Wit's end with underbiased super

[woodyboyd]

Very loud A/c hum, plates glow red after 20secs. Bad caps replaced, Bad output trans replaced, ALL new tubes. This is an early SF with was BF'ed. Bias is too low(-25) resistors check good, checked all the regular stuff, Great B+. What am I missing? I am at aloss on this one.

[MWJB]

Your bias (current) is way too high rather than too low, you need more negative voltage (-50v or more, depending on B+ & brand of tubes) @ pin 5. What value do you have as the dropping resistor in the bias supply (the one that runs from the bias diode to the bias pot)?

[woodyboyd]

TY for your reply. I have 57 volts

[soundmasterg]

Did you replace the backwards connected electrolytic in the bias supply?

You only have -57v max off the bias supply tap after the diode? Most schematics I see show that the tap was a 70v tap. You can always disconnect the bias supply from that point, make a divider off the B+ and connect a backwards diode with the rest of the bias circuitry after that to the divider and see if you can get things to bias up correctly. Sometimes the tap goes bad on the transformer and doesn't supply the required voltage.

[woodyboyd]

I have 57AC before the diode and -26DC after the diode which is too low. Sounds like you are right on the tap voltage going a miss. How do I go about this divider my B+ is 495. Any help would be great.

[Mark Black]

Before rushing to a re-design of the bias circuit there might be some more checking to do.

26VDC is about right on the money for 57VAC into a half-wave rectifier circuit so either the supply voltage is low (the aforementioned bad tap) or there is too much load on the bias circuit either from a leaky bias filter cap or possibly leaky coupling caps from the phase inverter. Or some other sneaky problem.

Are your measurements with the power tubes out or in? If in try with them out (in fact I would have them out until this bias thing is squared away).

Try lifting the power tube grid side of the phase inverter coupling caps and check for leakage (positive DC voltage present at the raised cap leads), and see if it makes an appreciable difference in the bias voltage.

Also as soundmasterg suggested what about the bias filter cap? Was it replaced and if so was it installed correctly? It's a very easy mistake to install one backwards since it's like the only cap in the amp to have its positive end to ground. If it was installed backwards then replace it - it probably has been injured. If that cap is leaky that could be dragging down (or up if you prefer) the bias voltage and possibly causing excessive AC ripple in the bias supply which would account for the loud hum.

How much AC voltage drop do you get across the series resistor between the transformer tap and the diode? That would give you an idea what kind of current is flowing in the bias circuit, and it shouldn't be a lot. Something in the single-digit milliamp range sounds about right.

[Enzo]

Scope the bias supply, it should be smooth DC. If the filter cap is not working, then you get very lumpy DC, and that results in a lower reading, since the meter reads an average voltage, and in the case of raw DC, it will be about half what it should be.

No scope? Set your meter to AC volts and measure. A good clean bias supply will show no AC volts. If you get 25VAC, then there is no filter action.

57VAC once rectified should net you 70 volts or more of DC.