74 Musicman HD130 From Hell (HELL I TELL YOU!)

[Helmholtz]

Schematic shows around -46VDC bias

Bias voltage depends on the setting of RV-2. For a cathode current of 25mA I would expect a grid bias of around-39V for a typical EL34. And yes, -30V seems off.

[g1]

Bias voltage depends on the setting of RV-2. For a cathode current of 25mA I would expect a grid bias of around-39V for a typical EL34. And yes, -30V seems off.

Yes, I see now that the -46V on the schematic is max. cold bias. But still -30V seems off as you say. The 6L6 versions can run around -46V at grid relative to cathode, but they run much colder.
Your -39V number sounds closer to what I would expect here.

All that being said, Frank, it sounds like the power tubes are working based on your pin8 numbers. Not sure why no 'electron party'? Maybe signal is not making it that far?

[fdesalvo]

Oh, the electron party was on once I replaced two bad diodes on the rectifier board. I’ve gone back and brought up the bias voltage a bit to see how it would impact the numbers.


V2-5: el34 hi
1 .4mvdc
2 13.7mvac
3 726vdc
4 365.5vdc
5 -40vdc
6 -40vdc
7 6.66vac
8 84.7mvdc

V1: PI (12ax7)
1 293vdc
2 20.6vdc
3 46.4vdc
4 39.4mvac
5 “
6 207.7vdc
7 .1mvdc
8 1.82vdc
9 6.72vac

I don’t understand what’s up with the heater circuit. The color codes on the schematic don’t exactly line up with what I’m seeing on this end. Can’t quite discern if there’s an CT on the heaters or not.

[fdesalvo]

Holy cow! I just noticed something that’s been staring me in the face this whole time. Can’t wait to get the images up. I think this is it. Ready to humiliate myself.

[fdesalvo]

Notice anything off???





The impedance selector. Yep.


If it was a snake...can’t believe I missed this! Hoping this explains the lack of signal on the output lol.

[Helmholtz]

And don't forget to adjust bias voltage to get 500mV at the EL34 cathodes as per schematics instruction. Exact bias (grid) voltage doesn't matter but cathode current does.

[Helmholtz]

I don’t understand what’s up with the heater circuit. The color codes on the schematic don’t exactly line up with what I’m seeing on this end. Can’t quite discern if there’s an CT on the heaters or not.

Heater voltage looks good now. What color codes? What schematic are you using? Heater winding doesn't have a CT - one end is grounded.

[fdesalvo]

And don't forget to adjust bias voltage to get 500mV at the EL34 cathodes as per schematics instruction. Exact bias (grid) voltage doesn't matter but cathode current does.

Sehr danke! Please disregard color code comment. It’s all sorted out now.

The amp is back to acting like a guitar amp on the normal channel. Now I have to debug the effects channel. Volume is almost nonexistent - about 1/10 of the normal channel. I shorted the pot wiper lugs and there’s no difference. It’s a dual ganged pot. Looks like it’s sending signal in to the reverb circuit right up front. Oh, the odyssey this amp is becoming haha.

[fdesalvo]

Ok was able to spend a few minutes on the channel two volume problem. The dual ganged pot measures 10k across one set of lugs and 3k across the other. Is it recommended to completely remove these from the circuit to test?

[Enzo]

No, look at the schematic. One side is the channel volume control. The other side is the reverb. Turn it to zero. Now the volume side should read about 10k. But the reverb side? The bottom end has a 1k resistor to ground. The hot side has a 10k reverb gain pot to ground. That puts the gain pot and 1k in parallel with the pot section. depending on the gain setting, that will cause a low read. Somewhere between under 1k to maybe 5k. But you are not having reverb trouble, you are having channel level, so even if the reverb is frotzed, ignore it for now.

The two channels are the same up to the gain resistors of the second stage. So feed the same signal into both inputs and center the tone controls. Now compare signal level at the treble pot wipe from each side. Same or different? First stage is good or bad. Same? OK move to top of channel volume controls, same or different?

[fdesalvo]

Thanks, Enzo.

I wish I could hang out with someone more experienced, so I could learn the thought processes. There’s always school or google eh.

Update soon.