost
all that much. Sort of a continuity of brands sort of thing more than
anything else.

Another question though, the phono input worked but it was very quiet and
sort of thin sounding. I don't have a schematic for the amp. I am guessing
that one 12AX7 is the phono stage and the other is the preamp stage. What
parts should I check first to get the phono stage working?

Do the caps I replaced need to run in for a bit before the sound will be
optimal?

TIA,
Bryan

"Fred Nachbaur" wrote in message
news:jI6_a.22$zE1.13@edtnps84...


Bryan McGivney wrote:
I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a

tube
amp in it and I wanted to pirate the amp for a fun project. Well I

pulled it
out of the console and fired it up and it hummed badly. It has two multi
part can style caps on the chassis. I found caps at the local supply

house
and wired them in below the chassis to the terminals on the original

caps.
The amp sounds fine now but I figured I'd ask if there are any issues

with
not having removed the original caps from the circuit?


There might be down the road, if the old cap decides to short. That
being said, this used to be a pretty common service practise, and I've
never heard of a problem because of it. In my old age I'm getting
marginally more careful about stuff like this, so I think I'd physically
disconnect the old caps and put in a couple small terminal strips to
which to wire the new ones.

I hooked the caps up
with what I think is about 18ga speaker wire, with the copper conductor

on
one side and silver conductor on the other. This shouldn't cause any

issues
should it?


Not other than being kind of ugly. ;-)

How hot might one expect the power transformer to get? I can touch it

but
it's hot enough you don't want to hold you finger on it long. My guess

is
around 110 - 120 degrees.


That sounds about right.

I have seen reference to adding a modern power socket like a computer

would
use and using the 3 prong grounded cord, grounding the chassis to help
reduce hum or noise. Is this usually OK or might I somehow get myself

into
trouble doing this?


You'll only get yourself in trouble if you wire it improperly. ;-)

I want to make a wooden enclosure for this with a metal screen top or
something like that. It seems that 17" is about the width of most
components?


Yes.

Reading the comments along with new tubes it looks like JJ/tesla might

be
the winner of not being the cheapest or the most expensive, just

inexpensive
yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work

well
or might I consider others?


What's wrong with the ones that are in there?

The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2.

There
are only one set of speaker outputs. Is this maybe a setting for 4 ohm

and 8
ohm? I think the volume goes up a bit in the 1 positions.


It's *probably* meant for 8-ohm speakers. You can measure the DC
resistance of the speakers that came in the console to get a pretty good
idea. 8-ohm speakers will usually measure around 6 ohms DC resistance or
thereabouts.

Are there voltages I should be checking or stuff like that?


Wouldn't be a bad idea (and would be educational also) if you have the
schematic showing the typical voltages at various points.

There is a little pot on the back marked HUM what should I do with this
adjustment?


Adjust for minimum hum.

Well it's late and I am sure there may be more questions but that's all

I
can think of for now. More than enough for one post anyways.


Have fun. Be careful. Enjoy the music!

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+



  #5   Report Post  
Fred Nachbaur
 
Posts: n/a
Default



Bryan McGivney wrote:
As for the tubes in it now, they sound pretty good but are mismatched, two
different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I
figured I might as well replace them while I was at it since they don't cost
all that much. Sort of a continuity of brands sort of thing more than
anything else.


Gotcha. One thing to check for - on the 12AX7's, look at the base, in
the center of the ring of pins. If you see a little diamond shape
hang on to them, they're Telefunkens. (These are considered "premium",
and are notorious for having the lettering rub off.)

Another question though, the phono input worked but it was very quiet and
sort of thin sounding. I don't have a schematic for the amp. I am guessing
that one 12AX7 is the phono stage and the other is the preamp stage. What
parts should I check first to get the phono stage working?


Your phono stage is *probably* working fine, but more info is needed to
be sure.

Are you using the turntable and cartridge that came with the console? If
so, and the sound is weak and thin, the cartridge is probably no good. *

If you're using a turntable with a magnetic cartridge, you've got a
different problem entirely.

* The phonographs provided with those old consoles were of the "crystal"
or "ceramic" type. The crystal type (usually you can tell those because
they're fairly large and have an aluminum "pan" under them) dry out over
the years and therefore sound weak and tinny. The ceramic type was less
prone to that problem, but can crack due to abuse - with the same result.

Both of these "piezo-electric" types of cartridges had low compliance
(i.e. they chew up records), high output (requiring little
preamplification), and needed no de-emphasis (cheap). That's why when
you play a magnetic cartridge into those inputs, it sounds quiet (more
gain needed) and thin/treble-y (needs de-emphasis). You need an outboard
"Magnetic phono preamplifier" to use with this amp, which will
accomplish both the gain and de-emphasis.

Do the caps I replaced need to run in for a bit before the sound will be
optimal?


I don't think so, but some people claim to be able to hear a difference
between new capacitors and ones that have "broken in."

