1970 Super Reverb: Low Plate Voltage and Very Low Plate Current

[Pinball]

Working on a 1970 Super Reverb. New JJ 6L6GC's and a NOS RCA 5U4.

Incoming Voltage is 117 VAC

Plate Voltage is 414 VDC
Plate Current is 9mA

Both 6L6 tubes are very similar.

I can turn the Bias adjustment POT and never get close to 50% of tube max before running out of adjustment.

Also Tried a JJ New GZ34 and didn't get much better.


Any help would be appreciated.

[sluckey]

What's the control grid voltage? What's the screen voltage? Screen resistors OK?

[tubeswell]

Also, did you replace all the filter caps including the bias supply filter caps?

[Pinball]

Control grid voltage is -49.3
Screen grid voltage is 427
Screen resistor is .495k
Control grid resistor is 1.528k


I have replaced all electrolytic capacitors.

[tubeswell]

Control grid voltage is -49.3

Where are you measuring this? At the socket pins?

[Pinball]

socket pins

I have -44.8 between the two 10k coming off the Bias pot. BTW the bias pot only shows 6.75k total across the two outer leads.

[tubeswell]

So you need to be hotter, so decrease the bias pot tail resistor. (Try tacking various parallel resistors in). -49V is too high for 414V. You want to be around 10% of plate voltage +/- 2%

[Pinball]

That was it. Thanks for your help on this. Dropped that resistor down to around 9-10k.