Ampeg B-25 Amp Problems

TheAmpNerd

Member
Messages
1,056
I've got this Ampeg B-25 that is giving me fits.

Anyone have an old service manual for this amp?

I've got the schematic, pasted in the amp....
just about everything checks out on it
from the voltage perspective.

Pre amp isn't functioning properly
I think in the PI and right before it.

Very weak tone...

Shotgunned it too, along with tubes
and anything remotely out of spec.

And additional tubes just to be sure and they all check.

Not sure where else to look.

Happy daze are here again.
 

SatelliteAmps

Member
Messages
6,186
I don't think there is one that will help. When you say "just about everything checks...." what does that mean? If one thing is out of spec it can throw the whole thing out.
 

TheAmpNerd

Member
Messages
1,056
I don't think there is one that will help. When you say "just about everything checks...." what does that mean? If one thing is out of spec it can throw the whole thing out.
Just what I said just about everything checks...

...voltages, values, etc.

...amp had been hacked.

...corrected most of it, that which I couldn't correct
has been disconnected (channel 2 hack [for now]).
Amp isn't too bad...still have problems with cross talk.
Hence the channel 2 disconnect.

Working on the motor boating, and some pre-amp squeal at times
nearing full volume and with treb/bass pots wide open....almost
wondering if those RC All in one packed things that Amped liked
to use has gone.

Wondering about the ground scheme on this amp.

Anyone have some chassis pics of what the grounding
scheme looks like?

My bass cab's jacks decided that this moment was the
right time to screw up on me. Go figure.

 

SatelliteAmps

Member
Messages
6,186
If you have motor boating, squeal, and one channel disconnected due to bad modifications, then there are a lot of different issues going on. Squeal can can be lead dress, bad mods, bad tubes, bad soldering, or even transformer issues, which could have mostly correct voltages.

Motor boating is usually a B+ issue somewhere. If you are having squeal in the preamp, look at the electrolytics there.

When you say cross talk in between the channels, what does that mean? There will be some bleed because they are in the same power amp. That might be a feature, not an issue.

How much of the second channel is disconnected? Just cutting the wires there can affect the voltages in the rest of the amp. The best thing to do is to try and get the amp schematically correct, and then troubleshoot, not the other way around.
 

TheAmpNerd

Member
Messages
1,056
There were some coupling and decoupling issues that I resolved. I also lowered the ESR of the electrolytics feeding the power tubes.

I relocated and increased the value on the power tube grids as well.

I removed all the signal wires from channel 2 in the preamp section
and left all the control resistors, etc in place along with the tube.
This keeps the amps voltages proper.

I'm really surprised at how much the amp is affected by one pre
amp tube. Your talking about 30 volts on the main B+ and over 100V
in the pre amp section.

It looked like the amp took a hit at one time so I resoldered every
joint on the boards.

The cross talk between channels was really bad, seems the 270K
resistors in the mixer don't isolate the channels enough and turning
the knobs in the channel two and or pressing the hi lo ultra switches
turn the amp to oscillating like crazy.

Yep, that is what I've been doing going through the schematic and
trying to get everything proper, voltages an all. When I got it amp
was running B+ about 575V with about 480 in the pre amp section.
All the little 400V coupling caps were roasted, some even popped
their ends off.

I've finally managed to get it under control and sounding good
though very noisy still.



If you have motor boating, squeal, and one channel disconnected due to bad modifications, then there are a lot of different issues going on. Squeal can can be lead dress, bad mods, bad tubes, bad soldering, or even transformer issues, which could have mostly correct voltages.

Motor boating is usually a B+ issue somewhere. If you are having squeal in the preamp, look at the electrolytics there.

When you say cross talk in between the channels, what does that mean? There will be some bleed because they are in the same power amp. That might be a feature, not an issue.

How much of the second channel is disconnected? Just cutting the wires there can affect the voltages in the rest of the amp. The best thing to do is to try and get the amp schematically correct, and then troubleshoot, not the other way around.
 

SatelliteAmps

Member
Messages
6,186
When you say you removed all the signal wires for channel two, does that mean there is nothing on the grid and the plate? Or did you just remove the grid wire? That might be your problem right there.
 

TheAmpNerd

Member
Messages
1,056
I removed the wiring from channel 2 that went from the pots to the board.
There are no additional wires from the grid or the plate, everything is on
the circuit board.

