The first 6A5G get*set*go is up!

[Shannon Parks]

Not referencing the filament should cause a lot more hum - by all means, try it.

Is the mechanical noise coming from the amp (choke? damaged rectifier?) or the speakers (damaged tube)?

[hembrook]

I think it was the power trans.

I tried powering it up, and while the rectifier powered up, the rest of the amp did not, so I am back to the beginning. No heat on the 6SL7 or the 6B4Gs. Maybe the heater windings on the tranny canceled each other by phase? I dunno. Its all in pieces on my workbench now.

[Shannon Parks]

I tried powering it up, and while the rectifier powered up, the rest of the amp did not, so I am back to the beginning. No heat on the 6SL7 or the 6B4Gs. Maybe the heater windings on the tranny canceled each other by phase? I dunno. Its all in pieces on my workbench now.

The three filament circuits were never directly connected. Attaching the 3rd set to TP1 or TP2 should have just put a ~50VDC bias on the AC circuit instead of giving it a DC reference of ground. And that through a couple 1k resistors. See if you can suss out if your voltages are in working range.

One more thing: how can your rectifier be working and not the 6SL7? I thought they were on the same filament circuit?

[hembrook]

I tried twisiting the wiring to the tube sockets above the board.

At this point I am just as willing to tie off the 3rd winding on the trans and just rewire the sockets to the board and give it up as a bad experiment.

I'll try to suss it out when I get back fgrom Japan. Ots 10:00 PM, my flight leaves at 0730, meaning I have to be up at 0300, and I have not packed yet...

[hembrook]

I just tied off the wires from the 3rd heater winding, and soldered the sockets back to the board. I fired it up and I still get a mechanical hum from the chassis. With the 6B4G(ish) tubes in it hums thru the speakers, but with the 6A5Gs in it is quiet thru the speakers, tho the chassis still hums.

[Shannon Parks]

I just tied off the wires from the 3rd heater winding, and soldered the sockets back to the board. I fired it up and I still get a mechanical hum from the chassis. With the 6B4G(ish) tubes in it hums thru the speakers, but with the 6A5Gs in it is quiet thru the speakers, tho the chassis still hums.

How quiet is this mechanical hum, since some is normal?

I remember us talking about the different wiring setups for the 6B4G and 6A5G. Do you have the UX4 socket wired up standard?

[hembrook]

How quiet is this mechanical hum, since some is normal?

I am going thru 95db or better speakers, so there is some hum in the speaker. I guess I need to get/learn to use and oscilloscope.

I remember us talking about the different wiring setups for the 6B4G and 6A5G. Do you have the UX4 socket wired up standard?

6A5G is pin-compatible with the 6B4G and is octal. I wired it standard, so I can also use 6B4Gs in the circuit (which hum considerably more).

[Shannon Parks]

How quiet is this mechanical hum, since some is normal?

I am going thru 95db or better speakers, so there is some hum in the speaker. I guess I need to get/learn to use and oscilloscope.

I remember us talking about the different wiring setups for the 6B4G and 6A5G. Do you have the UX4 socket wired up standard?

6A5G is pin-compatible with the 6B4G and is octal. I wired it standard, so I can also use 6B4Gs in the circuit (which hum considerably more).

Even with my 102dB horns, my GSG is one of my quietest amps - its totally silent. I think it might be time to put in hum pots. You could use two 100 ohm, 3W pots. No need to chassis mount them at first, while testing. You can remove the 47 and 51 ohm resistors and mount it with resistor leads from junkbox resistors. The wiper goes to where the 47 and 51 ohm resistors meet.

BTW, sorry if I've asked this before, Robert, but are you using a safety ground to the chassis?

[hembrook]

Even with my 102dB horns, my GSG is one of my quietest amps - its totally silent. I think it might be time to put in hum pots. You could use two 100 ohm, 3W pots. No need to chassis mount them at first, while testing. You can remove the 47 and 51 ohm resistors and mount it with resistor leads from junkbox resistors. The wiper goes to where the 47 and 51 ohm resistors meet.

OK, I'll get that. The 6B4Gs are really noisy, the 6A5Gs are quite a bit less.

BTW, sorry if I've asked this before, Robert, but are you using a safety ground to the chassis?

I have a wire going from one of the output transformer bolts to the IEC socket ground lug. The power transformer shield wire goes to the same place.

[EWBrown]

If I use a "third wire" ground, I generally connect it to one of the power transformer mounting screws, under the chassis. If there is a hum problem, then I disconnect the (green) wire and just tape up the ring lug to insulate it - this allows for easy future re-connection, if becessary.

/ed B in NH

[Shannon Parks]

I was going to suggest disconnecting the safety ground for test, but I forget how the 220V is wired up in Europe. In any case, if different tubes are causing different hum levels, a hum pot should do the trick.

[EWBrown]

I think European AC power is similar to our 120VAC power, with line, neutral and third wire safety ground. DElivered at 2X the voltage and 83.333% the frequency :o

US domestic 240VAC is L-N-L . OTOH, 208VAC, which is usually present in industrial 3-phase circuits, with 208VAC between the phases, and 120VAC from any of the phases to ground. in this kind of power circuit, phase-to-phase is 1.732 (sqrt 3) times phase to neutral.

For the serious "big time" industrial power systems, 480VAC three-phase yields 277VAC from each phase to neutral. Hence those "weird" 277VAC florescent lamp ballasts, etc...

/ed B in NH

[Shannon Parks]

Thanks for that info, Ed!

Hembrook, you might want to try disconnection safety ground during some of this testing.

[hembrook]

Worth a try.

[hembrook]

No change in hum. There is still a loud mechanical buzz coming from the amp chassis itself. Any clues there? Power transformer or choke is my guess. Can I disconnect the choke as a test?