I am not a tech but I can follow schematics and do repairs fairly well But
I am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in the
background
I have recapped the amp coompletely and retubed it. And tried to swap each
tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER REFERENCE"
Can anyone help me with resolving that or tell me where that may be in the
schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
Thank you in advance for any info tha can help

Kirk

www.kirkpowers.com
--


---
Remove the "NOOOOOO00" from email address to reply

"svtbass"

I am not a tech but I can follow schematics and do repairs fairly well But
I am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in
the background
I have recapped the amp coompletely and retubed it. And tried to swap each
tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER REFERENCE"
Can anyone help me with resolving that or tell me where that may be in the
schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif



** That is ONLY the power stage schematic.

If you short the "extension amp" socket does that stop the hum?



........ Phil

Thank you..
Here is the other preamp schematic
SVT Preamp http://www.drtube.com/schematics/ampeg/svtpre-jp.gif
SVT Poweramp http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
I have not tried checking to see if it does it when the EXt amp is shorted.I
will try that tonight.. Like I said I am not a tech and I appologize for
that. But I have been a tinkerer for 20 years and this has been my project.
Got it all working but have this little problem with the hum.

--
---
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"Phil Allison" wrote in message
...

"svtbass"

I am not a tech but I can follow schematics and do repairs fairly well
But I am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in
the background
I have recapped the amp coompletely and retubed it. And tried to swap
each tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER REFERENCE"
Can anyone help me with resolving that or tell me where that may be in
the schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif



** That is ONLY the power stage schematic.

If you short the "extension amp" socket does that stop the hum?



....... Phil

"svtbass"
Thank you..
Here is the other preamp schematic
SVT Preamp http://www.drtube.com/schematics/ampeg/svtpre-jp.gif
SVT Poweramp http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
I have not tried checking to see if it does it when the EXt amp is
shorted.I will try that tonight.. Like I said I am not a tech and I
appologize for that. But I have been a tinkerer for 20 years and this has
been my project. Got it all working but have this little problem with the
hum.


** Lemme know what happens.

Also any other details of what affects the hum level or hum tone.


BTW Please do not top post.


........ Phil

When I shorted the jack to ground on the "extension amp" the hum became
louder..
The hum that is now there is kind of low in volume in the background but it
is obviously present. It is above 60Hz. Sounds like a harmonic of about
120Hz and very mellow in tone. No extended upper harmonics like some ground
buzzes...kind of like a mellow sine wave at about 120Hz or so.

--
---
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"svtbass" wrote in message
...
Thank you..
Here is the other preamp schematic
SVT Preamp http://www.drtube.com/schematics/ampeg/svtpre-jp.gif
SVT Poweramp http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
I have not tried checking to see if it does it when the EXt amp is
shorted.I will try that tonight.. Like I said I am not a tech and I
appologize for that. But I have been a tinkerer for 20 years and this has
been my project. Got it all working but have this little problem with the
hum.

--
---
Remove the "NOOOOOO00" from email address to reply
"Phil Allison" wrote in message
...

"svtbass"

I am not a tech but I can follow schematics and do repairs fairly well
But I am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in
the background
I have recapped the amp coompletely and retubed it. And tried to swap
each tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER
REFERENCE"
Can anyone help me with resolving that or tell me where that may be in
the schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif



** That is ONLY the power stage schematic.

If you short the "extension amp" socket does that stop the hum?



....... Phil

** Lemme know what happens.

Also any other details of what affects the hum level or hum tone.


BTW Please do not top post.


....... Phil



I am sorry.. I will stop the top posting.. bad habbit...
Thank you very much

Define "re cap".

Then, does the amp hum when totally quiescent _and_ with nothing
plugged in at all?
Have you adjusted the bias?

Peter Wieck
Wyncote, PA

One more thing... Have you adjusted the hum-pot?

Peter Wieck
Wyncote, PA

"svtbass"
When I shorted the jack to ground on the "extension amp" the hum became
louder..
The hum that is now there is kind of low in volume in the background but
it is obviously present.


** How the heck can you hear it over the fan noise?



........ Phil

Remove the "NOOOOOO00" from email address to reply
wrote in message
oups.com...
Define "re cap".

Then, does the amp hum when totally quiescent _and_ with nothing
plugged in at all?
Have you adjusted the bias?

Peter Wieck
Wyncote, PA

If nothing is plugged into the amp it still makes that warm mellow hum in
the background... It does not sound like an open jack sort of hum.. that is
almost like a buzz to be.. this sound has less harmincs and is like a warm
mellow sine wave sort of sound...
I only use Channel #1 but The hum lessens when I turn the CHANNEL #2 volume
up to about 4 with no signal going into that channel.. I suspected that
channel 2 may be the culprit in some way...
I replaced the power supply caps and all the electrolytic caps in the power
amp section. also the Orange drop spragues on the Main Board. I replaced the
40u 600V Electrolytic in the preamp.. and almost all of the other orange
drops in the preamp section.
I got rid of all of the paper caps that were in there since 1972...
It almost sounds like a bad or leaky cap.
Retubed with new 6550 winged C and all new preamp and driver tubes.. 12 dw7
and 12ax7, 6C4 and 12BH7s...
I biased the amp to spec. and balanced it .. adjusted the Hum balance .
I have also tried swapping out all the tubes One by One just to see if maybe
I have a bad tube tube somewhere..
still there.