NOTE: The pre amp tube sockets are on the circuit board with plate and
grid wires going to the pots (4 wires) and 1 wire going to the pre amp
mixer, which feeds the PI.

The coupling caps have all been replaced and the resistors that control
the tubes operating points have been measured and are in spec.

Ampeg really screwed up when they did this amp. They have the speaker
jack located right on top of the output tube socket...1.5 mm clearance
between the jack, plug and the 7027 A pins!



When you say you removed all the signal wires for channel two, does that mean there is nothing on the grid and the plate? Or did you just remove the grid wire? That might be your problem right there.
 

SatelliteAmps

Member
Messages
6,186
You have the second channel wired wide open. When you removed the wires going to the tube, all you did is remove the volume and actual signal, but didn't stop the tube from working. That is going to be an insane source of squeal and hum. You have to remove the source of voltages being applied to that tube to stop it from affecting the circuit. The plate is wired on the circuit board to go to the B+. Check the schematic and you will see that it is all still wired up.


As for the output jack, get the amp working and see if it is an issue. It usually isn't with that amp.
 

TheAmpNerd

Member
Messages
1,056
You have the second channel wired wide open. When you removed the wires going to the tube, all you did is remove the volume and actual signal, but didn't stop the tube from working. That is going to be an insane source of squeal and hum.

How can it be? I also removed the wire going to Z, which is mixes the
signals from Channel 1 and Channel 2 to the PI. This is where the 270K ohm
resistors are.

If I do anything to shut off V3 (PI tube) I'll have way too much voltage
in the pre amp stage--as I mention in the prior post.

You have to remove the source of voltages being applied to that tube to stop it from affecting the circuit. The plate is wired on the circuit board to go to the B+. Check the schematic and you will see that it is all still wired up.

As for the output jack, get the amp working and see if it is an issue. It usually isn't with that amp.

Hmmm, I'm almost thinking that I need to isolate the B+ from each
gain stage, ala what's that tone guy from Canada?, O'Connor.

There is something else going on here that I'm not sure about.
Perhaps the power tranny is fried and just makes the amp hum
like crazy.

It could have been the cheap fix the hack tech tried to implement
by feeding the PI with a pair of 12AU7s, instead of 12AX7As....
That is, significantly reduced gain, therefore less hum. He actually
wrote this with paint marker on the metal circuit cover where the
pre amp tubes reside.
 

SatelliteAmps

Member
Messages
6,186
I am now staring at the schematic. If you are having these issues, and everything is fine except for what you are saying it is, you can pull V2 with no issues or harm to the amp, and that should eliminate the issues you are having, if that is where the problems are. Are you sure you pulled the wire from R21 (what you are referring to as point Z) and not R14? R21 should be on the circuit board, not a spare wire going to a different spot. Is R14 still hooked up to the B+?

I don't think I suggested nor would anyone suggest shutting off V3 to fix anything. You would get no signal.

Power trannies don't fry themselves and only make hum or create noise issues. Running 12AU7s will reduce gain (and a lot of bass players do this to Ampeg amps), but it's not really a fix for hum issues. The B-25 amps are not known for being extremely noisy amps.

The only step I think that will help is to get the amp to the specs of the schematic. If you are unwilling to do that, the only other thing to do is to check all the voltages on each pin of each tube, and post them up here so I, or someone else, can take an educated look at what you are getting.
 

SatelliteAmps

Member
Messages
6,186
I already have the exact same schematic as that. I need you to measure the voltages on each pin of each tube and list them here, so I can see how they compare to the actual schematic. That is the only thing that will help get anywhere else in this discussion.
 

TheAmpNerd

Member
Messages
1,056
Nothing special voltage wise:

Pin V5 V4 V3

1 238 218 308
2 - - 266
3 2.2 2.4 78
4
5
6 239 233 1.8
7
8 2.1 2.2 139
9 118

some a tad high in the B+
E C B A
342 460 529 535

Bias at -56

Outputs
3 528
4 526


Current approx 33mA to CT.
.033 x 528 = 17.4watts/tube
 

SatelliteAmps

Member
Messages
6,186
There are a few issues you might want to look at. In the filtering, if you have 535 and 529 on points A & B, then you might want to figure out a way to drop that. That is going to be over what those caps can handle. Could be causing noise issues as is, as well as not being able to filter correctly. Could pop.

Are you using a 5AR4? If so, you will want to look at that whole section to see why the B+ is so high there.

