Reasonable expectations for a 500C

[Al Taylor]

It's been about 3 months since I finished the restoration on my 500C. I had one cold solder joint that gave trouble when the unit heated up, otherwise I've been extremely happy with the unit's performance (used 1-5 hours/week, depending).
I've noticed a barely audible hum in the unit, I'm not sure if it's new or if I just never heard it before. It sounds like 120hz-ish, not sure. I can only hear it if my ear is an inch away from 92db woofers, and it's independent of the volume control. it's also the same in both speakers so I suspect it's power supply hum on the power tubes.

My question is whether this is worth chasing down. On the one hand, it's an old unit and I can't hear the hum at all if my ear isn't right on top of the woofer. On the other hand, it could be a symptom of a drifting resistor value in the PSU or something similar. Should this thing be absolutely dead quiet, or should I just not worry about it? Opinions?

 

[Naptown Rob]

I believe that changing your listening position will solve this problem.

 

[amphead4]

If you decide to chase this, it would only be because you like a challenge, not because it has any impact on normal listening.

If you can't hear it during normal listening, does it even exist? Like the tree falling in the woods...

 

[Fisherdude]

What work did you do on it?

 

[Al Taylor]

What work did you do on it?

So, this wasn't a museum piece really, there's a fair bit of pitting on the chassis and the original case got fairly chipped and banged up. It was kept in fairly good shape though electrically, all the passive components I measured are well within spec (elytics aside, of course). Even the Erofoil coupling caps are within 1% and have no leaks to speak of. Actually, the Fisher-branded power tubes even sound fairly decent still, ignoring the brown getters... preamp tubes are all original and it's got a matched quad of JJ 7591s in there at the moment.
So because of the condition, I tried to be minimal with signal path stuff. I replaced all coupling caps and electrolytic cathode bypass caps. The only resistors I swapped out were the power tube grid leaks, which went down to 200K from the original 330K.
The power supply got a complete rework. Cap cans were replaced with either Weber cap cans or radials mounted on perf board (with original power resistors). Selenium rectifier replaced with silicon, power cord replaced with grounded three-prong. I increased capacitance values on a couple PSU nodes, but only by a few uf, depending on what was available.
The only big difference spec-wise is in the bias/filament supply. The 1000uf caps are rated for like 150V or 250V instead of 50V, I figure there's no problem there though.
All that, half a can of deoxit, and about 200 Q-tips is all I've done so far.
Sounds like I should leave well enough alone and just listen to music, which is fine with me. I thought there was a small chance that a little buzz would send off alarm bells with people so I wanted to ask.
I'll try to keep my head more than 3" away from the speakers from now
Thanks for the input everyone!

 

[audiodon]

Do you use a sub with it?

 

[Al Taylor]

No sub, but it's in the works (not a big budget for this stuff, I did all the 500C electronics myself, and the $200 for my speakers is the most I've ever spent on home-listening audio equipment). I use the tritrix speakers right now for the most part, and am starting to really notice the lack of low end. This is my first HiFi system, 5 months ago it was sterile M-audio powered monitors and nothing else. It's been a nice process of slowly appreciating what I've been missing all this time, for sure.

 

[amphead4]

If you pull the power tubes and listen again, it will tell you if it's coming through the tubes themselves or from magnetic coupling between the PT and the two OPTs. If it's magnetic coupling, no amount of electronics work will reduce the hum.

If it's 120 Hz, it's PS ripple from the first stage after full wave rectification. Push-pull output stages tend to cancel this if they are perfectly balanced, but perfection is hard to acheive in the real world.

If it's 60 Hz, it could be filament hum or ground loop issue. Check the two 220 ohm resistors that make up the virtual ground of the power tube filament circut or even try a pot with the wiper grounded to see if you can get a better balance.

Or pull your interconnects from the inputs to see if a ground loop problem is present because of the 3 conductor power cord.

 

[audiodon]

Amphead4,
Yeah, my first thought was 3 prong cord. I've bought converted units that did 2 to 3 right and I've bought converted units that did 2 to 3 wrong.
Al,
I'd also try a 3/2 conductor cheater plug on the AC. You should be able to turn it up without a hum and you should be able to run a sub off the center channel taps and it'll respond properly to the volume control.

For example, I've got a bad ground in the AC in my workshop and I'll have to disassemble my workshop and fix the AC this fall.

I had a Fisher X-200 that i thought was working right and lent it to a friend and it blew up two tube TVs at his home connected on the same terminal strip!

It turns out that the X-200 had a ground problem. it only worked right with an incorrectly wired AC line in my workshop I must have troubleshot that X-200 on the bad AC.

 

[sgmlaw]

I've noticed a barely audible hum in the unit, I'm not sure if it's new or if I just never heard it before. It sounds like 120hz-ish, not sure. I can only hear it if my ear is an inch away from 92db woofers, and it's independent of the volume control. it's also the same in both speakers so I suspect it's power supply hum on the power tubes.

My question is whether this is worth chasing down. . . . Opinions?

I wouldn't worry about it. These instruments are never dead silent. An inch away, you'll often hear some tube rush at a minimum.

If it's anything, it's likely a very mild ground loop current from connected equipment. I've seen CATV drop differentials over 25v. Now THAT's ground loop hum. Drop an isolator on it and it goes away.

If you start hearing things from a normal listening position, then you have cause to dig deeper. If it's bothersome through headphones, then you may want to isolate the cause.

Otherwise, I wouldn't worry about it.

 

[audiodon]

What's an isolator? How would I drop one in? Where would it go?

 

[sgmlaw]

What's an isolator? How would I drop one in? Where would it go?

From a supply house, they cost $2 ea.

Holland makes a similar unit with built-in spike protection for slightly more money:

http://www.hollandelectronics.com/catalog/catalog.php?product_id=CISP-Cable-Isolator

Don't buy the snake oil/high end labels for 25x the price - there is no meaningful difference except in your wallet.

It is attached at the client/receiver end of anything coming off the remote drop, and severs the shield at that point, breaking the loop. If the installer did their job, effective single-ended shielding will still exist across the drop. Where local grounding is maintained, there is no meaningful loss of interference protection or signal strength.

Any device shared with any other connected to a long-distance drop with remote ground is subject to ground loop hum, which is simply a voltage differential in the ground potentials, creating a current. Most common with CATV, but I've seen it in apartment and office distributions. Strong enough, it can cause nasty video hum bars and destroy good audio. You shouldn't normally encounter it with a dedicated array drop to a single receiver.