--
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wrote in message
oups.com...
One more thing... Have you adjusted the hum-pot?

Peter Wieck
Wyncote, PA

Yes sir.. I did adjust the hum pot.. It does seem to work well too.. It
turns smoothly without and drop out it seems. when turned CCW or CW you can
immediatelt tell the difference.. somewhere in the middle is the sweet spot
... but the mild hum is there.. almost like a ground hum.. but
different...not as loud as an obvious ground hum if that makes sense..

By any chance does the amp have phenolic circuit boards (tan/brown in
color)? And have you checked the traces on these boards? Also, are you
dead-sure of the polarity of the new caps? Cold solders on the tube
sockets?

These may seem like silly questions, but it helps to get the obvious
out of the way.

Peter Wieck
Wyncote, PA

In article , says...

"svtbass"
When I shorted the jack to ground on the "extension amp" the hum became
louder..
The hum that is now there is kind of low in volume in the background but
it is obviously present.


** How the heck can you hear it over the fan noise?



....... Phil




Hi all,

Just my two cents here. Are you using a 3 pronged power cord?

-bart

Hi all,

Just my two cents here. Are you using a 3 pronged power cord?

-bart

yes sir..

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wrote in message
oups.com...
By any chance does the amp have phenolic circuit boards (tan/brown in
color)? And have you checked the traces on these boards? Also, are you
dead-sure of the polarity of the new caps? Cold solders on the tube
sockets?

These may seem like silly questions, but it helps to get the obvious
out of the way.

Peter Wieck
Wyncote, PA

ThanK you Peter.
It does have a Tan colored PCB on the component side.. but the underside
where the tracks are is green... I hope that is not bad... gulp!
I will go over all the traces.. and I have always been a little unsure of
one of the caps...C8 90u ~200V has the positive to ground..
here is a schematic i am working off of
http://www.kirkpowers.com/SVT_AmpSchem.jpg
It appears that C8+ goes to ground..

Thank you Peter.. I will go through that amp with a fibe toothed comb as
they say starting tonight ,, after i get in...

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wrote in message
oups.com...
It appears that C8+ goes to ground..

This is correct, as C8 is the filter for the negative supply (not
ground). SO!!

Check to see, if the negative side of this cap is at ground-potential,
then you have found your problem... I doubt it, but it may be possible.
Check that entire line and see.

On the phenolic board, not bad, it just makes it harder to check the
traces. But where you should be concentrating is at the tube sockets
and at any component poke-through and wire connections on the board.
Make sure that they are all 'good'.. which (while the unit is OFF and
UNPLUGGED) means wiggling them and looking for lift or cracks at the
board. Look for hairline cracks.

Also, be sure to do the obvious and clean the controls and adjustment
pots. Check for any chassis grounds that are riveted or screwed and
make sure that they are tight.

Peter Wieck
Wyncote, PA

Yes sir.. I did adjust the hum pot.. It does seem to work well too.. It
turns smoothly without and drop out it seems. when turned CCW or CW you
can immediatelt tell the difference.. somewhere in the middle is the sweet
spot .. but the mild hum is there.. almost like a ground hum.. but
different...not as loud as an obvious ground hum if that makes sense..


It appears the previous owner had changed C1, C3 and C4 to a .22 @600V
cap... it looks like the schematic calls for a .1 @ 600v. .....do you
think that may be part of the cause?
Thanks again

In article , "svtbass"
says...




Hi all,

Just my two cents here. Are you using a 3 pronged power cord?

-bart

yes sir..



There could also be a ground loop problem. I'll wait for the results of
Peter's questions before continuing.

-Bart

svtbass wrote

I am not a tech but I can follow schematics and do repairs fairly well But I
am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in the
background
I have recapped the amp coompletely and retubed it. And tried to swap each
tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER REFERENCE"
Can anyone help me with resolving that or tell me where that may be in the
schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
Thank you in advance for any info tha can help

Just a few thoughts to add.

"Heater reference" is presumably the hum pot, marked "hum bal". I assume that's
what you have already adjusted. Unless some heaters are elevated, that's the
only "reference".

Have you tried adjusting the bias slightly on one side, to see if the hum gets
better or worse? It is possible that the AC balance is slightly offset from the
DC balance point. I see that output valve swapping made no difference, so this
is an unlikely cause perhaps.

Are you sure your amp hums more than is normal for the model?

When you worked on it, did you put all the wires back in the same places? In
particular, the heater wiring should be properly twisted and tucked in close to
the chassis. A soft mellow hum suggests an unrectified source?

Or from an early stage perhaps. Are all the pin voltages correct on the little
valves?

Did you put all the ground connections back in the same places, and are they all
secure?

cheers, Ian

Kirk said

It appears the previous owner had changed C1, C3 and C4 to a .22 @600V cap...
it looks like the schematic calls for a .1 @ 600v. .....do you think that
may be part of the cause?

