'66 Blackface bassman choking from oscillation (??)

[doctorfate]

hello all, I haven't frequented the forums much for awhile but I could really use any help I can get on this problem with a 66 Bassman on my bench that has me completely stumped.
The volume can't be turned up more than two or so before it really just oversaturates and compresses (almost like an envelope effect) and with the amp dimed there isn't even a fraction of typical volume. I have been over and over this amp, all voltages seem fairly normal and all resistors are within tolerance. I've been leaning towards this being some sort of wicked oscillation problem that's partially 'shutting the amp down' but I just can't find it. Someone please throw me a bone. here's what I've done
-new power filters and bypass caps
-plate R's and checked all coupling caps
-tried raising the value of power tube grid r's (though i don't think it's an issue of blocking distortion)
-subbing all tubes w/ known good of course
-tried jumping in another known good OT... and plenty of other stuff I'm sure

One important thing that may help is that though this exists on either channel, even with the opposing channel's tube removed, the problem does seems to be more prominent in the Normal channel. In fact with the Normal Ch cranked about 3/4 or more, turning the treble pot to full will completely shut the volume down!!
This reeks of oscillation to me but I could be very wrong.

any opinions or experience with a problem such as this are very welcome....THANKS GUYS!

[Enzo]

Don't assume all problems are bad parts, often there are other reasons. Lead dress being a common one. Try moving the wires to the power tubes, poor dress can cause instability.

Scope the output, if it is oscillating, you'll see it.

No scope? Look up "RF probe" and you will see many versions of the same simple thing - a diode and a cap that allow you to measure very high freq AC with your basic meter. Use that to detect oscillation.

[Prairie Dawg]

After you have rechecked all your work it is worth hooking up your bias probe and monitoring it, if it oscillates it may make the bias current go crazy. I worked on an Ampeg a guy found in a barn and it took a while to get it back to where we could figure out why it ended up in the barn in the first place. It was oscillating like crazy and it turned out that I had to relocate a ground wire about two inches. But it took a while to get there. One thing you can do is advance the volume to the edge of oscillation and then start poking things looking for a pitch shift. That's what put me on the right track.

[Chuck H]

You say all voltages check out, But have you really checked all voltages? What about the preamp tube bias voltages? I've seen those boards go conductive (a lot) and this can do all mannor of bad things. A little voltage on the preamp cathodes is a lot of voltage on those circuits. You might also check for leaky coupling caps. A film cap can read the correct value and still leak enough DC to throw the bias of the following stage.

Chuck

[doctorfate]

very good ideas. I should've elaborated more on what I HAVE tried (not that I couldn't have missed something of course) I actually don't assume this problem from a 'bad part' which is why I'm posting, it does seem to be a case of one stage picking up junk from another and no easy to locate reason why.
I have checked all the coupling caps for leakage. Lead dress and ground have been my main returning points here based on the symptoms etc... I've actually checked/reworked all lead dress off of another 'untouched' bf bassman paying extra attn around the ot tubes but it couldn't be that easy i guess.

At one point it was much worse and the normal ch tone controls would pitch shift something awful. So yeah I'm quite sure there is oscillation going on. Actually so bad that with a dummy load on it , the signal was actually coming from the OT like a little radio speaker! I seem to have somehow improved it to where now the amp is just low volume, choking and gating out.

I was also looking at the preamp biasing for this but I think it'd have to be WAY off to cause a prob this bad and this highest voltage on any cathode I think was about 2.4v (changed the cap/res and stayed the same)
And yes my scope is on the fritz very unstable. Interesting idea with the rf probe enzo , I'll look into that.

finally the output tube bias is a good point. Espc when it was oscillating audibly I did notice that bias current was going nuts, getting way up into 100mA i think) I wish it was simple as moving a ground. Best I could do was check all grounds for continuity and removed the whole buss to check it. Also I have removed the bottom board and cleaned it suspecting possible conductivity.
sorry for the ramble but this thing's got me freaked. I have faith one of you can point me the way on this at least

[Pedro Vecino]

Disconnect feedback line and try again

[Euthymia]

Pedro's on the right track there. If you disconnect the feedback line and it goes away, your OT primary leads are probably swapped.

[doctorfate]

No I've had the OT leads( and plate leads) swapped several times going after this oscillation. At this point I'm trying anything so yeah I'll disconnect the neg fb res in a bit just to check once again. It's really strange , the way it's behaving right now the amp sounds like a strong envelope effect, very heavily compressed, when turned up very much.
I will say i had done the typical 864 phase inverter mods to this AB165 ( which sorry I forgot to point out the circuit in this amp but figure it was assumed based on the year) and it seemed that's when the problem worsened some although everything was done as I always do it, so right now I've put it back to stock aside from a couple reduced coupling caps to at least be a ground zero to troubleshoot.
I really appreciate the help guys

[doctorfate]

Ok I'm as lost as ever w/ this one. I tried disconnecting the neg fb which gives only a slight improvement on the choking up. No matter what the amp is very saturated at even the lowest volumes , either channel. SO I decided to rework the PI back to AA864 style again and see what I get, swapped the OT secondary accordingly.
The input has to be slammed harder to get a signal this way and again I hear the signal coming from the output transformer ! so I diconnected the neg feedback with this setup and hit it with a signal...Well, the screen resistor on the first power tube lit up like a sparkler!!!
This is a tube socket I even tried replacing, thinking it may be arcing. ( made no difference of course) I think it was this tube I noticed the bias going crazy on and not the second.
What could this be caused from !? hope this gives someone another idea cause I'm about ready to ask to buy just so I can mutilated the thing

btw...that 3w metal oxide screen r' still reads good although it sparked for like 5seconds and looks a tad baked. Dying to find out what gives with this thing.

[52 Bill]

What are you using for a dummy load?

Are you sure that it is hooked up to the amp correctly?