Series Filament Amp Hum

[bobloblaws]

My buddy brought me this amp, a Pepco 201 series filament/widomaker amp. An amp tech added a 3 prong plug and bypassed the tube rectifier and simply put a single diode inline for rectification. My buddy claims that when he got it back from the tech a few years ago it had only a low hum which was acceptable to him. When he went to use it recently he found it is making a very loud hum rendering it unplayable. I plan on adding an isolation transformer but I'd like to deal with the hum problem first. If I disconnect the green mains ground wire from the chassis the hum goes away. This seems counter intuitive to me, I always thought that grounding the chassis would reduce hum, not exacerbate it. Is this a case of a ground loop?
On the other hand, if what he told me is correct, that it was working OK and then developed this problem, I suppose it might suggest a failing component that only manifests itself when the chassis is grounded. Does that make any sense? Any insight, suggestions etc. appreciated, thanks.

B

p.s. here's a schematic for an almost identical amp. The power tube is actually 50L6 and the wiring for the for the mains power section is a little different, probably changed around by the tech when the rectifier was replaced with the diode (see attached photo). I'm guessing that big ol' 400 ohm resistor was put in to drop the heater voltage that would have been dropped by the rectifier tube.

[g1]

Probably when he got it back, he was using an outlet that wasn't grounded. That would have gave him the same result as what you are getting now when you lift the green.
What was the point of the diode rectifier? The guy who did that job is lacking in safety knowledge. The amp is still quite hazardous as there are still many places with incorrect or missing grounds at the outlets.
Good that you are adding the iso tranny, that is the only proper solution.

[bobloblaws]

Probably when he got it back, he was using an outlet that wasn't grounded. That would have gave him the same result as what you are getting now when you lift the green.
What was the point of the diode rectifier? The guy who did that job is lacking in safety knowledge. The amp is still quite hazardous as there are still many places with incorrect or missing grounds at the outlets.
Good that you are adding the iso tranny, that is the only proper solution.

I'm assuming the original rectifier went bad and the diode was put in to avoid the cost of a replacement. You could very well be right about the un-grounded outlet, he lives in a fairly old house which looks like it hasn't seen a renovation in a long time. I bought a 35W4 in case his funky diode solution was the problem but haven't tried it yet. So do you think this hum is something I can track down or is it expected with this type of amp when grounded (in other words, just move on to installing the iso tranny)?

[The Dude]

......in other words, just move on to installing the iso tranny)?

Yep. That's what I would do. You need to do it anyway and there's a good chance that will take care of the problem.

[bobloblaws]

Yep. That's what I would do. You need to do it anyway and there's a good chance that will take care of the problem.

Here's hoping! So I'm gonna go ahead and do this but I'm unclear about what I should do with the AC ground wire. Should I keep it connected to the chassis or will I likely have the same hum problem in that case? In other words, are you suggesting that using the iso tranny will provide a sufficient level of safety such that the chassis ground can be eliminated, thus eliminating the hum? Or is the idea that the iso tranny will likely inherently eliminate the hum and I still need the chassis ground to handle a situation of a shorted component making the chassis "hot"? Please forgive my ignorance of these matters.

[potatofarmer]

Neutral goes to power transformer. Hot goes to fuse, then switch, then the other lead of the power transformer. Safety ground gets connected to the chassis with its own dedicated bolt.

Start worrying about the hum - if it still exists - after the amp is safe to use.

[g1]

The safety ground connection to chassis is the most critical thing so it must stay connected.
The whole point is to try to get it so it won't hum with the chassis connected to AC 3rd prong ground.

Don't ever worry about trying to clear up 'ignorance' of safety issues.

[mozz]

For the price of a isolation trans, or 2 back to back 120/12v trannys, i think you can pick up a cheap power transformer. I myself would scrounge one out of something, record player, mono reel to reel, i have even seem some old radios which had enough parts for a champ.

[Enzo]

You could do that, but then you'd need to replace the power tube and the rectifier tube, and rewire everything. So you'd just be building a new amp in the old chassis with the old OT.

[nevetslab]

If the isolation transformer is mounted to the amp chassis, then it's frame will get grounded. If it's mounted elsewhere, like to the wooden cabinet, connect a grounding wire with a solder lug to the transformer mounting screw and attach that to the amp chassis as well, so it's also safety grounded. The amp circuit looks like something from the early 50's, but I noticed a date on the schematic from 2009.

[Enzo]

I think that date is when the drawing was made from an old amp chassis.

[bobloblaws]

OK, so I wired it all up, with a lot of alligator leads at this point as proof of concept, I didn't want to hard-wire everything until I know it is working as it should. So the hum is in fact greatly reduced. Once again lifting the AC ground wire from the chassis reduces the amount of hum but only by a small amount compared to before I hooked up the isolation xfrmr. I would say the signal to noise ratio as it stands renders the amp playable but still leaves something to be desired. I'm guessing part of it might be because of the various alligator leads running here and there and crossing over each other etc. I'm gonna wire it up properly ASAP and see where it stands. For now, if I still get a significant level of hum is there anything you guys would look at first with this scenario, or would you suggest just doing routine troubleshooting at that point? Thanks for all the tips! Potatofarmer, I followed your directions re: hot to fuse to switch etc.

[Enzo]

We wanted to add the iso for safety. it would also reduce hum from THAT source. But you have not yet rebuilt the amp itself have you? Leaky caps, etc?

[1ampman]

Scrap the amp. Get him a cheap solid state amp. It would be a lower cost than a transformer. Keep it simple...

[bobloblaws]

We wanted to add the iso for safety. it would also reduce hum from THAT source. But you have not yet rebuilt the amp itself have you? Leaky caps, etc?

Well, no, the objective was only to get rid of the hum and make it safe. But it sounds like you are saying the best M.O. will indeed be routine troubleshooting if there is still a problem. I just wondered if there might be anything obvious to look for that would cause hum in a series filament amp even after adding the transformer. I'm putting the cart before the horse anyway, I'm gonna go ahead and do it up right and see where I'm at.