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Old 21st February 2013, 09:24 PM   #1
shucky is offline shucky  United States
diyAudio Member
Join Date: Jan 2009
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
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ
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.
Eli D.
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Old 22nd February 2013, 02:13 AM   #3
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Join Date: Jul 2008
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
diyAudio Member
Join Date: Jan 2009
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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