Bottlehead Kits > Legacy Kit Products

Help with 60Hz hum in Foreplay I in custom chassis

(1/12) > >>

atalcott:
A little while back I picked up a Foreplay I kit that had been sitting in someone's closet for the last 30 years. I'm just getting starting with tube electronics, and putting it together was a great learning experience. It came with the Anticipation and Sweetest Whispers upgrades, though I only installed the former. Once I had it running in the original configuration I started thinking how I could make it better for my tastes and purposes. I built a new case for it out of thinner walnut with a lower profile, moved the PT underneath the chassis inside the case, moved the input and output jacks to the back of the case, replaced the dual volume pots with an Alps Blue Velvet stereo unit, added a choke in place of the second resistor in the CRCRC network, replaced the coupling caps with Solens, added 100k resistors at the outputs to attenuate the output level a bit (without them the gain was just ridiculous when matched with my Lafayette KT550 power amp) and got a matched pair of Tung Sol reissue 12AU7s.

The thing sounds great, but it's got a pretty pronounced 60Hz hum. I tried to replicate the grounding scheme as much as possible when moving to the new chassis, but with the move of the i/o jacks I did have to make some changes. Like I said, I'm more or less a complete beginner, so I'm not sure where to start with tracking down the hum. It’s in both channels, is unaffected by input source or volume control, though there is an additional, slightly louder 60hz buzz over the top of the hum when I touch the volume pot. I’ve also tried moving things around (various wires, the Anticipation circuit boards) with a chopstick and no movement seems to make any difference in the volume of the hum.

Any suggestions for things to look at? Any obvious mistakes I've made with the grounding scheme? Pics attached. Thanks very much for any input.

Paul Joppa:
One widely used mod from back in the day was to tie the signal ground to the chassis ground. If I recall correctly, that connected terminal 13 to 14 - but it's been at least a couple decades so I might have mis-remembered. It solved most hum issues.

atalcott:
Thanks Paul. I just tried that out but no luck I’m afraid.

In the original setup signal ground goes to the grounds for the volume pots, the CF/VA boards, the cathodes of the first section of each tube, and the heater center tap of socket B (pin 9), then to terminal 13, and finally a line ran from there to terminal 1. I put a jumper from 1 to 3, connecting the circuit to chassis ground at the first filter cap ground, and there only. With that configuration, I didn’t get any hum.

Now the signal ground goes to a jumped dual terminal strip near the volume pot. That strip is connected to the grounds for the volume pot, CF/VA boards, the cathodes of the first sections of each tube, and the heater center tap of socket B. Finally, a line runs from that jumped terminal back to terminal 1.

To my thinking this should be effectively the same as the original, but maybe I’m missing something. The only effective difference I can see is that now the output signal ground comes after the input signal grounds.

Paul Joppa:
Well, it was worth a try  :^)  Hopefully PB will join in; he has the most experience fixing these things. Meanwhile:

* If you haven't already checked the resistances and voltages, it's always a good thing to do first to be sure it's working properly.

* The second thing is to re-flow every solder joint, making sure the solder flows and wets both wires and terminal lug. It takes only a little time, and solves a great majority of issues without wasting a bunch of time figuring out where the problem is.

(N.B. If you are using the original single-crystal copper ground wire, it had an amazingly strong enamel insulation that makes soldering difficult worth checking each joint. Usually just re-soldering does the trick, but this wire sometimes needs serious scraping.)

* The third thing is to isolate the preamp from other components and cables as much as possible - shorting plug at the input are most helpful, but turning the volume all the way down is often nearly as good.

* Finally, posting some photos of the wiring is a big help.

I don't understand your 100K output resistor - can you clarify?

Paul Birkeland:

--- Quote from: atalcott on June 30, 2020, 06:33:20 PM --- With that configuration, I didn’t get any hum.

--- End quote ---
So...problem solved?

Navigation

[0] Message Index

[#] Next page