Silverface Bassman Hiss (Oscillation?) Problem

[AtomicMassUnit]

I'm working on a silverface Bassman head. It has been blackfaced, changed to a level bias setup, but I noticed it had a lot of hiss and just seemed unstable, so I left the caps to ground on the power tubes. Initially I thought the hiss was from plate resistors, so I changed those out and it lowered it a bit, also did the power supply resistors (caps are sprague atoms for the 1st pair, grey illionois for the rest).

Played the amp, all seemed fine, gave it back to my buddy. He played it a while, no problems, then jumpered the channels and kept playing. After a half hour or so, he said the volume started dropping and he heard a buzzing sound that got louder and louder until he shut it off.

I was thinking it was from the grids of the preamp tubes having unshielded wire, so I replaced them with shielded. Played it, and it still seems unstable, or on the verge of oscillation. Even with the volume controls down, there is a good amount of hiss, and it doesn't stop entirely until I pull the 2nd tube (gain stage for both channels).

I have a scope but don't know how to use it to find oscillations. How do I proceed?

[olddawg]

Put your scope across the output load. If it is oscillating you will see it. You will have to turn up the time base to see the waveform.

[AtomicMassUnit]

Put your scope across the output load. If it is oscillating you will see it. You will have to turn up the time base to see the waveform.

Don't have a camera to show you a pic at the moment, but the scope shows a fuzzy looking waveform with a few tiny spikes coming off the tips here and there. I also noticed the amp started motorboating for a second then stopped.

[olddawg]

Don't have a camera to show you a pic at the moment, but the scope shows a fuzzy looking waveform with a few tiny spikes coming off the tips here and there. I also noticed the amp started motorboating for a second then stopped.

What is the frequency of the waveform? If it is oscillating it will be above audible range. If it is a dual trace scope, run a 1kh tone into the input and compare the input and output waveforms with the two channels. Does it have the original OT? I guess you could say motorboating is a type of oscillation too. Does amp only motorboat when you put a scope probe across the output? Is it s 10:1 probe?

[AtomicMassUnit]

Thanks olddawg. Some of your questions are a little over my head, so I will do my best. I am new to scopes, and don't have a good understanding of them.

The OT is original. It is motorboating without the scope, just takes longer to start. This is a dual channel scope.

I am trying to figure out how to measure the frequency. Please bear with me as I am trying to learn how to answer your question! I am getting a horizontal division of 6, and it's at 2ms. So, if I'm doing this correctly, that would be 6x2=.0012 and

1/.0012 is 833hz? That doesn't seem right.

I don't think it's a 10:1 probe... doesn't say that on it anywhere, but I'm new to using a scope, still don't understand it! I think that part of it has me running in circles.


So, basically the amp is hissing at a fairly loud volume, and turning down the channels doesn't stop it. Pulling V2, the shared stage after the preamp, or shorting the grid to ground, silences it. Pretty much all the circuitry around that tube has been changed. If I turn up either channel, a normal sounding amount of hiss comes up in addition to the constant hiss already there.

***pulling the V3 (normal channel) AND V1 (Bass channel) tubes at the same time is now silencing the noise. With either one in, it's back. grounding any of the grids on either one doesn't stop it though.

[olddawg]

If you are using a 1k test tone tone, the source isn't calibrated, you didn't count the divisions right, whatever, 800-900hz is close to 1k. I would be suspecting an electrolytic cap somewhere. But I'm sure there are people on this board with a lot more experience on Bassman amps than me. They will probably want to know what you mean by "blackfaced" since it means different things to different people and there are different degrees of doing it. Personally, I would put the amp back stock and start over.

[Gtr_tech]

The hum noise sounds like a failing tube. But....you'll find that when you add shielded cabling in the front end, you introduce capacitance that the next stage might not be cool with.....therefore...oscillation. Small value grid stoppers on the following stage usually takes care of that. Somewhere in the ballpark of 2-5k. Helps to isolate the grid from the capacitance of the cable. I've had to do this many times when trying to cut hum pickup and the grid stoppers don't change the tone.

[AtomicMassUnit]

If you are using a 1k test tone tone, the source isn't calibrated, you didn't count the divisions right, whatever, 800-900hz is close to 1k. I would be suspecting an electrolytic cap somewhere. But I'm sure there are people on this board with a lot more experience on Bassman amps than me. They will probably want to know what you mean by "blackfaced" since it means different things to different people and there are different degrees of doing it. Personally, I would put the amp back stock and start over.

