New Electro Harmonix 12AT7 in reverb driver (Fender Deluxe), won't hold 410v plate?

[g1]

Throw one of your AX7's (from other amp) in the PI and try the AT7 from the PI position as the reverb driver.
Having an AX7 as the PI will change things a bit but work fine for testing.

[mozz]

PM me your mailing address and i will send you out a few tubes. I have about 200 mil spec 12at7's taken from 50's-70's test equipment, they have double thick mica's and are a great tube.

[Chuck H]

Pretty sure the current equation would be limited to the circumstances of the cathode resistor. So 9/2200 = .004 (ish). Or 2mA per side. So plate voltage (-cathode voltage) 420 x .002 = .84W per triode idling.

[mikepukmel]

Pretty sure the current equation would be limited to the circumstances of the cathode resistor. So 9/2200 = .004 (ish). Or 2mA per side. So plate voltage (-cathode voltage) 420 x .002 = .84W per triode idling.

Thanks I totally screwed up the first calc. Eek.

[mikepukmel]

Gotta be way wrong. 96 mA on an output tube would have its plates glowing orange. In a pre tube, white hot with the glass nearby melting into slag. That's not happening is it?

9V div by 2200R = 4 milliamps (lets forget multiple decimal point accuracy for now, ballpark figure is close enough) for both triodes, or 2 mA per plate / cathode.

Yeah, thanks,. no glass melting I totally screwed up the calc.

[Enzo]

Comment from earlier:

ANY tube can fail at ANY time.

[mikepukmel]

Comment from earlier:

ANY tube can fail at ANY time.

Got it, thanks Enzo. I spent a few hours downloading and inspecting a bunch more chassis images from vintage era Fender amps with reverb. Looking to see if I did something very badly. My wiring is no work of art, but I haven't been able to find something that sticks out, obviously (wires crossing other wires, wires parallel, etc). A few wires a little longer than necessary, but not horribly so. A melt in a small cap 0.003 uf to pin1 on the reverb recovery tube.

[soundmasterg]

PM me your mailing address and i will send you out a few tubes. I have about 200 mil spec 12at7's taken from 50's-70's test equipment, they have double thick mica's and are a great tube.

I'd be interested in some OS 12AT7's if you were in a selling state of mind and the price was right.

Greg

[mozz]

What have you got to trade, speakers, tubes, old electronic stuff?

[soundmasterg]

What have you got to trade, speakers, tubes, old electronic stuff?

In order to not bog down this thread with a personal thing, I'll send you a PM later tonight and we can talk about it there.

Greg

[mikepukmel]

12AT7's arrived today (Thanks Mozz, I sent you a PM re PMT!!!). Popped in a different 12AT7 and .... drum roll .... still crackles. Definitely needed extra 12AT's they will come in very handy in the next couple of builds and for spares.

So I was probing around with the amp running (between bites of my dinner mixed greens) and got a bit of a surprise. Getting DC betweenb the grid connection to the reverb driver, to ground. <frown> Kinda, sorta, kinda looks erratic. Bounces between 2 to 5v, then jumps to 9.somethig, then down to 3 to 5v again. Spit shined the probes, checked this and that, jiggled and ... very stable instability. So, to get DC there, it would have to go through the 500pf mica cap, and the 0.02uf blocking cap on the plate of V2. <frown> I had again done Chucks suggestion, poke the - probe as hard as I can into the board near a HV connection, then probe nearby HV connections. Got nothing. I don't think the board is leaking.

Checked the wire going from the grid to where its supposed to go in the ckt, get a short reading. COuld a missing ground wire someplace cause this?

[nosaj]

12AT7's arrived today (Thanks Mozz, I sent you a PM re PMT!!!). Popped in a different 12AT7 and .... drum roll .... still crackles. Definitely needed extra 12AT's they will come in very handy in the next couple of builds and for spares.

So I was probing around with the amp running (between bites of my dinner mixed greens) and got a bit of a surprise. Getting DC betweenb the grid connection to the reverb driver, to ground. <frown> Kinda, sorta, kinda looks erratic. Bounces between 2 to 5v, then jumps to 9.somethig, then down to 3 to 5v again. Spit shined the probes, checked this and that, jiggled and ... very stable instability. So, to get DC there, it would have to go through the 500pf mica cap, and the 0.02uf blocking cap on the plate of V2. <frown> I had again done Chucks suggestion, poke the - probe as hard as I can into the board near a HV connection, then probe nearby HV connections. Got nothing. I don't think the board is leaking.

Checked the wire going from the grid to where its supposed to go in the ckt, get a short reading. COuld a missing ground wire someplace cause this?

Is it possible the .02 is leaking?

nosaj

[g1]

Check for that fluctuating DC on the other side of that mica 500pf.
With the occasional reports of bad mica's showing up on the forum, I'd be inclined to try some other 500pf there to test.

[mikepukmel]

Unsoldered the lead to the grids of the reverb driver. With the grids floating, no crackle, nothing. Vol or reverb pot 1 to 10. Grounded the grid lead, cathode about 7.8v. Few other voltages look ok. Hmmm. Interestinger and interestinger.

[mikepukmel]

Thanks, working on that now!! Checking for leaking 0.02 and also fluctuating V on other side of mica cap. Of COURSE it COULDN"T be my wiring or soldering skills <harrumpf>