New project

[gui_tarzan]

*duh*

That center tap is supposed to be on the 6.3v winding, not on the 5v winding. I'll fix that too.

The PT and OT are from a Hammond M-100 organ and the OT tap I used is (from the schematic) the 8 ohm tap. My speaker (also out of the organ) is a 12" Jensen 8 ohm 30W.

[gui_tarzan]

Corrected schematic in post# 28.

This bias control is one one of you guys sent me when I asked about having an adjustable bias without a tremolo circuit.

[Tom Phillips]

It's nice to be working with the schematic rather than the wiring diagram / layout.
The learning experience for others in the future is one of the reasons we provide this information. Therefore, I think it would be better to just put the updated schematic in your reply post. When you go back and change the schematic in the old post the following discussion narrative will be very confusing because it was about the old version of the schematic. We want to preserve the process and the timeline for anyone who joins in later or reads this in the archives years from now.

[gui_tarzan]

Ok, I fired it up tonight and no loud buzz! Woohoo!!!

However, I have no sound at all.

Voltages:

Rectifier tube (GZ34) = 670v AC across the pins
B+ (C1 & C2) = 400v
(Choke is between B+ and C3)
C3 = 399v
C4 = 359v
C5 = 303v

V1 (7025)
1 = 187v
2 = 0v
3 = 1v
6 = 204v
7 = 0v
8 = 1v

V2 (12AU7)
1 = 64v
2 = 1v
3 = 1v
6 = 303v
7 = 64v
8 = 81v

V3 (7025)
1 = 231v and I get noise in the speaker when I contact the lug with the probe (like you've got a loose input jack)
2 = 8v and I get what sounds like DC in the speaker
3 = 29v
6 = 210v and noise in the speaker (same as pin 1)
7 = 9v
8 = 29v

V4 (6V6)
3 = 390v
4 = 400v
5 = -20v
6 = 401v (470/2w resistor between 4 & 6 - B+ goes to 6 from C3)

V5 (6V6)
3 = 389v
4 = 400v
5 = -20v
6 = 401v (470/2w resistor between 4 & 6 - B+ goes to 6 from C3)

Bias -25v at the 220K resistor pair. That's the highest (lowest?) negative voltage I can get, a resistor must need to be changed there.

[loudthud]

V2 doesn't have the correct resistors in the circuit for a 12AU7. The plate voltage is too low causing grid current. This is caused by the 820 ohm resistor in the cathode. Stick a 12AX7 in there to see if you can pass a signal. The other voltages look ok. The amp should be working unless the signal is not getting where it needs to be. That could be caused by a mis-wired input jack, missing wire, wiring mistake or solder splash shorting the signal to ground. Check along the signal path for continunity and shorts to ground with an ohm meter.

[g1]

Your V2 voltages seem way off compared to the V2 of 5F6A Bassman, check wiring, resistors etc.
To increase bias voltage, reduce 100K feeding bias diode.

Edit: missed Loudthud's post, he has the answer for the V2 voltages.

[gui_tarzan]

I put a 7025 in V2 and now I hear the normal hum of a tube amp in the speaker but still nothing out of the input. I'll start tracing wires again.

[gui_tarzan]

WHOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOBABYYYYYYYYYYYYYYYYYY!! !!!!!!!!!!!!!!!

It's working!!!!

It's dead quiet when not playing, clean clean clean until about 6 and it starts breaking up. Smooooooooth breakup from there to 10 (but not ear-splitting loud) and damn does THAT FEEL GOOD!!!

Hahahahahaha!!!

I can't thank you guys enough, this is frickin' awesome! What a sweet tone!



I piggy-backed another 100K on the bias feed 100K and I've got -46v now. That's in the range, now I can put some good tubes in it and fine tune it.

OMG you have no idea how good this feels. Well, I'm sure you do but...

Oh, the no input thing? The damned jacks! I put a pair of jacks out of my Bassman in it and it works great! The encased jacks I took out of the Yamaha (that's what this was, a SS Yamaha) must be wired funky or something.

[Tom Phillips]

Glad to hear the report of your success.
Does the presence control work well for you?

[gui_tarzan]

Nope, I might have to adjust that NFB resistor value and my tone controls aren't working real well, but this is great! To be clear, I have a pair of 6V6s in it because they are used and I could afford to lose them if something happened, but my original goal was to have 5881s in it. I'm sure that will boost the power a few watts anyway.

Which brings up another question, the Hammond organ PT originally drove a pair of 6V6s and nine other tubes, should it have the oomph to drive a pair of 5881s and three 12A*7s?

Another observation - the original rectifier was a 5U4, which if I remember correctly draws 3 amps and the GZ34 only draws 2, and the three 12A*7 I'm using draw 150ma per, where the original Hammond chassis had two 12AX7, three 12AU7 and two 6C4s so I should be fine.

[Tom Phillips]

In response to question “Does the presence control work well for you?”

Nope, I might have to adjust that NFB resistor value and my tone controls aren't working real well…

That is not surprising if the feedback circuit is really wired as you are showing in the schematic in your post #28. You show the signal feeding back to the bottom of the tone stack. The arrangement shown would mess up both the presence control and the tone stack performance. Reference the original Bassman 5F6-A schematic that you used as the basis for your design and you will see that the feedback signal and the presence control connect to the phase inverter section.

… another question, the Hammond organ PT originally drove a pair of 6V6s and nine other tubes, should it have the oomph to drive a pair of 5881s and three 12A*7s?...

I say that the PT probably will not have enough oomph to drive 5881s to significantly higher power levels than you are getting from the 6V6s. However, you can always give it a try.

… the three 12A*7 I'm using draw 150ma per, where the original Hammond chassis had two 12AX7, three 12AU7 and two 6C4s so I should be fine.

You are fine now but, for planning purposes, use ~1 mA per triode section vice 150 mA.

[gui_tarzan]

Here's the updated schematic. I put a 12AT7 in the PI and the V1 & V2 are 12AX7s. For some reason it's quieter now. Very quiet but has a lot of breakup.

[g1]

Can you post your voltages at PI tube with 12AT7, and with 12AX7.

[Tom Phillips]

Did you end up changing your as built circuit or was it just the schematic drawing that was incorrect?

[gui_tarzan]

Which part?