low B+ EL84

[allsavy]

I saw a schematic online somewhere for a headphone amp that uses an EL84 SE output rated in mili-watts!
B+ around 12 v.

Is this even possible?

Would the tube sound and react anything like normally expected when run this way?

[ThermionicScott]

I saw a schematic online somewhere for a headphone amp that uses an EL84 SE output rated in mili-watts!
B+ around 12 v.

Is this even possible?

Of course it is. Do you have a link to the schematic? (My guess would be that it's wired as a triode.)

Would the tube sound and react anything like normally expected when run this way?

Depends on what you expect! There are lots of things you can do with EL84s besides sticking them in Vox or 18-watt clones.

- Scott

[allsavy]

here is the 10-30 mili-watt el84 amp

Low Power (Apartment Level) Tube Power Amp 10-30 mW

[ThermionicScott]

Interesting that he kept them in pentode mode! That ought to be a pretty easy amp to build.

- Scott

[allsavy]

yeah 1/4 watt in PP pentode mode!

I hope to use push pull pentodes in a low power scheme as a switchable alternative to my 2x 6L6 PP fender amp so as to sound like a big amp only quietly as opposed to using a self split 12au7 as an output to get a low watt output.

I didn't realize that el84s could be operated at such a low B+ (12v) and sound ok (???)

Has anyone here done anything like this?

How does it sound?

Doesn't an el84 require a minimum voltage (60v) to be in any sonic ballpark?

[Hardtailed]

Wow... if that works then I'm intrigued!!

And come to think of it, people have been building tube pedals running on 9V for years using a 12AU7. So it would be possible to run this thing all tube.
Maybe the 12AU7 as the input stage, paralleled or cascaded depending on how much distortion you want. Use opamps or MOSFETs as the PI into a pair of EL84.

Gotta take into account the heaters current demand. Suppose you wire the 12AU7 in serie 12V, and treat the EL84s as a serie pair too. Then you get under 1A total, and you can use your 12V power supply for the whole amp.

Or... just wondering out loud here... make it a +/- 12V supply with the ground on the negative rail (might as well call it a 0-12-24V supply then). Run the tubes with the cathode on the ground and the plates on the 24V, and run the heaters between the 12 and 24V (instant DC elevation).

If it works on 12V, it will work even better on 24V and still be rather low-powered.

[Hardtailed]

yeah 1/4 watt in PP pentode mode!

I hope to use push pull pentodes in a low power scheme as a switchable alternative to my 2x 6L6 PP fender amp so as to sound like a big amp only quietly as opposed to using a self split 12au7 as an output to get a low watt output.

Yeah... self split 12AU7 is rather uninspiring imho.
I built a Firefly 2 years ago and never liked it. Power triodes don't do it for me.

What I did, and you may consider this: I kept the iron set of the Firefly (Hammond 269EX and 125C) and used them with a pair of 6K6GT. Those are like 6V6 but even smaller. With the variable impedance of the 125 series (and thank god I had bought the C and not the A), I set it at 12K (based on load lines simulations).

With 285V on the plates, theory says I should get about 8W. I don't have a scope, but I'm pretty sure it's a bit lower than that. It does exactly what the Firefly promised: sound like a big amp at small volume.
It's still rather loud, but thanks to the logarithmic nature of sound, it's subjectively not that much louder than a self split 12AU7.
Just like the 12AU7, it's still too loud for appartment use.

6K6GTs are not manufactured anymore, but are cheap and abundant in NOS variants. I paid 5$ a pop for mine. (Fender used them for stand alone reverbs back in the days)

I don't have access to Youtube from work, but search on it for "6K6 5E3" and you should find it. I was playing through Greenback speakers so it sounds really Marshally on the video.

[allsavy]

Thanks for the tip re: 6K6

have only one to play with.

sounds like Like a 6v6 but with earlier breakup right?

a bit more round and brown... vintage old school vibe.

Anymore interesting to me than a typical lack luster 6v6.... not sure....
they belong in a deluxe.

Thing is I went to 6L6s in place of my amps 6v6s to get more headroom and a bit mo spank and sparkle so my paf humbuckers get a chance at some psuedo-single coil tones.

The overdrivin powertube sound is all but gone in this amp (unless cranked) what with a 5751 in v1 and a12au7 LTP pi it rarely gets heavy dirt unless played at gig levels.

