Harmony H303A amp

[mtlbasslad]

Hello all - my first visit here, hoping someone can help me out. Straight up - i'm a bass player & amateur tech. So, sweet girl Petra gives me this funky old (1964, i think) Harmony H303A - 3 tubes, 6" Jensen, maybe 5 watts - & it's dead. No prob, sweetie, i'll just take it to my bud Gerd, in Montreal we call him 'The Amp Doctor', he's ex-Marshall factory-service guy. It took Gerd about 5 min to say - 'the output trans is fried' - unfortunately a replacement locally was the price of the amp - and far more unfortunately Gerd lost his battle with cancer & we went to his funeral, with the Harmony still on his workbench. With help from Chad at Triode Elec in Chicago, i got a replacement OT, installed it and - wow - we got sound - mostly 120 HZ. Gee i guess the 46 yr old caps might need replacing. Okay, now here's the problem. Original multi-cap is a Sprague Atoms TVA4529 labeled 60/40/40/20 mfd @ 160/160/160/25 V. No way to find the original, of course, but i could open up the cardboard shell & re-stuff it, retaining the original funkiness. BUT... i find conflicting schematics both online, in my 'Jack Darr's amp handbook' (circa 1965) & the barely legible one on the inside of the amp. if you have had the patience to read this far, maybe you can help me (& the lovely Petra). Thanks

[Enzo]

Modern caps will be a ton smaller than those. leave the can cap there for appearance, just unwire it. The schematics you find may differ, but they all should be fairly similar. The one at Schematic Heaven shows three sections in the B+, wired 40uf and 20uf in parallel for the first filter stage (making 60uf), then another 40uf for the screen stage. And that lonesome 20uf at 25v would be the cathode cap for the output tube. A 20uf 25v cap is tiny, and easily wired right across the tube socket. Don't go nuts trying to find the exact thing, 20-22-25uf are all going to sound alike. You might find 22uf easier than 20uf for example. And 25v is common, but nothing wrong with 35v if it makes the search easier.

There should be room under the chassis to mount modern caps and wire them to where the can cap wires went. The schematic says 150v, your old part says 160v. Fine, then 160-200v or whatever-v over 160. The original used three caps to make the two filter stages, but nowdays you could do it with two. 60uf might be an odd value to find, but 50uf would be common, or 47uf. And if 60 works, 100 ought to work too.

Having said all that, AES and other places do sell can caps that would likely work, but watch the dimensions. Oh, and they cost $30-40.

You will find that cardboard shell is just a cover glued over the aluminum can.

Be aware your tubes could be part of the problem too. And if any of the remaining smaller caps are the old wax coated types, expect to change them too.

[mtlbasslad]

Thanks Enzo, I'm honoured that a senior member had the time to reply to a newbie like me.
I expect that when you get to the pearly gates, Saint Peter will usher you to the front of the line.

I was just worried that to much cap would stress the rectifier & IT.
So I should be ok with 100uf@160-200V, 47uf@160-200V, & 22uf@25-35V?

Yes, might as well re-tube, not like it's an SVT that would set me back a weeks pay...

And I guess if i respect you & the other vets, I should install a 3-prong plug - & maybe a fuse...?
Next to "Chicago USA" it says "45 watts o.3 Amp" - would .3A fuse be suitable? - normal or slo-blo?

I was just trying to help out a friend when I ran out of tech-savvy, but you have filled that gap. Maybe if there were more
people with that attitude, the world would be a better place.

Thanks again, Lorne.

PS I'll be bothering you again soon, recently got a sweet Ampeg B15n that someone has made some weird mods to...

[mtlbasslad]

Status update - changed $2.15 worth of caps and the Harmony is sounding harmonious again.
Thanks again to Enzo, & Chad at Triode - Petra will be pleased.