Mixed Bias designs, anybody?

[CoolBlueGlow]

Anybody doing "mixed bias" designs out there?

Note that I don't mean selectable designs, e.g. a cathode OR a fixed bias selectable by switch. I mean mixed biasing, as in a class AB1 amplifier output section using a pair of EL84 and featuring cathode cap/resistor AND a B- supply, each sharing a portion of the function of setting the tubes' operating point.


Just curious. I'm cooking up something in the Utility Muffin Research Lab.

Cheers,

CBG

[uriah1]

Didnt THD do that once?
Been awhile.

wish you luck

[BiggerJohn]

Didn't Fender try that in the very early SF amplifiers, with poor results?

[CoolBlueGlow]

Yes, I believe the AA768 is a mixed bias design, but I wondering maybe it doesn't sound good because of other "improvements" in the circuit...and maybe not because of mixed bias?

Anyway, seems like introducing a limited amount of phase shift via a cathode bypass cap might be an interesting halfway point between fixed bias designs and full-on cathode bypass layouts? I guess I'm trying to find the grind somewhere in the watts above the cathode biased AC15, but somewhere below the output of the typical EL84 AB1 fixed bias design.

Dunno - I guess I'll mock up something and see what happens.

Didn't know about THD - I can check into that as well.

Thanks guys,

CBG

[StephaninMelb]

On the Fender Silverface Combination Bias:

http://music-electronics-forum.com/t26726/

Not what you asked for but Fortin in Canada in their amp have individual bias for every power tube:

http://www.fortinamps.com/BonesManual.pdf

Kustom in their Coupe (Legacy) and Defender amp series use a mixed bias they call "Autokruise." I don't know if the carried over that to all their newer designs.

http://www.guitarplayer.com/article/...36-Coupe/11704

http://www.kustom.com/Content/20082/...nderManual.pdf

www.kustom.com

Blackstar use a mixed bias in the HT20

http://www.musicradar.com/forum/show...=101179&page=1

[Prairie Dawg]

I think the reason Fender tried it was that even if you lose your bias supply your amp won't barbecue the power tubes.

[Billm]

I tried mixed bias in my early Blues Junior modding days. I had hoped to dial in a bit of cathode-bias natural compression, but the tonal effect was minimal and the loss in volume was noticeable, even when the plate current was correct. Perhaps I could have done more tweaking, but I was disappointed and gave up on it.

[jefrs]

There is little or no point in going grid-biased on EL84 because they were designed to be cathode biased.

It can be done, like the BJr, but you don't get the really noticeable difference you get from an octal. The EL84 was designed for cheap domestic appliances (picnic-box record players and radios), it is not actually a very good bottle, it is rather squishy by either method.

[CoolBlueGlow]

Hi Jefrs,

Ha! I was going to chastise you for calling the EL84 a "domestic appliance tube"... until I saw where you are from.

O.K., thanks to everyone for the opinions and info. Think I'll skip the innovation and just go with a conventional cathode bias. I have lots of other more productive ideas in this design.

cheers,

CBG

[printer2]

Originally Posted by jefrs

There is little or no point in going grid-biased on EL84 because they were designed to be cathode biased.

It can be done, like the BJr, but you don't get the really noticeable difference you get from an octal. The EL84 was designed for cheap domestic appliances (picnic-box record players and radios), it is not actually a very good bottle, it is rather squishy by either method.

So what does designing a tube for cathode bias entail?

[CoolBlueGlow]

Maybe it has to do with using only PIO caps? By the way, Celeste says he's done a dissimilar pentode design and gotten good results.

Now I'm torn. Celeste says "It's great", Jefrs says "It's pointless, the EL84 is crap" (that's a technical term) and some others say "why bother".

You guys are not helping me! I wanted to do something epic and weird, and all I'm getting is yawns!

:-) hehe

p.s. Over on the DIY amp forum, I'm giving away a passel of 6X8 and some Chinese 12AU7 and a few others, in case anyone needs some.

Cheers,

CBG

[jefrs]

The EL84 was a crap bottle for cheap stuff, but it works great in a guitar amp.

It is also unusual in that it was designed to have appreciable gain so that a pre- stage could be deleted, to save money.
This poses a small but not insurmountable problem if you want to mod an EL84 amp to substitute 6V6 or other octal (not to mention the OT).

The grid-biased BJ runs to "15W" but you can push a pair of EL84 to about 20W with cathode-bias (PaMax=12W). Any output increase from cathode to grid is insignificant on the EL84. Grid-bias may make it "stiffer" and less "squishy". Imo they are supposed to be a bit "squishy", making an EL84 amp stiff spoils the fun.

