Think I damaged my plate caps and/or rectifier? Duh!

[David Root]

Ran a new build for about one minute with out speakers plugged in. No signal in, vol and master zeroed. Speaker jack is shunted to ground Fender style.

Getting a "wheezing moo" sound for want of a better description, as soon as HT is switched in, now pulsing slowly like motorboating, plate voltage is only 500V instead of 530.

OT primary and secondary DCR OK. Power tubes test OK, no change. OT is a from a Bassman 100, big iron.

Took the plate B+ apart, balancing resistors OK, two 330uF/400V caps test good on my DMM. But my DMM doesn't put 530V on those caps.

I suspect the plate caps could be damaged by flyback voltages. The PT secondary AC is also out of balance, 215 one side, 175 the other, not sure which is cause & which is effect. It is 400-0 feeding a 1200V 32 A bridge rectifier. Maybe I fried the rectifier as well/instead of the caps?

Before I change the plate caps out, am I on the right track here?

[martin manning]

I don't know how you'd get high flyback voltage with the secondary shorted. Maybe run some AC through the OT to see if it appears to have the expected turns ratio. Being a new build I guess that means it has never ben in a working state? Maybe the PT wasn't right to begin with?

[David Root]

OK, Martin, I see what you mean, not flyback voltage then. Thanx for coming by.

The amp has been working OK for two weeks now, all stable voltages. Too much gain and a bit noisy. I have been gradually cutting the gain in different places.

The PT is a potted toroid and is putting out a big magnetic field. It worked fine for a long time in its previous life in a different circuit in the same chassis. I was about to move the OT 5" further away from the PT, as I had put a Strat pickup on the PT's pot and measured over 500mV out of the pickup's coil. Then I forgot to plug in the speakers.

The OT's DCRs have not changed. Primary blue to CT 52 ohms, brown to CT 48 ohms, blue to brown 100 ohms. Secondary 0.18 ohms. I would think that means the OT is OK?

It was not operating into a shorted secondary more than a minute. Neither the HT nor the mains fuse blew. This amp uses a driver tube as a cathode follower DC coupled to the power tube grids. Have not checked the coupling caps from the PI to the driver grids but the driver tube tests normally, same as before. Bias supply is from a separate PT.

[Cliff Schecht]

The transformers sound fine. I use good iron in all of my builds (old USA stuff is tough!) and have ran most, if not all of them, with no speaker plugged in by mistake for some period of time. None of these amps or transformers were hurt from this. It is hard on the transformer because you are now energizing the primary but giving the energy nowhere to go, so it ends up as heat. This eventually cooks the insulation enough to cause it to arc or melt. But again I'm yet to have hurt any of my iron running the secondaries open (I don't like wasting my precious stash of shorting jacks on output jacks ).

Usually I'm very anal about checking that everything is plugged in and secure but we all slip up sometimes, especially when swapping parts in moments of inspiration.

[rdjones]

The transformers sound fine. I use good iron in all of my builds (old USA stuff is tough!) and have ran most, if not all of them, with no speaker plugged in by mistake for some period of time. None of these amps or transformers were hurt from this. It is hard on the transformer because you are now energizing the primary but giving the energy nowhere to go, so it ends up as heat. This eventually cooks the insulation enough to cause it to arc or melt. But again I'm yet to have hurt any of my iron running the secondaries open (I don't like wasting my precious stash of shorting jacks on output jacks ).

You may be tempting fate
Consider a damper resistor across the secondary in lieu of a shorting jack, ala Traynor.
10:1 or more is sufficient to suppress flyback without effecting loading.
80-100 Ohms on an 8 Ohm secondary.

An amp with separate speaker jacks for multiple impedances could benefit from a secondary damper since a shorting jack can't be used in such a scenario.


rd

[rdjones]

OK, Martin, I see what you mean, not flyback voltage then. Thanx for coming by.

The amp has been working OK for two weeks now, all stable voltages. Too much gain and a bit noisy. I have been gradually cutting the gain in different places.

The PT is a potted toroid and is putting out a big magnetic field. It worked fine for a long time in its previous life in a different circuit in the same chassis. I was about to move the OT 5" further away from the PT, as I had put a Strat pickup on the PT's pot and measured over 500mV out of the pickup's coil. Then I forgot to plug in the speakers.

The OT's DCRs have not changed. Primary blue to CT 52 ohms, brown to CT 48 ohms, blue to brown 100 ohms. Secondary 0.18 ohms. I would think that means the OT is OK?

It was not operating into a shorted secondary more than a minute. Neither the HT nor the mains fuse blew. This amp uses a driver tube as a cathode follower DC coupled to the power tube grids. Have not checked the coupling caps from the PI to the driver grids but the driver tube tests normally, same as before. Bias supply is from a separate PT.

The shorting jack protects the transformer's primary by reflecting the short on the primary.
The load is then mostly the tube, the theory being a tube is easier to replace than an output transformer, the current value of NOS tubes notwithstanding.
An open secondary reflects a high impedance load at the primary dissipating all the power in the primary and very little in the tube, (and none in the secondary) creating the risk to the primary.

rd

[martin manning]

...The PT secondary AC is also out of balance, 215 one side, 175 the other, not sure which is cause & which is effect. It is 400-0 feeding a 1200V 32 A bridge rectifier. Maybe I fried the rectifier as well/instead of the caps?

This 20% miss-match on the PT secondary doesn't sound right. Can you maybe do a test to see if there are shorted turns in the PT? http://www.premierguitar.com/Magazine/I ... ester.aspx
However, if you are running it as a FWB, I'm not sure how this would produce the symptoms you described. Is the wheezing moo evident just idling? 1200V and 32A? I think that bridge rectifier would still be working after the rest of the amp became a smoking pile of ashes!

[David Root]

Thanx guys. I don't recall if the amp was on standby or play when I measured the mismatch on the PT secondary. I agree on the rectifier it should be bulletproof in this application.

The tubes are NOS Mesa 415-STR and don't appear to have been damaged. Cathode draw is as before. No visible internal damage.

Because of the pulsing sound, again, this is with no signal and vol and master zeroed, I keep coming back to the plate caps, even though they tested at about 305 uF each, which is what they were out of the box.

I'll check the OT turns ratio just in case, and take a more detailed look at the PT.

The pulsing sound is in play mode at idle.