Circuit diagrams you posted don't show part numbers AFAICS

You might want to change them back anyway...cheep enough to try.

cheers, Ian

"Heater reference" is presumably the hum pot, marked "hum bal". I assume
that's what you have already adjusted. Unless some heaters are elevated,
that's the only "reference".

Have you tried adjusting the bias slightly on one side, to see if the hum
gets better or worse? It is possible that the AC balance is slightly
offset from the DC balance point. I see that output valve swapping made no
difference, so this is an unlikely cause perhaps.

Are you sure your amp hums more than is normal for the model?

When you worked on it, did you put all the wires back in the same places?
In particular, the heater wiring should be properly twisted and tucked in
close to the chassis. A soft mellow hum suggests an unrectified source?

Or from an early stage perhaps. Are all the pin voltages correct on the
little valves?

Did you put all the ground connections back in the same places, and are
they all secure?

cheers, Ian


Yes.. I have heard that some of these early 70s SVT have hum.. I had another
that I never noticed doing this hum.

I have checked and rechecked all grounds and PCB traces.

If you like here is a schematic for your download pleasure
http://www.kirkpowers.com/SVT_AmpSchem.jpg

I don't know if I should start o replace diodes? TO be honest with you I am
not a tech.. But I know enough just to mess things up.. NO really i have
mil spec soldering experience from years ago, and understand components but
just not how they all work together in a circuit like this. If I were
directed to a part I can replace it and have it look like a NASA job---
pristine and all.. but I have no idea how to find the problem.. And I am
sorry for driving you all mad but thank you kindly for your help... I have
actually learned a few things in this short 24 hour thread... but I feel
like i may have to take it to a certified tech at this time... I don't want
to give up though.. I love the search.. and when I finally get it . it will
be Eureka!!!!

So I offer a real big THANK YOU !!!



So anyway, I am sure that all wires are replaced in proper places. the only
thing that i have not done is tension the tube sockets or check voltages at
the pins.. and I need to do as you said make sure the heater wire are tucked
in close to the chassis and twisted... i know that one set of wires in not
twisted right now.. and i will verify... again .. i cant thank yo all
enough.. I have been Lurking in this group for about 6 months or so and
finally have neede to call on help..

I am going to get o work on this thing and do as you said and should reply
tomorrow with results... this thing is heavy.. about 92 pounds.. it really
does sound good... the mother of all bass amps

could a problem with the filement transformer cause the problem

could a problem with the filement transformer cause the problem

If not shorted and if the voltage is correct, no. Transformers are
remarkably simple beasts.

Peter Wieck
Wyncote, PA

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wrote in message
oups.com...
could a problem with the filement transformer cause the problem

If not shorted and if the voltage is correct, no. Transformers are
remarkably simple beasts.

Peter Wieck
Wyncote, PA

Peter.. can you direct me to a beginning source of learning what you guys
know about tube gear.... i am 58 years old and facinated with stuff..
thank you
Kirk


www.kirkpowers.com

"svtbass" wrote in message
...
:
:
: --
: ---
: Remove the "NOOOOOO00" from email address to reply
: wrote in message
: oups.com...
: could a problem with the filement transformer cause the problem
:
: If not shorted and if the voltage is correct, no. Transformers are
: remarkably simple beasts.
:
: Peter Wieck
: Wyncote, PA
:
: Peter.. can you direct me to a beginning source of learning what you guys
: know about tube gear.... i am 58 years old and facinated with stuff..
: thank you
: Kirk
:
:
: www.kirkpowers.com
:

http://groups.google.nl/groups?as_q=...q=&as _oq=&as
_eq=&as_ugroup=rec.audio.tubes&as_usubject=&as_uau thors=&lr=&as_drrb=q&as_qdr=&as
_mind=1&as_minm=1&as_miny=1981&as_maxd=15&as_maxm= 3&as_maxy=2006

gives me 7000 hits ...
enjoy :-),
Rudy

"svtbass"

or 'quick fix' :
loads of downloadable, a tip from Sander recently, he
http://www.pmillett.com/Books
:
R.

loads of downloadable, a tip from Sander recently, he
http://www.pmillett.com/Books
:
R.



thank you very much

svtbass wrote

Yes.. I have heard that some of these early 70s SVT have hum.. I had another
that I never noticed doing this hum.

I have checked and rechecked all grounds and PCB traces.

If you like here is a schematic for your download pleasure
http://www.kirkpowers.com/SVT_AmpSchem.jpg

OK thanks.

I don't know if I should start o replace diodes? TO be honest with you I am
not a tech.. But I know enough just to mess things up.. NO really i have mil
spec soldering experience from years ago, and understand components but just
not how they all work together in a circuit like this. If I were directed to a
part I can replace it and have it look like a NASA job--- pristine and all..
but I have no idea how to find the problem.. And I am sorry for driving you
all mad but thank you kindly for your help... I have actually learned a few
things in this short 24 hour thread... but I feel like i may have to take it
to a certified tech at this time... I don't want to give up though.. I love
the search.. and when I finally get it . it will be Eureka!!!!

