motor boating on a twin

[Curranproducer]

no change.

[Stevem]

I have reread all back thru this string of post and way back when you had posted that when you pulled out V4 the amp still worked, but sounded muddy, and that should not happen, when you pull out V4 the vibrato channel should not work at all unless it's bleeding back thru the normal channel circuit way down stream at the PI input!
If you pull V4 out do the controls on the normal channel effect the sound at all?
If the amp just sounded like crap before you reworked it but it did not motor boat than at this point I would have to say that you have a new filter that is bad, or installed backwards!
I would get hold of a new or good filter at this point and jump out all the filters one at a time as that is why I think you have this issue, if not then what you have is a layout issue where the now working with more gain is making for the motor boat issue.
One other thing to try would Be to disconnect the normal channel down near the PI section at the mixer resistors and trouble shoot just the vibrato channel since that channel has more gain and is the more likely problem maker!

[Curranproducer]

this amp does have a tone of break up and crunch to it in BOTH channels, I have de-wired the entire vibrato circuit to use that tube for an FX loop. this is the 3rd set of filter caps I have ordered for this amp, and i went from JJ's to F&T's so that should be good. I will re-try the tube pull an check BOTH channels this time. Looking at the schematic, at what point in the circuit do I un wire the normal channel? coming from V1B off of the plate? or after the .047/220k section after the plate? ot after that, before it hits the MV?

[Colossal]

I would have to say that you have a new filter that is bad, or installed backwards!

I have been wondering this same thing...backwards electrolytic.

[Curranproducer]

the tremolo tube is not hooked up or wired for the FX loop yet? I want to get the motorboating fixed 1st. but the tube heaters are wired to this tube

[martin manning]

Which Twin is this?

[Curranproducer]

this is a silverface 1975 with a master volume that I blackfaced, and popped in a tube merlin fx loop(which worked but the level was to low). it then started to motorboat so I stripped out the loop, and started trying to figure out the issue, I have done a bunch of component replacement to this beast so either way it is going to be an awesome amp when this gets sorted out. it has that push pull master volume on it

[martin manning]

I suggest going through the power supply string carefully to make sure that you have the various filter stages connected correctly and that the grounds are distributed as they should be. Is your cap board configured like the AB763 Blackface? Can you post a picture of that board? Does it look like this: http://www.mojotone.com/Amp%20Kit%20Sch ... f?gc=clear

[Stevem]

So this is a added effects loop that uses the vibrato tube with no extra sections of filtering but for the stock ones?

[Curranproducer]

that was the plan, I used merlins schematic and it worked great, until you turn the amp up then the motor boating would start up. from there i removed the fx loop components and wiring so it is not getting any voltage at all to the tube, except for the heater. i replaced the filter caps with 80uf/500v(2) and 22uf/500v(3), I bumped up the wattage of the 1k and 4k7 up to three and 5 watts(these where the values I could find easily in my resistor box.

[Curranproducer]

and after reviewing the image from the above link I see that I have ground lines running from multiple points on the caps to different spots in the amp, I will make this a universal layout like the pic

[Richie]

Have you tried replacing the mica treble caps? Many of the new ones can be leaky.

[Curranproducer]

I did replace all the original ceramics pf caps with silver mica's.

[Richie]

I did replace all the original ceramics pf caps with silver mica's.

try putting the ceramics back in

[LOUDthud]

I didn't see any of those dual caps Fender used to use for cathode bypass in the preamp. I had an amp once that motorboated because of a bad ground on one of those caps caused coupling between the two channels. Are there any preamp stages where the cathodes are tied together and bypassed by a single cap? A weak cap might cause the muddy coupling between channels.