I think the heater voltage is 8V on the 8BQ5 instead of 6V. I don't want to stress out the power tranny.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
... yet hardly anybody addressed Zone47's main concern: will that stress the transformer ? Your suggestions (to rectify 6.3V rail and hang a truckload of capacitance onto it) will increase transformer load so transformer could overheat and perhaps also blow its thermal fuse if it was not overdesigned to begin with (= if it heats up significantly in normal operation with existing tubes in circuit). And this doesn't address the potentialy audible nasties from current pulses (buzzing) on each halfperiod when large capacitors are charging at all.
Simply replacing 6BQ5 wth 8BQ5 without altering the circuit (this is assuming current circuit uses AC heater supply) on the other ahnd will run tubes at lower emission but it won't put any additional tax on the transformer.
Don't worry, The transformer can't overheat: power will be drawn 8V*0.6A instead of 6.3V*0.76A according to specs for 6BQ5 and 8BQ5, plus a little bit will be lost on Shottky diodes. Not a big deal.
Capacitor does not draw power. It causes surge current, so cold filament does.
Buzzing? No buzzing, even when I power mic preamps using SS rectifiers. All depends on layout, but what's the need for bad layout, right?
Capacitor does not draw power. It causes surge current, so cold filament does.
Buzzing? No buzzing, even when I power mic preamps using SS rectifiers. All depends on layout, but what's the need for bad layout, right?
2x Schottky drop is 10% on top of the original load. 10% extra could be 10% too much (we know nothing about the original transformer and the nature of the question implies that OP is concerned about his transformer as it is).
Here I have a transformer that hums a bit at 50 Hz without any load attached to secondary and buzzes a bit with rectifier and load matching its nominal capability. It is an old TV transformer (low voltage mains type).
Here I have a transformer that hums a bit at 50 Hz without any load attached to secondary and buzzes a bit with rectifier and load matching its nominal capability. It is an old TV transformer (low voltage mains type).
Rectification will increase heating of the transformer secondary. This is because the current will become more spiky, so increasing the difference between mean and RMS. However, this won't be a big effect unless the capacitor is huge.
U didn't read my answer I guess. But my point is we all basically tell him no harm will be done. Some are more bored than others and go on about possible concequences and remedies.
No stress, it's fun to have discussions, but this thread could stop at the first few. I simply commented on how many of us, me included, pretty much say the same thing but in a different way.
How many of us do we need to change a light bulb?
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