In either case, I wouldn't worry about it. Enjoy the music if the sound
is good, fix it if it's not.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+



  #6   Report Post  
Jimmy
 
Posts: n/a
Default


"Bryan McGivney" wrote in message
...
As for the tubes in it now, they sound pretty good but are mismatched, two
different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I
figured I might as well replace them while I was at it since they don't

cost
all that much. Sort of a continuity of brands sort of thing more than
anything else.

Another question though, the phono input worked but it was very quiet and
sort of thin sounding. I don't have a schematic for the amp. I am guessing
that one 12AX7 is the phono stage and the other is the preamp stage. What
parts should I check first to get the phono stage working?

Do the caps I replaced need to run in for a bit before the sound will be
optimal?


I bet you either have the wrong catridge or a bad cartridge in your
phonograph. You probably should have a crystal cartridge. Either it has been
replaced with a magnetic one or it is just plain bad.

TIA,
Bryan

"Fred Nachbaur" wrote in message
news:jI6_a.22$zE1.13@edtnps84...


Bryan McGivney wrote:
I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a

tube
amp in it and I wanted to pirate the amp for a fun project. Well I

pulled it
out of the console and fired it up and it hummed badly. It has two

multi
part can style caps on the chassis. I found caps at the local supply

house
and wired them in below the chassis to the terminals on the original

caps.
The amp sounds fine now but I figured I'd ask if there are any issues

with
not having removed the original caps from the circuit?


There might be down the road, if the old cap decides to short. That
being said, this used to be a pretty common service practise, and I've
never heard of a problem because of it. In my old age I'm getting
marginally more careful about stuff like this, so I think I'd physically


disconnect the old caps and put in a couple small terminal strips to
which to wire the new ones.

I hooked the caps up
with what I think is about 18ga speaker wire, with the copper

conductor
on
one side and silver conductor on the other. This shouldn't cause any

issues
should it?


Not other than being kind of ugly. ;-)

How hot might one expect the power transformer to get? I can touch it

but
it's hot enough you don't want to hold you finger on it long. My guess

is
around 110 - 120 degrees.


That sounds about right.

I have seen reference to adding a modern power socket like a computer

would
use and using the 3 prong grounded cord, grounding the chassis to help
reduce hum or noise. Is this usually OK or might I somehow get myself

into
trouble doing this?


You'll only get yourself in trouble if you wire it improperly. ;-)

I want to make a wooden enclosure for this with a metal screen top or
something like that. It seems that 17" is about the width of most
components?


Yes.

Reading the comments along with new tubes it looks like JJ/tesla might

be
the winner of not being the cheapest or the most expensive, just

inexpensive
yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work

well
or might I consider others?


What's wrong with the ones that are in there?

The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2.

There
are only one set of speaker outputs. Is this maybe a setting for 4 ohm

and 8
ohm? I think the volume goes up a bit in the 1 positions.


It's *probably* meant for 8-ohm speakers. You can measure the DC
resistance of the speakers that came in the console to get a pretty good
idea. 8-ohm speakers will usually measure around 6 ohms DC resistance or
thereabouts.

Are there voltages I should be checking or stuff like that?


Wouldn't be a bad idea (and would be educational also) if you have the
schematic showing the typical voltages at various points.

There is a little pot on the back marked HUM what should I do with

this
adjustment?


Adjust for minimum hum.

Well it's late and I am sure there may be more questions but that's

all
I
can think of for now. More than enough for one post anyways.


Have fun. Be careful. Enjoy the music!

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+





  #7   Report Post  
Bryan McGivney
 
Posts: n/a
Default

I didn't try the turntable in the console. I had hooked up my Dual with an
Ortofon cartridge. Taking a look at the cartridge in the console it looks
like a small cylinder with the needle going into it, no metal plate or
anything. It's got a plastic tone arm. I will tinker a bit more and see what
I find out.

Thanks,
Bryan
"Jimmy" wrote in message
om...

"Bryan McGivney" wrote in message
...
As for the tubes in it now, they sound pretty good but are mismatched,

two
different brands or 6BQ5. The 12AX7's have all the writing rubbed off so

I
figured I might as well replace them while I was at it since they don't

cost
all that much. Sort of a continuity of brands sort of thing more than
anything else.

Another question though, the phono input worked but it was very quiet

and
sort of thin sounding. I don't have a schematic for the amp. I am

guessing
that one 12AX7 is the phono stage and the other is the preamp stage.

What
parts should I check first to get the phono stage working?

Do the caps I replaced need to run in for a bit before the sound will be
optimal?


I bet you either have the wrong catridge or a bad cartridge in your
phonograph. You probably should have a crystal cartridge. Either it has

been
replaced with a magnetic one or it is just plain bad.



  #8   Report Post  
Mikkel C. Simonsen
 
Posts: n/a
Default

Bryan McGivney wrote:

I didn't try the turntable in the console. I had hooked up my Dual with an
Ortofon cartridge.


So the amp is made for a crystal cartridge, which means the phono input
is basically a line input. You will need a RIAA preamp to use your
Dual/Ortofon.

