Reverb stopped working; Trouble shooting question

[Redfishbum]

Greetings.

I have an amp I build from scratch.  The reverb has been working fine for the last 6 months and then it just stopped working.  The reverb circuit is basically from a Princeton Reverb.  The tank is a 4AB3C1B by MOD.

I pulled the tank and inspected.  There is continuity within the wires within the "in" and "out" sides, respectively.  The Out also measured about 200ish Ohms and the In measured about 1 or 2 Ohms.

There is voltage at the B+ coming out of the Reverb Driver Transformer into pin 6 and 1 of the 12AT7.  When all is connected and powered up, if the tank is jiggled the springs make a sound out of the speaker.  I should also note that I made sure that the RCA connections going to and from the tank were clean.

The circuit seems fine.

My first thought is that the the tank is malfunctioning but it might be the tube.  I don't have a tube tester.

I have emailed MOD to ask them how to test the tank (sent several days ago) but have not received any response yet.

Any thoughts on troubleshooting?

Thanks in advance.

Bart

[sluckey]

The resistance measurements on the tank are correct and would indicate it is OK. Your other tests point to the reverb driver circuit. Change the 12AT7 first because that's easy. If no fix, measure voltage on pin 3 of the driver tube. It should be about 8 volts. Measure resistance from pin 3 to chassis. It should be 2200Ω.

[Redfishbum]

Thanks for the quick reply and your thoughts.  I did want to start just spending on replacing parts.  I'm glad that you are thinking tube.  I'll order a replacement  and go from there.

Thanks!!

Bart

[Redfishbum]

Wanted to update you on the what I found.  I was measuring the voltage at pin 3....read a few mV DC.  I then noted a white ash inside the tube.  Pulled the tube and it cracked at the base, right across pin 9.....all clearly point toward the tube is my problem.

Thanks again!

Bart

[sluckey]

Replace that 2200Ω resistor with a 2200Ω 3 watt.

[eleventeen]

"Wanted to update you on the what I found.  I was measuring the voltage at pin 3....read a few mV DC.  I then noted a white ash inside the tube.  Pulled the tube and it cracked at the base, right across pin 9.....all clearly point toward the tube is my problem."


Yep, a cracked tube will not always but in at least 70% of cases show that white crystallization should it lose its vacuum. Unmistakeable sign of a cracked tube. It is my experience that a very new tube (with low op hours) may not show the white crystals but the older the tube, the more the white stuff shows up if it's cracked.


Thanks for following up with what you found!

[PRR]

> 2200Ω 3 watt.

Is that the cathode resistor?

Why 3 Watt?

That's near as big as the 12AT7 (2*2.5W). Being a hi-Mu device we expect bias resistor dissipation small compared to plate dissipation. The computed heat at nominal 8V is point-oh-something Watts.

[sluckey]

Why 3 Watt?

Because that's the next size up from 1/2 watt that Doug carries.

I know that's overkill. 1/2 watt should theoretically be big o plenty. But that resistor will get hot. And I've seen several that were fried. I like to use 1 watt for that one, but Doug has no 1 watters. Doug uses a 3 watter for that resistor also.

[Redfishbum]

I've got a 1watt 2200 ohm cathode resister in there from the beginning.  I probably cracked the tube taking it in and out many times when I was originally troubleshooting my circuits which is probably the cause of the crack which caused everything else to fail.

Thanks again for the suggestions!

Bart