Yeah.
I'm worried about running it with 120R cathode.
I ran 3 different tubes(two used ones that should have been working) with that to see if it was the tube that was the problem.
But if running them with a 120R resistor on the cathode burns them... I have 3 dead tubes, and now that I have changed it to 220R, I will still have dead tubes and the problem will remain.
So... does 120R cathode kill the EL84?
The datasheet recommends a maximum anode power of 12W for the EL84. The numbers you measured indicate that the tube is dissipating over 20W with the 220R resistor. With the circuit as-is, this will shorten the life of the output tube significantly, possibly catastrophically. That said, 220R is probably not wrong if the rest of the circuit were working as intended. If C2 is really not in the circuit/shorted out, then that is your problem. Verify that C2 is installed correctly, and add a measurement of the voltage at the junction between C2 and the 1.5K resistor you added along with the other voltages if they changed, and we'll go from there.
An EL84, like all pentode output valves, will run in overload conditions temporarily. If grossly overloaded, the anode will glow red, (Red plating, as the Americans call it). This is not a problem if limited to just a few minutes etc.
What happens is; When the anode glows red, it no longer attracts electrons and limits the current drawn from the cathode. When the anode stops glowing, it makes a full recovery, unless left for an extended period of time.
Many years ago I made an amplifier using 813 type, VHF transmitting valves. The anodes are coated in carbon and do not allow the current to reduce, (Class C transmitting has no issue with a carbon anode as it is only running at a duty cycle of around 50%) the glass envelope collapsed with the heat!
The valves will perform as they should for many hours, if not years, to come.
What happens is; When the anode glows red, it no longer attracts electrons and limits the current drawn from the cathode. When the anode stops glowing, it makes a full recovery, unless left for an extended period of time.
Many years ago I made an amplifier using 813 type, VHF transmitting valves. The anodes are coated in carbon and do not allow the current to reduce, (Class C transmitting has no issue with a carbon anode as it is only running at a duty cycle of around 50%) the glass envelope collapsed with the heat!
The valves will perform as they should for many hours, if not years, to come.
Last edited:
Bingo!
But... Why?
I thought a cap in the audio path would mostly filter the signal?
What's the purpose there?
Last edited:
With C2 shorted, anode voltage of V1b will be applied to the grid of the EL84 causing positive voltage on the control grid, in turn causing the EL84 to try and sink all of the HT across it.
There again, if not long when powered up, no damage. Ensure there is a grid leak resistor fitted between the grid and ground, otherwise it will distort badly.
The capacitor AC couples or decouples DC from the previous stage; it will block any DC in the path.
There again, if not long when powered up, no damage. Ensure there is a grid leak resistor fitted between the grid and ground, otherwise it will distort badly.
The capacitor AC couples or decouples DC from the previous stage; it will block any DC in the path.
Thanks! I did not know that. Tubes seem fine, so... phew.
The tone stack has been removed as seen on my schematic. If I rebuilt that, would I need C2 and R5?
Excellent.
Yes you will not need R5 or C2 with the tone stack, follow the resistor path to ground, at the EL84 end; see the path to ground the grid?
C2 is only there if the tone stack, having series capacitors (that has the same effect), is not there.
The learning curve to start with is like a vertical line but it gets easier.
Keep a clear head, lots of coffee and don't be ashamed to ask the questions.
Best regards.
Yes you will not need R5 or C2 with the tone stack, follow the resistor path to ground, at the EL84 end; see the path to ground the grid?
C2 is only there if the tone stack, having series capacitors (that has the same effect), is not there.
The learning curve to start with is like a vertical line but it gets easier.
Keep a clear head, lots of coffee and don't be ashamed to ask the questions.
Best regards.
I'm glad to see Rasped is in capable hands, and his amp is now working. 
Getting help with technical problems via the 'Web is a wonderful thing. I still remember how frustrated I'd get in my early days with electronics, when something I'd just built didn't work as intended.
Back then, it was entirely up to me to figure it out, which often took days, and that felt like a lifetime when I was eight or nine years old. And if I needed a trip to the public library for more electronics books to pore over, that would take even longer - I'd have to wait until my mom or big sister took pity on me and took me to the library.
So thank you to everyone who helps strangers out via the 'Web - it's a wonderful thing you're all doing!
-Gnobuddy
Getting help with technical problems via the 'Web is a wonderful thing. I still remember how frustrated I'd get in my early days with electronics, when something I'd just built didn't work as intended.
Back then, it was entirely up to me to figure it out, which often took days, and that felt like a lifetime when I was eight or nine years old. And if I needed a trip to the public library for more electronics books to pore over, that would take even longer - I'd have to wait until my mom or big sister took pity on me and took me to the library.
So thank you to everyone who helps strangers out via the 'Web - it's a wonderful thing you're all doing!
-Gnobuddy
I know Rasped's amp is fixed now, but for completeness, especially if anyone is wondering how the problem was tracked down:
Rasped, you were right when you pointed out that pin 6 of the 12AX7 was at too low a voltage.
Pin 2 of the EL84 (the control grid) should have been at 0 volts. Your measurement showed it at 23.4 volts, close to the voltage you measured at the 12AX7 anode (pin 6), considering the amp was still warming up and DC voltages were drifting with time.
So there we have two big fat clues. Something is dragging the 12AX7 anode (pin 6) voltage down. And something is dragging the EL84 control grid (pin 2) voltage up. Hmm, could these two problems be connected?
And sure enough, that was exactly the problem. There was supposed to be a capacitor to block DC (and let AC through) between those two points. Instead, there was a short-circuit.
-Gnobuddy
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.