Harmony 304 quiet

[plook]

hi, i have a Harmony 304 which has a very low output even after checking all tubes, output transformer (tried another), and the speaker. Any tips on what else in the circuit could fail and cause low output? It has all the original caps. Voltages seem to be ok but i don't have a voltage chart.

[Phil_S]

Please post voltage readings and some pictures of the insides. Maybe someone will see something. Have you swapped tubes? How do you know the tubes are good?

Check all the obvious stuff, for example, is the connection between the OT and the speaker solid? Look for a broken or cold solder joint. Are tubes fully seated in sockets? That sort of thing. Maybe you've done all of this?

I'd be concerned about the old caps. They may simply be too tired to do the job. You might be surprised what a cap job will do.

[plook]

i'll record the voltage readings later this evening. Swapped several good tubes in all sockets, swapped speaker, swapped OT, swapped over all the electrolytic caps. Only thing i have not doe is swap out resistors and pots.

It was working fine a week ago and i went through it checking for dry joints to get rid of a very bad hum. the hum has gone but it's replaced with very low output and a bad farty tone you would associate with a blown OT, but i've ruled that out. It's a simple amp but this has me tied up.

[Structo]

Could it be blocking distortion?

[Phil_S]

...i went through it checking for dry joints to get rid of a very bad hum. the hum has gone but it's replaced with very low output and a bad farty tone...

What did you do to get rid of the hum?

[plook]

Blocking distortion?

Tightened all grounds and put fresh solder on every joint.

[plook]

Here is a voltage drawing. What should the missing readings on the 5Y3 be.

The 12AX7 has replaced the original 6C4 tube and socket.

Sometimes when i turn it on the output seems to be ok and then it rapidly recedes with a horrible distortion to almost nothing. the only tube i have not tested is the 6AT6 because i don't have a spare. Everything else has pretty much ben changed out apart from resistors. I am wondering if the secondaries on the MT are the problem.

[Structo]

OK, that is the 304A amp.

You replaced the 6C4 tube which has a gain of 16 with a 12AX7 which has a gain of 100.

Did you change any components on the V1 socket?

Also, until you can rule out the 6AT6, all bets are off.

And, those old cardboard electrolytic caps should be replaced as well as the cap can.

[Phil_S]

In addition to what Tom says, you may want to put a 12AU7 in that noval socket to restore the tube compliment to the proper equivalence.

[plook]

I didn't put the 12ax7 in, came to me like that. I'll look to see if components have been changed

I'll get a 6AT6 and try a 12AU7

thanks