Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Misc Schematics and Layouts => Topic started by: NSB_Chris on April 28, 2020, 05:39:43 am

Title: EL84 power amp with fixed bias - does it exist?
Post by: NSB_Chris on April 28, 2020, 05:39:43 am: For my next build, I was thinking about going EL84 on the power amp. It seems most "modern" lower wattage tube amps I see these days are built on an EL84 platform. Maybe because they allow for a compact construction, power amp distortion characteristics ...

My original thought was fixed bias with SS rectification. Sort of like a low wattage Marshall. I built a version Rob Robinette's RR800V, which is a JCM800 preamp with a 6V6 power section that was great. I thought, why not try something similar with an EL84! I thought I could find an AC15 schematic or a forum original, but I am coming up blank. In my research, I am finding only single ended or push-pull with cathode bias. Is there some reason for that that I am missing?

Any insight, knowledge, advice would be greatly appreciated!
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: sluckey on April 28, 2020, 10:37:02 am: Here's one...

http://sluckeyamps.com/november/november.htm
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: NSB_Chris on April 28, 2020, 05:04:16 pm: That is really great, Thanks!
Did you ever try the hammond PT?

I found references to the ax84 site, but it looked like it was defunct. They had a note saying something like "click here for our forum in its former glory". Spelunking through it now to see if I can find some of the schematics that inspired your amp.
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: NSB_Chris on April 28, 2020, 07:01:03 pm: I think I may have found one of the references:
LoWatt by Bryan James

OT:
The print out is difficult to read but it looks like the output transformer is Hammond 1650F. Hammond lists this as a 25W 7.6K c.t. OT - they don't list the EL84 among the suggested tube types.
The Hammond 1650E might be a good choice also. 15W 8K c.t.
Thoughts?

PT:
They have a Hammond 270FX, which looks like a 550V PT with ample 5V and 6.3V current capacity. With a SS rectifier, is 550 too much for the EL84. It looks from the data sheets that the EL84 is not as forgiving on the B+ voltage level. Should this be lower output voltage?
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: sluckey on April 28, 2020, 07:36:40 pm: I used a 270FX and 1650F in my second November amp.
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: NSB_Chris on April 28, 2020, 07:52:59 pm: What B+ voltage did you get on the second November amp? The data sheet for JJ EL34 says the max plate voltage is 300V, which I assume is reasonably typical for the tube type.

Now that I look at the LowWatt schematic, they put a 330 ohm resistor before the B+ tap!
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: sluckey on April 28, 2020, 09:46:19 pm: Voltages are on the schematic at the bottom of this page...

http://sluckeyamps.com/vox/vox.htm
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: tubeswell on April 29, 2020, 03:27:02 am: Traynor YBA-2B is another one
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: NSB_Chris on April 29, 2020, 05:41:48 am: Thanks Sluckey! So the voltages on both the November project examples are essentially the same.

Thanks Tubeswell for another example! This one even has a higher B+ voltage.

So all of these have EL84 plate voltages that are higher than the max listed for the EL84. Is the data sheet that conservative for the max voltage, or am I missing something?

Sluckey, the split chassis design is very interesting. I am currently building a split chassis design. Covid-19 has slowed the process because I have limited access to my shop. That is why I am thinking beyond it to the next project :-)
The motivation for this one is to have the head close to the pedal board with short 4 cable method runs and knobs close for tweaking, but have the speaker wherever it needs to be.
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: Bantam on May 14, 2020, 01:45:49 pm: Very interesting build. Love to see more. Is the tube running between filter can and O.T. between preamp tubes, and also right hand side wall for wire runs? I was kicking around the idea of the basic nice preamp sound and then different sized output power in separate form.
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: tubenit on May 15, 2020, 05:31:54 am: https://el34world.com/charts/Schematics/files/Carvin/Carvin_vintage16.pdf

fixed bias and 380v on the EL84
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: DummyLoad on May 15, 2020, 06:46:15 am: fender blues jr.

--pete
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: Johntb on June 08, 2020, 07:10:14 am: This is what I used for my ENGL Fireball clone.

https://www.gapcb.com/documents/SuperLite-II-2.7-Schematic-v3.pdf
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: sluckey on June 08, 2020, 07:30:23 am: Quote from: Johntb on June 08, 2020, 07:10:14 am
This is what I used for my ENGL Fireball clone.

https://www.gapcb.com/documents/SuperLite-II-2.7-Schematic-v3.pdf
But this is a cathode biased amp.
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: Johntb on June 08, 2020, 04:28:49 pm: Quote from: sluckey on June 08, 2020, 07:30:23 am
Quote from: Johntb on June 08, 2020, 07:10:14 am
This is what I used for my ENGL Fireball clone.

https://www.gapcb.com/documents/SuperLite-II-2.7-Schematic-v3.pdf
But this is a cathode biased amp.

Oops! Sorry.

Thats because I'm stupid and wasn't paying attention. :BangHead:
Title: Re: EL84 power amp with fixed bias - does it exist?
Post by: frank57 on July 19, 2020, 12:57:27 pm: Hiwatt used a fixed bias on their lead 20, 30 and custom 20 tube amps.Also Jet city amps and their Soldano clones like the 22h.(https://pasteboard.co/JipaTi2.jpg)(https://pasteboard.co/JipaTi2.jpg)