Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: toney on August 27, 2013, 06:37:05 pm
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I have been working on electronics for years..mainly semiconductor. I am now learning tube technology. I know tube amps must have a load..I.e. speaker. Can the the output tubes be pulled for testing of the preamp section??? A STUPID QUESTION IS ONE THAT YOU ASK AFTER YOU DESTROY THE REPAIR. THANKS FOR ANY HELP
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I would leave the power tubes in. Otherwise the B+ voltages will rise and may skew the results of your preamp test.
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Get a dummy-load.
Even for sand-state amps. They may not die un-loaded, but they can part-die in a way that they run fine no-load but quit pulling a load.
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Thanks Sluckey, your advice is greatly appreciated. in semiconductor I normally static test with a peak esr cap tester first. then I test with a huntron semiconductor tester. I will then test resistors with a fluke vom. finally I test small caps with a Beckman tester. only then will I power on test. I have repaired tube amps before. This amp is kicking my butt. 60 or 120 hertz output...extremely loud. I suspect it is a small coupling capacitor in the preamp stages. I guess I might have to get a load resistor or follow other advice on this website...disconnect caps and measure dc voltage?
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Welcome toney! :icon_biggrin:
Can the output tubes be pulled for testing of the preamp section???
Sluckey's right but if I might add to what he said, yes you can pull the power tubes to test the preamp section IF you really need too.
That is along as the B+ filter caps are rated high enough for the increased B+ dcv. As Sluckey said ;
I would leave the power tubes in. Otherwise the B+ voltages will rise and may skew the results of your preamp test.
When you pull power tubes, or any tubes, you are lightening the load on the B+ power supply (PSU) so the B+ dcv will rise.
The power tubes will probably in most amps pull ~90% of the total B+ current. If you pull a small bottle tube, like a 12AX7, or any other 12 _ _ 7 tubes, they only will draw maybe ~2mA to ~10mA compared to 40mA/60mA/80mA, etc. for power tubes, and that's at idle.
So you can see how pulling the power tubes will through off your test readings.
I am now learning tube technology.
Here's a couple of links to help you if you haven't already found them;
1. Our host Doug's "Tube amp Library of information."
http://www.el34world.com/schematics.htm (http://www.el34world.com/schematics.htm)
2. Lots of good info popular with many guys here;
http://www.valvewizard.co.uk/ (http://www.valvewizard.co.uk/)
Brad :icon_biggrin:
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Sluckey would it be possible to add your drawing/info on the light bulb current limiter some where here? Maybe it already is?
Our new member toney, as any new member, needs to build one.
Thanks, Brad :icon_biggrin:
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It would be interesting to know if the hum is 60 Hz or 120 Hz. Since most power supply hum would be 120 Hz (full wave or bridge rectifiers) and 60 Hz hum could be induced or could be from an ac heater circuit. Does your VOM have a frequency counter as one of its functions?