2204 Individual Bias Controls

[bcmatt]

One of the first amps i built from a kit was a 2 bottle JCM 800 but now my main amps are a Liverpool and a DC30 clone so I don't keep matched pairs of EL34s around. I lost one pretty new KT77 and am just left with some single EL34 types with no place to use them.
I'm wondering about why individual bias controls are so rare. Is there a reason why no one does this, even with the increased rarity of great tubes at reasonable prices?Are there negative consequences for an OT to separately bias an unmatched pair?
Then, I guess I'd want to know a good schematic for doing so, but I guess I could make some attempts myself and then google it when I'm not just on my phone.

[martin manning]

Probably the main reason that you don't often see separate controls is cost. There have been a few in the past, and some boutique amps now that have them. Fender tried a balance control for a while. Basically the same part count as an adjustable bias, but roughly half of that needed for separate trimmers. With this set-up you can balance, but not set level.

Then there is the question of what is "balanced" and what is the benefit? Matching idle current is good because it zeros out the net flux in the OT core, but that doesn't mean that the two sides will be balanced when there is signal present. There are ways to address that, though, such as trimming the phase inverter's plate loads.

I think the next one I do will have a level and a balance control, where one trimmer sets the level and a second one balances the two sides. I think this would be just a bit easier to set than a separate trimmer on each side.

[Reeltarded]

As long as I am in range with the normal setup and within 3-5mV showing on the meter between the tubes it's pretty dang good. As Martin said about the PI.. and balance there isn't the key to a good Marshall, I say.

Dual bias or separate bias works great if you have more variance in a pair to get them closer but again perfection isn't a real issue there.

[bcmatt]

OK, thanks guys!
Ya, I'm not overly obsessed with balance. Maybe it adds a bit of swirl if they are mismatched a bit. I'm really talking about the situation where one KT77 got banged and has gone microphonic, so thinking about using the other KT77 it was matched with, with an unused JJ EL34 that I happen to have in my box of tubes.

The guy I had lent it to (the one who received it from Fedex with a microphonic tube) put in his backup up pair of Russian EH EL34s, and I recently was in California and biased it for him. However, his tubes were not exactly a matched pair. I could get one at about 52% if the other was around 70% max dissipation. It seemed to sound fine... but it started me thinking about all the lonely unmatched tubes in the world, and how they long to be abused in a marshall-type circuit.

I do need some mods to tame the brightness though...

But first, I need to see if I can use this with the tubes I have, and then have it always able to take mismatched tubes... some amp has to take them!

If it was my main amp, I'd be a bit more willing to buy new matched pairs, but I don't exactly make any income aside from the pittance I get as a sub-par guitarist in an unknown 3-piece bluesy garage-rock band... and that ain't much! It all goes to gas, food, strings, and new sticks for the drummer!

[rp]

I got a question: is balancing out two fairly unmatched tubes the same, sonic-wise, as using a closely matched set?

[martin manning]

Not necessarily. A truely matched set would be matched both for idle current and transconductance. Lining up the idle current by adjusting the bias doesn't mean that they will produce the same AC current when a signal is applied. A miss-match in transconductance might be a good thing though, as that implies production of second-order harmonic distortion.

[Reeltarded]

I have heard an amp with mismatched tube types and dual bias that was totally awesome and hell no I don't need that kind of OCD. haha

[lumox0013]

I second that motion, theres already enough ocd around here !! but to each hiss own .