Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Pinball on March 09, 2023, 04:08:42 pm

Title: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: Pinball on March 09, 2023, 04:08:42 pm

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.

Title: Re: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: sluckey on March 09, 2023, 04:24:50 pm

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

Title: Re: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: tubeswell on March 09, 2023, 05:09:12 pm

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

Title: Re: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: Pinball on March 10, 2023, 09:52:59 am

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.

Title: Re: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: tubeswell on March 10, 2023, 11:07:01 am

Quote from: Pinball on March 10, 2023, 09:52:59 am

Control grid voltage is -49.3

Where are you measuring this? At the socket pins?

Title: Re: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: Pinball on March 10, 2023, 11:41:39 am

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.

Title: Re: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: tubeswell on March 10, 2023, 12:15:02 pm

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%

Title: Re: 1970 Super Reverb: Low Plate Voltage and Very Low Plate Current
Post by: Pinball on March 11, 2023, 05:45:47 pm

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