Tremolo troubles in a non-tremolo amp

[C Moore]

A friend of mine has a 1969 Fender Bassman.
It was in BAD shape when he bought it. Blown OT, weird mods, a fiber board that Laird Hamilton could almost ride on, etc. etc.
So I stripped it down and rebuilt it. It basically has the AB165 preamp, and the AA864 Power Amp, PI, NFB.
The Bass Channel has a switchable cathode, a choice between Two different values of Cat resistor//cap combinations. I utilized the old Deep Switch for this. I changed the value of the components in the tone stack to try and make it more pleasing to his type of playing, but the circuit itself is pretty much stock.
The Normal Channel also hits V2, as in the AB165 schem. This channel is also pretty much stock BF Bassman specs.The problem is what sounds like a rather fast Tremolo. You can play the amp with no indication of this, but once you start to dig in with some chords in the "Bassier" register, it sets of this Tremolo effect.
If I pull the Phase Inverter it will stop. But what is strange (to me anyway) is that if I am plugged into the normal channel, and I pull V1, the problem stops.
So I do not even know what is happening, or how to correct it. And I am confused about what seems to be an interplay between the two channels. Other than the fact that they both share one half of V2 as a gain stage, I do not believe the channels should mix.....should they.?
So.....what can cause a Tremolo sound in a non-trem amp.?
I have a video my friend made. The amp was really overdriving the mic on the camera, and in the video, it sounds much more like "motor-boating". To hear it in person, it sounds much more like a fast tremolo. But I am more than happy to link the video. It starts at about the 25 second mark.
And, as in the video, when it starts, it just keeps on oscillating......
AS ALWAYS.....Would appreciate ANY help you can give me.
Thank You
https://www.youtube.com/watch?v=OSC83qd9Vjo

http://i111.photobucket.com/albums/n143 ... dan008.jpg
http://i111.photobucket.com/albums/n143 ... dan007.jpg
http://i111.photobucket.com/albums/n143 ... dan006.jpg
http://i111.photobucket.com/albums/n143 ... dan005.jpg
http://i111.photobucket.com/albums/n143 ... dan004.jpg
http://i111.photobucket.com/albums/n143 ... dan003.jpg
http://i111.photobucket.com/albums/n143 ... dan002.jpg
http://i111.photobucket.com/albums/n143 ... dan001.jpg
http://i111.photobucket.com/albums/n143 ... dan009.jpg

[thejaf]

Isn't that what you'd call classic "motorboating"? Pulling the PI would certainly kill any signal going to the power tubes, but pulling V1 really narrows this (IMHO, without looking at the amp in person). I'd double check your component values and ground connections V1.

[C Moore]

Isn't that what you'd call classic "motorboating"? Pulling the PI would certainly kill any signal going to the power tubes, but pulling V1 really narrows this (IMHO, without looking at the amp in person). I'd double check your component values and ground connections V1.

It might be motor-boating. But like I said, if you hear the amp in person, it sounds much more like a volume swelling/tremolo than a putt-putt, chain-saw, motor-boat kind of sound.
Yeah.....V1 stooped it, but that is while he is plugged into V3.

[Cliff Schecht]

Dude I bet Pink Floyd would pay big $$$ for an amp that does that.

You are definitely dealing with a motorboating issue. It's hard to tell sometimes because this is one of those things that shows its head in different ways in different amps. Two amps I recently finished together both motorboated because the OT primary leads were flipped. The first amp was a high pitched whine type motorboating and the other one sounded very similar to what yours is doing. Have you verified that the wires are the right polarity on the output transformer? I've learned my lesson with wire coloring codes too, you gotta get in there and either disconnect the negative feedback or flip the OT primary leads to see if this is the issue.

Could also be a bad electrolytic. Did you use all new parts?

Are the wires going from the main filtering e-caps to the circuit board really long? This can cause instability..

[Alexo]

Few months ago I was working on my 100W bass amp build... I added a gain stage and forgot that it would change that negative feedback to positive feedback - flipped it on and it was like the gates of Mordor had been opened.

Motorboating often comes on as some sort of oscillation at the resonant frequency of the RC network formed in the power supply. Some fixes are de-coupling PSU nodes that are currently shared, or just increasing the resistance between existing nodes. Also can happen if successive stages share a cathode bypass cap...

