The 2204 is down.

[ViperDoc]

So I decided to crank my 2204 build in defiance of the elements of the day (master volume around 7-8) and, after a decently long high gain performance, it stopped working. I pulled the chassis and put it on the bench and found the following:

No obvious carnage
No blown fuses
Powers on normally.
PT secondary: 350 V per side
Bias supply voltage: -50 V
Heater supply: 6.3 VAC
Preamp turrets: no voltage
Tubes: no red plates, all lit
Filter caps: voltage on one, no voltage on the other
No sound.

Diagnosis: Blown filter cap?

[pdf64]

HT choke / dropper resistor / standby switch failed open circuit?

[ViperDoc]

Thanks for the suggestions. I'll check it out. How do you diagnose a choke failure?

[xtian]

Thanks for the suggestions. I'll check it out. How do you diagnose a choke failure?

Measure DC resistance.

[frankdrebin]

most probably first resistor after the choke open,just check voltage on screen and PI supply,maybe a broken soldering

[ViperDoc]

Here's the final layout that I used to build this amp, many thanks to Martin.



The PT is fully functional. The first filter cap is fully charged up. That cap goes to the HT fuse which is fully functional--I tested it through the holder and outside the holder and I get connectivity. Then it heads to and out of the choke to the second filter cap which has NO VOLTAGE on it. There is also no voltage on the pins 4 or 6 on the power tubes. Also, when I turn the amp off, the first filter cap holds a full 450 V until I drain it, which it didn't used to do.

Is this a nuked choke?

[martin manning]

Seems to be the case. Disconnect the choke leads and check DC resistance.

[ViperDoc]

Choke leads disconnected and there's resistance. Dead choke.

Here's the one in the amp: https://www.mojotone.com/transformers/B ... -CHOKE.pdf

If I need a beefier one, how would I spec that? I mean, turning up the master to 8 should be an option, yes?

Thanks.

[sluckey]

Choke leads disconnected and there's resistance. Dead choke.

You should measure 106Ω between the choke leads. If so, the choke is not your problem.

[ViperDoc]

Choke leads disconnected and there's resistance. Dead choke.

You should measure 106Ω between the choke leads. If so, the choke is not your problem.

Thanks, Sluckey. WELL, I have 109.2 Ω between the choke leads. That means I have continuity.

If the choke is fine, why would I have no voltage on the second filter cap and no voltage on pin 6 of either power tube? The lead out of the choke into the power tubes is continuous.

[Roe]

Choke leads disconnected and there's resistance. Dead choke.

You should measure 106Ω between the choke leads. If so, the choke is not your problem.

Thanks, Sluckey. WELL, I have 109.2 Ω between the choke leads. That means I have continuity.

If the choke is fine, why would I have no voltage on the second filter cap and no voltage on pin 6 of either power tube? The lead out of the choke into the power tubes is continuous.

just check the wiring from the choke to the screens, including the screens filter cap

[ViperDoc]

Standby switch works.
Wire between choke and power tube screens is good.

[pompeiisneaks]

check where the voltages seem to 'disappear' if that's on the other side of the choke, look for a loose wire that's touching ground or even just with the amp off, test for continuity to earth along the entire power rail and see if you find a spot where it goes to near 0 etc. I've found an accidental ground where I got a wire under a standoff and crushed it to ground heh.

~Phil

[dorrisant]

Hook the choke up, turn it on and check your DC string from the rectifier on. It could be a faulty switch.

[ViperDoc]

I checked the switch and it works fine. I disconnected the second filter cap and both sides measure 54 uF. I’ll hook it back up tonight and take another look.