Crate Vintage Cub 20 VC-2112R Amp Bias

[TomCarlos]

MEF Members:

I want to double check on something before making any recommendations to a friend. He has a Crate Vintage Cub 20 VC-2112R amp. It is similar to the VC-2110. I am showing the output portion of the amp below.

This amp has two EL84 tubes. According to the schematic, TP11 (at the Cathodes) is 9.6 volts. There is a single 120 ohm resistor on the Cathodes (R22).

9.6vdc / 120 ohms = 80ma total, 40 ma through each tube.

I am estimating the plate voltage (will measure tomorrow) to be ~ 300vdc.

300vdc – 9.6vdc = ~ 290vdc from plate to cathode.

290vdc x .040ma = 11.6 watts in each tube.

This tube is rated at 12 watts maximum dissipation.

Would the 70% rule apply to the EL84? (Run at tube at 70% of max dissipation). If so, I calculate as follows:

Ia of 48ma x 70% = 33.6ma

33.6ma x 290 volts = 9.57 watts dissipation.

Thus 9.6vdc / (33ma + 33ma) = 144 ohm cathode resistor

Is Crate pushing the (current/power) limit on these tubes? I have read these particular amps have Cathode resistors that overheat (perhaps due to the placement near the EL84s) and I have seen retrofits using Dale Aluminum resistors mounted onto the chassis. But that still doesn’t tell me if the problem is placement or nearing the maximum power dissipation for the tubes.

Thanks, Tom

[alexradium]

99% of the time you will find cathode bias amps running at least 90% of dissipation and up,like Voxes which go up to 130%.
Is it right?
I won't go in class A or AB explanations,it is like that and forumers of the world will tell you that the amp has to run hot to deliver a HOT sound otherwise it will be cold and sterile...

[Helmholtz]

The difference is in the cathode bias. Other than with fixed bias, total tube dissipation does not increase when driven. This means that cathode biased tubes can be safely biased up to 100% plate dissipation.

The 70% rule only makes sense with fixed bias.

[TomCarlos]

Thank you for the replies!!

[TomCarlos]

Epilog: My friend brought the amp to me today. You could barely hear any output- it was totally distorted. I focused on the Power Supply and output stage. Power Supply voltages were a skosh high. 335 volts on the EL84 plates. O volts on the EL84 Cathodes. 0 volts on the screens. Each of the screen resistors were open (R25 and R25). Quick swap out of these two resistors and everything was back to normal. Cathode voltage was now at 9.8 Vdc. Before closing the amp, I also took a look at a very weak reverb. After pulling the tank out (a small tank indeed), I noticed one of the springs had been dislodged. After reconnecting and cleaning the RCA jacks, the reverb was happy. Yeah for this relatively easy repair.