Why i should buy a scope ( maybe not ) ?

[tepsamps]

So a few years ago, i built 2 identical amps, i mean same trannies, same power supply, same power stage, the preamp is different though higain both and power tubes are different, one is 6V6 the other is el84.

The power tranny is intended to do single ended, i did 2 push pull, but the rectifier are both L first, so the HT voltage is 325VAC (130mA), the B+ is 280VDC.

And the 6V6 is working like a charm and the el84 has a big hum, with no guitar plugged, that comes with the gain and the MV, and it recalls me the noise of power tubes biased way too hot, though the el84 are not that hot.

I made some tests, and the hum would come from the preamp, but it's a preamp that should work, like all the ones i've made so far, classical cascading gain stages...

Both are cathode bias.

I don't know where to search ...

[nsubulysses]

Are all the grounds wired identically the same in both amps?

What about if the heater winding is not balanced on one PT compared to the other. a hum balance pot or just artificial center tap for the heater supply could test that.

There must be a wiring error or the layout of the wires is different I would guess.

[Chuck H]

This does not need a scope to trace (but a scope is good to have anyway ) A schematic would probably help a lot before this thread is done.

Do you know if the hum is 50Hz or 100Hz? Do the tone controls affect the hum?

If you unplug preamp tubes one at a time, starting with the first in the signal chain, does pulling any one tube reduce the hum dramatically?

Have you tried swapping a known quiet preamp tube into each socket? I've had preamp tubes new from the box hum like they didn't know the words.

If the hum is 50Hz then it could be as nsubulysses says and there may be a wiring error in how the filament circuit is balanced. But if you can answer the questions above and provide a schematic or inform us of circuit topology if we ask then this should be traceable. Once you isolate the problem it's much easier to affect a solution.

[Enzo]

Plug a guitar into the hummy one, and turn the guitar volume control to zero. Still humming?

[Chuck H]

I would love it if this turns out to be a ground fault on the input jack Here we are going through these scenarios and isolation techniques, and Enzo starts with what is REALLY step one.

[tepsamps]

Plug a guitar into the hummy one, and turn the guitar volume control to zero. Still humming?

Yes still humming :-) !

[tepsamps]

This does not need a scope to trace (but a scope is good to have anyway ) A schematic would probably help a lot before this thread is done.

Do you know if the hum is 50Hz or 100Hz? Do the tone controls affect the hum?

If you unplug preamp tubes one at a time, starting with the first in the signal chain, does pulling any one tube reduce the hum dramatically?

Have you tried swapping a known quiet preamp tube into each socket? I've had preamp tubes new from the box hum like they didn't know the words.

If the hum is 50Hz then it could be as nsubulysses says and there may be a wiring error in how the filament circuit is balanced. But if you can answer the questions above and provide a schematic or inform us of circuit topology if we ask then this should be traceable. Once you isolate the problem it's much easier to affect a solution.

First tube : they are the 2 first gain stages, i tried this to reduce the overall gain of the preamp, it's like a plexi now. If i remove it, i still get the hum.
Second tube : 2 cascading gain stages after the gain pot. If i remove it, no hum at all, just a little noise.
Third tube : Cathode follower for the tone stack, and fx loop. If i remove it, even not a noise, like if the amp were of.
Fourth tube : The PI. If i remove it, it's like if i remove the power tubes, i get like previously .

Tone controls don't really affect the hum, they voice it just.

So they are 3 gain stages with 2 // first gain stage, one gain pot. And the amp has still a lot of gain. It's noisy... The preamp voltage is only 250 V that is not really enough for hi gain. The tone is a little muddy compared with 350 or 400 V in the preamp which would be less noisy i think. But i put a lot of preamp tubes, i would like to use them. So with 3 or 4 gain stages, the hum is there.

For 10 watts, it's a very loud amp, very reactive.

[Chuck H]

First tube : they are the 2 first gain stages... If i remove it, i still get the hum.

So the hum would be after the first tube.

Second tube : 2 cascading gain stages after the gain pot. If i remove it, no hum at all, just a little noise.

So the hum is either before or within the circuitry for the second tube.

Tone controls don't really affect the hum, they voice it just.

It's not clear from your post where the tone stack is in the circuit. But if the hum is being voiced by the tone stack then it is being generated before that in the signal chain.

You can further isolate the hum. With all tubes installed and the amp set up to induce hum, ground the grid of the next gain stage after the first preamp tube. If the hum goes away then the hum is either between the first tube and the very next gain stage or within the circuitry of that gain stage. If the hum remains then it is being generated between that gain stage and the next or within the circuitry of that next gain stage. This must be, because removing the second tube stops the hum.

Let's call the second tube V2a (next triode stage after the first tube) and V2b (the triode stage after that). Once you have the hum isolated to before or within V2a or before or within V2b you will have isolated the problem to a single gain stage. Then we can examine that part of the circuit and (hopefully) solve the hum problem.

Questions you did not address:

Have you tried a known quiet tube in the V2 position? This would not be a matter of just swapping tubes around in the amp because if there IS a hummy tube in there you would simply be relocating it. You would need to substitute ONLY V2 with a tube that is working without hum in another amp.

Do you know if the hum is 50Hz or 100Hz? This answer would tell us if we're dealing with hum from the AC mains or the rectified power supply ripple.

[tepsamps]

It seems to come from v2b. With v2b grid to the ground, no hum at all, just a sympathic noise. With v2a grid to the ground i stil get the hum, attenuated, but still there.

