Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: six el six on July 14, 2012, 10:58:54 pm
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Is there anything wrong with doing this?
I know the voltages will be high but if there's no tube rectifier is there any problem with doing this?
What about voltages going over the rating of the filter caps while doing this?
I think I just answered my question.
Thoughts?
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What about voltages going over the rating of the filter caps while doing this?
I think I just answered my question.
Yep.
Brad :icon_biggrin:
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What about voltages going over the rating of the filter caps while doing this?
A good design would use caps with a sufficient voltage rating to handle no tubes plugged in. For example, you have a PT rated for 300-0-300. The max unloaded dc that a conventional full wave rectifier can produce is 300 x 1.414 = 424. Filter caps should be rated for 450 or higher. Then you can never have that over voltage problem.
If you have a ss rectifier it's very important to use caps rated above the no load voltage because you will have that no load condition every time you power up with cold tubes unless you use a STBY switch.
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I see many well know amps without corect rating caps when no tubes load. Higher voltage capacitors cost more money ...and the amps are not made for working with no tubes
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Is there anything wrong with doing this?
Cap issues were covered.
My concern: what useful troubleshooting info do you gain that would not be had by having tubes in the amp?
Except for misrating of filter caps, you shouldn't do any harm though.
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My concern: what useful troubleshooting info do you gain that would not be had by having tubes in the amp?
On first power up of the few new builds I've done, I usually plug in the tubes except the recto, turn on and verify filament operation. Then I pull all tubes and insert the recto, power on and check for B+ in all the right places, ie, node caps, plates, screens, etc. Not looking for exact voltages, just that B+ is where it should be and ain't where it shouldn't be. This just gives another layer of confidence before powering up with all tubes in place.
It could be helpful sometimes when troubleshooting an amp that has a high B+ current related problem. Getting the tubes out of the equation, especially power tubes, is quick and easy to do and may help you to quickly get on the right trail rather than chasing a goose.
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I hear you on that.
I was thinking, but didn't state, that I thought this might be related to SixElSix's other thread about the Ampeg preamp. So I took the question to be about a previously functioning amp rather than a new build.
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I was thinking, but didn't state, that I thought this might be related to SixElSix's other thread about the Ampeg preamp. So I took the question to be about a previously functioning amp rather than a new build.
No. The Ampeg preamp I didn't remove the tubes to check voltages. It just has a severe hum problem that I can't run down.
Checking voltages without tubes is something that I've considered when an amp is in a serious state of disrepair and I want to see where the voltages are at before putting any tubes in.
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In that case I'd probably do the same.
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Yes, but with a current limiter. You might save a questionable filter cap or prevent a major B+ malfunction.
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Yes, but with a current limiter. You might save a questionable filter cap or prevent a major B+ malfunction.
Always.
I built mine with a normal/limit switch and a fuse.
I recently got a KILL-A-WATT I use in conjunction with my limiter. It's educational to see just how much current I'm pulling.
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In working on Bias issues, I never have the tubes in place. Variac set on about 100vac will remove the concern of overloading. Some do not like Variacs. I do not know why. This will give you a voltage rating enough to tell if you have a problem.