DIY Audio Projects Forum

Motor run caps are designed with only three requirements in mind. First they must impart the appropriate phase shift to auxiliary motor windings dependent on the motor speed requirements. Second they must support high current draw without damage. And third, they must be robust enough to stand large voltage and current spikes (as can occur with step torque loads on the motor) without failure. There are no formal requirements for leakage, capacitance drift, or uniformity throughout the capacitor. The net effect is that many of these caps, while meeting the requirements for their intended use, fail in regard to what we would consider audio performance. This may be due to non-uniform plate material and dielectric properties, inexpensive bonding techniques within the caps, or even non-uniformity in physical construction.

I am not saying that some motor run capacitors can't be used in an audio amp with acceptable results. All that I'm saying is that these caps are not designed for this type of use and you have no way of knowing what tradeoffs the manufacturer used to meet his cost and yield targets. I had a bad experience and after a discussion with the manufacturer decided that it is simply not worth the risk to save a few dollars on an amp. I've used JJ, F&T, Jensen, Sprague/Vishay, and IC all with excellent results at a reasonable cost. So as a matter of personal preference I simply stay away from motor capacitors in anything other then a motor application.

Don't worry about the extra filter cap or two, I ordered a couple extra which all arrived yesterday. The only one I don't have is the 10uf. But I'll find a work around or trouble a few tube friends for one.

Not to interrupt your design discussion but hopefully add to it, one general comment and question about the 300B:

After the cathodes have warmed up and the B+ is applied you can hear the tubes making noise as they thermally expand, and this noise also comes through the speakers. This happens with my Electro-Harmonix 300B Gold tubes at least. My KT88 indirectly heated tubes don't exhibit this behavior.

I'm just wondering if anyone else has the same experience with directly heated power triodes. I'm also trying to figure out what exactly inside the tube it is that's making the noise. Thermal expansion of the plate, of the cathode filament wires rubbing against the supports, etc?

Perhaps better quality tubes don't do this. It stops after 30 seconds or so and is dead silent after that.

This is a little fly in the ointment. I was just getting ready to post the final PS design. It is based on an Edcor XPWR077. But that's not what the customer wants.

So... For the output transformers, thats easy. Go with the Hammond 1630SEA. It meets all requirements and we already considered it in our design. For the primary PS choke, I've already settled on the Hammond 193J 10H choke (or the 193M or 193Q if you can't get the 'J').

The power supply transformer is another matter. Hammond only makes two transformers that meet the PS requirements for this amp. These are the 282X or 382X ( 1000v(500-0-500)@200mA, 5v@3A C.T., 6.3v@6A C.T.). And I couldn't find either of these on the web page you linked. If I need to go to the lower voltage (400-0-400) transformer it will mean an entirely new design from the ground up.

That will work I got (by chance) 3 X 47uf.

Want to change the PS schematic and re-send. Just so we are all on the same page if we have to fault find. NEVER!!

We actually already settled on the PTFWF460 (460-0-460)@200mA transformer.

However, the link you posted is great for anyone ordering Hammond transformers in the US. Free shipping on orders over $80! I think that's every Hammond order I've ever made.