Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: catnine on February 28, 2013, 07:23:17 pm
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My line was 119.7 VAC
The B+ is 415
plate 412
P-P JJ 6V6S @idle 19 mA bais
preamp 12ay7 pin 3 146.3 V pin 8 2.28 V plate load resister 100K cathode 1.5K 25 uf bypass
second gain and PI 12ax7 pin 1 202 V pin 3 1.55 V pin 6 239 V just PI at 1K and 56K 60.5 V pin 8 61.8 V plate load res 100K cathode 1.5K 25uf bypass .
rect sovtek 5Y3 tried GZ34 added 5 VDC B+ .
30uf cap
1st dropper 1K = 2 V drop also screen node = 2 mA
16uf cap
2nd dropper 22K = 334 v drop = 3 mA
16uf cap
3rd dropper 10K = 300 v drop = 3 mA
16uf cap
Total rail = 33K 110V drop = 3 mA
PT 326 VAC 326 VDC secondary @ 150 mA 6.5 VAC heater
PT secondaries pin 5 @ rect to ground 65 ohm pin 3 @rect 62.5 ohm total 127.5 ohm
OT 7K prim to 8 ohm load blue plate to red CT 134.4 ohm Brown plate 121.5 ohm , total 256 ohm V drop 3 VDC B+ to plate.
Is there any way to measure the 6V6 screen current? If I plugged in my strat and turned the amp up to 7 vol and hit a low E chord the plate 412 dropped to 382 and the screen across 1K dropper from 410 to 374 volt . can't say that means a thing.
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HBP has left the building.
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For output tube plate current in fixed bias tubes, you can measure the resistance across each side of the OT primary (with the amp off) and then measure the voltage drop across each side of the OT primary with the amp at idle. (You won't get much voltage drop across the OT primary, so use the 10s of volts setting on your meter, but be careful not t short anything/yourself). Voltage/Resistance = Current. The multiply the plate current by the plate-cathode voltage for the plate dissipation.
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I use a bias rite to check the mA current draw for the putput tubes. I guess when measuring the voltage drop on each dropper I should have measured across each resister instead of from each dropper to ground. I didn't consider the affect the filter caps have plus the current draw all the way to the preamp tubes on the rail. I imagine that makes a difference how you take the measurments.
I also got the shock of my life . I touched on of the droppers to see how hot it was and I only had one hand in the amp . I never had that happen before . Rubber souled shoes on a dry hardwood floor .
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I also got the shock of my life . I touched on of the droppers to see how hot it was and I only had one hand in the amp .
Never ever put your hand in an amp when its on. No matter who else does it, or says they do it.
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I also got the shock of my life . I touched on of the droppers to see how hot it was and I only had one hand in the amp .
Never ever put your hand in an amp when its on. No matter who else does it, or says they do it.
Yes , I know . that's the last time I will do that. At least I did not have my other hand on the chassis . I guess what what happened was just moisture in my left hand middle finger was enough to conduct and the V3 6V6 screen grid resister is only a 1/2" away from the last preamp dropper. My finger felt numb for a while after.
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Ok I measured across the 1K screen dropper in the rail I get a 4.5VDC drop across the 1K dropper at idle so that is 4 mA screen current at idle . I also checked the drop across the 470 ohm screen resisters and dropped 0.265 VDC across 470 ohm = 0.0005 amp. If I check from the screen side of the 1K dropper to chassis ground to either side of each 470 ohm screen grid resister the voltage shows no drop.
4 mA current on the screen at idle does not seem like a lot . I didn't check with a signal to the amp since it's a weekend. I have the JJ 6V6s at 19mA each at idle.