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Topic: Vintage Peavey Festival amp |
Keith Cordell
From:
San Diego
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Posted 13 Sep 2012 5:11 pm |
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I want to lower the power output of a Peavey Festival with 4 6L6's. I have removed 2 6L6's from other amps of this type with the result being a lowering of output power. Can I do this with the Festival? And if so which 2?
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 13 Sep 2012 8:09 pm
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Normally the four output tubes are paired on each side... you need to remove one from each side to leave one on each side.
When you do this, you've *increased* the effective impedance... the tubes run in parallel, so pulling half of them doubles the impedance. If you don't want to run with a mismatch then you need to double the speaker impedance too... if you have two speakers in parallel (as in a Fender Twin) you can pull one lead from one speaker and double the load impedance. If the speakers are in series this doesn't work, it is your responsibility to verify that your amp is like a Twin Reverb in this regard! Of course, if your amp has more taps (like a 2ohm tap) you can plug your 4ohm system into the 2ohm output (or 8-ohm into 4, 16 into 8, etc) to get this ratio. From what you've written I'm guessing that you never paid attention to the impedance match when you pulled tubes before... at least the match you created (8ohm output into 4ohm load for instance) is the better mismatch... it only creates more heat and works the remaining tubes harder; a mismatch the other way (4ohm output into 8ohm load, say) causes output voltages to climb, perhaps enough to create arcing (Fenders not so much, other amps you take a risk, Marshalls etc don't handle this well).
As I've mentioned in other threads this gives results that may not be as pleasing as the original sound... your high voltage will rise, perhaps changing your bias settings and certainly changing the voltages your preamp tubes operate at (can get harsher). For standard guitar the effect may be worse than for steel... not many steelers are looking for break-up or sag in an amplifier and these are reduced when you 'stiffen' the power supply (as we're doing here). OTOH, the higher HV can give more parasitic oscillations if the preamp section was on the hairy edge of oscillation before this and can also result in higher bias current through the final tubes than desired (this can be re-adjusted).
The amp doesn't get much quieter... the tone changes, perhaps for the worse... it doesn't get that much lighter and it doesn't get *any* smaller when you remove one speaker... and that hole will want to be plugged or a new baffle installed. Your matched quad of 6L6's will un-match when only one pair is worn... same with the speakers (rotate every 6mos for both to lessen this). Most folks that find their Twin too heavy end up best served by putting it into a head cabinet, separating the head and speakers so you can carry them separately.
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Too much junk to list... always getting more. |
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Keith Cordell
From:
San Diego
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Posted 13 Sep 2012 8:14 pm
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| Thanks Stephen. Yeah I knew about the speaker impedance, but a reminder is always good. |
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Jim Cohen
From:
Philadelphia, PA
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Keith Cordell
From:
San Diego
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Posted 13 Sep 2012 9:09 pm
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| So I could use an 8 ohm 15" speaker. I have revisited the idea and decided to go with the 4 ohm and just pay up. 8 ohm 15's are just easier to find. |
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