Best regards,

Mikkel C. Simonsen
  #9   Report Post  
Bryan McGivney
 
Posts: n/a
Default

So I still want to replace the 6BQ5/EL84 since they are not even the same
brands but I looked at some web pages and tried to identify the 12AX7's I
have. On one tube I can make out a bit of the writing. What I can make out
seems to say in white paint
ECC83/12AX7
Made In
Holland
915
and in gold paint really faint
mCo
49B
that's all I can see.
I looked on the web for 12AX7 made in Holland and about all I come up with
is Amperex. I found this page with pictures
http://www.raretubes.com/XEECC83.htm
if you go there scroll down to the first three entries for Amperex they say
sq. getter, the insides of mine look like those with the very top piece
being a D shape with the little flat plate as shown. I am guessing that is
the square getter, I am not up on all the tube terms. So going by the
insides I would say that is what I have but what's left of the paint doesn't
look like it could have ever been that logo. Did any other tubes use guts
that looked like that? This guys got them listed at $150 - $175 a pair so I
guess I'll keep those. Someone mentioned that they last almost indefinitely?

Having fun,
Bryan
Durham, Oregon.
"Bryan McGivney" wrote in message
...
As for the tubes in it now, they sound pretty good but are mismatched, two
different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I
figured I might as well replace them while I was at it since they don't

cost
all that much. Sort of a continuity of brands sort of thing more than
anything else.

Another question though, the phono input worked but it was very quiet and
sort of thin sounding. I don't have a schematic for the amp. I am guessing
that one 12AX7 is the phono stage and the other is the preamp stage. What
parts should I check first to get the phono stage working?

Do the caps I replaced need to run in for a bit before the sound will be
optimal?

TIA,
Bryan

"Fred Nachbaur" wrote in message
news:jI6_a.22$zE1.13@edtnps84...


Bryan McGivney wrote:
I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a

tube
amp in it and I wanted to pirate the amp for a fun project. Well I

pulled it
out of the console and fired it up and it hummed badly. It has two

multi
part can style caps on the chassis. I found caps at the local supply

house
and wired them in below the chassis to the terminals on the original

caps.
The amp sounds fine now but I figured I'd ask if there are any issues

with
not having removed the original caps from the circuit?


There might be down the road, if the old cap decides to short. That
being said, this used to be a pretty common service practise, and I've
never heard of a problem because of it. In my old age I'm getting
marginally more careful about stuff like this, so I think I'd physically
disconnect the old caps and put in a couple small terminal strips to
which to wire the new ones.

I hooked the caps up
with what I think is about 18ga speaker wire, with the copper

conductor
on
one side and silver conductor on the other. This shouldn't cause any

issues
should it?


Not other than being kind of ugly. ;-)

How hot might one expect the power transformer to get? I can touch it

but
it's hot enough you don't want to hold you finger on it long. My guess

is
around 110 - 120 degrees.


That sounds about right.

I have seen reference to adding a modern power socket like a computer

would
use and using the 3 prong grounded cord, grounding the chassis to help
reduce hum or noise. Is this usually OK or might I somehow get myself

into
trouble doing this?


You'll only get yourself in trouble if you wire it improperly. ;-)

I want to make a wooden enclosure for this with a metal screen top or
something like that. It seems that 17" is about the width of most
components?


Yes.

Reading the comments along with new tubes it looks like JJ/tesla might

be
the winner of not being the cheapest or the most expensive, just

inexpensive
yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work

well
or might I consider others?


What's wrong with the ones that are in there?

The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2.

There
are only one set of speaker outputs. Is this maybe a setting for 4 ohm

and 8
ohm? I think the volume goes up a bit in the 1 positions.


It's *probably* meant for 8-ohm speakers. You can measure the DC
resistance of the speakers that came in the console to get a pretty good
idea. 8-ohm speakers will usually measure around 6 ohms DC resistance or
thereabouts.

Are there voltages I should be checking or stuff like that?


Wouldn't be a bad idea (and would be educational also) if you have the
schematic showing the typical voltages at various points.

There is a little pot on the back marked HUM what should I do with

this
adjustment?


Adjust for minimum hum.

Well it's late and I am sure there may be more questions but that's

all
I
can think of for now. More than enough for one post anyways.


Have fun. Be careful. Enjoy the music!

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+





  #10   Report Post  
Mikkel C. Simonsen
 
Posts: n/a
Default

Bryan McGivney wrote:

So I still want to replace the 6BQ5/EL84 since they are not even the same
brands but I looked at some web pages and tried to identify the 12AX7's I
have. On one tube I can make out a bit of the writing. What I can make out
seems to say in white paint
ECC83/12AX7
Made In
Holland


So you have Philips tubes - aka Mullard, Valvo, Miniwatt, Adzam, Mazda,
Amperex.

So going by the insides I would say that is what I have but what's left
of the paint doesn't look like it could have ever been that logo. Did
any other tubes use guts that looked like that?


Just look at the brand list I wrote above - all those company
names/brands were used by Philips, and they all had different logos. The
tubes are the same...

This guys got them listed at $150 - $175 a pair so I guess I'll keep those.
Someone mentioned that they last almost indefinitely?


I have never replaced one so far. The ones I have in equipment from the
late 50's still work fine...

Best regards,

Mikkel C. Simonsen
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