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Old 21st February 2013, 09:24 PM   #1
shucky is offline shucky  United States
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Question Safe use of half of a stereo PP amp?

I have a stereo push-pull power amp but I only want to use one side, and it is not a dual-monoblock, ie I can't just power down half of it. Is it safe to just pull the tubes on one side without a load connected? Or should I disconnect the B+ from the output trans also?

The negative of the speaker terminals is grounded if that matters. Also I'd be pulling the PI for that side. The heaters are A/C powered so I expect they will still be within spec. I ran sims in Duncan's PSU designer and the only side-effect is half the ripple.

Forgive me if I'm asking a dumb question, but I'd rather live to ask two stupid questions than die because I didn't ask one.

EDIT: The schem shows one of the two identical channels.
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File Type: jpg Mesa 2x100W amp.jpg (146.0 KB, 82 views)

Last edited by shucky; 22nd February 2013 at 02:47 AM. Reason: wanted title to clarify that I'm asking a question
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Old 22nd February 2013, 01:05 AM   #2
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Use a shorting RCA plug at the I/P of the "slack" channel. Connect a resistor, instead of a speaker, to the "slack" channel's O/P. ALL tubes are left in place.

Operating voltages will be incorrect, if a channel is "disabled", by removing tubes.
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Old 22nd February 2013, 02:13 AM   #3
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if all you want is to reduce the number of output tubes, check with the mesa boogie forums to see if you can remove one tube from each side.
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Old 22nd February 2013, 02:28 AM   #4
shucky is offline shucky  United States
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Well Mesa themselves say you can remove a pair from each side to make it a 2x50w amp but that also would mean the tubes would be presented with exactly half of the originally intended load impedance, and that doesn't appeal to me. I don't really need a stereo rig and 2x100w is WAY too much power for my needs, so really my goal here is to turn it into a single 100w amp, and not have four 6l6 and a 12ax7 just sitting there wasting electricity and creating heat while not being used.
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