Is it safe to use a 8BQ5 in place of a 6BQ5 in a guitar amp? - diyAudio
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Old 16th August 2010, 05:34 PM   #1
Zone47 is offline Zone47  United States
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Default Is it safe to use a 8BQ5 in place of a 6BQ5 in a guitar amp?

I think the heater voltage is 8V on the 8BQ5 instead of 6V. I don't want to stress out the power tranny.
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Old 16th August 2010, 06:06 PM   #2
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It would work fine, but you would probably have to create a power supply for the heaters. As-is it may work, but not optimally.

Ideally your amplifier is point-to-point wired. That would make wiring in an extra transformer for the job easier. If you're looking at a PCB then you'll likely need to cut traces on the board to do it.
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Old 16th August 2010, 06:25 PM   #3
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If to rectify 6.3V by a shottky bridge and a cap say 10,000 microfarad you will get about 8V of DC. I powered such a way 8AL9 tubes in one hybrid amp.
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Old 16th August 2010, 10:59 PM   #4
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It's like running 6.3 volt tubes on 5 volts. No harm to the tranny, perhaps slight low emission but perhaps not enuff to make an issue.
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Old 16th August 2010, 11:23 PM   #5
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I'm with Wavebourn, Bridge rectifier and a nice juicy electrolytic will give you 8v's, I just played with a 6.3v tap on one of mine, got up to about 4,400uf on capacitors and I started seeing 8volts idle, and with a 35v 10,000uf I was holding 8volts @ 350ma quite comfortably.
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Old 17th August 2010, 12:51 AM   #6
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I have a big bag full of 8BQ5's. Most of them work just fine in a Simple P-P in place of EL84's. If your amp is old and was designed to run on 110 or 115 volts your 6.3 volt heater voltage is probably 6.8 volts or more. The tubes that don't work either distort or don't make full power. Nothing will blow up.
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Old 17th August 2010, 02:04 AM   #7
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What George said.
FWIW,I've been told by a couple of 'old-timers' that 8BQ5's will usually work in place of 6BQ5's 'just fine'. The emission will be a bit lower (not usually a problem,depending on the circuit),but that the tubes would,in turn,last longer.
I haven't actually tried it,but it seems to make sense. Give it a shot,It shouldn't hurt anything.
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Old 17th August 2010, 02:17 AM   #8
wa2ise is offline wa2ise  United States
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If you have available a second 6.3V secondary (either from the power transformer or a 2nd transformer that has 12.6V or 6.3V) connect up to get 12.6VAC, then use an unfiltered rectifier diode (no capacitor) from the 12.6VAC source to feed in series the 8.4V heater. What this does is cuts the power consumed by the heater by half that had we run it directly off the 12.6VAC. Power = Vsquared/R If R is constant, then we can ignore it for the moment: 8.4^2=70 nearly = (12^2)/2=72. Here the diode drop of 0.6V helps us to get it closer to 70, which is close enough. Be sure to have half the 8BQ5s heaters operate on the negative cycle, and the other half the positive side of the 60Hz waveform, to avoid a DC load on the transformer.
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Old 17th August 2010, 04:29 AM   #9
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Note that, contrary to intuition, low filament voltages may also negatively affect tube life. Numerous OEM technical documents from the 1940's - 1960's indicate that both over and under voltages (especially on thoriated filaments) are deleterious to tube life.

In addition (and to your benefit if using 8v filament tubes) you should note this. If your amp was manufactured in the 117 volt era, (1950's - 1960's) then today's 121 volt mains supply, common in many US cities, means that your filament voltage may already be a few tenths higher than the expected 6.3 volts. It could easily be 6.7 volts. That means you would be operating an 8v filament tube at 16.25% below rated voltage. This would probably reduce the power output of the tube by some small but measurable amount.
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Old 17th August 2010, 08:02 AM   #10
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Kind of funny how all of us can say the same thing...
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