I don't know if it's 50 or 100 Hz, it's deep so i would say 50Hz but i'm not sure i can hear 50 Hz.

I try swapping tube 2 with another tested one, but it's the same. As soon as i switch on the amp, there is a noise from the power tranny, normal noise, the tranny's noise i guess, and this noise (i guess it's 50 Hz) seems to be amplified.

[Chuck H]

So a few years ago, i built 2 identical amps, i mean same trannies, same power supply, same power stage, the preamp is different though higain both and power tubes are different, one is 6V6 the other is el84.

What about the layout? Are the transformers located in the same place on the chassis? What about the filament circuits? Are both amps using a twisted pair with a CT? You mention that the PT for this amp is for a single ended amp. I don't understand this because rectified voltage and current capacity don't care what class of operation an amp is. I've seen some PT's indicated as "Class A", whatever that's supposed to mean with regard to a PT.?.

I think your assessment of the hum frequency sounds right. If the problem was the filament supply I would expect significant hum reduction when the first tube is pulled. So it's odd (to me) that you have significant 50Hz hum on the SECOND tube in the signal path. My first thought is that you should make sure the power supply filter for that tube is grounded with that tubes amplifier circuitry. NOT the filament supply. Also check any "daisy chained" grounds for the signal between V2a and V2b and the amplifier circuit for V2b. perhaps re do them as individual leads to the circuit ground.

Can you supply a schematic of the circuit from V2a plate to the grid following V2b? Including associated filters and ground locations. Even hand drawn is fine as long as it's legible.

[tepsamps]

What about the layout? Are the transformers located in the same place on the chassis? What about the filament circuits? Are both amps using a twisted pair with a CT? You mention that the PT for this amp is for a single ended amp. I don't understand this because rectified voltage and current capacity don't care what class of operation an amp is. I've seen some PT's indicated as "Class A", whatever that's supposed to mean with regard to a PT.?.

I think your assessment of the hum frequency sounds right. If the problem was the filament supply I would expect significant hum reduction when the first tube is pulled. So it's odd (to me) that you have significant 50Hz hum on the SECOND tube in the signal path. My first thought is that you should make sure the power supply filter for that tube is grounded with that tubes amplifier circuitry. NOT the filament supply. Also check any "daisy chained" grounds for the signal between V2a and V2b and the amplifier circuit for V2b. perhaps re do them as individual leads to the circuit ground.

Can you supply a schematic of the circuit from V2a plate to the grid following V2b? Including associated filters and ground locations. Even hand drawn is fine as long as it's legible.

The PT is for a single ended project and clean tone, a fender twin style of preamp going into an el34. It's intended for L first rectification, so that the B+ is low, at 280V.
Yes both amps use twisted wires and a faux CT, with 2 100R resistors.
Doing tests, i noticed that the hum begins with gain to 0. With gain to 0 et MV to max, the hum is max.
Other thing, i tried a 3 gain stages circuit which does not involve V2b and it's still humming.
But the main problem is not the hum, it's the sound of these 2 amps. With or without hum, the distortion sound is hugly, i first tried to clone a diezel VH4 CH4 but with 280 v instead of 400 in the preamp. And the result is very bad. With 4 gain stages, it's very muddy, Unplayable... With 3 it's ok, but i've made far better amps before and after these ones.

[tepsamps]

So once my idea was to put a C first filtering, i would get 455 V at the plates. Can an el84 stand this voltage? Can a 6V6 stand this voltage ? In this case, i drill my chassis to accept octal sockets, put 6V6 instead, and have a preamp at 400, ideal for higain...
Because there are the first gain amps i 've built with L first filtering and with so low voltage in the preamp...
Is this possible ?

But the PT won't stand this new current draw, 130 mA max...

[Chuck H]

DUDE!!!???

Where is this going? I thought we were solving a hum problem. That is how you introduced the thread. Eleven posts in (time I'm never getting back) you're saying the main issue is the distortion tone.

If the input choke is dropping 175V (not likely) then the current through the choke would be equal to (actually greater than) the current through the tubes without the choke. This current has to go through the PT. So the current through the PT would be equal or less without the choke.

I don't recommend putting 455Vp on any el84 or 6v6 tubes.

Please provide a schematic of what you have. Please provide a schematic of what you wish to do,.. Or something. And please decide what problem you are taking other peoples time to solve and consider that these other people are investing time on your behalf.

You're dancing on a line I think you're familiar with in your travels. This doesn't seem likely to end differently if some consistency doesn't develop. Quickly.

[tepsamps]

Sorry to waist your time, but playing on that amp i had to hear that bad distortion tone, and thought it was more important.
I saw 6V6S datasheet, i did not know this tube, and it's rated at 500V for the plate and 450V for the G2. And could stand 30 watts for a pair, as many guys put them in their marshall instead of el34, and it's OK.
Don't forget that what you told me about the hum is not lost in my mind.

[tepsamps]

So i've done the mods, i have 6V6S actually(they seem to be the new 6V6), now i get 455 at the plates, but 35V at the cathode, i've to increase the cathode resistor to bias correctly, and the tone of the distortion is nice, and no hum. The preamp is at 375V.
I've to check the PT to see if it's not hot, i've made only a quick test, and it seems ok, i'll tune it better tomorrow, with 4 gain stages, and my new homemade humbucker.
Thanks for the help anyway!