Unknown circuit picked up at estate sale

[madkatb]

Have you tried chopsticking it? There seems to be a lot of signal wires running close to one another.

[davohilts]

thanks for the reply. yes i've used a chopstick and moved wires all round and poked this and re-soldered that and looked for open grounds. i must be missing something but I don't see it.

[jmaf]

That 1M resistor on the phase inverter, right across the center, is a probable hum source. The unshielded wire that connects those 2 grids back to the board, crossing several other leads on the way is also a likely hum source. I'd use shielded leads there.

The power tube cathodes are grounded at a different place than the reservoir cap grounds, that's another probable hum source, just extend their connection a few more centimeters back to where the reservoir caps are grounded.

Just some ideas, hope this helps.

[davohilts]

Thanks for the reply jmaf!
I have a couple of questions to clarify what you suggest to make sure I do the correct thing.
On the 1M resistor on the phase inverter, you say it is a probable source of hum, should I remove it? Also if I install shielded wires on the 2 grids, should I ground the shield mesh to a ground lug near the tube?
On the power tube cathodes grounding, this appears to happen through the cathode resistor. Should I remove the ground from the cathode resistor to the ground bus on the pot side and reroute it over to the ground on the cap can?
Dave

[jmaf]

Thanks for the reply jmaf!
I have a couple of questions to clarify what you suggest to make sure I do the correct thing.
On the 1M resistor on the phase inverter, you say it is a probable source of hum, should I remove it? Also if I install shielded wires on the 2 grids, should I ground the shield mesh to a ground lug near the tube?
On the power tube cathodes grounding, this appears to happen through the cathode resistor. Should I remove the ground from the cathode resistor to the ground bus on the pot side and reroute it over to the ground on the cap can?
Dave

Hi davohilts, yes I would dress that 1M resistor differently. I do it via a shielded cable, but some folks have told me it's overkill. My own build policy is that all grid circuits must go via shielded cables. Only ground one of the ends of the shielded cable to your star ground point.

Yes I would route the power tube cathodes straight to the reservoir capacitor ground. I see there's another capacitor up at the top, not just the two sprague atoms....that ground should also be routed back to the sprague atoms' ground IMO.

Basically pull every ground from the sprague atom ground, that should reduce your hum. Test each change and listen so you know if any of the changes caused the amp to malfunction.

[jmaf]

No, I'm sorry, it seems to me the reservoir cap is the one above...not the spragues....so don't disconnect the cathodes, they are at the correct spot..... I would then make that one spot the star ground center and route other grounds there.

[davohilts]

Okay, thanks.
So what you are saying is to route the grounded shield mesh to the star ground point on the ground bus by the pots?

[jmaf]

Okay, thanks.
So what you are saying is to route the grounded shield mesh to the star ground point on the ground bus by the pots?

See this pic:



The potentiometer buss is grounded left and right, and your main circuit ground is way at the top, so there you can see there is potential for a ground loop.

I would redress all preamp grounds to that same point. I would place a thicker bare copper wire such as the one you used at the pots from the same place the tube cathodes are grounded, across and all the way in front of the main board.

The other end of this copper bus must not be grounded. Just affix it somewhere isolated, don't ground it...the only reason to drive it all the way across is to find an isolated spot to affix it, if you won't use any grounds ahead, there's no need to have it that long either.

This copper buss is where you'll ground everything on the preamp. A single path drives all the preamp current down to the main reservoir ground.

That would be my first step and only then proceed to look for further hum sources. Hope the image is somewhat clear: all the preamp grounds get detached from the pot buss, the pot buss is just there to equalize the 3 pots, don't use it for ground anymore because between the pots, the screw to the chassis and the path on the chassis to the upper reservoir cap there are plenty of paths for current to flow, that is a potential hum source. Hope this helps.

[davohilts]

thanks jmaf, I will take your advise and do as you outline. i will take a photo and post once I'm done.

[jmaf]

Nice. Test the amp with every change you make and listen. I know it's a PITA to turn on, turn off, discharge caps, but it'll keep you from getting lost in case one change breaks the amp. Remember to only ground one side of the ground buss, only ground it to the same place the power tube cathodes are grounded too. Let us know...

[davohilts]

Okay, I think I have gotten rid of about 70 percent of the hum problem by using the shielded cables on the grid wires to the board. I found that a cap on the board had a broken wire at its base and since I didn't build the board I wasn't sure what value it has. It was labeled .1K 240volts so I replaced it with an vintage cap I had laying around which was a .022 mf 600v out of a silverface deluxe reverb. I tied the two grounds from the shielded wires and ran them over to the ground bus wire that the cathode resistor is grounded to. I alsomoved some of the wires around and twisted up some others. The circuit sounds pretty good with a lot of power and breaks up about a third of with way up. I will attach a pic of the mods I did that you suggested.

[jmaf]

Glad to hear about the progress! The components on that board are good quality, there is no reason this amp shouldn't sound great once you finish the revamp.