I measured my amp's power output - it doesn't seem right.

[imaradiostar]

I wouldn't waste my time on sending it to Mercury. Order a transformer from Triode electronics and don't look back. Mercury has fine products but I think the Magnetic Components transformers Triode sells are just as good!

It's funny- for all the times people on this forum say "it's gotta be the output transformer" and it rarely is, this one is an exception.

jamie

[Enzo]

Since it is so rare, we need to make a large effort to be sure when it is the case. How many times has someone written in that they changed out a transformer and their problem remained?

[Chuck H]

Right. I wouldn't part with the OT before putting in a new one proves good. Also, I think there are places that will actually repair that OT using as much of the original as possible and vintage correct materials. Sorry to say I don't have a link right now. I may research it later.

[Sonicboom]

I agree, which is why I was going to have MM test it. They have the equipment to do it AND it only costs postage both ways, so I don't have to buy a xfmr. just to test it.

Right. I wouldn't part with the OT before putting in a new one proves good. Also, I think there are places that will actually repair that OT using as much of the original as possible and vintage correct materials. Sorry to say I don't have a link right now. I may research it later.

Thanks, Chuck. My plan was, in fact, to have it repaired. The only one that I know of that will repair it is MM. If anyone knows of someone else that is better/cheaper, please let me know.

[Bruce / Mission Amps]

I agree, which is why I was going to have MM test it. They have the equipment to do it AND it only costs postage both ways, so I don't have to buy a xfmr. just to test it.



Thanks, Chuck. My plan was, in fact, to have it repaired. The only one that I know of that will repair it is MM. If anyone knows of someone else that is better/cheaper, please let me know.

I'm curious what the MM charge is to rewind the output transformer. If it is less or close to the same as a new one from any well known vendor, then I would probably rewind it. If not.... Mag Comp.

[Sonicboom]

Update - Good and bad news

Well, I replaced the OT with one from Mag. Comp. The good news is that the output signal across a resistive load is about 20v RMS, so the power is back to normal. The bad news is that now I have parasitic oscillations in the form of a very high pitch squeal as soon as I flip the amp off of stby.

I triple checked the OT wiring, swapped all the tubes with known good ones, checked the grounds, re-tensioned the tube sockets and poked around with chopsticks. The oscillations are present regardless of whether anything is plugged in to the input or not, and none of the controls has an effect on it.

I didn't have this problem before. Literally, the only thing different is the replacement OT. Could the new OT be bad?

[Chuck H]

Try swapping the OT primary leads on the tube sockets.

[km6xz]

Are you testing with a dummy load or speaker? There primary of the transformer is OK. You know that because the rupple you saw on the scope is normal but it is not normally heard if the transformer is balanced on either side of the center tap. The ripple would be fed to both so the fields from the two halves would be out of phase and result in low ripple from the speaker due to cancellation. If one side is shorted, even only one layer of turns, the hum would be audible in proportion to the amount of imbalance. That common mode rejection is why we use balances lines in pro audio.

Are you getting the same sine-wave voltage, measured with the meter, not the scope, with the speaker or with a power resistor?
There is a possibility of a problem with the transformer secondary however. Shorted turns there would change the ratio so a lower resistance load would allow a better power transfer.
Make sure in all your tests that you are not clipping the front end and driver or all your output measurements will be wasted.
If you have not used a dummy load instead of the speaker, a shorted speaker would cause all these symptoms. What is the ohm resistance measured of the speaker when disconnected from the amp? Your 8 ohm speaker should measure 6-7 ohms.

[MarkusBass]

The bad news is that now I have parasitic oscillations in the form of a very high pitch squeal as soon as I flip the amp off of stby.
......Could the new OT be bad?

I don't think so. The problem is usually caused by incorrect phase of negative feedback from the output of the amp (back to the input). Simple solution is to swap secondary windings of the output transformer. Have you tried this?

Mark

[Sonicboom]

Thanks, Gents. I just swapped the leads. Mag. Comp. wired the color leads wrong so the phasing was backwards. This thing absolutely ROARS.

Thanks for all of the help. I'm still bummed that the orig. OT is toast.