However, what you describe also sounds like it could just be ghost notes - the 60hz wall signal creeping into the audio path via the power supply when the filter caps are drained out. Increasing those caps, the resistance between them, or adding a choke are possible fixes..

[C Moore]

Yeah, all new parts.
I will have some questions regarding some of the above comments.
Maybe it is picture time, let me get the camera.....

[Alexo]

I just watched the vid - holy crap - definitely not ghost noting.

[Lynxtrap]

The AB165 circuit has a funny loop between grid and plate in the third stage. Plus it has negative feedback connected to the PI input grid, not to the ground side. Those could be possible sources for positive feedback if somethings not connected right or a component is bad.
Just looking at the schematic without seeing the actual amp, that's where I would start looking.

[C Moore]

The AB165 circuit has a funny loop between grid and plate in the third stage. Plus it has negative feedback connected to the PI input grid, not to the ground side. Those could be possible sources for positive feedback if somethings not connected right or a component is bad.
Just looking at the schematic without seeing the actual amp, that's where I would start looking.

Certainly some intelligent places to take a look. I was wondering the same about the 220k that separates the two channels at V2.
I added some pictures to my first post. Just in case somebody sees an obvious concern.....
Thank You

[Prairie Dawg]

Troubleshooting 101.

The last person in there caused 95 per cent of all the problems. To remedy this take a schematic, blow it up to twice size, and tape it to the wall next to your workbench. Make all your friends go away. Close the door. Turn off your phone.

Now. Start by positively validating every circuit path and every component value in the amp. While you're at it, replace all the stuff you thought you didn't need to in the capacitor line. As you go, trace out your validations on the schematic.

I would probably start by building it back to what it was supposed to be instead of modifying it.

Also remember that the problem is right there staring you in the face in a fairly small piece of real estate. And let us know what fixed it.

[Lynxtrap]

You seem to have the negative feedback connected the usual way, to the ground side. It doesn't matter as long as it on the right phase compared to the OT.

A couple of things I don't get. Where's the signal going after the last triode, physically closest to the PI? I can only see its blue B+ wire.

It's easy to make a mistake with that circuit, with the common triode and the funny layout Fender used. Trace the signal path again, and then once more

I like the funny cap that looks like a melting smurf!

[blam]

I was recently overhauling my class 5 and one of the things I was changing was the 220 ohm resistor for a hammond choke. (100mA 10H 195ohm)

the choke was supposed to be 262ohm, but I got the wrong choke and it measured 185 ohm. I got a VERY pronounced tremolo effect with this choke in (sounds exactly like your video). others who used the 262 version did not have this problem.

hopefully its something as simple as that?

[C Moore]

Lynxtrap -
You have good eyes. I do not like, and do not like to use "Under Board" wiring. But I did in this case. There is a yellow wire that runs under the board, right along the edge under, that connects Pin 6 of V3 to the grid (is it Pin 7) of V2.
I "know" I have done it twice already, but I will diligently trace the circuit again.
Just to reiterate. This problem seems to be, not only frequency dependent, but also amplitude dependent.
My friend played this amp "at home" for a week, and he said it was great. But when he took it to their rehearsal studio, turned it up, dug into some chords on the lower notes.......all the stars would align and the amp would oscillate.
Same thing happened at my house. He played for 10 minutes with no problems. But as soon as he digs into some low chords, the amp goes off in a Tremolo/Motor-Boat fiasco.
Thank You
btw....I got those little Blue Smurf caps from Mouser. I believe they are ceramic disc.

[Firestorm]

Could we see pix of the power supply (under the doghouse)? As Alexo said, if stages are not decoupled adequately, they will talk into each other. I think I would try disconnecting the feedback loop and see what happens. It may be that you need to follow the AB165 feedback scheme. Beginning with AB165 (the first amp that wraps the V3 signal back to V2) and continuing until AA371, all Bassman amps use that type of feedback network. Before and after they used the one you have. I can only assume they were fixing something.

[Lynxtrap]

turned it up, dug into some chords on the lower notes.......all the stars would align and the amp would oscillate.

I think that's consistent with motorboating or oscillation. Lower notes contain more energy and load down the stages more. I think - correct me if I'm wrong. Turning it up, digging in... it all points in the same direction, IMO.