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?
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?
I was at a "hifi" shop and they were demonstrating a pair of Valve amplifiers for me, (not what I went ot enquire about, I am a solid state buyer).
One started to glow blue for a couple of seconds and then blew up taking out the speaker as well.
Knowing the compnay dealt mainly in expensive gear, I think that demo gone wrong cost them thouands, soon after I walked out never to return. I didn't buy anything from them.
One started to glow blue for a couple of seconds and then blew up taking out the speaker as well.
Knowing the compnay dealt mainly in expensive gear, I think that demo gone wrong cost them thouands, soon after I walked out never to return. I didn't buy anything from them.
What value is cathode resistor ? and 390k for an anode load sounds pretty high tbh. All the evidence should be there in the form of the voltage readings. Is the 390k OK ? and is there any voltage dropped across it ?
Only a few volts, Mooly - presumably because the stage isn't conducting anything - cathode to ground is 0V.
The 390k SHOULD be okay - it worked before I redid the socket connections...
The 390k SHOULD be okay - it worked before I redid the socket connections...
Plate voltage really high? OK, and is cathode voltage about zero? Look at the tube heaters, are both sides glowing inside the 12AX7? High plate voltage and missing cathode voltage is a sure sign the tube is not conducting current, and most common reason for that is the heater not coming on.
Typically a bad or missing solder joint. Note that such a failed connection could be right at V1 socket, but also could just as easily be at V2 socket where the wires to V1 attach.
Typically a bad or missing solder joint. Note that such a failed connection could be right at V1 socket, but also could just as easily be at V2 socket where the wires to V1 attach.
It might be worth putting 150k or whatever in its place, just to test, just to see what happens.
The other clue is to measure on the actual pins of the valve and not just the socket. A simple common cathode stage has to work at a basic level if the conditions are correct.
The other clue is to measure on the actual pins of the valve and not just the socket. A simple common cathode stage has to work at a basic level if the conditions are correct.
Enzo, you nailed it! Thank you!! Bad solder joint on either pin 4 or 5 - the heater for that side wasn't coming on, which I obviously didn't spot testing those pins on the underside of the socket... V1a plate is now reading about 175V and the cathode is now about 1.2V... thanks for the suggestion - very much appreciated!!
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