1970 Twin Reverb Oscillation is Motorboating?

[bbmade]

Working on an amp that had mods done in 1972.  It's a 1970 Twin Reverb and there are two push/pull pots in the vibrato channel.  The volume control when pulled seems to bypass the reverb circuit and send it to the second half of V4.  The treble when pulled appears to put another capacitor in series with the 250pf treble cap increasing capacitance to sort of "shift" the sound a little.

6/20/22:  Voltages are as follows with (schematic voltages):

V1:  P1 233V (260V), P3 1.7V (2V), P6 243V (270V)
V2:  P1 221V (250V), P3 1.8V (2V), P6 220V (240V)
V3:  P1 440V (440V), P3/8 9.2V (8.6V)
V4:  P1 230V (275V), P3 1.9V (2V), P6 222V (280V)
V5:  P1 437V (can't read schem), P6 389V (380V), P8 18V (17V)
V6:  P1 245V (245V), P3 92V (100V), P6 252V (235V)
6L6 Pin 3 - all are around 446VDC with the bias adjusted to -52V as shown on schematic

It seems where I've replaced coupling caps in the power section, the voltages are higher.  The brown "turd" caps in the preamp, reverb and tremolo were lifted on one end to check for DC leaks, and I didn't see anything higher than .01VDC.  I'm considering replacing the coupling caps to the tone stack to see if the lower voltages go up.

I thought for sure these two mods created a low frequency oscillation.  When I turn the volume pot on the vibrato channel all the way up it will oscillate and then draw more power.  My variac will show it run away up to 12 or more watts as it oscillates.

The volume pot in question also is dirty and it's an old sealed push/pull pot so there's not a good way to clean it.  When I turn it all the way up it measures 30 ohms compared to a new alpha which is 1 ohm or less.  Not sure this could cause the oscillation either but I'm just starting to problem solve it because the mods weren't the culprit.

UPDATE 6/21/22 - Thanks to some advice off forum, I think it's motor boating and have some things to try today.  I've heard of motor boating but never "heard" it in person.  Great learning experience, lots to check and a little hope for the day.

Any insights are welcome.  Images attached are filter caps then first half of the chassis circuitry and then second half.

[pdf64]

Maybe the ecaps are weak, no decoupling properly?
V4 cathode bypass is a favourite for causing oscillation problems as it fails.

[bbmade]

I did put in all new cathode bypass caps.  I think I’ll add pictures to show what I’ve done.  Resistors on the power tubes, coupling caps to the power tubes.

All the brown “turd” caps are still there.  Contemplating changing them but don’t want to shotgun parts for no reason.

[pdf64]

Do you know if this behaviour is new, or might it always have been like that?
Is the D node HT ecap that decouples all the preamp stuff definitely good?
If so, maybe try doubling up on it, 47uF or 2 x 22uF in parallel.

[bbmade]

I’m not sure.  It’s probably been there the whole time I’ve owned it but I’ve played it very little and never cranked it all the way up like that.

I did filter caps, electrolytics, the power tube resistors and a few other things several years ago to take it to an outdoor gig.  It sounded pretty good at home but at the gig didn’t sound good.  I did not hear this oscillation though.  I assumed the RCA 6L6’s were worn out and set it aside.  Now that I’m working on it again I tested those tubes and they all test strong, no leaks no shorts.  Right now I have a new set of JJ’s in their place while I work on it.

I don’t think the push/pull mods will stay but, I’d like to get rid of this oscillation to hear what it was intended to be or, like it sounded in 1972 when the mods were done.

I’ll be back at it on Monday.  I might post a video of the problem and some pictures of the work I did.  I really appreciate the forum and all the knowledgeable people willing to offer advice, thank you.

[bbmade]

"Is the D node HT ecap that decouples all the preamp stuff definitely good?"

Yes I replaced all the filter caps and dropping resistors under the doghouse.  I'm going to measure all the voltages and record a short video to share the problem more efficiently.  There are still a lot of sketchy solder joints to fix.

I can't tell if this oscillation is related to low frequencies only or, if they are somehow triggering the tremolo circuit.  It occurs when I crank either the channel volume all the way up (quickly), the master volume which was added in 1972 or the bass control in the vibrato channel tone stack.  The intensity control will alter the oscillation and the speed control doesn't seem to affect it in any way.

I need more experience to better my troubleshooting skills. 

[bbmade]

Do you know if this behaviour is new, or might it always have been like that?
Is the D node HT ecap that decouples all the preamp stuff definitely good?
If so, maybe try doubling up on it, 47uF or 2 x 22uF in parallel.

On advice from another tech I tried adding filtering on the D electrolytic and the problem went away.  I'm going to try a 47uf instead of the 20uf and see if that tames the problem.  If not, I just might throw in the towel trying to get rid of a problem that only happens when the amp is cranked wide open.

[bbmade]

The problem has not gone away and I’m not convinced it’s motorboating.  It really seems like a low frequency oscillation perhaps bleeding into the tremolo circuit.  So, perhaps it’s a lead dress issue?

All the wires feeding the controls are together with a purple ground wire around it.  Looks like 1970 factory approach to lead dress. 

I’m running out of options to try.  Amp sounds awesome but the next owner might crank it wide open and hear that which worries me.

[kagliostro]

Motorboating ......


http://www.youtube.com/watch?v=pwSplfKL4qQ#


I could not resist

Franco