Transformer hum?

[txstrat]

One thing I didn't mention before: the power tubes are supposed to be matched but they are a couple mAmps away from each other. While the first one is at 36mA the second is at 44mA. Do you guys think this could be the culprit for the remaining hum? How close to each other should they be?

[Tom Phillips]

That power tube mismatch could be causing some hum. However, your remaining hum increases with the volume pot setting. Right? Therefore, I would surmise that the power tubes are not causing the residual hum you are now tracking down.

Many people are very particular about exact idle current match wanting it to be withing a couple of mA. But I currently have an amp that is more mismatched than yours and it does not hum. Remember that when these amp were originally manufactured they did not even try to use matched tubes. I think matching is a good idea. (Or using separate bias adjustments) But...it I don't think it's causing your current hum.

Cheers,
Tom

[Tom Phillips]

Matt,
Better balance can't hurt. Especially in a bass amp better balance will improve the PA low end response.
Tom

[txstrat]

...your remaining hum increases with the volume pot setting. Right? Therefore, I would surmise that the power tubes are not causing the residual hum...

Yeah, wasn't thinking of that. Thanks

[Tom Phillips]

I have a suggestion for an easy test. Based on your report in posts 92 & 95 maybe the first pre-amp tube is picking up hum from an external field. The test is to put a temporary shield around the tube and determine if it reduces the hum. The temporary shield can be something simple like an empty soup can grounded to the chassis.
If this works then a permanent fix can be devised. If it doesn't work then not much time wasted and external field interference from above the chassis is ruled out.
Just an idea. What do you think?

[txstrat]

I have a suggestion for an easy test. Based on your report in posts 92 & 95 maybe the first pre-amp tube is picking up hum from an external field. The test is to put a temporary shield around the tube and determine if it reduces the hum. The temporary shield can be something simple like an empty soup can grounded to the chassis.
If this works then a permanent fix can be devised. If it doesn't work then not much time wasted and external field interference from above the chassis is ruled out.
Just an idea. What do you think?

Yeah, that's great. I'll try that.
Nice to know you're still keeping this in view. Thank you so much.

Matt
b.t.w. I added a scheme of my actual grounding, whcih is the quietest I could manage

[txstrat]

Took a tin can (not enameled) and put it over the first and second tube (one after the other, with a clip lead to connect the cup to the chassis).
No difference, but I found something weird. When I flip the ultra hi switch ON and there is no cable in any input jack, A BAAAAD buzz occurs. It disappears as soon as I put a guitar cable in any input OR flip the switch back OFF. Then there's the "normal" hum again.

edit:
Buzz also stops when I touch the sleeves of the input jacks (remember they are isolated from the chassis).
The bad buzz comes up only when the treble pot is full up AND the volume pot is NOT full up or full down.
If I turn down the treble pot a little OR turn the volume pot full to either end the buzz disappears and the normal hum is there.
All this (as I said before) only when the ultra high switch is on.

[txstrat]

Found a layout pic on the net and rewired the preamp almost completely. No change in hum/buzz (it's the same level like in the sound clip I posted - with a little more buzz when the treble is full open).
Difference is, I'm using only 1 shielded cable now - from vol pot wiper to 2nd grid (looks way more tidy).
When I ground the signal chain after the second triodes coupling cap, the buzz disappears. Turn the treble pot down and there it is again.

[R.G.]

Buzz also stops when I touch the sleeves of the input jacks (remember they are isolated from the chassis).
The bad buzz comes up only when the treble pot is full up AND the volume pot is NOT full up or full down.
If I turn down the treble pot a little OR turn the volume pot full to either end the buzz disappears and the normal hum is there.
All this (as I said before) only when the ultra high switch is on.

What happens if you put a 0.001uF ceramic cap from the input jack sleeves to the chassis/enclosure right at the input jacks?

Could be RF buzz riding in on the input cables, and this trick sometimes gets rid of it.

[txstrat]

What happens if you put a 0.001uF ceramic cap from the input jack sleeves to the chassis/enclosure right at the input jacks?

Could be RF buzz riding in on the input cables, and this trick sometimes gets rid of it.

Would this sort of buzz stop if I ground the first grid with a clip lead? Actually if I do ground the first grid nothing changes.
I'll try your proposal.
Somehow I think it might be some easy thing I haven't thought of yet (maybe a defective component - tube socket?).
Sort of slap your forehead and say "aaahh yes".

[tubeswell]

Would this sort of buzz stop if I ground the first grid with a clip lead? Actually if I do ground the first grid nothing changes.
I'll try your proposal.
Somehow I think it might be some easy thing I haven't thought of yet (maybe a defective component - tube socket?).
Sort of slap your forehead and say "aaahh yes".

This is to block any RF that your guitar lead cable shield might be picking up. (See diagram 13.18 on page 15 of Merlin's grounding article: http://valvewizard2.webs.com/Grounding.pdf )

[txstrat]

I tried the 1nF from input jacks sleeves to grund to no avail.
Have also changed the first preamp tube to a 12AX7 and tinkered around with the grounding path.
I found that this design is incredibly sensitive to changes in the grounding. Even moving one ground wire for two inches was noticeable.
I'm gonna rewire the amp back to where I got it the quietest and then gonna feed a different preamp to the PI of this amp.
Will report later about it.

[Tom Phillips]

Hi Matt,
How soon do you plan to re-wire? I have some further ideas for you but currently am involved in lots of weekend chores including leaky plumbing burred underground so I can't write more now.
Tom

[txstrat]

Tom,

I really appreciate your help.
I'm at my wits end and be desperately poking around in my amp just on the long shot that I accidentally find the culprit (it might not be a culprit but more a fault of my own).
Since the amp is built on terminal boards I can re-rewire it any time I like if you find the time to write more.
Thanks alot.
Currently I'm thinking about rewiring it like the TUT3 Portaflex layout. That should be quiet cause it's a proven design. I could then try to change to the '68 design step by step and might find the cause of my hum that way. What do you think?
Actually I'm not stuck to the '68 design but I want to keep the ultra low switch and don't know if I can insert it after the second stage of the TUT3 design.
Think I'm gonna leave it as fixed bias. I don't think the hum is caused in the power stage, cause if I pull the first tube the amp is dead quiet.

I wish I could help you with your leaky plumbing. I'm sort of used to it, cause I had to fix it several times at my house too.

Matt

[w0ttm]

This might help.........
http://www.antekinc.com/