1970s Ampeg SVT refurb

[xtian]

I used the diode test setting on my multimeter. It shows from 512 to 540mV for each of the eight diodes in the bridge, with zero mV with the probes reversed. Any other test I can make?

[xtian]

My time scale on that scope photo is 2ms, making the period ~8.2ms, and the cycle 122Hz. My mistake.

[martin manning]

Back to the heater circuit.

[xtian]

Back to the heater circuit.

Well, per Leo, I removed the heater balance pot and added two 100R resistors to ground. Each leg, with respect to ground, reads equal. No change in hum.

Out of ideas. You?

[martin manning]

You say it hums with the PI tube out... put your scope on the power tube grids and see if the hum is on one or both sides.

[xtian]

Here is a trace with one probe on each grid. The traces are very nearly identical, and in phase. Heaters again?

Screen supply? I did replace the six screen resistors with 10R non-inductive 5W, as Stevem suggested.

[martin manning]

That looks like stray 60Hz noise. Was the PI tube in or out when you took that pic? Did you look at the bias supply to see if it is clean?

[Leo_Gnardo]

Leo, I replaced ALL of the e-caps with CE multisection cans and F&Ts.

There is only one 12ax7--it is the first two triodes of Channel 2. The PI is a 12dw7, which feeds a pair of 12BH7 before hitting the power tubes.

Right you are & my bad. In my mental library of complicated power amp schematics, I swapped in a McIntosh 240 for your SVT. Yes 12BH7's in place or else bias voltage could be unpredictable. (The Mac also has 12BH7 plus an extra set of 12AX7 here thus my cornfusion.) Some SVT have a 12AX7 instead of 12DW7 inverter. No biggie.

On to tracking down the mystery hum... hmmm?

[xtian]

Thanks, guys!

Martin, the PI was in, with it's input grounded.

You asked about the bias supply: I see the typical 1v p-p sawtooth at the bias filter cap (can we also call this the bias reservoir?). But scoping the power tube grids (where I'm seeing about -46vdc), I see far less AC than that, more like 0.2 or 0.1 vac.

BUT, I see a full 1v p-p sawtooth wave at the power tubes screens, pins 4! This sawtooth gets amplified to 10v p-p at the plates (pins 3). (Well, I don't know for sure this gets amplified. The electrons are supposed to scream past the screens, right? But sure seems suspicious.)

[xtian]

All right, guys, what do you think? Is the sawtooth ripple on the screens being amplified by the power tubes?

Most amps have the OT CT attached to B+, and the screens at a SECOND R/C node, providing much greater ripple reduction for the screen supply. This SVT has only the one filter cap before the screens.

[xtian]

Well, let me add the next bit of evidence to the pile. I decided to try adding an R/C node in the screen supply. I inserted an 220uF/100R node before the existing 100uF and screen resistors and powered up. I observed that my screen ripple dropped way down, to nearly undetectable at the grids. Great! BUT I STILL saw 10v p-p ripple on my power tube plates! Same old hum.

And, in the 60 seconds it took to observe these things, I noticed a lot of heat radiating from the first dropping resistor, the 15 watt 8K2, so I shut it down. I don't understand why I'm suddenly pulling a lot more current. I still saw -48vdc at the grids, so the tubes weren't running away. [EDIT: after removing my extra R/C filter stage, I saw that the 8K2 is dropping 220v and 6 watts, meaning it always gets hot and I didn't notice before.]

[xtian]

I guess I shouldn't be surprised to see the sawtooth ripple on the plates, because the OT center tap is fed by B+. Therefore, any ripple present at B+ will also be present at the OT primary. Normally, does this noise get cancelled by 180-degree push-pull magic?

[martin manning]

Yes anything that is on both sides of the power amp and in phase will cancel, and the peak-to-peak ripple you are seeing doesn't sound excessive for 600V. I'm still thinking the hum is related to the filament circuit since it's still there when you switch off the main power transformer.
- Look at the PI plate and cathode with the PI input grounded.
- Look at the plates of the driver stages (12BH7 pin 1's).
- What does the hum look like on the speaker output?
- What happens if you disable the fan?

[xtian]

Thanks, Martin. Traces shown. I note that one side of the PI has more 120Hz ripple (top trace in image).

There is no fan. (The connector is there, but owner did not provide.)

[martin manning]

Do you mean 2 milliseconds? I can't really see a repeating waveform there so can you find a setting the shows more peaks? 5ms? (Edit: it's 120 and 60Hz mixed, but from what I can see the output is showing predominantly 120Hz with 50mV amplitude) Also, how much ripple is on the supply node feeding the PI?