If you want clean, stiff and sparkly, then 6V6 (with which you can also play with the bias, and have a greater range of effect)

Btw - an ultra-linear EL84 amp is done because you can, not because you should. Again there are better bottles for this hi-fi application.
Guitar amps are not hi-fi, they are based on olde public address amps, they are supposed to be loud and brash, and then we run the output into a high efficiency lo-fi speaker.

[dgr888]

Been there done that with excellent results. I design and build 84 based amps and not "British/brash/squishy/clangy/crunchy/overly bright" sounding at all. Smooth sounding with a tight fat bottom can be had with these bottles in a properly designed circuit. Copying vox/marshall/fender blah,blah,blah circuits are NOT going to get you there. It takes a very lot of experimentation/trial and error and you can make tubes do what you want them to do. Most companies and cloners don't innovate they regurgitate. Go for it and good luck.

[celeste]

Originally Posted by CoolBlueGlow

...By the way, Celeste says he's done a dissimilar pentode design and gotten good results.
....

With all due respect, she

[printer2]

Originally Posted by jefrs

The EL84 was a crap bottle for cheap stuff, but it works great in a guitar amp.

It is also unusual in that it was designed to have appreciable gain so that a pre- stage could be deleted, to save money.
This poses a small but not insurmountable problem if you want to mod an EL84 amp to substitute 6V6 or other octal (not to mention the OT).

The grid-biased BJ runs to "15W" but you can push a pair of EL84 to about 20W with cathode-bias (PaMax=12W). Any output increase from cathode to grid is insignificant on the EL84. Grid-bias may make it "stiffer" and less "squishy". Imo they are supposed to be a bit "squishy", making an EL84 amp stiff spoils the fun.

If you want clean, stiff and sparkly, then 6V6 (with which you can also play with the bias, and have a greater range of effect)

Btw - an ultra-linear EL84 amp is done because you can, not because you should. Again there are better bottles for this hi-fi application.
Guitar amps are not hi-fi, they are based on olde public address amps, they are supposed to be loud and brash, and then we run the output into a high efficiency lo-fi speaker.

I have a Scott hifi amp that runs EL84's and they sure do not sound like crap bottles. Maybe it is the circuit and not the tube that determines how clean, stiff and sparkly the amp sounds?

[celeste]

jefrs is just being dramatic. everything he says is technically correct, the EL84 was developed with the idea that it could cut production costs in inexpensive equipment. It has 2-4x the voltage gain of otherwise similar bottles, and it is not always the most well behaved tube.

[dgr888]

printer2... you are absolutely correct. ANY tube can sound bad or good depending how the circuit around it is designed. There are numerous factors involved in a good circuit which starts at the input jack piece by piece and ends at the speaker. It takes a lot of experimentation between these points to get everything behaving as you want it to. Big companies and cloners don't have the patience/and or knowledge to do it.....

[CoolBlueGlow]

Sorry Celeste. My sincerest apologies! I meant no slight - I was merely thinking "celeste" as in the musical instrument, not "Celeste" as in the feminine nom de plume.

My abject apologies - I will go and sort unlabeled small signal transistors from bins for several hours as penance.

:-)

CBG

[CoolBlueGlow]

O.K. LADIES and gentlemen, I love you all, but you are not helping me. :-)

Think EPIC and WEIRD! Mixed Bias - mismatched pentodes - triode/pentode push pull mismatches...that kind of stuff.

I have over sixty of the little dutch-made buggers in boxes out in the Utility Muffin Research Lab, along with two dozen classic and vintage NOS EL84 transformers just begging to be used, but the last thing the world needs is another VOX knock off. So, AGAIN - the question was "is anyone doing mixed bias designs...perhaps using EL84?"

Remember, epic and weird.

Come on Jefrs... get in the spirit of it. You seem to have the right stuff for this kind of journey.

:-)

Cheers,

CBG

[dgr888]

Originally Posted by CoolBlueGlow

Anybody doing "mixed bias" designs out there?

Note that I don't mean selectable designs, e.g. a cathode OR a fixed bias selectable by switch. I mean mixed biasing, as in a class AB1 amplifier output section using a pair of EL84 and featuring cathode cap/resistor AND a B- supply, each sharing a portion of the function of setting the tubes' operating point.


Just curious. I'm cooking up something in the Utility Muffin Research Lab.

Cheers,

CBG

I've stated in my previous posts it can be done and I have done it with a combination of fixed and negative bias and build el84 amps with this design. As far as mismatching pentodes that can be done also such as a 6v6 and el84 push pull but each one will require it's own separate biasing scheme separate from the other. Each one could have a combination of fixed and negative biasing. It's more difficult to do with mismatched pentodes because both require a different amount of drive signal. What we be good for one might be too much for the other etc. 84's require less to drive them to full output than most other tubes so designing a phase inverter to compensate would be a pain. You'd be better off mismatching pentodes with similiar drive requirements. There also would be a slight mismatch on the output trans. Have fun..