So I offer a real big THANK YOU !!!



So anyway, I am sure that all wires are replaced in proper places. the only
thing that i have not done is tension the tube sockets or check voltages at
the pins.. and I need to do as you said make sure the heater wire are tucked
in close to the chassis and twisted... i know that one set of wires in not
twisted right now.. and i will verify... again .. i cant thank yo all enough..
I have been Lurking in this group for about 6 months or so and finally have
neede to call on help..

I am going to get o work on this thing and do as you said and should reply
tomorrow with results... this thing is heavy.. about 92 pounds.. it really
does sound good... the mother of all bass amps

It is customary to warn ppl about lethal voltages. The equipment is only safe if
you have unplugged it and discharged all the high voltage caps. Otherwise it can
kill in an instant. If you have to work on it live, for testing voltages for
example, work with only one hand. Some say you should put the other in your
pocket, but bear in mind that many injuries from shock arise when you are flung
into something hard or sharp, so keep a clear space around you. The important
thing is that the other hand should be electrically floating, so don't hold the
chassis or anything grounded while you have the other hand inside the amp or you
may fry your heart.

Changing those coupling caps may well exacerbate a hum problem because bigger
caps extend the low frequency response of the amplifier. However, there is a
global feedback loop which encloses them all, and makes their effect on hum
quite complicated...it depends where the hum originates. If there were no bad
effect of using bigger coupling caps, then the manufacturer would probably have
fitted them.

I should explain my comment about rectification. Rectifiers are switches, and
introduce sharp corners to the waveform. Sharp corners mean high frequencies, so
the sound tends to be harsh. If your hum is mellow, that suggests it is either
coming from an unrectified source such as the heater supply, or that the high
frequency content has been filtered out, suggesting an early stage. But this is
only a vague and relative indication.

The power amp is a beast!

If you want to know how it works, in basic circuit outline it is similar to the
iconic Williamson amp. There is a lot of discussion and guidance on that circuit
about if you look. The details are all different, but the basic structure is the
same, but for a few respects as follows.

It has an extra stage comprising the second halves of V2 and V3, set up as two
cathode followers driving the output stage. This allows the 6550 grids to be
driven positive, so they begin to pass current. As they do, the current will
cause a voltage drop across the 47k resistors (R26, R29, R34, etc.), giving soft
clipping. I like the way they have gone to the trouble of ensuring that the
resistance into the grids is linear. For a hi-fi amp it would seem paradoxical
to create a low-impedance driver and then add a load of series resistance to its
output.

What makes it particularly beastly, in addition to the massive grunt of half a
dozen 6550s, and in combination with the ability to drive the grids positive, is
the low impedance supply to the output valve screens. At the point where the
grid begins to conduct, the screen resistance falls equally dramatically, but
there is no limit to the screen current in this circuit. Do these things eat
valves? I bet they need good quality, rugged 6550s. It is common for pentode or
beam-tetrode amps to include a choke in the screen supply, and use screen
resistors of several hundred ohms. Yours have sod all. Good for smooth clipping
though. I suppose tone isn't cheap.

Whilst on the subject of the screen supply, there is precious little smoothing
there. Do the manufacturers recommend matched output valves? In some ways they
are not necessary, because you can adjust the DC idle current via the two bias
controls such that the DC balances through the two halves of the output
transformer. Similarly, AC balance with respect to the signal can be adjusted at
the cathode of the concertina phase splitter (2nd half of V1). That leaves hum
to consider (power supply rejection, or PSRR, is a key phrase to look for). The
idea is that, assuming the hum voltage on the screens has an equal effect on
both sides of the push-pull arrangement, the resulting hum output will be equal
and opposite through the transformer, and the hum will cancel.

However, if the valves react slightly differently to hum signals on their
screens, the total may not cancel. If you have a DMM that will tolerate a high
voltage on both leads, you can set the bias as per instructions and then measure
the voltage drop across the 5.1 ohm anode resistors (one handed...use a clip on
one test lead), using the results to calculate the current through each
individual valve. It should be fairly closely the same through each. If not, you
may gain some benefit by intelligent rearrangement of the valves, so that as far
as possible each has a similar partner on the other side of the push-pull
circuit. You will need to readjust the bias and AC balance afterwards. I would
appreciate the comments of others on this particular point.

The two circuits you have posted are different in important hum-related
respects! One has a point marked G which should be at or near ground but it has
a label like it should be a power supply connection, but those stop at F. The
other shows this as a test-point for the bias, and a ground connection. The
first shows a 10 ohm resistor (R53...only 1/2 W, is that wise? Anybody?) between
the signal ground and the chassis, the second shows a connection to the centre
of a transformer winding instead. Which is yours? If you mix them up you could
end up with hum.

It's a monster to work on for your first project.

I'm not an expert, BTW...just keeping the discussion going.

cheers, Ian

Kirk:

What a question. I came to tube audio via vintage radios, and there via
building SS stereo kits through the 60s & 70s. I have never been
trained in this, it is 100% a hobby.

