Rocket Filament Winding Center Tap Vs Artificial One

[Matt D]

For the record:

In my rocket build I used RJ's em0708 power transformer, which has a center tap for the filament winding. Originally I used the center tap and grounded it to the top of the cathode resistor of the power tubes. The amp had a slight to moderate hum.

Tonight I removed the center tap, installed a 470 ohm hum balance pot and grounded that to the top of the cathode. Using the hum balance I was able to make the amp MUCH quieter.

[rooster]

OK, you see this come up recently, where it is suggested that you tag your center tap of the filaments to the top of the cathode resistor. This is old school hi-fi stuff and it's treated like it's been recently invented, OK, coolio. But along those lines, the treatment here is to use a single 100 ohm resistor - and not a balancing pot. And this may be all you are experiencing - removing the center tap from the cathode by 250 ohms.

All of this said, if you wrap the center tap (don't use it) and instead use two 100 ohm resistors off each leg of the filament supply (and ground it near your PT, away from your DC ground(s)), your amp will be quieter still. Sometimes regurgitating old ideas is basically discovering cost savings ideas from the various manufacturers, whether labor or parts count. Interesting stuff but maybe more interesting to me to see who jumps on the bandwagon. Try everything, then you know what's best.

[Matt D]

Thanks for the reply Rooster. I certainly am under no illusion that I have figured out something new in grounding the center tap to the top of the power tube cathode resistor!

Perhaps I didn't make it clear enough that I had de-soldered and wrapped the center tap and used a pot to make an artificial one. I used the two sides of the pot in place of the 100r resistors. The pot is a 400r so I guess it would be in place of two 200r resistors.

Anyway, I thought I would post my experiences in case they might be useful for someone building a Rocket with the same transformer. When I started the build I figured I would use the filament center tap from the transformer and if it was noisy change to an artificial one. It was a little noisy, so I changed it up and with the artificial center tap the noise is much improved.

I'll try grounding the hum-balance pot wiper near the PT and see if the amp gets any quieter.

[RJ Guitars]

Great job Matt! It's nice to know that you conquered the problem and the solution is very affordable. Sometimes we get so caught up in the cloning business to the point that we forget to apply those fundamentals that folks sorted out a long time ago. I've got a couple places I'd like to try this and see if I can get similar results. It's funny how many times and places you might find this reference but I don't think I've seen it suggested in place of a center tap before (I could have missed it). Most of all I am really appreciative that you shared this tech tip. rj