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I've got a B-160 back in the shop. Pair of 7591's with a 5Y3 rect. Owner said bad hum and blew fuses. After putting in a new fuse, I noticed issues with the Sovtek 5Y3. I removed all tubes and tried another rectifier tube. Still has a bad hum even when powered up through a current limiter. I suspect bad pwer supply caps, even though they test OK with an ESR meter. Is it possibly a bad transformer? What is a simple test to check for that? Thanks for help, Rod
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I'd replace the 7591s first.Originally posted by retrorod View Post -
I mean the amp hums with all tubes removed. Just the rectifier tube installed. 7591's are out of the equation. This amp was working fine a couple of months ago. The Sovtek 5Y3 was added at that time. After replacing the blown fuse, I fired it up and noticed some arcing in the 5Y3 and a hum that is unaffected by vol. pot controls. I have ran with a RCA 5Y3....no arcing...but hum still there. I assume that the Sovtek is shot and will not use it any further. I need to find the source of the hum...Comment
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Probably a shorted output transformer then. WIth no power tubes, there is very little that will make output to the speakers. It pretty much has to come through the transformer. And that mostly suggests that between the B+ at the center tap, something is conducting to ground. SO with power removed, and the output tubes removed, measure resistace to ground from the primary side of the OT. If the thing is shorted to frame, then it is bad. It could also be arcing, which will only show up with high voltage applied - you can;t find that with an ohm meter.
The poor 5Y3 will be freaking out because the power supply it serves is shorting to ground.
A simple test is to find that center tap lead from the OT primary and disconnect it. Does that kill the hum and stop the fuses from blowing?
If that seems to be the case, disconnect the wires to the plate pin of each power tube socket, and leave the center tap wire off. Now take most any output transformer you might have, and clip its center tap wire to the B+ in your amp, and the primary wires to the two power tibe socket plate pins. COnnect its secondary directly to a speaker with wires. Does the amp work that way?Education is what you're left with after you have forgotten what you have learned.Comment
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Thanks ENZO, I checked resistance to ground on OT primary. I get 334ohms fron CT to Ground; 516ohms from left tube plate to ground and 1600ohm from right tube plate to ground. Resistance from CT to left side is 184ohm and 1784ohm from CT to the right side. When I disconnect the center tap, the hum is gone.
I may try to sub another OT. I think the only loose one I have around is for a '62 Princeton.
I did not find any ground issues. How do those numbers look? A little lopsided I think? Please advise further...RodComment
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Hey again, I just subbed the Princeton OT on the Premier and did not get the hum. I only powered up thru the current limiter. Seems as if the Premier OT might me shot! If so, what type of replacement am I looking for? The amp runs 5Y3,Pair7591,12AX7,EF86 into an 8ohm 15" Jensen. Thanks, RodComment
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