motor boating on a twin

[Curranproducer]

yeah each stage is coupled by a single cap resistor set

[Stevem]

Ok here's what I want you to do at this point, take all the components of the vibrato channel ( caps and resistors and even the plate output cap )tone stack off the board and solder them to the tone stack pots, even if you need to extend some of there leads with wire and shrink tube them up!
This will shorten up on the lenght of grid wire for that channel by a bunch and if that does not kill your motor boating issue than you must have a new bad filter that you changed out!
This is the way these amps should have been made anyway for the best tone and no interaction between stages, but it's time consuming in a production setting!

[Curranproducer]

then this is what I shall DO!! as always SteveM you and the rest here are a great bunch of guys!

[martin manning]

That seems like a lot of work to find a bad filter that might not be there?? Is this the amp that had the conductive board issue? You are sure the filter cap board wired correctly?

[Curranproducer]

it is the conductive board twin, I slipped a new piece of fiber board between the two old boards and the issue I was having with that is now resolved. I was planning on going through the fixes listed before the major lift and re-wire from the above suggestion. I have also been told that the lead dress in this amp is deffinatley wanting in the area of clean and neat as well as out of phase heater dress and such, I have also been told to shield a bunch of the lines as well. This amp is quite a bit gain-ier than what I would consider for a normal twin type of amp, so I ma sure that this is a part of the problem for the amp

[Curranproducer]

I was also thinking of re-building the entire power filter board on a fiber board with turrets. but this might be overkill.

[Richie]

I was looking at the pics you had, in some pics the trem bug is there.. in the later pics its removed and not on the board.

[Curranproducer]

correct, I removed the roach out of curiosity, since the tremolo didn't work when I got my hands on the amp, and I am taking the tremolo tube to use it for an fx loop. I was thinking maybe the roach was causing the motor boating

[Stevem]

Great news! , but keep in mind for your own fender amps or any other make amps that they will always sound better with the least amount of grid wire lenght which also as more gain is made keeps the amp more stable!
Do not bother with remaking a filter board as there is no signal there.
That amp has more gain due to having no trem circuit.
What I do a lot for folks with those amps who at times still want the tremolo is disable the pull on gain boost of the master volume control and wire it to open and close the left hand lead on the trem intensity pot, this seems to add at least 3 DB more signal level.

[martin manning]

Do not bother with remaking a filter board as there is no signal there.

Except that motor boating oscillations are often related to power supply decoupling. Are the boards under the filters as bad as the main board?

[Curranproducer]

The plot thickens!!!! so I went back to the amp last night with grim determination and purpose! I pulled the reverb tube again and the motor boating stopped BUT the vibrato channel wen tall muddy and dropped in volume. I then plugged into the normal channel and NO MOTORBOATING AT ALL. with all the controls pegged at 10(which is where the issue comes into play) I had no motor boating and great tone!! Then it hit me that all the gain in this amp from making everything 12AX7 and blackfacing the amp I have probably made this thing supper crazy gainy with a loose fender layout. I popped an ECC81 in the reverb and the motor boating was less, I also replace two more caps in the amp. Another suggesstion was to toghten up the heater wiring and get all of the pins in phase. the .003(changed too orange drop), and the 560pf(500pf silver mica) in the reverb section of the amp thinking that there may have been issues with the old ceramic's that where in there. I Have a layout of some mods that people have done to this version of the twin and there is an additional 6k8 and .001 cap added to the reverb footswitch connector going to ground, could the reverb part of the amp not be getting grounded enough?
After the changing of the caps in the reverb section the vibrato channel is now non motor boating BUT it is not nearly as aggressive or thick sounding as the normal channel and the volume is pretty low. It sounds very clean and fender-esque but the normal channel is getting into an awesome rawk end of things

[Stevem]

Where getting down to " not much left to try point " in light of this try swapping around from end to end the position of all the tone stack and plate uncoupling caps in the vibrato channel that are of the tubular type as caps like this have a input and output side so to speak and getting them in the right way that may help?
On a lot of caps the output side is to your right with the lettering on them is facing you, but without a cap meter and a section of tin foil to prove this out it will just be a trial and error type deal.
Another thing to try out is that bundled harness of wires that comes up to the face of the amp from the parts board and connects to all the pots of the vibrato channel can make for that issue also and if you open up that bundle and spread them out some that may cure it also!

[Curranproducer]

ok, there is more progress!!!! found out how important voltage is to tone!!! I had wired one of the "D" connections to the wrong spot in the vibrato channel and that is why the tone was so low and clean sounding, nice but not awesome like the normal channel! I have gone through and shielded the input's and the grids for each tube stage from input to volume control and the motor boating is gone from the normal channel BUT it is still present in the vibrato channel. This leads me to believe it is all about the lead dress in this amp!!! SO I am thinking of shielding the tone stack in the vibrato section to try and eliminate the final strands of motor boating, does this sound like an idea that may pay off for me! I am so close I can taste the victory of tone!!!

[Stevem]

In the time frame it will take you to do that you can like I posted move all that tone stack stuff onto the pots, remove a bunch of unneeded tone sucking grid wire and have the issue done with, where as with what you want to do may not end the issue with the motor boating!

[Curranproducer]

got some advice to add another filtering stage to decouple a section of the preamp, and VIOLA!!! the motorboating is gone, AND the normal channel is sounding like a true fender and the vibrato channel is very angry and crunchy cool when you turn it up all the way!!! neat-O. just so every one understands what I ended up doing. I added another 20uf/500 JJ cap tp the filter section with a 1k/3watt resistor to keep my voltage drop to a minimum and separated the 1st and second preamp stages from the "D" node of power. NOW everything is very nice. I will move onto making the fx loop work next!!!