MEDIC ! My Carvin just crapped out on me

[Nuuska]

OK - meanwhile you could test the filament voltage with all tubes pulled out and if it collapses - then start pulling out those parts I listed in post #55

 

[JohnW63]

I went ahead and re-heated the solder connections on the tube sockets. It hums, but not as much and it doesn't have a pop and loud hum. It barely amplifies yet is distorts in a nasty way.

 

[JohnW63]

" remove rectifier D2-8-9-10 "

Do you mean pull all 4 diodes ?

 

[Nuuska]

" remove rectifier D2-8-9-10 "

Do you mean pull all 4 diodes ?

Yes - all 4 at same time

 

[JohnW63]

Without the rectifier, won't have have won't have have AC instead of DC going places ?

 

[Nuuska]

The idea is To REMOVE NOT REPLACE In order To find out if that part of the circuit causes your mains transformer 6,3V collapse

Again - read post #55

 

[JohnW63]

So, you want to remove the rectifier as a way of not having the part of the circuit beyond the rectifier in the mix.

 

[JohnW63]

Green wires on there own, volts across them 5.3vac

I'll pull the rectifier, next.

 

[JohnW63]

OK, I have to ask this since it seems obvious.

If I do not have the right voltage right out of the input / power transformer, why hasn't anyone said, THAT looks like your problem and suggested I replace it ? It seems like you have me checking voltages with and without tubes, removing caps and diodes to troubleshoot incorrect voltage and voltage drops, but what we are starting with is already too low. If it was good at the start, but but gets pulled down when I plug it into the board, I would understand looking for issues down stream, but ..."Volts across the red wires, unplugged from board = 168 VAC " tells me that it's bad even before I plug power into the board. Yes, the green pair starts good and drops, but isn't the transformer already suspect with the high volt input being down ? If I were to put a 100ohm resistor across the green wires, if that section of the transformer were healthy, I should see 6.3vac on the resistor.

 

[Default]

Filtercaps.

 

[GAD]

Try not to get frustrated. Debugging an amp over a forum is a challenge.

 

[DThomasC]

OK, I have to ask this since it seems obvious.

If I do not have the right voltage right out of the input / power transformer, why hasn't anyone said, THAT looks like your problem and suggested I replace it ?

I don't think the power transformer is the problem.

The fact is, you measured 407 VDC after the rectifier but measured 168 VAC before the rectifier. It's impossible for both to be correct. It's extremely unlikely that a bad transformer would output 168 VAC. Usually, when they fail they output 0 volts. At the end of the day I just don't trust your measurements which is why I can't offer any further advice. However, I do not recommend that you replace the power transformer.

 

[JohnW63]

"At the end of the day I just don't trust your measurements which is why I can't offer any further advice. "

So, I've just been handed the dunce cap. Great. Can't be trusted to stick probes in two red wires.

 

[Default]

If it's a voltage doubler circuit, John's measurements could very well be correct.
168+168 = 336, and voltage is increased more due to the ac peaks.

 

[Default]

Not a voltage doubler, looks like.

https://el34world.com/charts/Schematics/files/Carvin/Carvin_vintage16.pdf

 

[Default]

If John is measuring only one lead from the tranny, that sounds close enough. The schematic is calling for 360 volts on the plates. The listed voltage on the amp is 407, almost fifty volts over. Maybe a cathode resister on one of the tubes is burned open, and that might be the issue?

 

[Default]

Without the rectifier, won't have have won't have have AC instead of DC going places ?

Actually, you won't have ac on the board, because removing the diodes breaks the power circuit. Does anything look burned or discolored?

 

[Default]

Also, measure the voltages on pins 3 & 8. They should only be a couple of volts. If the voltage is high, and there isn't amplification, something isn't grounding properly in the circuit.

 

[Default]

12ax7 Tube Datasheet

The 12ax7 is a miniature high-mu twin triode, each section of which has an individual cathode connection. It is especially suited for use in resistance-coupled voltage amplifiers, phase inverters, multivibrators, and numerous industrial-control circuits where high voltage gain is desired. A...

guitarkitbuilder.com

 

[fronobulax]

"At the end of the day I just don't trust your measurements which is why I can't offer any further advice. "

So, I've just been handed the dunce cap. Great. Can't be trusted to stick probes in two red wires.

Just observing that everyone here is trying to help so if there is some unfortunate phrasing it is not necessarily a personal attack.