Low DC Voltage Unloaded after Power Tube Failure

[jlatrace]

Tonight my ODS 183 copy blew an EL 34 power tube. Afterwards, I replaced the tube with a fresh one noting that the B+ voltages were low. So I pulled the power tubes and checked the voltages throughout the amp. All of them are low. For example, the B+ voltage "unload" (no power tubes) is 252 vdc. Every other voltage is low but the drop is pretty much the normal voltage drop at each stage - for example the B+ voltages on the v1 12ax7 (v1) were 108 vdc & 107 vdc. I do not see any burned resistors, but the chassis did have an ozone smell when I opened it up.

I'd really appreciate suggestions on where to start trouble shooting the amp. For example, what issue resulting from a blown tube would cause so much voltage to be dumped to ground before getting to the power tubes?

I'm pretty sure the issue is in the power section, or the solid state rectifier. Should I de-solder the power transformer high AC leads and verify that the high AC voltages are correct? Or is there a better place to start?

And please excuse my ignorance, I have a hobby builder and this site has helped me build two 102's and the 183 that I'm currently fusing with.
Thanks,
Les

[xtian]

I'm pretty sure the issue is in the power section, or the solid state rectifier. Should I de-solder the power transformer high AC leads and verify that the high AC voltages are correct?

Exactly. You could have a shorted diode or filter cap. It would be best to disconnect ALL PT secondaries and observe power consumption--the PT should draw 8-12 watts with no load.

[jlatrace]

Thank you, your advice solved (albeit unfavorably) my problem. My amp was producing approximately 1/2 the design voltages, because 1/2 of the windings are DOA.

I took disconnected the secondaries and read the voltages, 1/2 of the secondaries read the proper voltage, the other 1/2 read -0- volts, confirming that the power tube failure took out my Mercury Magnetic's power transformer. I guess I install some protective measures (maybe from Rob Robinette's site) so that the Classictone replacement (I had one on the shelf) survives the next tube failure.

Thanks again for your help!
Les

[xtian]

I urge you to do a little further testing, because "1/2 of the windings are dead" does not explain the observed low voltages on your HT chain.

Which PT secondary windings are good, and which are dead? How, exactly, are you measuring?

[Charlie Wilson]

Yeah, not sure about that. The entire winding is either open or shorted( a little or a lot). I agree, how are you testing this?
CW

[Richard1001]

I urge you to do a little further testing, because "1/2 of the windings are dead" does not explain the observed low voltages on your HT chain.

Which PT secondary windings are good, and which are dead? How, exactly, are you measuring?

It actualy does explain the low voltage... The highvoltage winding is made by two coils in series. Where they are connected together is called the center tap. When one coil fails you're left with only a half wave rectifier. This causes the lower voltage.

[jlatrace]

I finely had time to get back to this problem. I wanted to post this to help other uninformed hobby builders like me.

So as a recap this was a 2013 183 build. I changed tubes to some SIE EL34s in 2002. I did not realize that those tubes cannot handle more than 450vdc on the plates. A tube failure occurred. Afterward, without me realizing, the 3.9k resistor in the bias circuit was apparently stressed, but not fried. I assumed it was my PT, so I replaced it, but the low voltage problem continued.

Today I rechecked every circuit, took voltage reading, etc. As a result of consistently too negative bias voltage (-103), I pulled the bias board to learn that the 3.9k resistor on the board was reading below 3k.

So I replaced the resistor with a 5 watt, 3.9k resistor reading 3.89k, problem solved!

I hope this helps others whose el34s fail.

[rccolgan]

This is great and thanks you for posting the findings! I wish everyone would follow up like this! Most of all, I'm glad you resolved your issue and we all learned something.

[timrobertson100]

Yes, very helpful. Thank you Les.