Yes, the input of the lower tube is the cathode. Pin 2 gets the input signal, the signal on the plate (pin 1) is out of phase. The signal developed on the cathodes is in phase so too the output on the plate (pin 6). There may be some signal on pin 7 in "this" case as the capacitor is not grounded directly. You probably won't be able to see it as the probe will load it down. If and when this gets straightened out try looking or measuring pin 7 and you'll what it does on the output of the amplifier. The output will not be good.
Have you tried taking the AC up to full 115/120. The phase inverter is biased by the DC on the plate of the preceding tube, maybe it will correct itself at full voltage. Worth a shot. Either there's a mistake in the power supply, grounding, or the same thing on both channels. What's on pins 3/8 with no tube installed? Should be zip. In fact check all of the pins of the 6CG7 socket with no tube installed. Pins 1 and 6 should full B+ for that section, pin 2 should be the same as pin 5 of the 6BH6, pins 3/8 should be zip, pin 7 "might" have a low voltage. Is there any DC voltage at the junction of C5/C6?
Craig
Have you tried taking the AC up to full 115/120. The phase inverter is biased by the DC on the plate of the preceding tube, maybe it will correct itself at full voltage. Worth a shot. Either there's a mistake in the power supply, grounding, or the same thing on both channels. What's on pins 3/8 with no tube installed? Should be zip. In fact check all of the pins of the 6CG7 socket with no tube installed. Pins 1 and 6 should full B+ for that section, pin 2 should be the same as pin 5 of the 6BH6, pins 3/8 should be zip, pin 7 "might" have a low voltage. Is there any DC voltage at the junction of C5/C6?
Craig
Hi Raymond -
So, you have voltage on the plates, current through the cathode resistor, and a signal on the driven grid?
Maybe there is an AC short to ground on the plates (like on the output tube grids)?
Try pulling the output tubes... and then disconnecting the coupling caps and see if there is any AC signal on the plates.
Pete
Hey Pete,
I don't know if checked the other thread but there is something wrong in the phase inverter section as there is a 50-60 VDC difference between the grid (pin 2) and the cathodes, 10VDC is normal in this case. And at the same time the cathodes have 166VDC on them, should be around 100VDC.
Craig
I don't know if checked the other thread but there is something wrong in the phase inverter section as there is a 50-60 VDC difference between the grid (pin 2) and the cathodes, 10VDC is normal in this case. And at the same time the cathodes have 166VDC on them, should be around 100VDC.
Craig
Hi Pete,
I'll measure things without the output tubes, but the odd thing is that the error is in both channels, which usually points to the power supply. All that seems to be working tolerably well with B+ where I expect it. The fact that there's no signal accompanying the DC through the tube puzzles me.
Hi Pete,
I ran the B+ up to 400 for testing without the output tubes in their sockets. With 1V 1KHz as a signal, I get 35V on pin 2 of the 6CG7. This is where it goes all wrong. I'm getting the same measurements in both channels. Pin 1 = 0.25VAC, pin 3/8 = 1.7VAC, pin 6 = 0.26VAC, pin 7 = 0.01VAC, and pin 9 = 1.7VAC. I have 0.45VAC on all four EL34 grids and 1.7VAC on pins 1 & 8. Are both 6CG7s defective? They test as good on an emission tester and they have B+ and cathode voltage so they're passing DC. Any ideas? I'm stumped.
I think I would get the DC levels corrected before worrying about the signal. If the DCVs are still as bad as before the tube CANNOT pass a signal. The relationship between the grid (2) and the cathodes (3/8) needs to be about 10VDC for the tube to operate correctly, the grid is negative with respect to the cathodes.
Craig
Craig
Thanks
Well, DUH! I finally located the problem, and it's as I expected from day one, a wiring error, a disconnected ground lead. The unconnected wire was tucked behind a bundle of other wires and escaped my attention. the amp now plays music, doesn't sound half bad, but the output tubes seem to require too much negative voltage to indicate correctly on the meter. More testing to do and more adjustments. No smoke, so all is well. Thanks for all the input.
Well, DUH! I finally located the problem, and it's as I expected from day one, a wiring error, a disconnected ground lead. The unconnected wire was tucked behind a bundle of other wires and escaped my attention. the amp now plays music, doesn't sound half bad, but the output tubes seem to require too much negative voltage to indicate correctly on the meter. More testing to do and more adjustments. No smoke, so all is well. Thanks for all the input.
In tech school they sort of skip over the subject of how important ground iz because so few components are involved..... It's a bit like the end of an exhaust pipe on an internal combustion engine - normally not a big deal - but try and get an engine to run without it....
I've been listening to the 8B for a couple of weekends now, and it seems to be behaving. No smells, no magic smoke, no buzz, no hum, sounds good. The replacement meter seems to work ok. My only issue at this point is that the B+ is low, about 385 vs 420 as per the voltage chart. Should this be a concern? If so, what could cause this. I have measured the cathode current at 100 mA per pair of EL34s depending on the bias voltage adjustment. This amp had been very hot before I got it. It had some of the transformer tar melted around the wire opening. Could that be the source of this symptom?
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