B-Verb Mk II underway (1-channel AB763 6V6)

[joulupukki]

The latest hi-res photos, latest schematic, and layout diagrams...

What schematic are you using?

@tele_savales Attached are my latest schematic and layout diagrams in PDF format.

Reviewing the schematic & layout I noticed I put an unneeded yellow wire going from V2 pin 7 to the 500pF cap on the board. Because I have the reverb dwell control that wire shouldn't be there. I'll remove it while troubleshooting the reverb hum. I pulled V2 this morning to see if the reverb hum would be reduced. Nada.

 

[joulupukki]

Ok, the reverb hum is MUCH quieter, nearly 100% gone when it's not in the combo cabinet! I am thinking that the lay down transformer isn't as good at preventing hum from the reverb tank proximity as a stand-up one is (based on what I'm observing). I removed the extra wire going between the 500pF cap and V2 pin 7. I also swapped out three wires with shielded runs:

- from V2 pin 7 to middle lug of Reverb Dwell pot
- from reverb pedal RCA jack to V3 pin 2
- from tremolo pedal RCA jack to tremolo circuit on the board

I connected ground to the tremolo RCA jack in hopes it'd stop this odd oscillation I get with the MV @ max & Intensity Pot. I did this based on wiggling the footswitch jack which seemed to have issues before I had opened the chassis (when it was plugged in). I also re-routed the tremolo footswitch wire to be more isolated from the V3 reverb recovery circuit which was causing some oscillation with the reverb up high. Also moved around the V3 cathode wire to reduce that same oscillation.

I also switched back to the Black-White primary, though I'm not sure if I'll keep it this way or go back to Black/Blue-Black-Green.

How do these voltages look?

Wall voltage: 122VAC
Heaters: 6.56VAC
Rectifier Heater: 5.19VAC
PT Main Primaries: 721VAC

B+: 459V
B+2: 457V
B+3: 397V
B+4: 392V

Bias supply: 50.5VAC
Rectified bias: -58.6V
Bias after bias pot: -43.9V

	Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6	Pin 7	Pin 8
V1 7025 Preamp	260		1.963			258.8		1.974
V2 Reverb Driver	452	14mV	8.6			452	14mV	8.6
V3 Reverb Recovery	259.7		2.176			257.1		2.176
V4 Tremolo	189.2		1.633			457	189.1	193.2
V5 12AT7 Phase Inverter	241.6	50.7	83.2			240	52.6	83.2
V6 6V6 Power Tube	-43.9		459	457	-43.9	457		19.7mV
V7 6V6 Power Tube	-43.8		459	457	-43.9	457		19.9mV
V8 GZ34 Rectifier	459

And here's some updated pictures with a new layout diagram showing what's current.

(Mk I on the left, Mk II on the right)

 

[joulupukki]

Update: I was wrong about Mk I in the head cab. If I turn the MV up past, let’s say half way, and then turn the reverb up past half way, the amp squeals. I’ve tried other tubes (that’s not it). What does stop the squeal is moving the reverb tank completely out of the cabinet. So it’s a proximity thing. I’ll have to put the Mk II in the head cab and see if if does the same thing.

 

[joulupukki]

I’m beginning to think the head cab was maybe not the best idea. Mk II does not squeal in the head cab, but the hum because of tank proximity is way too much. I’m guessing Mk I is probably in need of some lead dress and maybe that will at least cure it from squealing. The reverb is at least usable at lower gigging volumes.