Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: isaac_teller on January 10, 2022, 10:32:00 am

Title: DIY Princeton reverb won't develop negative bias voltage
Post by: isaac_teller on January 10, 2022, 10:32:00 am

Hello. I have built a AA1164 exactly as per the schematic with the exception of a full wave bridge instead of a tube rectifier. All the voltages are where they should be except the bias voltage which jumps between 5-12 positive volts. I have tried both these bias circuits but neither one gives a steady negative DC voltage. What could be the problem? Thank you.


https://robrobinette.com/images/Guitar/Bassman/High_Voltage_Bias_Tap.pdf


https://el34world.com/charts/images/biascircuit2.gif


(https://el34world.com/charts/images/biascircuit2.gif)


(https://robrobinette.com/images/Guitar/Bassman/High_Voltage_Bias_Tap.png)

Title: Re: DIY Princeton reverb won't develop negative bias voltage
Post by: sluckey on January 10, 2022, 10:41:18 am

That bias circuit will not work with a FWB rectifier. See my Scrapbook (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf) pages 8 and 9 for the correct circuit to use.

Title: Re: DIY Princeton reverb won't develop negative bias voltage
Post by: isaac_teller on January 10, 2022, 10:46:33 am

I'll try it(the one on the bottom left on page 8) and report back.

Title: Re: DIY Princeton reverb won't develop negative bias voltage
Post by: SILVERGUN on January 10, 2022, 10:58:54 am

Quote from: isaac_teller on January 10, 2022, 10:46:33 am

I'll try it(the one on the bottom left on page 8) and report back.

Read page 9 if you haven't already

Title: Re: DIY Princeton reverb won't develop negative bias voltage
Post by: sluckey on January 10, 2022, 11:08:35 am

Quote from: isaac_teller on January 10, 2022, 10:46:33 am

I'll try it(the one on the bottom left on page 8) and report back.

Yes. Use the same values. The .047µF should be a Class X or Y cap, but you can use any .047µF @ 500V until you get a Class X.

Title: Re: DIY Princeton reverb won't develop negative bias voltage
Post by: isaac_teller on January 11, 2022, 04:26:28 am

I hooked up the capacitive-coupled bias circuit and the amp works just fine now. Thank you Sluckey.