Hum after output transformer replacement

[Phil_S]

Did we resolve if this is 60Hz or 120Hz? I'm thinking this is a lead dress problem. If we know the frequency, we know where to look. 60Hz is about A# below the low E. 120Hz is ~A# on the A string. A is either 55Hz or 110Hz. For 60Hz, we look at the filament wiring, especially the supply from the PT, which is my suspicion.

[paulster]

It's 120Hz and a single-ended amp.

The level of hum on the primary side of the OT is probably exactly the same but the additional bandwidth of the larger OT is letting far more of it through to the secondary I'd guess.

I'd just increase the first filter cap significantly if it's a bother. It's not likely to make the amp particularly stiffer since the power supply in a single-ended configuration isn't going to have a whole deal of sag in the first place.

[mwickett]

It sounds very plausible that the new transformer is simply letting more 120Hz through. Looks like the filter cap is a 22/450. What would you recommend I increase it to?

[paulster]

Try a 47 as you should easily find one that'll be broadly the same dimensions.

100 would definitely be worth a try but it might be difficult to find one that'll fit.

[daydreamer]

Just a general question,
when the value of the capacitor increases as you suggest, what is the effect on the AC from an electrical perspective? I'm learning about these myself and I thought I had it, but this seems counter intuitive. It's probably because I'm learning it all based on electron flow and the caps are labeled the opposite maybe?

I don't mean to hi-jack the thread, but i will most likely be going through this same drama soon with my own amp. (New step down trannie didn't solve my hum issues). I want to understand the why as well as what!!

carry on!!

Andy

[selloutrr]

This might help explain

http://www.electro-tech-online.com/gene ... -caps.html

[daydreamer]

Ok, cheers. So by increasing the size of the cap, it's the discharge TIME that increases not the voltage. I had in my head that it was dealing with the frequency of AC, but it's DC at that point. Slowly, slowly...

[mwickett]

Though this thread is long quiet, I thought I'd post my (hopefully) final results.

I couldn't give up on this amp so easily, and from the suggestions here, it just seemed that more power supply noise was getting through with the new transformer. So I ordered a Hammond 156R filter choke and a JJ Electronics can capacitor and built a pi network filter. There just happened to be a jumper on the circuit board right where I needed to install the filter, so this turned out to be easy. With this in place the hum has gone.

To summarize my changes to this amp:
- Hammond PT125DSE output transformer
- Hammond 156R filter choke with JJ can capacitor
- Eminence Rajin Cajun 10" speaker

All fits in the original cabinet. This amp had very little low end before, and the OT changes that in a big way. The amp was also only moderate in volume, but the OT and the new speaker have made it quite loud.

Thanks everyone for their comments and suggestions. It's satisfying to have made this work out, and I'm very happy with the final results.

[dynaman]

...and thanks for the follow-up. These threads are far more valuable when a proven "fix" is reported.