Since you asked, however, you might try looking in your area for a
vintage (antique) radio club. If such exists, its members will have a
vast knowledge of the basics of tube gear (it _ALL_ crosses over from
radio to audio to performance gear), diagnosis, repairs, equipment
needed and the like. If you keep in mind that a cheap tube radio with
five (5) tubes still includes a tuner, pre-amp, AVC and power-amp,
adding more tubes simply makes them more sensitive, more powerful or
just larger, then you can see that the basic knowledge does not change.


If you want only an understanding of tubes and how they work, there are
several books on the subject. I cannot recommend one over another as I
have not read any of them except in passing and for very specific
needs, I am sure others here will give you titles and authors. If you
want some very basic troubleshooting guides, I can only suggest
radio-related titles, as I have relied on crossover information from
such to audio. You have missed the heyday of magazine articles, or even
manufacturors' guides that gave up a huge amount of information in
their time. I have the owners' manual for an AR 60s-vintage turntable
that goes deeply into hum-loops, residual hum sources and similar
information that you would _NEVER_ see today in such a manual. One of
my later Dynaco kits had a photo-copied flyer inside warning of
possible CB reception based on oxidation around a certain run of rivets
they used for a period to attach the phono-inputs to the chassis. As I
was having a similar problem with a non-Dynaco product at the time, I
went to *its* rivets... cleaned then... the problem went away. And
learned a great deal about grounds at the same time. I chased my tail
for days with a Dynaco SCA-35 until I found an intermittent trace on
one of the boards. Since then I have found dozens and have learned a
deep and abiding distrust of phenolic boards coupled to tube equipment.


Gathering knowledge at the hobby level is like that. Bits and pieces
come from everywhere and from experience. Best-yet is observing others
at it. Vintage radio clubs are best for that.

If you happen to get to Kutztown, PA in the spring (May 12/13), I will
be doing a basic clinic on the diagnosis, repair, care and feeding of
vintage radios. I will have appropriate safety equipment and I will
illustrate its uses.

Where are you?

Peter Wieck
Wyncote, PA

I found those caps, they are the only 0.1 caps on the diagram, and all
around V1. I would suggest that you replace all of them or none of
them, and make sure that the caps that go in are screened past just the
basic specs. If you can get them within 5%, it can only help. But, yes,
DO try to replace them with 0.1 caps ILO the 0.22 that are in there
now. If you do not have immediate access to these caps (they are mighty
cheap, BTW), I can send you a set. Just tell me what style they a

__ __
---[__]--- This way [__] or this way.
I I

BTW, just confirm: This problem has been with you as long as you have
owned the amp, or is it a post-acquisition development?

I am assuming that you do not have a lot of specialized equipment to do
other than the most obvious VOM-related tests. I also assume that you
understand that there are lethal voltages on this amp when in
operation, and that you do not poke around inside without full care and
knowledge of the risks....

Peter Wieck
Wyncote, PA

I found those caps, they are the only 0.1 caps on the diagram, and all
around V1. I would suggest that you replace all of them or none of
them, and make sure that the caps that go in are screened past just the
basic specs. If you can get them within 5%, it can only help. But, yes,
DO try to replace them with 0.1 caps ILO the 0.22 that are in there
now. If you do not have immediate access to these caps (they are mighty
cheap, BTW), I can send you a set. Just tell me what style they a

__ __
---[__]--- This way [__] or this way.
I I

BTW, just confirm: This problem has been with you as long as you have
owned the amp, or is it a post-acquisition development?

I am assuming that you do not have a lot of specialized equipment to do
other than the most obvious VOM-related tests. I also assume that you
understand that there are lethal voltages on this amp when in
operation, and that you do not poke around inside without full care and
knowledge of the risks....

Peter Wieck
Wyncote, PA


Oh yes.. I know about the voltages.. many years ago when I was about 19
years old i got a shock from one of these... it hurt allot.. a real lot...
i am older now.. 48 and have learned some safety techniques.. and i never
hold onto the chassis with one hand while working with the other... but i am
more careful and my heart is not as resilient or lucky as i was when i was
19....



it has not been plugged in for 2 days now and i will discharge anything just
in case...

Thank you for the offer for the caps.. I can get them as you said real
cheap at the www.tubedepot.com or tubesandmore.com

the caps that are in the amp now are those Orange Drop type Sprague's... I
see them now a days but the Sprague logo/ name is not on them it seems...

they are of the 715series http://www.tubedepot.com/cp-od715-1-600v.html

there is also a 716 series http://www.tubedepot.com/cp-od716-100-600v.html

i do not know what the difference is except for OD size...

But i live in the Denton, Texas area just north of Dallas.. And frequent
Austin, TX alot. Originally form New York City.. Boy do i miss the bagels
and Pizza....

The problem seems to have been there since I got the amp.. I had a Fretless
Pedulla bass that I have not used in years and my friend had this amp that i
really wanted.. so we traded.. i think i got the better deal actually.

