Bias help

[joesatch]

Got this Robinette jj 6V6S jcm800 put together. No issues , sounds great.  The bias is cold, very cold. With the bias pot at the hottest setting, i'm getting -70v at the Grid, the other way gets it to -80's.  My plate/screen voltages are 565V.

[sluckey]

Increase the 470Ω bias range resistor. I suggest 1K to begin with. Then increase/decrease for the exact voltage you want.

I would set the bias and balance pot to middle of rotation and change the bias range resistor to give -35V on the grids of the 6V6s.

[joesatch]

the only 1k i have on hand is one of these. Physically it will fit with no issues. It measures 1k exactly.

https://www.tubesandmore.com/products/resistor-5-watt-cemented-wirewound-5-tolerance-1k

[sluckey]

Well, whatcha waiting on? 

[joesatch]

got it down a very small amount. -67.1

[sluckey]

So, what's your next move?

BTW, I would not trim resistor leads until you find the magic value.

[joesatch]

So, what's your next move?

BTW, I would not trim resistor leads until you find the magic value.

I'm thinking of changing the bias pot or the resistors on it

[sluckey]

You're not paying attention! THAT ONE RESISTOR IS THE ONLY THING YOU NEED TO MESS WITH.

Increase the 470Ω bias range resistor. I suggest 1K to begin with. Then increase/decrease for the exact voltage you want.

I would set the bias and balance pot to middle of rotation and change the bias range resistor to give -35V on the grids of the 6V6s.

[joesatch]

You're not paying attention! THAT ONE RESISTOR IS THE ONLY THING YOU NEED TO MESS WITH.

Increase the 470Ω bias range resistor. I suggest 1K to begin with. Then increase/decrease for the exact voltage you want.

I would set the bias and balance pot to middle of rotation and change the bias range resistor to give -35V on the grids of the 6V6s.

how high can i go with it? why is it going to be so far off from the layout/schematic?

[sluckey]

There are no limits. Take it as high as needed. How much AC voltage do you measure at the PT lead where it connects to that resistor?

[joesatch]

There are no limits. Take it as high as needed. How much AC voltage do you measure at the PT lead where it connects to that resistor?

63.6vac

[sluckey]

Rob only used 50VAC. Try 5K, 4K, 3K, 2K. If voltage does not come down to -35V, then you probably have a wiring error on the pots.

[joesatch]

crap crap crap. I got the bias pot and the balance pot mixed up. I assumed the larger pot was the bias. No!

[joesatch]

got the 5.6k in there. range is -32 to -55 on the bias pot we are good there. I have them at -38 now and it all seems to be good. How do i measure ma on these tubes?

[sluckey]

Easy way... Remove the jumpers between pins 1 and 8 on both output tubes. Move the ground wire from pin 8 to pin 1 on the left tube (looking at layout). Install a 1Ω 1W 1% resistor between pin 1 and pin 8 on both sockets. Measure millivolts on pin 8 of each tube. The millivolts corresponds to milliamps, ie, 25mV equals 25mA.

[joesatch]

Easy way... Remove the jumpers between pins 1 and 8 on both output tubes. Move the ground wire from pin 8 to pin 1 on the left tube (looking at layout). Install a 1Ω 1W 1% resistor between pin 1 and pin 8 on both sockets. Measure millivolts on pin 8 of each tube. The millivolts corresponds to milliamps, ie, 25mV equals 25mA.

would that stay in permanently or just to measure?

[sluckey]

Easy way... Remove the jumpers between pins 1 and 8 on both output tubes. Move the ground wire from pin 8 to pin 1 on the left tube (looking at layout). Install a 1Ω 1W 1% resistor between pin 1 and pin 8 on both sockets. Measure millivolts on pin 8 of each tube. The millivolts corresponds to milliamps, ie, 25mV equals 25mA.

would that stay in permanently or just to measure?

You don't really need an answer to that, do you?