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I am building my EICO HF86 clone, and did the power supply which puts out about 450vdc with no load and a SSR instead of 5u4. So, i am wiring up the power tubes and just got through the cathode bias circuit. It is 1 265 ohm 5 watt resistor and 100uf x 50v cap per el84 pair. Besides this i have connected the plate transformer leads and the G2 supply. These are the only connections besides the heater circuit that is on the tube sockets. At this point i decided to energize the circuit to see what kind of plate voltage i would get. I put in 4 closely testing el84 and ramped up the voltage.
The Voltages are about 240vdc Anodes of all plates, 230 vdc G2, and 32vdc on the cathodes. The Kv is about 2x expected of around 15vdc, and i expected the Av to be around 350vdc.
Since i only have the power supply and the power tubes, partially wired, it doesn't appear that i have any errors. Thank you for your help. Dak
Follow Ups:
Do you have grid resistors connected to ground?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hey Tre, no i have not put those in yet. I was wondering if that may upset the operation of the tube, but why would it? So, nothing on G1 yet.
The voltage at the cathode is only bias voltage if the grid is referenced to ground.
Otherwise the tubes will just run away, they have no bias voltage to stop them from doing so.
Bias voltage is how much the grid is negative WRT the cathode.
With the grid not grounded (referenced to ground through a grid resistor) the tube is not "seeing" the bias voltage.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I did think that i needed a grid resistor to ground but only if there was the coupling cap from the previous stage. So, i actually learned an important point about the operation of a tube which is all the elements needed to be in place for proper operation. Thank you very much, indeed. regards, Dak
I was re-building an Zenith SE EL84 amp many moons ago. I neglected to re-install this resister...the plates went rosy red post hast on start up:> )
This was one of my learning experiences..
Stuben
In my case i was only at 70% of wall voltage where i was checking the plate and cathode voltages when i noticed the problem. So, before any damage was done i powered down.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Playing with this resistor's value can effect the overall performance of the tube. Quite dynamic, along with the cathode resistors & bypass caps.
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