motor boating on a twin

[Curranproducer]

So I have a friends fender twin here and I have recapped the power section, and the all the old blue caps and silver mica'ed most of the ceramics out of the amp. I am motorboating when I turn the amp all the way up and have the tone controls pegged, I can control this by turning the bass down to 4-5. Amps sounds great, this happen on both channels. I have looked for DC on the bass pot and have found about 45-55mVDC on the legs, I have also probed the tone stack caps and found the same small amounts of DV on one side of the Caps in the tone stack. is this leakage? and is this causing the motorboating? or is this an acceptable amount of DC to have on the other side of the cap? this is really the last thing i need to get worked out for this amp to be done and back at my buddies studio

[Jana]

You took all the tone-meister caps out and replaced them with leaky caps? Please work on some old Marshalls and take out all the mustards and send them to me.

[Curranproducer]

hey now! don't bust balls too hard!!!! the blue caps where in a bad way, this poor amp had sat in a basement for 35 plus years here in the wonderful weather land of ohio. I only did what I had too to get her running for my buddy. It sounded like ass before i re-worked it, now it sounds great IF I don't turn the bass up over 6 on either channel! HAHAHAHAHA so you guys are telling me leaky caps huh? there should be ZERO DC on the blocked side of the cap?

what part of Minnesota do you hail from I come from Minneapolis!

[sluckey]

45-55mVDC is pretty close to zero.

[Stevem]

A good new blocking cap shoud be .025 volt dc or less, at least a orange drop type.
Lift the down stream end of the plate cap feeding the tone stack, does the issue go away, it takes all of 10 seconds with a hot iron to find out!

[tony hunt]

I agree that level of mV DC is tolerable for now.
Re. Motorboating: if you pull the reverb driver tube, do the symptoms persist?

[Curranproducer]

so I pulled the reverb tube and it did not help, I also come to find that I can cause the amp to motorboat and change the speed of the motorboating by manipulating either bass controls regardless of the channels Im plugged into, and the volume also changes the character of the motorboating.

[tony hunt]

I had exactly that situation after recapping a British amp that is fairly similar to the twin reverb. Both channels. Like yours, one could actually dial the controls in to vary put-putting, like a mad early teutonic synth. Also mostly gain and bass related.
It was the reverb circuit coupling to a preamp socket, which is why I suggested to pull the reverb driver. (Divide and conquer)

After endless chopsticking, I stumbled upon a simple tool to find where the stage coupling was going on:
Get a cheap large screwdriver and insulate the metal. With the amp dialled in to "motorboat", walk the screw driver slowly between each wire, component and socket until the sound changes. It should physically deflect the stage coupling revealing where the issue lies.

[Firestorm]

If it's reverb coupling related, it's not the reverb tube V3, but the reverb recovery tube V4. Check all the ground connections and chopstick there, too. If the reverb return has an open ground, it's bad.

[Mark]

Ultimately motor boating is an oscillation and an oscillation needs a path to allow feedback. You need to find the loop it is operating in, then the problem will be apparent. If the bass control effects frequency then that is a part of the loop. The question is how is the signal getting back into the input of the amp?

Don't discount the signal traveling through the power supply. Keep plodding and let us know how you are getting on.

[Stevem]

One of your new power supply caps may not be acting new as it should, and if they do not decouple the gain stages they power from each other than that could be your motorboat issue!

[Curranproducer]

well I pulled V4 and the sound dropped and got super muddy. Put that one back in and pulled V3 things actually got worse with the motorboating. please re-explain this screw driver tool idea, would I insulate the entire metal shaft including the tip so no metal will come into contact with anything? or just insulate it enough so only the tip is exposed?(this feels oddly sexual in some way, I hope my wife isn't creeping on me right now! HHAAHA) and do I just go from point to point on the board or am I trying to jumper connections?

[tony hunt]

No, your not supposed to jump connections or short anything. Insulate the metal part so you don't get sparks flying or zap yourself.

I found this trick by chance and have never read about it, but it makes some sense: if you have two stages somehow coupling due to proximity (bad lead dress) a metal object coming between them will slightly alter the field in which the coupling occurs.
If you set your bass and gain controls just at the point when the motorboat sound begins, then it will be at a kind of threshold point. A metal object placed in the offending area should disrupt the field enough to pause the motorboat sound. At least that is how I discovered it.
Hold the screwdriver vertically and move it inbetween your stages, then wires, then the components.

The motorboat sound can drive you mad, so I would suggest checking the power supply filter caps again first as mentioned in a post above. Easy to overlook a simple mistake. We all do it at some point.

[Structo]

What value grid stoppers are on the power tube grids?

[pdf64]

Motorboating is almost invariably due to an problem with decoupling of the B+.
Most likely one of the (new) B+ caps is bad, or there's a bad connection to it.