Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: six el six on October 23, 2011, 12:14:08 am
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I just posted my voltages on a BF Vibrochamp. The amp sounds bad (anemic, weak, farts out a little, e.t.c.). All caps are new (all of 'em). I put some more details on the Voltage chart. The speaker checks out fine. All tubes have been tested in another circuit. If anybody has time to take a look I'd appreciate it.
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You may want to kill the trem when measuring voltages. If you don't have a footswitch for the amp, you can turn the oscillator off by using a jumper to connect the hot and ground sides of the footswitch jack.
Measure pin 5 of the 6V6 again, to verify the 0.1v reading. V1 pin 6 leads to a 0.02uF cap on the board. Lift one leg of that and measure the voltage at pin 5 of the 6V6 again.
Right now, 6V6 pin 5 shows 0.1v on your chart. It should be 0v (or extremely close), and this might indicate leakage through this cap. Make sure to measure this voltage on a low range (if your meter isn't autoranging); the reading may be an error if set for a several-hundred volt range.
It may just be the cap leakage, but something is way wrong with the bias of the 6V6. 38v/470 ohms = ~81mA! (432v-38v) * .081A = 31w. To be fair, screen current wasn't subtracted from the cathode current for this figure, but we don't have a screen resistor across which to measure the implied screen current.
Coupling cap leakage could be causing the increased 6V6 current. While you're in there, you might as well verify the value of the 470 ohm cathode resistor. A failing cathode bypass cap could cause excessive 6V6 current, but that doesn't seem to be the case in your situation: you stated you replaced the caps, and the cap would normally fail short and reduce the voltage seen across the cathode resistor.
Good chance your trouble is the 0.02uF coupling cap.
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It was the old cathode resistor. It measured 1.9 K. Once replaced the plate current came up to 51 mA.
Thank you HotBluePlates.
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Is the grid measuring 0v now?
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I've got 36. mV dc on that pin. All the caps were replaced in this amp not just the electrolytics and I've double checked all caps for leakage with a recently refurbished "magic-eye R-C-RATIO bridge" type old-school checker. Is the 36mV OK?
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Yeah, that's 1microamp and some change of leakage, perfectly fine.
And 36mV is meaningless in the face of the tube's bias. The previous reading of 1/10th volt or so was a little more worrisome.
As long as you're getting a good range of bias now, and the amp is sounding fuller, everything's cool.