It needs work here and there.. But it is a collectors item as well as a
staple in R&R history and the sound is simply Huge... it is rated to be 300
watts into 4 or 2 ohms. It is theeee bass amp sound for R&R... they
originally came out about 1969.. i see them on Ebay lately and some in great
condition go for up to $3000 at times and more...

I n 1971 i bought an original 1969 SVT used ... and sold it to get a
lighter amp a few year later...it used six - 6146 tubes in the power amp but
seemed to be unreliable and they switched to 6550s... it is only 300 watts
but sound as loud as my 1000 watt crown macrotech 1200..

The SVT was supposed to be on Amp with 2 cabinets.. each cabinet has 8 x 32
ohm 10" speakers wired in parallel to give you 4 ohms each.. so the total
would be 16 10" speakers.... I had one of those way back when big stage gear
was needed....

It is a beast!



So when i traded the bass for this amp i knew it would need some work but i
wanted one to get that sound that i used to have.. so i re-tubed and
replaced the old paper caps with new caps... and did a bias...and balanced
the two sides out...

but it has always made that hum since i got it. like i said the num is not
dominant, but it is there.. and is has a smooth sine wave tone. no bright
harmonics, or square wave sound..

Here is a pic http://www.kirkpowers.com/gear-pics_7.html and another
http://www.kirkpowers.com/gear-pics_11.html



I really don't mind if i have to put some money into it..it is sort of a
hobby in a way.. i prefer ot fix my own gear unless i just cant figure it
out after months and months. .this is one of those projects that may take me
a year to restore... i only have about $200 into parts since i did the
recap myself and the tubes....it works good enough to gig with but not quiet
enough to use on a recording session...

since i really dont know what I am doing but can follow a schematic i like
to work on tube gear that went bad because when something goes I can see
it.. where as transistor stuff i have no idea...

but i really enjoy the ownership of a vintage SVT.. playing bass through it
is pure joy.. it sounds like Buttah... Butter.... Silky with a Punch...



To answer your question.. i really don't have alot of test equipment and
this amp may eventually have to go to the shop..

I do have 2 DVMs which i am pretty good with. I have a load bank that i Bias
it on and a tone generator.... and yes a variable heat soldering
station..... i can solder real well.. back in the early 80s before i went
full time in music i worked as a QC QA inspector for an aerospace company
and learned NASA solder and Mil Spec solder.. I dont remember the details
but I do get a real nice flow without excess solder, shinny like a mirror
and no peaks....i have silver solder that i still have after 20 years...
they used it on some projects back then... i cant remember what though.. i
dont know if it is good for audio but looks real nice on the flow..



Sorry about going off on the history of the SVT, but it is a tube amp Icon
in the music player world...

I will order those .1 caps but would like to ask you your opinion first if
the ones listed above are OK???

thank you Peter

If you happen to get to Kutztown, PA in the spring (May 12/13), I will
be doing a basic clinic on the diagnosis, repair, care and feeding of
vintage radios. I will have appropriate safety equipment and I will
illustrate its uses.

Where are you?

Peter Wieck
Wyncote, PA

I am in Denton,TX just north of Dallas..
It is almost time for a road trip..
man.. most of what you guys would talk about would be over my head.. but I
am pretty sharp and would catch tid bits... step by step is what i used to
say... before you know it your there...
Thank you for the information too.. I have an older Dyanaco PAS preamp and
tube FM tuner.. and got a Dynaco 400 poweramp back in 1977 or so.... used it
on a bass rig for a while then did not want to ruin it so i retired it to my
home studio playback... its the one with the big blue analog meters..and
Dynagauard... that amp sound great.. needs work too.. seems like everything
i have needs work lately..
I have a newer Ampeg SVT2 from 1989. it is a rack mount type.. same thing as
the one you guys are helping me with but 19" rack mounted.. some little
extras in it.. but I just retubed it and recapped it last night... Spring
cleaning time i guess...
It had a hum but I found I had a bad 12AU7 driver tube... swapped it out,
rebiased and it is going to work with me tomorrow on my gig in Fort Worth
Texas...

--
---
Remove the "NOOOOOO00" from email address to reply
"Ian Iveson" wrote in message
k...
svtbass wrote

Yes.. I have heard that some of these early 70s SVT have hum.. I had
another that I never noticed doing this hum.

I have checked and rechecked all grounds and PCB traces.

If you like here is a schematic for your download pleasure
http://www.kirkpowers.com/SVT_AmpSchem.jpg

OK thanks.

I don't know if I should start o replace diodes? TO be honest with you I
am not a tech.. But I know enough just to mess things up.. NO really i
have mil spec soldering experience from years ago, and understand
components but just not how they all work together in a circuit like
this. If I were directed to a part I can replace it and have it look like
a NASA job--- pristine and all.. but I have no idea how to find the
problem.. And I am sorry for driving you all mad but thank you kindly for
your help... I have actually learned a few things in this short 24 hour
thread... but I feel like i may have to take it to a certified tech at
this time... I don't want to give up though.. I love the search.. and
when I finally get it . it will be Eureka!!!!

So I offer a real big THANK YOU !!!



