New project

[Tom Phillips]

The feedback and presence control connections.

[gui_tarzan]

I fixed them per your instructions and re-drew it. I flipped the wiper on the presence pot just to make the drawing clearer.

[Tom Phillips]

OK.

Next you can experiment with the 27k resistor in the feedback line and experience the difference in tone and feel of the amp as the amount of negative feedback is changed. With the 27K value hooked to an 8 Ohm OT secondary your amp currently has more negative feedback than the original Bassman. This is because there is more voltage on the 8 Ohm secondary at a given power output than there would be with the original 2 Ohm output impedance. I suggest that you experiment with higher values such as substituting a 56k or a 100k resistor in place of the 27k. Or you could put a 100k pot wired as a variable resistor in series with the 27k. Then you will be able to vary the overall value between 27k and 127k. This could just be a temporary setup to experiment. Changing the resistor value, or just leaving it at 27k, will not hurt anything. It's all about the tone and response feel when we are dealing with a guitar amp.

[gui_tarzan]

New NOS National 12AT7 - (in case there was something wrong with my used GE 12AT7)
1 = 196
2 = 19
3 = 67
6 = 192
7 = 19
8 = 67

[gui_tarzan]

Weirdness... when I turn the mid control all the way down it starts fluttering. Still very low volume.

[gui_tarzan]

Couple of updates: I lifted the ground off the mid control and I have a little more volume (I originally had that lug going to the presence control by mistake), but the weird thing is when I take the presence control off 10 I lose most of the volume.

[Tom Phillips]

Just a thought...
You are using a blackface style tone stack. Are you aware that if the Treble, Mid and Bass are all set to low values then the volume will be low no matter what? I have had people bring me BF amps with a mid control and say that the channel isn't working. Turned out that they had T, M & B all set all the way down. In that case even with the volume turned up high there was barely any signal getting through and that is just how the tone stack works.

[Tom Phillips]

...but the weird thing is when I take the presence control off 10 I lose most of the volume.

Yes.That is weird. My first impression is that something is connected wrong. I.e. not as shown on your schematic.

[gui_tarzan]

I resoldered the connections on the P control and now it's just flat. It doesn't increase or decrease anything. That's a new pot but I think I'll change it tonight and see if it's bad.

[Tom Phillips]

...That's a new pot but I think I'll change it tonight and see if it's bad.

I suggest that you don't jump right to parts changing. Just troubleshoot by using your Ohm meter to measure the actual value of the pot. Also double check the actual way the circuit is wired in addition to the individual connections.

[gui_tarzan]

Yeah, you're right. I went back through the wiring again and verified that everything is what my schematic shows (updated with pin numbers on the tubes and pots) and I still have almost no volume. One thing I question, not that it's not in the right spot now, but when I mistakenly had the NFB incorrectly into the tone stack I actually had volume. Once I put it where it goes I have little volume. That to me means either the tone stack or the NFB isn't acting right.

[Tom Phillips]

... I still have almost no volume. One thing I question, not that it's not in the right spot now, but when I mistakenly had the NFB incorrectly into the tone stack I actually had volume. Once I put it where it goes I have little volume. That to me means either the tone stack or the NFB isn't acting right.

Yea. I've been thinking the same thing. Certainly a strange situation. I don't see anything in the schematic that would cause the low volume. If I had it on my bench I'd inject a signal at the input and use the oscilloscope to trace it through the circuit looking for the section when it lost gain / had excessive attenuation. Without the test equipment you can re-trace your steps to when it was working. Other ideas are: Check that the presence cap is good (I.e. not shorted). What is the measured value of your presence pot? Lift the 27k feedback resistor and see it the amp comes back to life. Re-install the tubes you were using when it last worked. Check voltages again to determine if something changed. ...

[gui_tarzan]

Voltages:

V1 - 12AX7
1 = 189
2 = 0
3 = 1
6 = 206
7 = 0
8 = 1

V2 - 12AX7
1 = 171
2 = 0
3 = 0
6 = 306
7 = 171
8 = 180

V3 - 12AT7
1 = 187
2 = 21
3 = 65
6 = 183
7 = 18
8 = 64

V4 - 5881
3 = 392
4 = 403
5 = -31
6 = 404

V5 - 5881
3 = 394
4 = 403
5 = -31
6 = 404

Bias = -38

[Tom Phillips]

You reported 0V on V2 pin 3. No voltage on the cathode of that stage indicates that no current is flowing through the tube so it isn't amplifying. Check the connections there. The problem could be caused by an open cathode resistor, a shorted cathode capacitor a bad solder joint, a bad tube or a bad tube socket. Check these things out with your Ohm meter after the amp is powered off and caps discharged.

Edit: Since the V2 pin 1 plate voltage is about right there must be current flowing through the plate resistor. I'm now thinking that a shorted cathode bypass cap is a likely culprit. Try just removing the 25uF cathode bypass cap and see if the amp comes to life. When all is right you should measure ~820 ohms between V2 pin 3 and ground.

[gui_tarzan]

I lifted the cap on V2 pin 3 and there was no change and the resistor read just shy of 820 ohms. I bypassed the tone stack with a jumper and there was no change, which means the problem is still in the V1 or V2 area.