Hum in Bandmaster

[Oddvar R]

I get 0.003 and 0.004v AC on the pt tubes
and 21.2 and 64 mV DC.

[martin manning]

I get 0.003 and 0.004v AC on the pt tubes and 21.2 and 64 mV DC.

Way out on the current. If you swap the tubes does the high current follow the tube? What do you see for DC voltage on pin 5's?

[Oddvar R]

It seems to follow the tubes, the 21.2mV lowers to 13 mV when tubes are swapped. (I know the tubes arne't matched, I have bought new ones.)

[martin manning]

When you swap the tubes I would expect the readings from pin 8's to be the same, just reversed, so there may be something else going on. Start with the new tubes and see where you land.

[angelodp]

Have you checked that the two 220k's off of the bias circuit are correct?

[Oddvar R]

Have you checked that the two 220k's off of the bias circuit are correct?

I will...

[Oddvar R]

I tried to disconnect the primary from the th V6 and the hum stopped, when I did the same with the other tube, the noise came back? Is that natural or?

[martin manning]

That's the same as removing one of the tubes. Still seems odd to have lower noise with one tube removed.

[pdf64]

I suggest to avoid disconnecting the anode whilst its screen grid is at HT. The screen grid will tend to draw a lot of current and may overdissipate / wreck the valve.

[martin manning]

Oh yes, very good point!

[Oddvar R]

When I measure the ohming on the output transformer I get 87.5/99.8 ohm between the primaries and the B+. But between any of the plate leads and the secondaries I measure 9m ohm? Is there some info there?

[Stevem]

You should not be seeing any resistance between the primary and secondary side of the OT.

With the output tubes removed and the primary center tap unhooked do you still hear hum.

[Oddvar R]

You should not be seeing any resistance between the primary and secondary side of the OT.

With the output tubes removed and the primary center tap unhooked do you still hear hum.

No, doing that, no sound...

[martin manning]

You should not be seeing any resistance between the primary and secondary side of the OT.

There should be infinite resistance (OL) between the primary and secondary. No resistance would indicate a primary-secondary short.

If the center tap and the secondary are connected, measuring across them will be essentially like measuring from the CT to ground, which would include the power supply filters, balance resistors, and feedback loop in parallel.

[Oddvar R]

It seems like all the voltages on the board are a little above the schematis values, the voltage after the rectifier is 462. Now, even biased as in the schematics, the V6 is in the verge of redplating.