Fixed bias EL84 circuit?

[azatplayer]

Just tooling around on a mule with a high gain EL84 thing. Want to try it fixed bias. Any bias circuit values out there for me?
I recently finished one of the most amazing amps ive yet built, a Naylor superdrive. Man that thing is wicked. Got me to wondering what the circuit could sound like with EL84's. SOunds pretty damn good!
Im still figuring some stuff out with it, but wanna try fixed bias out.

[ampgeek]

The Soldano Atomic 16 is a nice little beast that uses 2XEL84s with fixed bias at ~-21VDC. Unfortunately, it comes off of a dedicated PT secondary.

You can get close by replicating the TW Express bias circuit. It will likely overbias (low tube current) but that is a good way to start. Adjust R29 higher and/or R31 lower to hone in ~-21VDC at the middle of the adjust pot range.

Might also need to look at the PI. That typically needs adjusting when switching from cathode to fixed bias. Again, check the Atomic to see how different the PI is from your cathode biased plan.

Do a Google on Atomic 16 or Astroverb to get the schematic. I don't have it to share on this computer.

Are you able to share your Naylor schematic? I am still trying to decide what plan to follow for an 18 watt'ish build. Yours sounds like it might be right up my alley!

Good luck!
Dave O.

[azatplayer]

Thanks Dave! Checked that out, heard a few decent clips. Some not so too..
I modded it to fixed bias, only had a 25k pot on hand but managed to manipulate it to give me between a wide range.
Now this is all naylor front end, inc the PI which has same values as that atomic for cathode and plates but the naylor has lower grids on the PI as well as bias feed resistors. So ive not yet attenuated the preamp to suit the EL84's yet.
The naylors run the power section pretty cool, its all preamp gain, so with that in mind, you could likely hit the power section harder and get it to clip, whether good or bad sounding. The SD60 i built is just perfect tho stock.
So i set this up with fixed, dialed in around 22ma, which i now have higher plate voltage as a result, about 375V, so around 65% ideal right?
Far out! This has gotta be cranking 50 watts at least!!!! Freaking loud!!!!!
As i stated the preamp has yet to be tamed before hitting the PI, but man compared to cathode biased this thing has twice the output, insane.
I have a 50 watt plexi, and the SD60, JTM50, Concorde and Express, and this thing is giving nothing away to any of them! Quite startling actually.
So, it doesnt sound terribly great yet, but the higher gain AC/DC chord crunchy stuff is astoundingly pushfull.
Now i gotta get to tweaking the signal down and see where we get to.
I started this thread some time ago. http://ampgarage.com/forum/viewtopic.ph ... highlight=
And theres a link to the place where the designer posted his schems for the naylor. http://freestompboxes.org/viewtopic.php ... r&start=40

[Gaz]

Check out the AX84 Novermber, which has fixed/cathode bias switching. Of course just ignore the cathode bias portion if you wish.

[azatplayer]

Ya i did see the november Gaz, cheers mate.

[Colossal]

Azatplayer,

Here is a drawing I worked up when this came up a while back on another thread; maybe it will be of use. It may require some adjustment of the range resistor and source voltage but you get the idea.

FWIW, I have a 4xEL84 high gain build but is cathode biased. It sounds massive however, very tight with a lot of squish.

[jon]

What primary should you run with fixed bias EL84's?

[azatplayer]

Im using a 4k3 with this one.
Hey Colossal, how long you think EL84's will last running at 377V plate, 355V screen and 22ma bias? I get around 66% so maybe ok? I punished this thing today, man its loud. Just going thru it now attenuating small areas, see if i can bring it down nice.
Pulled two tubes tonite, volts went up to 395V , bias was up as well, pulled it down. I have near 400V at 20ma. Sounds very pretty. I know theres a bunch of EL84 amps running higher B+, i guess as long as the screens are protected somewhat, they may be fine..

[jon]

Fender ran +-400V with 375V on the screens. They also claimed 25W from the amp. 6G9 Tremolux.

[azatplayer]

Wow! I didnt know they made an EL84 tremolux, way cool! Wonder if they fixed biased the thing.
Im playing very late here right now, master is so damn good on the naylor circuit. Got some delay in the loop, very low volume with some grind, killer tones.
Hope i get it sorted tomorrow.

[jon]

Here is the Schem

[azatplayer]

Man they sure didnt treat the screens with much love.
Interesting. After hammering this thing out today, zero redplate anywhere, not even a hint. They are hot tho, of course, higher volts and all. But they look like theyre behaving themselves ok. Diggin this one.

[Colossal]

Im using a 4k3 with this one.

Same here (for 4xEL84). 7k6-8k for 2xEL84.

Hey Colossal, how long you think EL84's will last running at 377V plate, 355V screen and 22ma bias? I get around 66% so maybe ok? I punished this thing today, man its loud. Just going thru it now attenuating small areas, see if i can bring it down nice.

No problem. I think that's about where I'm running mine, right around those voltages. I am using some 80's NOS Former Sovietsky 6N14N-EBs and they are very stout tubes and are rated to 400V on the plates. I am pretty sure it is biased around 11W a tube. The amp is capable of pretty crushing volume. It has a Trainwreck power supply but higher filtering in the preamp.

[jon]

I have a nice 25W 6.6K transformer i am going to use I should be fine.