Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chrisw on June 10, 2017, 06:12:41 am
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I've been building an amp - my own design, but using the usual building blocks, and I've hit a problem that I really can't find the solution to. I'm hoping it's something simple, but I'm at a loss...
I put the circuit together, and it was okay (sounded great!) apart from what I suspect was parasitic oscillation on the EL34s - the screen grids were glowing blue, although the bias current at idle seemed to be fine. So I tidied up all the wiring, made sure that there was plenty of space between the wires, etc.
Since then, I can't get anything out of the first preamp tube. Voltage is getting to the pins, the cathode resistor is correct with reference to ground, and I know it's not the tube, as swapping the first two 12AX7s doesn't alter the results - the V1 plate readings are in the region of 360V, whereas the plate for V2a is about 240V. i get the same readings without the tube in V1 regardless of whether I test the top or bottom of the socket, so the socket itself seems to be okay.
The signal can go from "input -> V1a -> volume -> V2a", or "input -> V1a -> gain -> V1b -> volume -> V2a" depending on how a relay switch is set. The relay is working as expected.
V1 and V2 are separated on the HT line by a 10k 5W resistor, and each tube has its own side of a dual 50uF + 50uF cap can. The plate resistor for V1a is 390k, for V1b it's 100k and for V2a it's another 100k...
Can anyone offer any insights into why V1a isn't conducting anything at all - I'm not seeing any output wave on my scope if I connect after the decoupling cap on V1a?
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What voltage do you measure on the grid and cathode? Measure directly on the tube socket to chassis.
What resistance do you measure from cathode to ground? From grid to ground? Measure directly on the tube socket to chassis.
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Both grid and cathode on V1a are 0V; the cathode on V2a is about 1.2V
Cathode to ground is 2.68kΩ; grid to ground is 1.02MΩ (Using a 1MΩ resistor on the input socket and a 33kΩ grid resistor...)
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The plate resistor for V1a is 390k
Temporarily change that to 100K. Any better?
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Temporarily change that to 100K. Any better?
I'll have to try that later - got a gig that I have to get ready for! But thank you so far - I'll try it as soon as I get back...
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Both grid and cathode on V1a are 0V
You should a volt or so on the cathode, I believe . Just for yucks, measure the AC voltage on your filament pins.
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Both grid and cathode on V1a are 0V
You should a volt or so on the cathode, I believe . Just for yucks, measure the AC voltage on your filament pins.
Nailed it, John - thank you! Must have been a bad solder joint on pin 4 or 5 - one side of the tube was heating up, the other wasn't, but as the wires are connected, I obviously didn't spot it from the underside of the socket... the plate is now running at about 175V, and the cathode at about 1.2V
Thanks for your help, and thank you too, sluckey - very much appreciated!!!
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Hot dawg. That's the second time in 3 years I've figured something out. :laugh:
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Hot dawg. That's the second time in 3 years I've figured something out. :laugh:
:l4:
Nice one!