Need help wiring a pot for cathode adjustment

[cjkpkg]

I am wiring up a PP EL84 based upon JB's posted circuit. I am also trying to integrate a bias adjustment for the EL84's based on the one at DIYParadise HERE

I have a Bournes multiturn 500 ohm pot that I can not figure out how to wire it.

I came out of the cathode pins with a 1.2 ohm and then the 15ohm connecting the 1.2 ohm to be placed in parallel with the pot.

The pot is like this...

----^^^^^^-----
1.........2..........3


Which two get the cathode connections and which one goes to the shared cathode resistor or CCS if I choose to do that?

TIA

 

[Frank Berry]

You can use a multimeter to measure the pot.
My guess is that pins 1 and 3 go to the cathodes.
Pin 2 goes to the shared cathode resistor.

 

[richardburak]

If 2 represents the wiper, then yes, 2 will go to the common resistor or CCS. As Frank pointed out, this can easily be checked with DVM: it will be constant between 1 and 3 and will vary between 1 and 2 or 2 and 3.

BTW, I've only recently completed the SimpleEL84 as a Proof of Concept, where the concept is I can build a valve amplifier. I recommend the CCS: it's easy and gives better bass, which in my situation turned out to be good for WAF.

But an issue with the 500R pot. Bang in the middle that would mean 250R to each valve and with 40mA to each valve: V = IR = 40/1000*250 = 10V drop across each side the pot. The LM317 is meant to maintain 1.25V so that should mean 11.25V total. But 11.25 won't run an EL84 at 40mA. I think what ends up happening is that the LM317 ends up operating outside of spec. while still maintaining 80mA. This became an issue when I started using 6p15p-ev because of it's different operating point. For me the answer was adding a resistor (~50R, don't recall atm but much smaller than 250R) across pins 1 and 2 and across pins 2 and 3. Having a resistor across 1 and 3 didn't seem to work as the drop across the LM317 was always well below 1.25V.

 

[cjkpkg]

Awesome! thanks for the replies....So far i have completed one channel and it passed the smoke test...I am using a standard resistor for the EL84 cathode bias...I have the parts for the CCS though...I wanted to give it a listen...Sounds absolutely fabulous...

I broke one of the legs of the pot off so I have to regroup on that...

I splurged for matched tubes though...so it should not be an issue for some t ime...