So anyway, I am sure that all wires are replaced in proper places. the
only thing that i have not done is tension the tube sockets or check
voltages at the pins.. and I need to do as you said make sure the heater
wire are tucked in close to the chassis and twisted... i know that one
set of wires in not twisted right now.. and i will verify... again .. i
cant thank yo all enough.. I have been Lurking in this group for about 6
months or so and finally have neede to call on help..

I am going to get o work on this thing and do as you said and should
reply tomorrow with results... this thing is heavy.. about 92 pounds.. it
really does sound good... the mother of all bass amps

It is customary to warn ppl about lethal voltages. The equipment is only
safe if you have unplugged it and discharged all the high voltage caps.
Otherwise it can kill in an instant. If you have to work on it live, for
testing voltages for example, work with only one hand. Some say you should
put the other in your pocket, but bear in mind that many injuries from
shock arise when you are flung into something hard or sharp, so keep a
clear space around you. The important thing is that the other hand should
be electrically floating, so don't hold the chassis or anything grounded
while you have the other hand inside the amp or you may fry your heart.

Changing those coupling caps may well exacerbate a hum problem because
bigger caps extend the low frequency response of the amplifier. However,
there is a global feedback loop which encloses them all, and makes their
effect on hum quite complicated...it depends where the hum originates. If
there were no bad effect of using bigger coupling caps, then the
manufacturer would probably have fitted them.

I should explain my comment about rectification. Rectifiers are switches,
and introduce sharp corners to the waveform. Sharp corners mean high
frequencies, so the sound tends to be harsh. If your hum is mellow, that
suggests it is either coming from an unrectified source such as the heater
supply, or that the high frequency content has been filtered out,
suggesting an early stage. But this is only a vague and relative
indication.

The power amp is a beast!

If you want to know how it works, in basic circuit outline it is similar
to the iconic Williamson amp. There is a lot of discussion and guidance on
that circuit about if you look. The details are all different, but the
basic structure is the same, but for a few respects as follows.

It has an extra stage comprising the second halves of V2 and V3, set up as
two cathode followers driving the output stage. This allows the 6550 grids
to be driven positive, so they begin to pass current. As they do, the
current will cause a voltage drop across the 47k resistors (R26, R29, R34,
etc.), giving soft clipping. I like the way they have gone to the trouble
of ensuring that the resistance into the grids is linear. For a hi-fi amp
it would seem paradoxical to create a low-impedance driver and then add a
load of series resistance to its output.

What makes it particularly beastly, in addition to the massive grunt of
half a dozen 6550s, and in combination with the ability to drive the grids
positive, is the low impedance supply to the output valve screens. At the
point where the grid begins to conduct, the screen resistance falls
equally dramatically, but there is no limit to the screen current in this
circuit. Do these things eat valves? I bet they need good quality, rugged
6550s. It is common for pentode or beam-tetrode amps to include a choke in
the screen supply, and use screen resistors of several hundred ohms. Yours
have sod all. Good for smooth clipping though. I suppose tone isn't cheap.

Whilst on the subject of the screen supply, there is precious little
smoothing there. Do the manufacturers recommend matched output valves? In
some ways they are not necessary, because you can adjust the DC idle
current via the two bias controls such that the DC balances through the
two halves of the output transformer. Similarly, AC balance with respect
to the signal can be adjusted at the cathode of the concertina phase
splitter (2nd half of V1). That leaves hum to consider (power supply
rejection, or PSRR, is a key phrase to look for). The idea is that,
assuming the hum voltage on the screens has an equal effect on both sides
of the push-pull arrangement, the resulting hum output will be equal and
opposite through the transformer, and the hum will cancel.

However, if the valves react slightly differently to hum signals on their
screens, the total may not cancel. If you have a DMM that will tolerate a
high voltage on both leads, you can set the bias as per instructions and
then measure the voltage drop across the 5.1 ohm anode resistors (one
handed...use a clip on one test lead), using the results to calculate the
current through each individual valve. It should be fairly closely the
same through each. If not, you may gain some benefit by intelligent
rearrangement of the valves, so that as far as possible each has a similar
partner on the other side of the push-pull circuit. You will need to
readjust the bias and AC balance afterwards. I would appreciate the
comments of others on this particular point.

The two circuits you have posted are different in important hum-related
respects! One has a point marked G which should be at or near ground but
it has a label like it should be a power supply connection, but those stop
at F. The other shows this as a test-point for the bias, and a ground
connection. The first shows a 10 ohm resistor (R53...only 1/2 W, is that
wise? Anybody?) between the signal ground and the chassis, the second
shows a connection to the centre of a transformer winding instead. Which
is yours? If you mix them up you could end up with hum.

It's a monster to work on for your first project.

I'm not an expert, BTW...just keeping the discussion going.

cheers, Ian


Thank you Ian.
YOu know I just got a schematic from Ampeg today for my newer SVT2.. which
is different from the amp we are working with ... for that newer amp, they
say there is a mod.. the mod is to remove the 10 ohm R53 resistor and put a
jumper in there.. to go straight to ground...I have not done that yet... i
dont know if that is a mod for safety or for noise problems...
This weekend I am going to try your suggestion to test the voltage drop
across the 5 ohm cement resistor.

