Power Transformer spec questions. Will it work or burn up?

[flysig]

Three quick questions on choosing a power transformer for a guitar amp. The design is 14 watts RMS maximum power using 6V6 power tubes and an assemblage of preamp tubes. The transformer will be either 300-0-300 or 285-0-285.

1) The quiescent current for all the tubes totals 235mA. Will a transformer rated at 250mA be sufficient? Is there a rule of thumb for the idle current draw vs transformer rating?

2) Can unused secondaries be left floating open, or should there be some load to ground or between the wires of that unused secondary? For example, a second heater secondary that is not used.

3) How much DC voltage drop is normal in a P-P output transformer primary? The 6V6 will draw about 24mA each at idle. Is 4 volts a reasonable ballpark number?

[kheper]

Originally posted by flysig
Three quick questions on choosing a power transformer for a guitar amp. The design is 14 watts RMS maximum power using 6V6 power tubes and an assemblage of preamp tubes. The transformer will be either 300-0-300 or 285-0-285.

1) The quiescent current for all the tubes totals 235mA. Will a transformer rated at 250mA be sufficient? Is there a rule of thumb for the idle current draw vs transformer rating?

In a Class A amp, the difference between the plate current at zero signal and the plate current at maximum signal should be very close.

In a Class AB1 amp, the plate current draw at maximum signal will higher than at zero signal.

A total current draw of 235mA for 2 6V6 tubes and the associated drive circuitry sounds a bit high to me. 35ma x 2 (70ma) is an average zero signal current for 2 6V6 tubes. I do not think that even 95ma for both 6V6 tubes will be reached at maximum signal. (But I do not know that much about guitar amps.)

2) Can unused secondaries be left floating open, or should there be some load to ground or between the wires of that unused secondary? For example, a second heater secondary that is not used.

Yes, provided they are truely separate secondaries/windings. The transformer will run cooler with a secondary not used.

3) How much DC voltage drop is normal in a P-P output transformer primary? The 6V6 will draw about 24mA each at idle. Is 4 volts a reasonable ballpark number?

It depends on the output transformer. I'd expect more than 4 volts.

[flysig]

Thanks. I should have specified that this is an AB1 amp.

I re-ran my sums and found that the total quiescent current is 114 mA. Some kind of operator error last night on the calculator or something, I just added the series of numbers incorrectly. Using the numbers from the graphs of current production specs the correct total is 114mA, and using the "typical" numbers off of a 1953 GE spec sheet it comes out to 124 mA. That's total for the amp at idle. Max signal current would be 156mA.

Should the transformer have a certain percentage of extra current capacity over the expected average or max current draw? If I assume 124mA idle and 156mA peak current, what should the transformer be spec'd at?

How much voltage drop is normal in the output transformer? I can't find a DC resistance spec for the transformer and need some kind of ballpark guess so that I can decide what voltage power transformer I need. Using the 1.2x rms voltage estimate under load, the 285-0-285 transformer leaves me with 6 volts for the DC drop across the output transformer. Going to the 300-0-300 transformer will bump up all the voltages and temps and require re-calculating a bunch of resistors and capacitor values, but if it has to be done it has to be done.

[kheper]

Originally posted by flysig
Thanks. I should have specified that this is an AB1 amp.

<B>I re-ran my sums and found that the total quiescent current is 114 mA. Some kind of operator error last night on the calculator or something, I just added the series of numbers incorrectly. Using the numbers from the graphs of current production specs the correct total is 114mA, and using the "typical" numbers off of a 1953 GE spec sheet it comes out to 124 mA. That's total for the amp at idle. Max signal current would be 156mA.</B>

250ma will be fine. ~100ma is fine cushion. Most commercially produced amps do not provide such a soft cushion.

<B>Should the transformer have a certain percentage of extra current capacity over the expected average or max current draw? If I assume 124mA idle and 156mA peak current, what should the transformer be spec'd at?</B>

See above.

<B>How much voltage drop is normal in the output transformer? I can't find a DC resistance spec for the transformer and need some kind of ballpark guess so that I can decide what voltage power transformer I need. Using the 1.2x rms voltage estimate under load, the 285-0-285 transformer leaves me with 6 volts for the DC drop across the output transformer. Going to the 300-0-300 transformer will bump up all the voltages and temps and require re-calculating a bunch of resistors and capacitor values, but if it has to be done it has to be done.

I do not know off of the top of my head what my st70's voltage drop was across the output. I think it was less than 10 volts, but I cannot swear to it.

[kheper]

The voltage drop from the B+ on the primary of the output transformer to the plate of the output tube is 7 volts. 425Vdc -> 418Vdc

[chrish]

I am building a similar guitar amp. An AX84 20watt with 2 x 6V6 + AX7 with the 'lead' preamp using 3 12AX7s with a 150mA transformer with the appropriate heater current ability. I would think that the maximum continuous current capability of the transformer should have an adequate margin already built in if it is a reputable manufacturer. Tubelab regularly exceeds the recommended maximum ratings for power transformers on guitar amps using the Allied transformers.

Cheers,

Chris

[flysig]

Thanks, that helps a lot. Time to order some parts and see what this whole thing does. Hopefully no smoke....