Hum balance not working on one of two power amps

[nevetslab]

MEF Post EH Scott 510 Power Amp

I’m having an issue with the hum balance control added to a 50’s vintage EH Scott 510 power amp.

A client recently bought a pair of EH Scott 510 Power Amps, which were 50’s vintage mono hi fi amps, in which the power amp chassis contained the power supply to run an external preamp via long extension wires 7 12-pin octal-style plug. The two he obtained were somewhat different from each other, one chassis being aluminum, the other chrome plated steel. The power transformers also were considerably different, with the steel chassis having a small lamination stack than that in the aluminum chassis. Slight difference in the output transformers too, with one having an 8 & 16 ohm tap, the other just an 8 ohm tap.

All that was inside the power amp chassis was the phase splitter, the output tube stage, and two different rectifier tubes and local / remote DC power supplies…the remote one runs the external preamp/control console. The tube compliment was a 6SN7 phase splitter, a pair of cathode-biased 6L6GC’s, a 5Y3 rectifier and a 25Z6GT rectifier for the remote power supply.



510 SCHEMATIC-2.pdf

Mechanically, I had 5 octal sockets to work with. As luck would have it, the two rectifier tubes were on one side of the power tubes, and the phase splitter was in the opposite corner, furthest away from the power transformer. I first tried to install the added preamp section, built from a cascaded 6SN7 tube in the corner where the 25Z6GT rectifier tube was. Getting it into the opposite corner looked very tedious. But, after working the circuits and lowering the overall gain, the amount of hum & buzz forced the issue. I had folded up a brass housing to place over the added 2-stage preamp which did work, but it seemed a better idea to cut the rivets out, unsolder all the wires that needed to be moved, and move the rectifier, power tubes and driver over one hole, and re-build the preamp in the far corner where one would have placed it in the first place.

I had help on this project….the chap who brought the project to me was working along side me to aid in all the busy work and duplication, so we did get the two chassis’ re-built in that fashion. I still have some homework to do on the 2-stage preamp ahead of the phase splitter, since I ended up adding an 8dB pad between the 1st and 2nd stages, which I’d like to eliminate, lowering the overall gain without having to attenuate the signal.

But, after adding a hum balance control to both amps, only the one in the aluminum chassis works correctly, and the one in the steel chassis won’t balance. It will vary the harmonics, but will NOT cancel as the other one does. I’ve played with the grounding for the pot, duplicated all of the power supply filtering so both are identical in components and how they are grounded, yet can’t get it to balance. I’ve yet to disconnect the 6.3VAC winding and power the heaters externally to see if it’s something with the power transformer.

I just composed the schematic this afternoon, believe it is accurate. I don’t yet have the DC voltages on the stages. The power tubes are running with cathode bias, around 62mA per tube. The unloaded HV Secondary on one xfmr is 794VAC CT while the other one is 812VAC CT. The output power is 20W @ 8 ohms.

This following morning, I've added the DC voltages thruout the schematic and a minor correction on the filter caps C1 & C3 values. I also added an image of the original construction prior to revisions.

[g1]

Maybe some other source of hum than the heaters, like output tube balance or something? In any case, your test with the external heater power should tell you.

[loudthud]

The schematic shows a grounded center tap on the 6.3VAC. That will burn up any hum balance pot when you rotate it to either end. Where is the 25Z6GT rectifier? Interesting tube. Separate cathodes allow it to be used as a Voltage Doubler.

[nevetslab]

Look closer......that 6.3VAC winding does NOT have a Center Tap. Only the 800VAC HT winding has one. My appologies in running the power supply lines in that fashion. Of course you're correct in the statement of it burning up a hum balance pot with a grounded CT winding. I was concerned about the heater winding provisions on my HP 712C Power Supply, and just checked to see if the heater CT was grounded or not. It's not...can be if desired.

I haven't checked the two chassis' heater wiring, but was wondering if on the amp that has the hum balance problem....if the two tubes V1 & V2 had the heater wires swapped (Pin 7 on V1 going to Pin 8 on V2 & corresponding swap with the other side of the heater). Would that make a difference?

The 25Z6GT tube was removed, and it's winding not shown on this xfmr. That was part of the original circuit, and it ran the preamp voltages, with all that circuit in a separate chassis. All the tubes got moved down one space to put the V1 tube into the corner as far from the rectifier tube and power transformer as possible.

[nevetslab]

Today, looking at the heater wiring of the two amps, I found the hum balance pot of the amp that DOES balance connected AT the heater terminals of V1 tube, whereas the pot of the amp that is NOT balancing connected at the power tube heater, where the Xfmr heater winding connects. The heater wiring of the preamp and driver tubes was the same. So, I moved the Hum Balance pot wiring over to the heater for V1, just to see if that made any difference. Maybe a tiny bit closer to balance, but, I still had to crank the pot all the way to one end...not normal behavior for a hum balance pot.

So, I disconnected the local heater winding, and connected the 6.3VAC heater from my HP 712C Tube Power Supply, with it's heater CT connection floating. I got the same results as before...no balance at mid-position, and only approaching balance with the pot at one end. I restored the local heater winding.

I swapped the two 6SN7 tubes V1 & V2, with no difference. I then swapped the V1 tubes (both 6SN7GTA's) between the two amplifiers, and found with the tube from the amp that balances, it now balances on this amp, whereas the other V1 tube required the pot to be cranked all the way to one side, yielding about the same hum level on the analyzer. So, it is related to the tube itself, it seems. Is this normal?

[Leo_Gnardo]

So, it is related to the tube itself, it seems. Is this normal?

Unfortunately yes. I've noticed sometimes after a preamp tube change I'll have to adjust the humdinger to find a minimum hum position. Furthermore the amp might be silent with the volume control at zero, but hum picks up when the volume's dialed up to an ordinary play position. Well that's not a good solution, so with a twist of the humdinger we may have to suffer a bit of hum with the volume at zero BUT have hum pretty well nulled with volume at normal play positions. Who uses the amp with the volume at zero? So the second situation is better. The wonderful world of tube amps... still, ya gotta love em! Well, most of 'em anyway.