Do these things eat
valves? I bet they need good quality, rugged 6550s.

well i guess it would depend on how loud i play.. my first SVT that i got
back in 1971 was used.. i had no idea about Bias or replacing tubes or
anything except turn it on, turn it up and rock out back then... i ran it
through 2 x SVT cabinets . each cab had eight 10" speakers in a sealed cab
with a total of 16 speakers between the two.. it was loud.. but it had alot
of speaker surcae area too... i dont think i ever ran it past 11 oclock
though... those tubes last me about 2 years and were most likely the
originals from 1969 or so..
eventuall i remeber one tube went south.. it took out the ressitor and i saw
flames on the main board through the grill cloth... i blew it out. the amp
kept going it was just a little lower in volume... since it was running on 5
tubes i suppose..
My other SVT that i got in 1989 was used on a world tour.. i had 2. one was
a backup.. it was on from sound check at 6 pm an dthrough showtime which was
anoy 9 pm to 11 pm.. 4 days a week for a year..... when i got home i
replaced the tube set only because when they packed the gear in austrailia
to send back to the states a tech just wound up cables in the back ofthr
rack and a metal jack went through and broke the bottle of the 6550s.. so i
guess i got my moneys worth from a set..
i hear you need to replace tube sets about once a year in med to heavy
useage and maybe every three years under casual player usage.. as they say
... milage may differ...hahah

--
Thank you for all the help so far..
Just for fun . I ordered a new Hum balance control today.. I am sure this
one is good.. but i still want to put it to rest..I should have it by
monday.. and drop it in ... i dont think it will help but ya never know..

wrote in message
oups.com...
I found those caps, they are the only 0.1 caps on the diagram, and all
around V1. I would suggest that you replace all of them or none of
them, and make sure that the caps that go in are screened past just the
basic specs. If you can get them within 5%, it can only help. But, yes,
DO try to replace them with 0.1 caps ILO the 0.22 that are in there
now. If you do not have immediate access to these caps (they are mighty
cheap, BTW), I can send you a set. Just tell me what style they a

__ __
---[__]--- This way [__] or this way.
I I

BTW, just confirm: This problem has been with you as long as you have
owned the amp, or is it a post-acquisition development?

I am assuming that you do not have a lot of specialized equipment to do
other than the most obvious VOM-related tests. I also assume that you
understand that there are lethal voltages on this amp when in
operation, and that you do not poke around inside without full care and
knowledge of the risks....

Peter Wieck
Wyncote, PA


Peter. It gladdened my old heart to see your post - friendly, helpful and
informative. Just what RAT should be about:-) There are still a few
gens amongst the shale.

Regards to all
Iain

"Iain Churches" wrote in message
...

wrote in message
oups.com...
I found those caps, they are the only 0.1 caps on the diagram, and all
around V1. I would suggest that you replace all of them or none of
them, and make sure that the caps that go in are screened past just the
basic specs. If you can get them within 5%, it can only help. But, yes,
DO try to replace them with 0.1 caps ILO the 0.22 that are in there
now. If you do not have immediate access to these caps (they are mighty
cheap, BTW), I can send you a set. Just tell me what style they a

__ __
---[__]--- This way [__] or this way.
I I

BTW, just confirm: This problem has been with you as long as you have
owned the amp, or is it a post-acquisition development?

I am assuming that you do not have a lot of specialized equipment to do
other than the most obvious VOM-related tests. I also assume that you
understand that there are lethal voltages on this amp when in
operation, and that you do not poke around inside without full care and
knowledge of the risks....

Peter Wieck
Wyncote, PA


Peter. It gladdened my old heart to see your post - friendly, helpful and
informative. Just what RAT should be about:-) There are still a few
gens amongst the shale.

Regards to all
Iain
Gens? Gens? I meant of course gems.
Iain

For the postage alone from AES, I could send you a dozen caps and be
ahead of the game.

Seriously, I bulk-buy my caps from either Mouser or Santech, so three
0.1uF @ 630V would cost me about $0.94, then postage. For less than a
couple of bucks in total.

Let me know.

Peter Wieck
Wyncote, PA

OK Peter i will take you up on that offer... I emeailed you my address...
Thank you and thank you again... you have been a huge help...

--
---
Remove the "NOOOOOO00" from email address to reply
wrote in message
oups.com...
For the postage alone from AES, I could send you a dozen caps and be
ahead of the game.

Seriously, I bulk-buy my caps from either Mouser or Santech, so three
0.1uF @ 630V would cost me about $0.94, then postage. For less than a
couple of bucks in total.

Let me know.

Peter Wieck
Wyncote, PA

"svtbass" wrote in message
...
:
: loads of downloadable, a tip from Sander recently, he
: http://www.pmillett.com/Books
: :
: R.
:
:
:
: thank you very much
:
found this, while looking for the 6J5, as possibly a nice candidate for
flipper's SE hybrid opamp-of-sorts at 12 mA,

http://www.du.edu/~etuttle/electron/elect27.htm