Modern bias circuit with balance or individual trimmers?

[xtian]

I'm looking for a modern bias circuit for a pair of EL34 (building a JCM 800 2204). HT will be taken from 310vAC winding. I looked at Merlin's Bias page, but it has no balance feature: http://www.valvewizard.co.uk/bias.html

Also looked at Rob's: https://robrobinette.com/RR2104_Master_ ... JCM800_6V6

I'll build it on vero board using radial caps and trimmer pots.

[luix]

For an EL34 HiFi amp I built time ago I've used this schematic:

bias balance.png

[sluckey]

If you have separate bias pots for each tube you don't really need a balance pot. I guess you could call one of the adjust pots a balance pot if you like. Here's the dual bias I used on a Sunn amp. You would need to tweak the resistor values to provide a negative voltage suitable for EL34s.

http://sluckeyamps.com/sunn/sunn.htm

Schematic at bottom of page.

[roberto]

I often use a similar system to what sluckey proposes:
I share the resistor between the raw bias and the two trimmers, but:
- instead of having separate resistors to ground, I share also that resistor:
- instead of two capacitors on the wipers, I use one single capacitor between ground and the most negative side of the trimmers.

[xtian]

I prefer to avoid designs that fail if the pot's wiper lifts from the tracks. I appreciate the input, friends!

[sluckey]

It's very easy to put a safety resistor between the wiper and top side of the pot.

But, I've never replaced a failed Fender bias pot. Have you?

[xtian]

Roberto, is this what you mean?

IMG_0135.jpg

[roberto]

Exactly that. Add one resistor from each wiper to the point where you inject the bias and it's done. I've used it for Hi-Fi built as well.
If you feel safer, add a resistor (10 times the value of the pot, so 220k in this case) from wiper to top, to ensure the connection.

Just remember one thing when dimensioning the circuit:
the voltage divider is (25k || 25k) to 10k, so 12.5k to 10k: you'll be able to regulate from 100% to 44% of the negative voltage you have after the 15k.
Set the 10k resistor in order to optimize the control of the bias, so widen the excursion of the pot in the spot of the right biasing voltage: it will simplify the regulation.

[roberto]

I explain it better: if you use a multiturn potentiometer you can do whatever you want and it will be ok, because with n turns you have n times the accuracy in regulating the bias. With a single turn pot I'd chose the values in order to be around the expected bias voltage with pot at noon.

This pot is cheap and perfect for the purpose: https://www.mouser.it/datasheet/2/427/t93-1763205.pdf
It's what I usually use: you have 21 turns of accuracy, so you can set the bias resistors around it in order to use standard values.

This is not needed in our DIY world, but I consider it a good engineering practice to standardize values even when it's not cost impacting.

[xtian]

Add one resistor from each wiper to the point where you inject the bias and it's done.

Confused. Why add another resistor? Not shown are the 220K resistors connected to the grids of the EL34s.

[pompeiisneaks]

That's the safety resistor they're talking about. allows some bias voltage to go through to the wiper in case it loses connection to the main resistive path of the pot.

~Phil

[xtian]

That's the safety resistor they're talking about. allows some bias voltage to go through to the wiper in case it loses connection to the main resistive path of the pot.

~Phil

No, I get that. Sure seems like Roberto is talking about TWO different resistors in his post.

[sluckey]

This shows the safety resistors. These are not the same as the grid leak resistors.

[roberto]

Add one resistor from each wiper to the point where you inject the bias and it's done.

Confused. Why add another resistor? Not shown are the 220K resistors connected to the grids of the EL34s.

You got it, it's exactly what I was talking about:
- one 220k "safety bias" resistor (10 times the value of the pot) for each pot;
- one 220k "PI load" resistor from each wiper to each power tube's grid.

I will sketch it if you need.

[xtian]

No prob, all good.