MKIIA Grounding

Has anyone had success changing the grounding scheme on the MKIIA? I have one on he bench that has the usual hum and noisy reverb. I noticed that the preamp tubes all ground to the input jack while most of the filter caps for the preamp section are grounded through the mounting posts of filter cap/power supply board. My thought it to move the "C" and "D" filter cap grounds to the bus on the preamp board. On the reverb circuit I have hum coming in on the plate of the reverb return. I've heard the placement of the reverb transformer can cause hum but the hum seems to be coming in after the tank (therefore after the reverb transformer). Looking for any suggestions for grounding/lead/dress/ hum reduction.

Thanks!

Grounding the caps for nodes "C" and "D" seems to help with the hum in the overdrive channel. In the MKIIa there's a cathode follower on V2 feeding the master volume pot that has 260v on the cathode. That's way too much heater-cathode potential and pretty much guarantees heater noise in the stage. They cobbled together a fix in the MKII-B by turning that stage into a regular gain stage and taking the signal that feeds it off the cathode of the preceding stage. Kind of a mess overall. There's a 3 inch long long lead to the grid of the reverb driver that sits on top of the board and runs right on top of and in parallel to one of the traces with the heater voltage. Moving that helped the hum in the reverb.

Rewiring the Master 1 as a pot rather than a rheostat is a definite improvement. The sense I'm getting is that the MKIIa was a disaster, the MKIIb was kind of a band-aid over the problems, and the MKIIc was the big redesign

Now the relay chatter is back for some reason! It seems like the relay needs its own power supply.

allsignalmustpass said:

Rewiring the Master 1 as a pot rather than a rheostat is a definite improvement. The sense I'm getting is that the MKIIa was a disaster, the MKIIb was kind of a band-aid over the problems, and the MKIIc was the big redesign

Click to expand...

If you want to view the IIA as a disaster, have at it.

Talk it down all you can, and I'll keep buying them - I have four atm, but wouldn't mind a few more :lol:

The grounding scheme of the MKIIA and B are total nonsense and mess ! But once redone correctly + some other mods, the MKIIA is unrivalled by its successors, trust me !



But yes, as is, it's a nightmare : full ridden of hum and hiss, coarse settings...

Cheers,

Any chance of posting a schematic of these changes?

Or better diagrams of the grounding changes?

Thanks!
-Dale

Well, I have no schematic of the grounding mods - it would be useless as a schematic doesn't show this kind of arrangement.

All my detailed and commented pictures have been lost with the steal of my previous laptop, but here are three views of the complete chassis once modified, which I took with an acceptable resolution :







Unfortunately, these pics can't show the exact details, but it matches the description of the modifications. Note that as the MKIIA and B were rather "case by case" basis, your amp may be different than my Boogie MKIIA Rev Eq 60 100 in its wiring or grounding arrangement...

A+!

Ok, II'll compare those images to my IIa

A question though: what is "larson" ? as in:

"On lead mode, the reverb is attenuated to probably avoid Larsen effect ..."

Maybe "feedback" ??

Thanks,
-Dale