'65 BF Twin Reverb(AB763) scratch build

[Twin]

Hey all, Just finished up my scratch build(Mojo small parts kit, 65 BFTR Reissue chassis & MM transformers) & I have a few issues:

1. Tremolo(speed & intensity pots) do not function at all

Note - Footswitch is plugged in & respective tube is "known good". Also triple checked all solder joints & all A-OK..?

2. Reverb is oscillating(gnarly high pitch squeal that increases when dialed up).

3. When the Vibrato volume pot is dialed up past 7 the volume for channel 2 cuts out completely

Note - Brand new Mojo pot with all proper connections & good continuity to ground(chassis).

Any ideas on what to do here or where to look? I've already tried the "drumstick method" to search for loose wires, bad components, to no avail.

Any help is much appreciated!

[Tom Phillips]

Can you post a series of detailed high resolution gut photos? Whole chassis shots plus closeups.

[Twin]

Tom,

Thanks for the speedy reply! Yes, I can post some gut pics as soon as tomorrow since the amp is still at my buddy's house. We were trouble shooting it there in his well lit shop.
I do have some pics of the amp just BEFORE I finished soldering all the grounds & the main filter caps(doghouse) so I doubt they'd be of any help since incomplete, but I will post anyway...

[Twin]

UPDATE: The bonehead(me...) plugged the tremolo & reverb footswitch in reverse! So that may explain why I had no sound on Tremolo & oscillation on Reverb. Now they both work BUT the Tremolo is pretty weak(maybe Intensity pot?).

Also after the pics below were taken(yesterday) I moved the CT & Filament(red/yellow & orange wires) to a bolt on the PT & the amp is much quieter at idle, but a hum or buzz is still there after switching off Standby. Would anybody recommend any specific delineation or placement of grounds? Thanks for all the help!

[loudthud]

How may grounds do you have coming out of the cap box? There should be at least two. I see one comes to the terminal strip where the red/yellow striped wire is grounded to the chassis. There should be another for the preamp filtering caps that grounds somewhere in the preamp. These should not be connected together in the cap box.

[Twin]

Loudthud,

Thanks for the reply
I have a total of 4 grounds coming from the doghouse:

- 1 blue wire(100uf caps) that goes to the terminal you spotted next to the PT.
- 3 green wires(22uf caps) twisted together & soldered to the rear of the chassis over next to V1 & V2. Each one of these caps in the can are grounded individually with homeruns. Not sure if their placement was optimal in relation to noise, but seemed like a wide open spot to fit the huge soldering iron in there without torching a lot of the work I already did... BTW, I had a bunch of extra green "heater" wire that was furnished with the Mojo kit & ended up using it for most of the grounds(pots, tagboard, caps, etc.).

If you have any suggestions on which grounds I should be particularly careful about separating or moving in order to achieve a quieter idol, I'd much appreciate it.

Also, any ideas why the Tremolo in this amp isn't near as clout as in my '73 SF Twin Reverb?

Thanks again!

[Twin]

Can you post a series of detailed high resolution gut photos? Whole chassis shots plus closeups.

Tom,
You got any ideas how to reduce hum/buzz? I posted the pics you requested.
Thanks,
Bill

[loudthud]

If you have any suggestions on which grounds I should be particularly careful about separating or moving in order to achieve a quieter idol, I'd much appreciate it.

There may be some disagreement on this, and I can't say if it will work or not but here goes: The ground for the screen grid filter, "B" should probably go to the same place as the first filter or near the power tubes. The Node "C" ground should go to the phase inverter ground at the 100 ohm resistor. The Node "D" ground should go to one of the cathode resitor grounds in the preamp.

Also, any ideas why the Tremolo in this amp isn't near as clout as in my '73 SF Twin Reverb?

Probably something to do with the tremolo bug. If you can dissassemble it, try changing the position of the neon bulb or photo cell.

[Tom Phillips]

Tom,
You got any ideas how to reduce hum/buzz? I posted the pics you requested.
Thanks,
Bill

It's difficult to comment without hearing the actual level and character of hum/buzz. Fender's traditional method of random grounds works better than one would expect after reading the literature on ground loops. Your construction work is very nice and it looks like you have attempted to improve on that by using the terminal strips as star ground points. Obviously the result isn't good enough ...Yet.
My thoughts are the same as those loudthud has already provided. You can add some wire temporarily to the ground leads and experiment with different locations. If you can visualize the ground currents you want to make sure that the curretns from later stages of the amp do not flow through, i.e. share the same conductor (be it wire or chassis metal) with more sensitive earlier stage ground currents.
One of the cardinal rules, already mentioned is that the first stage filter cap is grounded to the same point as the PT center tap and nothing else. For long term reliability I solder these to the chassis rather than using a PT mounting bolt.
Regards,
Tom

[Twin]

Loudthud,

I will give your recommendations a shot & report back... Thank you very much for donating your wisdom, it is much appreciated!

[Twin]

Tom,

Thanks for the compliment, as I have tried to make it as similar to the original Fender layout as possible. I'll try the temporary grounds to see if location is as vital as I assume that it is.

"One of the cardinal rules, already mentioned is that the first stage filter cap is grounded to the same point as the PT center tap and nothing else."

When you say first stage are you referring to the 2 100uf caps in the doghouse? Also should I separate CT & Filament to ground?

Also, thanks for the suggestion of soldering these grounds... Kinda figured that's why Leo did it in the first place

Will get to work & report back. Many thanks!

[Tom Phillips]

When you say first stage are you referring to the 2 100uf caps in the doghouse? Also should I separate CT & Filament to ground?

Yes to both and you can experiment by moving the heater CT ground to different lications.

I’d also suggest deleting the new thread you started and keeping all the discussion about this project in this thread. This thread is very fresh, it’s nice to have the continuity and I think you will get better participation & results.

Cheers,
Tom

[Twin]

Here are the updated pics including the cap can:

[Twin]

OK, so I tried the power tube swap & it had zero effect on the hum. I tried pulling each of the preamp tubes 1 by 1 & found V2 to be the source of the hum... I switched out the blue wire coming from the 100uf caps in the doghouse to its own ground on the top right lug of the PT & tried grounding the CT & the Filament ground separately with no effect on hum... Any other ideas here?

Thanks

[Twin]

Tom,

I tried to delete the new thread but I may be too new to have that option...? Can't seem to find any way to do it.

Thanks for the tip!