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66' Bassman OPT Whistle?

4/29/2000 10:15 AM
66' Bassman OPT Whistle?
Was playing my Bassman tonight, and every once and awhile I started to get this high pitch noise. Not quite a whistle but more of staticy whine. It was kinda coming and going, at first it seemed to go away when I played with the treble control. But after awhile it just wouldn't stop. So I was thinking either the pot is bad or the tubes. I just put in a new set of Svet 6L6 biased at 30ma. So I start tapping on tubes and basic troubleshooting(smack the top of the amp). Don't know how I stumbled on this but if I push the output trannie to the left--the pitch of the noise goes up, and if I push it to the right the pitch goes down.  
Trouble in Mudville? Anybody run across anything like this before? OT go bad?  
4/29/2000 4:10 PM

I haven't a clue on that but I just wanted to get this back up to the top for you. Just out of curiosity have you put the old tubes back in to see if it does it with them. New tubes can be bad (seen this too many times). Just a thought.  
Good luck.  
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4/29/2000 4:24 PM
Gil Ayan

Hard to say without being there, but there are a couple of clues there. If the treble control affects the problem and so does moving the transformer, my suspicion would be that you have a HF oscillation after the PI.  
Let me ask you, are you running the amp in open loop configuration? If so, try some feedback and that may do it for you. If not, do you have a presence control in the amp, and are you running it wide open?  
Lastly, if poking around doesn't seem to point to anything in particular, try the following. From the input to the P.I., hang a very small cap to ground, could be something like a 20, 47 up to 100pF. If you find that nukes the noise, leave the cap there. It has been my experience that up to 100pF in that position can do wonders in terms of getting rid of parasitic oscillations without actually killing "audible, useful" treble content. Of course, you can do something similar across the plate load resistor of one half of the PI (try around 120pF there), or 47pF from plate to plate, etc. All these tricks are "relatives." ;/  
4/29/2000 4:26 PM
Ray Ivers

It sounds like high frequency power-amp instability to me. I think that moving the transformer either a) changes the transformer case ground situation or b) changes the lead dress inside the amp, although it could be something else (don't forget the effect the presence of your hand can have on any hum or oscillation problem). First off, I would make sure the OT's mounting bolts are tight.  
Is your amp the AA864 circuit, or the AB165? I have one of these amps, too, and I don't really know which mine is. A wire from the speaker jacks to a 47K resistor in series with a .1 uF cap indicates an AB165; an 820 ohm resistor to a 22K / 100 ohm to ground indicates the (more stable, I think) AA864 circuit. If you pop this wire off, the problem will either go away or get much worse, both of which will help with figuring out the next step.  
A troubleshooting tool for this might be a wooden-handled screwdriver wrapped with electrical tape and clipleaded to the chassis. With the amp out and on the bench running, move this probe around the power amp and speaker jack areas (no need to touch anything, of course) and see if the oscillation changes in frequency or goes away altogether - I've found a frequency decrease to mean 'things are getting better' and an increase to mean the reverse.  
Tell me what you find!  
Ray Ivers
4/29/2000 5:45 PM
Steve A.

Don't know how I stumbled on this but if I push the output trannie to the left--the pitch of the noise goes up, and if I push it to the right the pitch goes down.  
    You could hook up a foot-controlled lever to push the OT back and forth for spacy sound effects! < grin >  
    On a more serious note (C# is about as serious as they get!) I'd try a 50pf to 150pF "snubber" cap across the PI plates. Either right on the tube socket between the #1 and #6 pins or trace those leads to the circuit board (the upside down "y" with the typical 100k and 82k resistors going to B+).  
Steve Ahola  
P.S. Is this a stock amp or have you been "rumbling" around again? Just wondering because high gain preamps can do a lot of weird things...
4/29/2000 9:32 PM
A little different today--More Details
"You could hook up a foot-controlled lever to push the OT back and forth for spacy sound effects! < grin >"
You goofy Bastard--you slay me. You can always count on Steve for a upside to any problem. I'm still roflmao!!!!!  
More details:  
The Bassman was the AB165 I did the Andy Ruhl Blkfacing mod on. It's never done this whine thing before so I did a bit of experimenting trying everybody's advice.  
First off--the amp wouldn't go into oscillations without help, except once right after I put a 100pf across the plates of the PI. If I grabbed the OT nothing would happen, if I pulled on it to the right it would start oscillating, moving it to the left had no effect. All of this was with the volume between 4 and 5. The OT temperature never changes either--cool to the touch.  
But's here is what I did.  
1. Put the old tubes back in--other than it wouldn't osc. without help no change. Old tube were 6L6WGB's.  
2. 100pf Snubber across the PI. This was the one time it osc. on it's own right as I turned it on. Played with the OT and it went away and worked like it did before. Only osc. when manually induced.  
3. While pulling on the OT pushing/pulling/separating wires that come from the OT. I should mention--when I mean pulling on the OT it isn't moving more than 1/8 either way at the end away from the chassis. I just assuming this is flex in the transformer mounting or chassis. It is bolted tight.  
The only difference I can think of is the size of the tubes, the svets are easily twice the length of the WGB's, maybe somehow interfering with the OT somehow(mag field or something--out of my league at this point).  
any more ideas?  
4/30/2000 1:04 AM
Gil Ayan

"any more ideas?  
Let me repeat what I wrote above: try a 20. 47 up to 100pF from the input cap to the PI to ground. See if that does the trick without killing audible tone.  
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