The new dumble modded bassman build video series... begins!

[pompeiisneaks]

2.8VDC V3B cathode

1.7VDC V3A cathode

Oddly I was watching one of them and under measurement it seemed to climb up to 1.7V stay there for a bit, then plummet to 0 and then climb back up? but I don't seem to see that repeating now, could be user error

~Phil

[Aaron]

The excess wire poking through the wiper lug on the volume pot looks very close to the ground lug, check its not touching it.

[pompeiisneaks]

I can definitely check. I also tested resistances to ground on all the pots to be sure none were shorted but it'll check again

[pompeiisneaks]

yeah it's likely a trick of perspective in the photo, there's a very large gap between the wire and the ground.

~Phil

[Aaron]

Do you have a scope?
Is there signal getting to the tonestack recovery?
Can you hear if the the tone controls have any effect?
Could be the coupling cap before the 470k mixer?

I can relate, I currently have a JMP Master Volume on the bench with low signal as well.

[pompeiisneaks]

I have a scope yes, that's my next step, just hoping someone was going to see something obvious. The signal is definitely getting to the grid of V3A, just doesn't come out the other side, and that tube seems to not be conducting due to the high anode voltages.

~Phil

[sluckey]

You only need your meter and eyes to fix this. The V3A issue should be a quick fix. Verify that both heaters are lit. Change the cathode resistor/cap, and plate resistor. If not working now, examine any wiring between the board and tube socket.

[pompeiisneaks]

Ok, I'll give that a try...

I did hook it up to the scope, all that did was confirm what I already know, I have signal at the grid of V3A and then it dissapears...

Should I remove one component at a time, or replace all three? I'm pretty sure the wiring is fine, as I've done continuity tests 3x now.

~Phil

[didit]

Assuming your known-working 12AX7 tested as faulty in the socket then would suggest you lift out one end of the cathode resistor to see if it has expected 1k5 value.

Best .. Ian

[pompeiisneaks]

I think I have a theory of what's wrong, I have an error in my layout that doesn't match the schematic.

In the layout, I have a dropper resistor of 100k between B+4 and the plate... then there's B+4 to the anode of the second triode of the PI the cathode follower side... BUT I realize I have both anodes connected to the raw B+. I need to remove the B+ line from the top of the 100k resistor and send it from the bottom. In other words, the layout doesn't even use the 100k resistor as a plate resistor, and instead is being used as a signal path resistor, likely causing one of two issues:

1. the triode is at a bad spot in the curves without that anode resistor putting it into cutoff
2. that 100k inline instead of above the anode is causing signal attenuation...

I'll be wiring it 'right' and see how that works.

~Phil

[pompeiisneaks]

That fixed it...

I rewired it, and just used my ac voltmeter and I'm getting about 20VAC out of the other side now. So the basic issue was a translation from schematic to layout... I'll fix the layout and update it now.

~Phil

[pompeiisneaks]

Well, just got my first ever high voltage DC shock... powered it up with the speaker, was playing, sounded good, went to toggle the bright switch, heard the difference, went to toggle it back off, and got hit hard... I shut it down and made sure it was safe, but know that the rectified DC is near that on the top, in the 'doghouse' I've setup, but the cover isn't on it, but oddly that switch is nowhere near it, like almost 3 inches away, I can only guess I somehow still slid my hand too far down that side and boom...

not a happy feeling. I'll be getting my doghouse cover on asap, and I'll also confirm my latest mods didn't somehow end up putting DC to chassis... ugh.

I am very lucky as it went through my left arm, to my right hand into the guitar... which means It could have hit my heart and killed me...

And I'm so careful normally... but man I'll be even more so now.

~Phil

[Matt J]

Well, just got my first ever high voltage DC shock... powered it up with the speaker, was playing, sounded good, went to toggle the bright switch, heard the difference, went to toggle it back off, and got hit hard... I shut it down and made sure it was safe, but know that the rectified DC is near that on the top, in the 'doghouse' I've setup, but the cover isn't on it, but oddly that switch is nowhere near it, like almost 3 inches away, I can only guess I somehow still slid my hand too far down that side and boom...

not a happy feeling. I'll be getting my doghouse cover on asap, and I'll also confirm my latest mods didn't somehow end up putting DC to chassis... ugh.

I am very lucky as it went through my left arm, to my right hand into the guitar... which means It could have hit my heart and killed me...

And I'm so careful normally... but man I'll be even more so now.

~Phil

Yikes!! I hope you're ok, Phil!!
I got my first high voltage DC shock a number of years ago when checking the caps on a Kendrick amp. I let my guard down for one moment, brushed something, and BAM! Electricity is wonderful, but boy can it teach a harsh lesson.
I've been quietly following this since I love Blackface Bassmans (I'm working on my own AB165/11864 hybrid now) and have really enjoyed watching you work on it. Best of luck with getting everything straightened out and I can't wait to hear a full demo of it!
- Matt J.

[pompeiisneaks]

Thanks Matt, yeah I burned my finger a bit that was on the guitar strings, and had some tingly sensation going on in my hands, wrists and elbows of both arms for a bit, but it finally is gone.

The amp did sound great but I won't be firing it back up until I can get the doghouse cover on it so it's safe.

~Phil

[norburybrook]

Phil

Glad you're OK

Been following your thread. Is there a Layout anywhere ?



M