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Old 17th February 2007, 05:52 PM   #1
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Default power supply question.... too many Volts?

I have two amplifiers i am concerned with in this post... one is an eico ST70, the other is an old motorola amp.

I would like to run both with less tubes, as stereo power amplifiers only. I'm wondering if removing tubes is going to create too much of a rise in voltage, and throw things out of whack?

The ST70 would be lacking only 4 preamp tubes in this configuration. I don't imagine that would be a huge problem.

The motorola however, is a 3 channel 6bm8 amp which has a total of 4 output tubes, the bass is a PP mono setup, the R/L channels are single ended. I want to use this amp with only the pair of single ended tubes. I think that might be more significant.

Any ideas? Should I worry about adding resistors?

Aaron
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Old 18th February 2007, 05:16 AM   #2
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^^^^
I doubt that it will amke much difference if the PS was designed to have anywhere near a reasonable regulation. Even if the voltage rises somewhat, remember, VT circuits are a good deal more "forgiving" than SS.
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Old 18th February 2007, 01:55 PM   #3
Klimon is offline Klimon  Belgium
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Quote:
The motorola however, is a 3 channel 6bm8 amp which has a total of 4 output tubes, the bass is a PP mono setup, the R/L channels are single ended. I want to use this amp with only the pair of single ended tubes. I think that might be more significant.
If you want to keep close to the original design (= not using it as a parts donor) I would suggest to measure B+ voltages and current draw for the SE output stages, then redesign the power supply (reusing the original trafo and possibly chokes / caps) using Duncan's PSUDII simulator (can be downloaded on duncanamps.com) which is an excellent tool + easy to work with.

edit: the single-ended opts will probably lack low-end bandwith if they were designed to work together with a dedicated low-end PP amp.

good luck!

Simon
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Old 18th February 2007, 04:12 PM   #4
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the OPTs for the single ended outputs on the motorola are lousy, but I have a pair from another amp which are much larger.
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