Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jojokeo on November 06, 2010, 12:31:17 pm
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I was thinking that we could test our already built amps' PT & OT placement locations for how much induced hum there is using the headphones on OT technique (from link below).
We wouldn't be able to move the OTs around for best placement like could be done before mounting them for a new build, but an existing amp could be "listened to" anyway. The info could be used for comparrison based on knowledge from prior tests done or to simply get an idea of how much that's heard w/ this test vs. the actual amount of noise emitted while in circuit at idle. This also could help troubleshoot or assist others that may be thinking that their heater wiring, grounding, or anything else that could be causing unecessary hum or noise could be checked, verified and/or targeted. With all of these various possiblilities, knowing that the amp's noise floor is as quiet as it could or should be would be helpful?
So, in order to do this wouldn't the only thing needed to do would be to pull the the rectifier tube or w/ ss rectification - simply pull the power tube(s)? Then power up and plug into the OT's output speaker jack w/ the headphones and listen for the PT noise if any?
Do you think this will work? Any issues I'm not considering?
*This is continued from: http://www.el34world.com/Forum/index.php?topic=10378.0
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Sure why not. Pull the piower tubes, make sure its on standby. I dont use headphones, I use a scope - juts because I have one. But you can use a DVM on mV AC. That way you will be able to compare results from one amp to the next.
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Thank you - induced hum is sm AC voltage - makes sense.
Secondly, you're onto what I was concerned with regarding any possible damage to the OT. What exactly creates the condition which could damage it? Is it when B+ is applied to it when there's no power tubes (or if they're blown) - meaning there's no longer a load across it?
However, I don't quite get this because take a single ended schematic and look to see that B+ goes into the tranny primary but on the other side of the coil if there's no power tube where the plate tag would be, then there's no circuit completion for the other side of the primary winding. Therefore, how could the tranny "be loaded or see a voltage" whether or not there's B+ to it?