'65 BF Twin Reverb(AB763) scratch build

[Tom Phillips]

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!

The edit /delete option is only available for ~1 day after you post. Then the "edit" button no longer appears. You can only edit your own posts of course and only when you are signed in.

You mentioned that you have a friend that has a real Fender Super Reverb. Correct?
If so you could carefully document the grounding in his amp. This is one area that is not really clear if you just reference the Fender schematic and layour documentation.
Tom

[Twin]

Hey Tom,

OK, will keep in mind for future posts.

Yes, my buddy has a Super Reverb & as far as I can tell there is no major difference between the amps other than rectifier(tube), power tubes(2) & transformers. I will copy his ground scheme exactly & see if that will solve my hum issue... Thanks again!

[Twin]

My buddy sent me pics of his Super Reverb & I've made a couple more changes:

Changed the three 22uf filter cap grounds to a single lead (via jumpers between the caps) & grounded it at the front of the chassis near the pots(like vintage Fender ground scheme) but still have the hum...?

1. Has anyone used a couple of 100ohm resistors at the lugs on the pilot lamp to ground? Does this help to get rid of the hum? I think I've seen this on later Twins.

2. MUST each of the power tubes be separately grounded(soldered to the chassis) or has anyone successfully used a terminal to ground the four power tubes?

3. Could the hum be caused by a bad component in the circuit? If so, which component would seem the most suspect for said symptom?
-Remember that when I pull V2 & the hum goes away.

Also just FYI, I have tried disconnecting all the jacks(foot switch, guitar & reverb cables) but that doesn't help either...

Thanks for any input.

[Tom Phillips]

...Has anyone used a couple of 100ohm resistors at the lugs on the pilot lamp to ground? Does this help to get rid of the hum? I think I've seen this on later Twins.

I thought this was already covered in one of your threads but, if not, it could be the source of your hum problem.
The 6.3V heater supply should be balanced with respect to chassis ground. This is often done by grounding the center tap of the power transformer's 6.3V heater winding OR installing the 100 Ohm resistors you described. Make sure you use one or the other method but not both.
Note that there are other methods to do this but if your amp doesn't have any heater balancing then you WILL have high hum level.

[Tom Phillips]

Following up on the heater CT you said "CT & Filament are grounded to bottom right PT bolt."
The wires connected to that bolt are Red/Yel (HV CT) and an Orange wire. I suspect that the orange wire is the PT electrostatic shield connection which should be grounded. However, I don't see a heater winding center tap on your PT. That would usually be a green/yellow lead but many transformers do not have a center tapped heater winding. Therefor, you would need to add the 100 Ohm heater balancing resistors.

Note that you can always check for balance heater by measuring the AC voltage reading from each side of the heater circuit with respect to the chassis. You should read ~3.15 VAC from each side to ground.

Please check and let us know. This may be it!

Tom

[Twin]

Tom,

The 6.3V Filament is the Orange wire that is grounded along with the Center Tap(Red & Yellow wire) at the bottom right bolt of the PT.
I did NOT install the 2 100ohm resistors at the heaters(running lamp). Thanks for clearing this up!

[Twin]

Tom,

The transformers are Mercury Magnetics & are faithful replicas of the original Blackface Twin Reverbs. Here is the diagram from MM:



Will check the heater voltages tomorrow & get back to you with results...

Thanks again for all the help!!!

[Tom Phillips]

OK. Interesting that MM uses the unconventional wire color for the heater CT. I thought I was on to something.

[Twin]

OK All,

In the other thread I started, Alan0354 walked me through a bunch of different steps to troubleshoot aforementioned issues.
He came up with a simple solution - Use copper tape to simulate the brass ground bus. This worked perfectly. No hum, dead quite!

Thank you for all your input, I have learned a bunch of valuable tips from this sweet forum!!!

[Twin]

Here are the pics of the finished project:



Thanks for all your input!!!