Rebuild or rewire to stomp out hum and buzz in 1-channel AB763?

[joulupukki]

A number of you have followed along with my two 1-channel AB763 6V6 amp builds (now dubbed the Mk I and Mk II). They are both good amps, but by far the Mk II has a much lower noise floor (nearly no hum and no buzz). Mk I has more hum at full volume and quite a bit more when you turn the reverb way up.

The Mk I uses a 40/20/20/20 cap can and the Mk II uses individual radial power filters. The Mk II also has improved layout on the turret board.

I've tried and tried doing different lead dress in Mk I with some improvement, but not near what it ought to be.

A couple different ideas are going on in my head right now and I'm curious, if these were your amps, which one you'd go after?

Idea 1:
Replace the can capacitor with identical radials as what I used in Mk II and adopt the same grounding scheme (each circuit grounds at the filter cap negative for that circuit in Mk II). I would mount the main reservoir cap to the far left of the chassis and the next two filter caps just off the left side of the board.

Idea 2:
Buy a new 3mm board blank and a full set of turrets, abandon the original board, and do the exact same wiring as I did in Mk II, including separating the bias circuit to it's own tiny board. I think I'd be able to re-use many of the board components with no trouble.

Thoughts?

P.S.
In terms of white noise/hiss I actually don't really notice any difference between the cheap carbon & metal film resistors in Mk I vs the more expensive Dale resistors in Mk II. It seems like they both have similar white noise and at this point I figure any white noise in both amps is likely a result of the preamp tubes.

Pictures for reference (Mk I first and Mk II second). PDF of latest layout diagram and schematic also attached for reference.

 

[pinchegil]

Did you do the headphone trick on both amps? Its probably just me but I have issues with hum on standup transformers. On my GA5, the only way I could remove the hum was mount the OT sideways inside the chassis.

 

[joulupukki]

Interesting. Glad you got that solved. I did do the headphone trick and there was no more hum in the headphones in its current Location. Next time I make an order from Mouser I’m going to get some radial caps and redo the power supply and the grounding to match as close as I can to what I did with Mk II. I’m hopeful that will solve the issue.

 

[NickK_chugchug]

Those heater wires will act like transmitters to any wire nearby. Keep them next to the chassis and preferably in the corner between the top and edge. If you do need to cross a signal and heater wire do so at 90degrees from each other.

 

[joulupukki]

I put the amp back in the combo cab this morning. Strangely, the same thing I had to do in the head cab amp (where I took this PT from) I’ve had to do here…

I had to rotate the reverb tank so it’s oriented exactly 90° instead of laying flat (so the RCA jacks of the tank are pointing straight up). If I don’t do this, there is significant hum and nasty buzz with the tank plugged in. Kinda weird, but I’ll do it if it means the hum and buzz from the reverb circuit are extremely reduced.

 

[NickK_chugchug]

What is the orientation of the the transformers?
How close is the reverb tank to a transformer?, are all the leads to/from shielded? Also is the reverb tank open on any side?

 

[joulupukki]

Whoops my previous post I thought I was responding to my other thread. In any case, an update to the Mk I … I ended up going with Idea 1 and re-wired things to be more like Mk II with separate filter caps for each B+ stage (instead of using the cap can). I definitely like the cleaner look to it but it didn’t solve all the issues, in fact it only barely made any difference. What did make a huge difference is using a different power transformer (see that here: https://www.tdpri.com/threads/found...b763-build-a-noisy-power-transformer.1133507/).

 

[Ellen Faye]

I have built a good number of scratch amplifiers. Maybe a bit more than 16. Need to check some of my serial numbers. But initially I have some hum issues that I had to work out, but I did settle on a few hard rules that I follow. 1) separate power and preamp grounds. Preamp is always grounded at an input jack. 2) every single pot and jack has a serrated washer behind it and are tight. 3) Heaters are twisted tight and elevated away from the tube. And of course, keep an eye out for wire routing. And always shielded cable for signals. I am sure you also use most of these, but keeping the preamp ground away from that of the power amp is the most important single thing.