Pushing over 500v on the grids (pin 3) of the 7027A's is also pushing things a bit much. You might want to adjust the bias a bit to compensate.

The voltages you are getting on V4 are showing that it is still playing an active role in the circuit. It is amplifying noise, but not input signal. Pull the tube and see if it helps with the noise and crosstalk. This really is the source of the squeal and oscillation when adjusting the controls for this channel.

Try these and get back to me.
 

TheAmpNerd

Member
Messages
1,056
There are a few issues you might want to look at. In the filtering, if you have 535 and 529 on points A & B, then you might want to figure out a way to drop that. That is going to be over what those caps can handle. Could be causing noise issues as is, as well as not being able to filter correctly. Could pop.
I have, new ones are mounted in series.

Are you using a 5AR4? If so, you will want to look at that whole section to see why the B+ is so high there.
It is just cranking it out. If the tranny was wound a tad hot from the
factory...and in this amp....550v is still with in spec...thats 10 percent.

Pushing over 500v on the grids (pin 3) of the 7027A's is also pushing things a bit much. You might want to adjust the bias a bit to compensate.
I could bump it down some more.
perhaps around 55 - 60 percent.

The voltages you are getting on V4 are showing that it is still playing an active role in the circuit. It is amplifying noise, but not input signal. Pull the tube and see if it helps with the noise and crosstalk. This really is the source of the squeal and oscillation when adjusting the controls for this channel.
Now when I pull V4, things get out of hand voltage
wise. While V4 is in there, and the signal wires removed channel 2
pots have no effect. With the signal wires in the amp and the tube
removed, the pots send the amp into oscillation and add more noise;
yes it is even worse with the tube in V4!


Try these and get back to me.
 

SatelliteAmps

Member
Messages
6,186
Just trying to understand exactly where you are with this...

I have, new ones are mounted in series.
What values in series?


It is just cranking it out. If the tranny was wound a tad hot from the
factory...and in this amp....550v is still with in spec...thats 10 percent.
550v is over spec, and will destroy the original filtering. Transformers are not usually rated as a 10 percent off spec. That would give a 100v swing either way with this circuit, which is not something that would happen. None of the values would make sense schematically.

There are ways to drop the voltage that might be worth looking into in the end, assuming everything else checks out. Are the resistors in the B+ section all reading correctly? What about the AC on pins 4 & 6 of the 5AR4? What are you getting for your line AC, and for your heater voltages?

I could bump it down some more.
perhaps around 55 - 60 percent.
I would, at least until the preamp and power sections are figured out. No reason to slam the tubes the whole time.

Now when I pull V4, things get out of hand voltage
wise. While V4 is in there, and the signal wires removed channel 2
pots have no effect. With the signal wires in the amp and the tube
removed, the pots send the amp into oscillation and add more noise;
yes it is even worse with the tube in V4!
When you pull V4, what voltages exactly get out of hand? Do you have a variac to drop the overall voltages to get them into spec when this happens?

If the amp goes into oscillation with the wires and the tube removed, then not all the wires are removed from that part of the circuit. The B+ connection is still going to both the V5 and V4. If you disconnect R14 and R18, then you should be able to remove all the voltages from V4, and isolate the channels. I highly doubt it will come to this, but you may have to isolate out the high voltage to V4 and V5 to keep them separated.

This is the fun stuff.
 

TheAmpNerd

Member
Messages
1,056
Thanks, the fun stuff:

Added capacitance a la Kevin O'Connor, to this and used his method
of raising heaters above ground etc. This helped the noise threashhold
considerably.

The amp is receiving 440 VAC to each side.

All the electrolytics have been replaced along with the coupling caps....
Most of the couplers were gonners too. This settled the amp down too.

Heater voltage is still high, need to put some voltage drop resistors in
there to bring it down to 6.3 VAC. A couple of .1 ohm/25 watters
should get me there, or just be a tad much...6.2 VAC perhaps.

At some point I plan on doing O'Connors outlined method of using
PI filters in the B+ to further noise reduction and quiet the amp down.

Channel two signal wires are removed from the board and taped off.

with V4 tube removed, the pre amp voltages rise more then 100 Volts
and the V5 and V3 tube bias voltages rise. I'm sure the heaters rise
another .2 - .3 Volts.

The amp sounds pretty good though. Probably should play guitar
through it. : )

That is kind of where I'm at on this, thank for checking in.
 




Trending Topics

Top