Dual bias adjust

[islandamp]

Has anyone had any luck with adding a dual bias adjust to a 50watt Plexi type amp? I am interested in trying mismatched and 2 different type power tubes and am looking for a good working schematic.

[jimipage]

There's a layout at www.sonicdeli.com that is their version of a 50W Plexi with dual bias, but not for dissimilar output tubes.
hope this helps

[Firestorm]

Get Kevin O'Connor's TUT 3. It has lots of examples of individual bias mods for classic amps, including the Plexi. Individual bias does let you use mismatched tubes, but I'm not sure about mixing different tube types in a two tube output stage. I've done a lot of that in 4-tube types where each side of the transformer sees the same thing, but two tubes...I'm not saying it can't be done, but it seems "problematic" somehow.

[Andy Le Blanc]

Ive built amps with a bias control for each output tube
it can be done for any number of tubes
trying to mix tube types in a push pull circuit will be problematic
tubes in the same family ie: 5881 and 6L6GC sure
you should be able to mix pairs sharing a opt
as long as voltage conditions and impeadance issues are
resolved for both sets
but with tubes with drastically different plate curves might
demand too much of a compromise of each to be practical
even if voltage and load matching is worked out
one side of the push pull set up could develop much more power
than the other for the given conditions and create a nasty imbalance

[John_P_WI]

Andy,

I have had great success with mixing individual 6v6 and 6L6 tubes. I think the imbalance actually adds dimension and warmth. Try and run 'em in triode mode too. Seems to loosen up "50" watt amps.

Just another color of the sonic palatte.

John

[Andy Le Blanc]

awesome.....
I do know that creating an imbalance makes more harmonic distortion
not good for HI-FI but perfect for an instrument amp
how was that accomplished
what are conditions that allowed you to do that
and how is the reliability?....

[benoit]

I talked to someone who had done this a while back and I think it was just a matter of making sure each tube is getting the B+ voltage it wants to see and having separately adjustable bias.

[John_P_WI]

Andy,

Nothing special was done. The amp was a silver face bassman chassis and xformers, with a new circuit card ala John Mcintyre's blues maker meets a jcm 800 with dual bias pots.

425 v on the plates. The only way I can descirbe it is "cream with the crunch".

Try it, its fun.... and it is still darn loud too.

John

[Firestorm]

I build a jazz amp that mixes a pair of KT88s with a pair of 6L6s (one of each on each side). The KT88s are biased on the cool side for clean power and the 6L6s a little warmer for "body." That works pretty well. I can also see doing the same thing with 6L6s and 6V6s, or even EL34s and 6V6s (which at about 400V bias up almost the same).
I've also tried mixing two of the same type tubes, matched for current but with wildly different transconductance. That arrangement is very harmonically rich.
But I still can't get my head wrapped around mixing a 6V6 and a 6L6 in a two-tube amp. The current would have to be way different, right? You'd wind up with a significant variance in the plate voltage for each tube, and the DC balance would be off enough to "uncancel" the 120Hz buzz. The B+ node off the first filter has a LOT of ripple. Did you (John) notice any problem with that?

[John_P_WI]

Firestorm,

I did not have much induced noise due to imbalance, but my first power supply capacitance was huge, in the range of 200 uF as I originally built the amp for drop d tuning. I also used a number of small poly caps and a small amount of series resistance to help decouple and limit the diode noise. Running the tubes in "triode" mode also seemed to help the noise. If one was to run the tubes individually in pentode or tride mode and not as a pair, then noise could be an issue.

I also decouple each stage in the pre amp as KOC details in his TUT series, so noise has not been a huge problem.

John

[Andy Le Blanc]

a 6L6 wired triode is similar at first glance to a 6V6 as far as
load and transconductance goes but the plate resistance of a 6v6
is way out line
I do think your large cap values in the power supply are your saving grace

ive seen many configurations for different purposes...except that
its not common.... so when it pops up.... you struggle with convention

a solution that came to mind....I found some time back in a old text book
some pentodes can be wired as a triode
or a triode with a medium mu, a high mu, or as a couple variations
of screen grid tube..... 6v6 and 6l6 cant......but el34 can
so Im curious enough to look at it..

[Firestorm]

ive seen many configurations for different purposes...except that
its not common.... so when it pops up.... you struggle with convention

a solution that came to mind....I found some time back in a old text book
some pentodes can be wired as a triode
or a triode with a medium mu, a high mu, or as a couple variations
of screen grid tube..... 6v6 and 6l6 cant......but el34 can
so Im curious enough to look at it..

Wait a minute, I've got to get my Maine-to-English dictionary out.....I think you mean that unconventional applications make you stop and scratch your head. I totally concur.
As to alternate pentode wiring, I know that EL34s and other true pentodes can be wired as triodes (or close, since the physical structure is different), but are you saying the actual mu of the tube can be changed by different wiring. That would be way cool (and seems plausible.) Tell us more.
Sorry about the Maine crack. Maine is probably my favorite place is the US. Really terrific people there.

[Andy Le Blanc]

"ELECTRONIC AND RADIO ENGINEERING" McGRAW-HILL, 4th ed. 1955

sec. 6-14 (pg. 209) ,. fig. 6-26,.

"Special Connections for Conventional Tubes"

"It is possible to rearrange the connections and potentials of the various electrodes of ordinary tubes, and thereby modify the the characteristics."

It goes on to describe the various screen and suppressor grid connections.
some are familiar with the triode hook up for common pentodes.... there are examples that go futher... but you need a tube where you can get at the screens to try it out .....the article has no citations
its a text book and the follow up question reffers to the fact that there are no commercial tubes with a variable mu. ... so how do you adjust your design....

I know it just a snippet... but it s one that has stuck in my mind ...
trying to wire dissimilar tubes in push pull might be an appropriate application

[Firestorm]

Thanks Andy. That is a very cool idea to think about. It also has neat applications for the small pentodes that are sometimes used in preamp circuits. I'm going to have to visit a serious library.

[Andy Le Blanc]

back before the golden age of the internet came and went
I used to scour used book stores and flea markets
even the librarys have to dump un-utilized text from their collections
so the information is hidden until some fool starts diggin