Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: clyde on July 06, 2016, 03:59:03 pm

Title: Low Voltage- Cathode Follower
Post by: clyde on July 06, 2016, 03:59:03 pm

I have a CF in circuit, 820 ohm cathode resistor on the input side, pin 3, 39k cathode resistor output side, pin 8 (12DW7), 266v B+ to pin 6, 100k resistor to pin 1, pin1 to pin 7.  The 100k resistor is dropping 200v so that I measure only 66v at the junction of the 39k cathode resistor, tone caps and slope resistor.  The voltage at pin 3 (820R) is 1.6v.  I've lifted all the tone caps and get no change.  Where is the voltage going?  I have an identical amp right beside this one that I'm using for reference voltages and see 142v on the output, about 255v in, .7v at the top of the 820R cathode resistor.  I've changed tubes from one amp to the other, no difference.  I'm stumped.  All resistors measure close to stated values. 

Title: Re: Low Voltage- Cathode Follower
Post by: sluckey on July 06, 2016, 04:10:00 pm

The first triode (pins 1, 2, 3) is conducting too hard. What is the voltage on pin 2? Probably should be zero.

Title: Re: Low Voltage- Cathode Follower
Post by: clyde on July 06, 2016, 04:47:20 pm

Thanks for the reply.  Yes, zero.
Well, of all the idiotic things, I have the triode halves reversed.  Input should be pin 7. 

Title: Re: Low Voltage- Cathode Follower
Post by: sluckey on July 06, 2016, 05:08:58 pm

Let us know if you get it untangled.

Title: Re: Low Voltage- Cathode Follower
Post by: clyde on July 06, 2016, 06:29:03 pm

All good now.  That's what happens when you rotate the tube socket 180 degrees and don't do the same with the wiring.  It's always 20/20 when you finally see it.  Thanks very much.