Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on October 16, 2021, 01:12:58 pm
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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.
(https://i.ibb.co/qyKWpYX/bias.jpg)
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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.
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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 (https://www.tubesandmore.com/products/resistor-5-watt-cemented-wirewound-5-tolerance-1k)
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Well, whatcha waiting on? :icon_biggrin:
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got it down a very small amount. -67.1
(https://i.ibb.co/hV1pWJP/bias2.jpg)
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So, what's your next move?
BTW, I would not trim resistor leads until you find the magic value.
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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
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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.
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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?
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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?
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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
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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.
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crap crap crap. I got the bias pot and the balance pot mixed up. I assumed the larger pot was the bias. No!
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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?
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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.
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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?
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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?