Dear colleagues,
I've finished test build of RH84 rev.2 amp. I am using EL84 replacement tube, 6p14p. B+ is fairly close to the desired one 315VDC however, in one channel I got really high cathode voltage. After powering amp ir slowly raises to around 3-4 volts and than peaks up to 45-50V DC!
So what I did for tshoot: checked wiring 10 times, swapped tubes, swapped cathode capacitors and checked LM317 which somehow is not blown yet, checked zener diodes. Second tube has bias voltage around 8V DC so all other values will be checked later but this one makes me to turn off amp quickly to prevent any damage. I am out of ideas which component might cause this behaviour.
A link for amp schematics for reference.
RH Amplifiers: RH84 amplifier - revision 2
Thank you!
I've finished test build of RH84 rev.2 amp. I am using EL84 replacement tube, 6p14p. B+ is fairly close to the desired one 315VDC however, in one channel I got really high cathode voltage. After powering amp ir slowly raises to around 3-4 volts and than peaks up to 45-50V DC!
So what I did for tshoot: checked wiring 10 times, swapped tubes, swapped cathode capacitors and checked LM317 which somehow is not blown yet, checked zener diodes. Second tube has bias voltage around 8V DC so all other values will be checked later but this one makes me to turn off amp quickly to prevent any damage. I am out of ideas which component might cause this behaviour.
A link for amp schematics for reference.
RH Amplifiers: RH84 amplifier - revision 2
Thank you!
You could check if the coupling capacitor (C1 = 220 nF) in the faulty channel is leaky (or just change it for a known good one). If it is leaky, than the control grid would go positive (the more leaky, the more positive). Maybe the LM317 tries to compensate that by highering it's voltage drop to keep Vgk fitting to the current you set the LM317 for.
But I only know little about solid state technology so maybe I'm completely wrong on this one.
Edit: You could check if it is leaky by removing the EL84 of that channel, and than measure the voltage on the contact for pin 2 of the EL84 on the tube socket. If the capacitor is leaky, you will measure some voltage. If the voltage is 0, than the capacitor is not leaky.
But I only know little about solid state technology so maybe I'm completely wrong on this one.
Edit: You could check if it is leaky by removing the EL84 of that channel, and than measure the voltage on the contact for pin 2 of the EL84 on the tube socket. If the capacitor is leaky, you will measure some voltage. If the voltage is 0, than the capacitor is not leaky.
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If the coupling cap is good, then another possibility would be that
- either the LM317 is not connected with the cathode at all (wire missing, bad solder joint)
- or the LM317 is connected backwards (input/output swapped)
- or the 27ohm resistor on its output is something different (like 27k)
- or the 317 is oscillating (the National data sheet states that 25uF or larger electrolytic on the output terminal insures stability ... avoid long wires between out and cap / load)
- or the 317 is simply broken (open circuit)
- either the LM317 is not connected with the cathode at all (wire missing, bad solder joint)
- or the LM317 is connected backwards (input/output swapped)
- or the 27ohm resistor on its output is something different (like 27k)
- or the 317 is oscillating (the National data sheet states that 25uF or larger electrolytic on the output terminal insures stability ... avoid long wires between out and cap / load)
- or the 317 is simply broken (open circuit)
Thank you for reply, I've learnd another thing to check if is faulty. But decoupling capacitor is fine. Voltage was around 0.15V DC so very little.
I've found however stupid mistake in attaching ground to LM317 segment, it was between output and R7 and not between R7 and COM. I've change that and now issue is gone .
I've found however stupid mistake in attaching ground to LM317 segment, it was between output and R7 and not between R7 and COM. I've change that and now issue is gone .