It is sweet for sure but I need to bring the OD back and preferably at bdrm level.

1/4 watt or 1/8 w with a tube seems to cry "attenuator" to boot for conversation level playing.

[Hardtailed]

Thanks for the tip re: 6K6

By the way, here's the video I was talking about (8W wonder)
YouTube - Broadcast Yourself.

Back then it had a 5E3 front end, and the channel are not jumpered on this video. It's very gainy, cause the power tubes are driven like mad!

This amp is a bit short for playing with a band though, it would have to be mic'ed. But that's sort of the magic here... completely cranked it's still not annoying (my "50W" Marshall clone is just painful when driven)

By the way, you want to make your Les Paul do some single-coil impressions? I put a 470pF Silver Mica cap as a treble bleed on mine. I'm always impressed at how "spanky" it can get this way. I used to hate the neck pickup, now I love it.

[allsavy]

Ya know... those 6k6s kinda remind me of el84s with the front of the amp driven.

and that is different than a 6v6. A bit grainy but I wont hold that against it.

I suspect a slightly tighter OD would be needed for mili-watt operation.


Sub miniature pentodes like the 6111 or 6021 look interesting.

zvex and rockblock mini amps sound articulate yet lush.

And are 12v tubes !!! If i'm not mistaken ....run of the heater supply.

[allsavy]

Getting back to my earlier post... Even though cathode biased, how can an el84 run with 12v B+ get cooking like with 300v plate voltage normally encountered?

Me thinks the tube would be easier to drive into dist. easier when run at 12v.
But the same as a tube driven to its upper limits.... cant be!

I'm not convinced yet.

So yeah perhaps sub-mini might be the way to go.

Or I could build an analogue ss marshall amp sim and send that OD through a low watt PP el84 thing to tube it up just a tad and settle for that. add an attenuator any I'm done.

Check out the LXH2 marshally all solid state amp sim sounds convincing real

Check out the the EVH sound clip...

http://hem.passagen.se/amps/

[ThermionicScott]

Getting back to my earlier post... Even though cathode biased, how can an el84 run with 12v B+ get cooking like with 300v plate voltage normally encountered?

Me thinks the tube would be easier to drive into dist. easier when run at 12v.
But the same as a tube driven to its upper limits.... cant be!

I don't think anyone is saying that an EL84 running at 12V WILL sound exactly like one at 325V -- otherwise, guitar amp designers would run their tubes conservatively, and replacing the output tubes would be a foreign concept.

But you should get a similar blend of overtones. For the cost of a couple ICs, EL84s, and some iron you might already have on hand, you could have a neat low-volume amp!

I built a push-pull 6AU6 a year ago that has some neat sounds in it. Something like 1/8 watt of pentode-overtoney goodness. It's running the tubes closer to their limits than this EL84 thing, so maybe that's part of it.

- Scott

[allsavy]

I like the looks of my 6au6s (actually they are 3au6) mostly because they are small I have two on hand LOL!

Good to know they have potential.

Found a good (and almost unbiased) report on london power's pwr scaling.

The sound clips are convincing as 380v does sound the same as 20v.

A drive control was added to make "the kit" work. see article:

guitar bruce | psk report

[allsavy]

Jeez...I should start a blog!

but while I'm here...

Someone in this forum mentioned that an EL84 when run in a low watt mode (ie: cathode biased with 12V on the plate) can be run higher (230v) at the plate to gain that sonic character and the screen grid somehow controls maintaining a low watt output.

Anyone remember how that post went?

[BackwardsBoB]

I wish I could tell you who posted it. It's in the huge power scaling thread.

There's no need to scale the plates/B+. You can drop only the screen voltage and track with the bias. In the nice link, you read that the kit MOSFETs get hot. It's a matter of whether you want the heat dissipated in the MOSFETs or the power tubes. I'm surprised that Kevin doesn't ship at least a cascode of two MOSFETs stock. At lower power, B+ current is greatly reduced, so the amp runs cooler either way. Worst case for the MOSFETs is somewhere around half power, where current and voltage drop are still fairly large.

The NFB loop has to be treated thoughtfully. O'Connor almost always uses an ultra-linear screen supply, and doesn't use NFB for linearity improvement. You can implement NFB as usual if you do a pre-PI MV and power scale the PI (easiest if you're dropping B+). This helps maintain the character of the PI.