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cancon 15th August 2012 10:13 PM

Harmony H-305 over conducting 5y3?
I've just finished re-capping a Harmony H-305. Here is the schematic and parts list.


Parts list

The amp played fine for about 5 mins during testing, then all of a sudden the signal dropped out and the speaker started to emit a loud buzz. I noticed the plates of the rectifier (5Y3) starting to glow. I tested all the tubes - no shorts and good emission values. I swapped the 5Y3 for a NOS, same problem. The buzz/hum even happens with the pre and power tubes removed. With the rectifier removed I get normal values on the PT (255V per tap, 5.2V on the heater). With the rectifier I get ~ 4.7vac on the heaters and about 120vdc on the B+, but the plates start over conduct immediately.

Any ideas?


Enzo 15th August 2012 10:50 PM

Yeah, any chance one of your filter caps is wired backwards?
COme to think of it, with the power tubes removed, a bad filter would still make no sound. If you get sound from the speaker with NO power tubes, then something is dragging current through the output transformer, most likely a short from primary to frame or from primary to secondary

disconnect your output transformer B+ lead. Does it stop?

cancon 15th August 2012 11:30 PM

Thanks for the reply.

With the OT CT removed from the first filter, the hum stops and I get my B+ back (~450), power/pre tubes still removed.

The replacement filters are a CE (mallory) multi-section can.

Enzo 16th August 2012 02:20 AM

OK, so the cap is probably fine, unfortunately I suspect your output transformer is not.

Power off, power tubes removed, CT still disconnected. Now measure DC resistance to chassis from that center tap wire, and also from either plate wire. Are any of those three primary winding wires showing resistance to ground? If one end were shorted to frame, then the resistance of the winding wire would be put from B+ to ground. And check for resistance to the secondary too. A good transformer has no continuity from windings to frames or between windings.

cancon 16th August 2012 02:31 AM

CT to chassis: 825 ohms.
Primaries to chassis; 1K and 660 ohms.

With the negative lead of the meter to chassis, touching the pos meter probe to either the CT or 1K primary causes some barely audible static to be emitted by the speaker. Again, no tubes installed. Secondaries show infinite to ground. I should mention that when the amp did work, it was quite noisy unless the tone knob and both volumes were completely turned down. Even though the inputs are grounded (I cleaned the contacts and they show continuity) you'd get noise as soon as you turned up the volumes with nothing plugged in.

Enzo 16th August 2012 10:58 AM

Circuit noise is a spearate issue from leaky OT. I am afraid your transformer is shot.

Your meter resistance scale runs a small current through a circuit to read the resistance, that small current causes the transformer to induce a little voltage in the secondary, which you hear out the speaker.

cancon 16th August 2012 04:44 PM

Thanks for your help Enzo. Could the replacement cap have caused this to happen? The amp played fine for about 5-10 min after the cap job.

Tubelab_com 16th August 2012 05:02 PM

Before condemning the OPT, unsolder the wires that connect the OPT to the 6V6 sockets, and recheck the resistance readings. I have seen sockets fail due to moisture absorbtion. This type of failure usually results in smoke, stink, fireworks, or all of the above though.


Could the replacement cap have caused this to happen?
Probably not, although the B+ voltage probably went up a bit with a new cap helping the failure along.

Enzo 16th August 2012 10:47 PM

Sure, we could have arcing sockets or other leakages, worth checking, unsolder two wires and find out. But I have to think the 600 ohms to frame wasn;t the sockets.

I agree the cap is probably not the cause. If you had played it a while longer with crummy caps it probably would have gone ahead and failed too.

Tubelab_com 16th August 2012 11:25 PM


But I have to think the 600 ohms to frame wasn;t the sockets.
The older black phenolic sockets were prone to arcing which led to carbonization. This usually happened with dirty sockets that were stored in a moist environment. 600 ohms is a very real resistance reading for a carbonized socket, but there is uaually a tell tale smell. A socket could have arced years ago due to being operated without a speaker and short out later in life. I have seen this happen, but it is usually in higher powered amps using 6L6GC tubes.

The arc starts from pin 3 (plate) to pin 2 (heater) which is often grounded. Look carefully with a magnifier for a burned track. It will be black on black which is hard to find. The burnt stuff can be scraped away and the socket cleaned with WD40 and a toothbrush, but it may fail again.

I agree the OPT is probably the culprit, and moisture related arcing is probably the cause. Still, the tube sochets should be checked, since you wouldn't think to look there unless you have seen it happen. The black plastic/phenolic sockets and the brown wafer sockets are the bad guys.

I have never seen a white CERAMIC (not plastic) socket fail, including the new Chinese stuff. They can be made to arc, but they don't carbonize and burn leading to shorts.

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