Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on December 13, 2010, 01:50:52 am

Title: Cathode Bias Power Tubes Bias Voltages
Post by: TIMBO on December 13, 2010, 01:50:52 am

Hi guys, Had a thought brought on by Dougs new voltage chart. The usual practice is to have two 1ohm resistors to be able to measure your bias voltages when you have two power tubes.My question is, could one resistor be used to record the two tubes voltages combined? Is this something that would be of use, if so would it be worth adding to Dougs new voltage chart? Thanks

Title: Re: Cathode Bias Power Tubes Bias Voltages
Post by: LooseChange on December 13, 2010, 05:08:22 am

This is done all the time... For example, Fender does this on all (most) of thieir Hot Rod series amps.

Title: Re: Cathode Bias Power Tubes Bias Voltages
Post by: Fresh_Start on December 13, 2010, 07:28:46 am

Personally, I prefer to look at each power tube's bias separately.  Even if I were building a cathode biased poweramp, I would want a 1 ohm resistor between each tube's actual cathode and the shared cathode resistor. 

"Matched" pairs in someone's tube tester ain't necessarily matched well in my amp.  Looking at combined cathode current might hide the fact that one tube is near red-plating while the other is cold.

Separate cathode current measurements also would be more useful for troubleshooting, especially if one of the power tubes is nearing the end of its life.

Respectfully,

Chip

Title: Re: Cathode Bias Power Tubes Bias Voltages
Post by: tubeswell on December 13, 2010, 09:08:36 am

I never bother with 1R resistors in a cathode biased output stage. I just measure the actual resistance of the Rk before installation and then divide the cathode voltage by that once its up and running (to get the overall tube current)

Title: Re: Cathode Bias Power Tubes Bias Voltages
Post by: sluckey on December 13, 2010, 09:28:06 am

Quote

Personally, I prefer to look at each power tube's bias separately.  Even if I were building a cathode biased poweramp, I would want a 1 ohm resistor between each tube's actual cathode and the shared cathode resistor.

Me too, 'cause I'm a tinkerer.  :grin:

I don't think adding a single 1 ohm resistor to measure shared cathode current in a PP cathode biased amp is useful. Just do as tubeswell suggests.

Title: Re: Cathode Bias Power Tubes Bias Voltages
Post by: TIMBO on December 13, 2010, 01:24:09 pm

Thanks guys , I think i'll stick to having two resistors and checking the bais with them. Using your calculations is a bit beyond my pay grade. Thanks

Title: Re: Cathode Bias Power Tubes Bias Voltages
Post by: HotBluePlates on December 13, 2010, 06:59:31 pm

The full value of the 1 ohm resistors is had when you have meter tip jacks (1 for the "ground side" of the resistors, and 1 each for the other side of the resistors), so you can measure the tube current from the back panel of the amp rather than pulling the chassis.

Title: Re: Cathode Bias Power Tubes Bias Voltages
Post by: LooseChange on December 14, 2010, 05:06:14 am

Quote from: HotBluePlates on December 13, 2010, 06:59:31 pm

The full value of the 1 ohm resistors is had when you have meter tip jacks (1 for the "ground side" of the resistors, and 1 each for the other side of the resistors), so you can measure the tube current from the back panel of the amp rather than pulling the chassis.

Exactly and don't forget to place the bias adjust pots on the chassis too. One for each tube is optimal.