Bias supply problem

[Sonny ReVerb]

Troubleshooting a new amp build and the bias supply doesn't seem to be working properly.


The PT bias winding provides 53VAC. After rectification, it produces -73VDC. The bias pot wiper shows -38VDC at one end. The amp passes a somewhat distorted signal at this bias setting. However, when I try to move the wiper more negative, the power tubes cut off and produce no sound. The 1 ohm resistors on the cathodes show no current. I scoped the bias supply and it shows a sawtooth waveform. It should be more flat (DC) with some ripple, no?

[shooter]

your scope shot - depends, what's the scale?  if it's 37mVac on -38DC it's good
if it's 38VAC on no DC, that's bad

power tubes cut off

adjust end to end without tubes, the socket bias pin don't care if the tube is there, your tube REALLY cares if the bias is NOT there

[sluckey]

Scope pic don't mean much without knowing the peak to peak amplitude of the sawtooth and the dc level it's riding on. But, there are only 4 components plus a pot. Probably can change all of those in 15 minutes.  Be sure things that should be grounded have a good connection. I would change the cap first.

And pull the output tubes until you have a proper dc bias voltage on the socket grids.

[2deaf]

What type of tube are you trying to bias?

[Sonny ReVerb]

Thanks. I figured it out. The bias voltage was too negative to begin with. Going more negative just mis-biased the tube more. I changed the 1 ohm resistor to a 4k7 and re-biased. All is good now. Sounds very good. Slightly too much screen voltage, though. This amp uses 7591A's and the plates are at 460V, but the screens are at 452V or so. Supposed to be max screen voltage of 440V.

[shooter]

so what did you decide on the scope scale?

[Colas LeGrippa]

i just can't understand the reason of the 1 ohm resistor where you put it.The 1 ohm resistor has to be located between the K of the power tubes and the ground to read in mV what's goin' on in mA.

[Sonny ReVerb]

I thought that might be confusing. This amp is based on an Amplified Nation Wonderland. Taylor's bias supply used the 1 ohm resistor. I don't really understand its purpose, except as a place holder for the usual bias range resistor. The 1 ohm resistor would give you the max negative voltage possible, I guess.

My amp also has 1 ohm resistors between the power tube cathodes and ground for setting the bias.

[Sonny ReVerb]

so what did you decide on the scope scale?

I rechecked the bias voltage after the resistor change. The scope shows a smoother waveform (but still a sawtooth, I'd say). It shows 200mV of ripple on top of -36VDC. That seem reasonable?

[sluckey]

very reasonable

[PRR]

> 200mV of ripple on top of -36VDC. That seem reasonable?

5% is usually commercially reasonable. That would be near 2V. Your 0.2V shows you could probably squeeze-out another part-penny of cost in mass production.