Oh, that was without any tone going through the amp, just the hiss.

I found a cathode bypass cap that wasn't soldered well, just twisted around the other one next to it on the ground side, and that seems to have quieted that stage a bit, but there is still a lot of hiss when I turn up the volume controls on either channel. I have a Vibrolux Reverb that makes nowhere near the noise this one does side by side. For an amp that's relatively clean and low gain this thing is just so noisy.

[AtomicMassUnit]

Ok, here's what I know for sure. I can play an instrument into V2 and get a nice clean, strong signal. The noise is before that.

If I go into the grid half of V3 (normal channel recovery stage), I get the loud hiss. I am guessing it's the same on the Bass channel. So, that leaves only that one gain stage to check out, but ALL of the components for it are new. Cathode resistors, cap, plate resistor, 220k feeding the next stage, the 470k that goes from there to the plate of V2. I suspected the filter caps for the preamp, and changed them, but it made no difference. If it were further upstream in the power supply it would be heard from V2 on, right? This just isn't making sense. I don't get why I'm getting the same amount of noise on both channels, either. The only shared parts are the power supply, and I changed those! ARG!!

[loudthud]

Eyelet boards have been known to cause noise, allegedly from moistue absorption. The cure is to gently heat and dry the board with a hair dryer or a couple of hours sitting in the sun.

[AtomicMassUnit]

Eyelet boards have been known to cause noise, allegedly from moistue absorption. The cure is to gently heat and dry the board with a hair dryer or a couple of hours sitting in the sun.

Thanks for the idea! I will try this.

[AtomicMassUnit]

Dried it out with a hair dryer, but it didn't make any difference.

*update*

If I pull V2 out of the amp, everything works and plays clean, with no noise on either channel. Apparently the noise is somewhere in that tube stage. I have changed every part for that tube. Any ideas?

I'm getting 279v on the plate, .016 on the grid, and 2.2 on the cathode of that tube stage.

[AtomicMassUnit]

The hum noise sounds like a failing tube. But....you'll find that when you add shielded cabling in the front end, you introduce capacitance that the next stage might not be cool with.....therefore...oscillation. Small value grid stoppers on the following stage usually takes care of that. Somewhere in the ballpark of 2-5k. Helps to isolate the grid from the capacitance of the cable. I've had to do this many times when trying to cut hum pickup and the grid stoppers don't change the tone.

I will try adding some grid stoppers. When I got the amp, the channel 2 volume control output had a shielded wire, so obviously someone tried quieting this thing before.

I am going to a try a couple other things. I'm going to check every part and connection one by one, and if nothing shows up I'll try disconnecting one channel completely and see if I can get the other quiet. Could it be the 68k or 1m resistors on the input jacks have picked up some moisture? The thing that's so puzzling is that it's on both channels, but before the stage where they combine.

[AtomicMassUnit]

I will try adding some grid stoppers. When I got the amp, the channel 2 volume control output had a shielded wire, so obviously someone tried quieting this thing before.

I am going to a try a couple other things. I'm going to check every part and connection one by one, and if nothing shows up I'll try disconnecting one channel completely and see if I can get the other quiet. Could it be the 68k or 1m resistors on the input jacks have picked up some moisture? The thing that's so puzzling is that it's on both channels, but before the stage where they combine.

Ok, I verified all of the parts values in the preamp, then disconnected the Normal channel completely, so I'm just troubleshooting one channel.

On the preamp tube I'm getting
Pin 1-235vdc
2-.003vdc
3- 1.8vdc

Same on pin 6,7,8.

All the grounds are solid. I tried replacing the volume pot. It's not a tube, I tried many different tubes. It's got new plate and cathode resistors and cathode cap.



I made a quick recording of the noise. This is from about a foot away, turning the amp from 0-4 on the volume control. I do that 3-4 times and then turn the tone controls, treble, then bass.

Tell me if my logic is faulty here... The volume control turns up the hiss, so it's before the volume controls. The tone controls don't change it, so it's after the tone controls... Where is it? The tone controls do work, but they don't do anything to the hiss.

To add the grid stopper, do I put it after the volume control?

[Gtr_tech]

Thats just hiss, not oscillation. The grid stoppers are for oscillation control...so they will be of no benefit here.

To me, this is just normal carbon comp resistor noise.....there's no crackling or zipper" noise associated with failing resistors. Maybe if you *need* it quieter you could replace all the plate loads with metal film types.