Greetings all,
I'm finishing an amp with an LTP made of ECC88 cascodes. There's a CCS in the tail, with a 500 Ohm balance pot between the cathodes.
I was adjusting the balance with my scope and a 1 kHz sine. Right channel went fine, identical sines on the scope, top anode voltage on both sides 80V (B+ = 300V)
Left channel, same sines, nice. Top anode voltage 80V on one side, 230V on the other!?!
I blamed the tubes, swapped a few, but I still get the same result.
Cathode voltages are all in the same area.
I have NO idea what's going on, I hope someone can shed some light on this?
The amp seems to work fine, but this just seems wrong.
Thanks in advance.
I'm finishing an amp with an LTP made of ECC88 cascodes. There's a CCS in the tail, with a 500 Ohm balance pot between the cathodes.
I was adjusting the balance with my scope and a 1 kHz sine. Right channel went fine, identical sines on the scope, top anode voltage on both sides 80V (B+ = 300V)
Left channel, same sines, nice. Top anode voltage 80V on one side, 230V on the other!?!
I blamed the tubes, swapped a few, but I still get the same result.
Cathode voltages are all in the same area.
I have NO idea what's going on, I hope someone can shed some light on this?
The amp seems to work fine, but this just seems wrong.
Thanks in advance.
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With cascodes the anode voltage of the bottom tube is set by the grid voltage on the top tube. Have you checked all those?
Yes, all the same 50V. I was talking about the top anode voltage, I'll edit my post.
You say the resistors are the same. The same type or the same value? Have you measured them after putting them in circuit?
Regards, Gerrit
Regards, Gerrit
Same value. AND same type btw, but I didn't think that was important 😉
And I measured them now in the circuit.
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Will do. My scanners effed up at the moment, so it will be a photo of a hand drawn one.
Do you have grid stopper resistors on the bottom triodes?
The ECC88 can be a very good RF oscillator, even if you do not want it to be oscillating.
A DMM meter lead can either start or stop such an oscillation; DC voltages change when a stage is oscillating.
Please post the schematic.
A blindfolded archer can not hit the target.
The ECC88 can be a very good RF oscillator, even if you do not want it to be oscillating.
A DMM meter lead can either start or stop such an oscillation; DC voltages change when a stage is oscillating.
Please post the schematic.
A blindfolded archer can not hit the target.
Okay, I feel silly now...
I drew a schematic, then checked the amp again, and saw I connected the top grid to ground and not to 50V.... That must be it.
Regarding oscillation: at first I had used 100 Ohm gridstoppers. There was oscillation.
So I replaced them all with 2K2, that helped, but it was a difficult hard to reach operation. And yes it went wrong.
I'll correct it.
Thanks for the help anyway!
I drew a schematic, then checked the amp again, and saw I connected the top grid to ground and not to 50V.... That must be it.
Regarding oscillation: at first I had used 100 Ohm gridstoppers. There was oscillation.
So I replaced them all with 2K2, that helped, but it was a difficult hard to reach operation. And yes it went wrong.
I'll correct it.
Thanks for the help anyway!