Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Altec, Acrosound & Dynaco

Eico, Grommes, HK & Heath

Pederson thru Stromberg

Notes About Resistors

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For the most part I think what types of resistors to use should be decided by the builder. There has been a lot of debate on this I am not going to repeat here. I can tell you a few things:

Carbon compostion resistors have the disadvantages of not being flameproof, a positive temperature coefficient, and a tapering response at high frequencies, e.g. higher the frequency, the lower the value of the resistor to the signal. But many people claim they like these best.

Carbon film is pretty much a waste of time in my opinion. You might as well go the rest of the way and use metal film.

Metal film has the advantage of not being flammable (but they can get hot enough to make other things burst into flame if overheated!), very even frequency response and very good temperature coefficient. I am not going to discuss brands except to say they are all good, even the cheap ones.

The only other caveats regarding resistors are the calculations of the wattage rating. Remember when you do the usual Ohm's Law calculation to figure what rating should be to get at least double what your figures say, example, if you caculate you need at least 1/6 watt, get a half-watt, if you need 4/10ths use a one watt.

Remember that if there is a significant AC signal present that you must add the AC and DC dissipation together to figure the rating. For example if we have a 10,000 ohm resistor in a cathode follower, and there's 5 ma of DC current, that's a 50 volt DC drop, thus DC dissipation is .005 X 50 = .25 watt, if there's another 50 volts of AC signal at one end of the resistor, that's another .25 watt, thus we need a 1 watt, not a 1/2 watt.

When people calculate the dissipation of a bleeder resistor paralleling an electrolytic capacitor (usually used when 2 are run in series) they usually only calculate the DC dissipation and forget about the residual AC ripple current that appears across these.

Rather than trying to calculate the ripple current, it's easier just to make the resistor twice the wattage that you normally would, except in extreme cases this gives plenty of safety margin.

Notes About Transformers

Any power transformer or combination of transformers that supply the requisite voltages at the required current may be used instead of the ones listed. There is no problem using a transformer with current capacity well in excess of what is required, just bear in mind that a load of at least 10% of the transformer rating is required to maintain good voltage regulation.

Choke input power supplies usually require a fairly large bleeder resistor for proper loading except on straight Class A amplifier or other arrangements where the current does not change much. The Radio Amatuer's Handbook which is found in most public libraries, has a good deal of information regarding high voltage power supply design. Older copies are the most useful for arrangements using vaccum tube rectifiers.

There are a number of makes of output transformers both new and old which are suited for use in these projects, provided they are close to the electrical parameters required. Used transformers are as good as new ones for all practical purposes, so long as the insulation is in good condition, good quality transformers pulled off otherwise junk units are OK.

Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Altec, Acrosound & Dynaco

Eico, Grommes, HK & Heath

Pederson thru Stromberg

Back to Dusty Files Online