Self Split output stages in tube amps. Some questions.

[frequencycentral]

I've become interested in self split largely due to Doug H's Firefly, which I've modified for subminis with my Superfly. I've also be PM'ing back and forth with kurtlives about other possiblities using self split output stages.

So a couple of questions:

- parallel self split: possible? Basically doubling up/duplicating the output section of Firefly for example, for more dB, presumably at half the impedance?
- self split but using pentodes? But how would the 'lower' tubes' grid 2 be configured? A mirror image of the top tube? And what about negative grid 1 biasing - presumably not possible with the lower tube as grid 1 is tied to ground?

[slacker]

I've been reading up on this quite a bit because I'm just about to start ordering stuff to build a Firefly.

There's a post over at the ax84 forum about using parallel output tubes http://ax84.com/bbs/dm.php?thread=360832, from that it sound like you're on the right track with halving the impedance, that post only mentions a small volume increase though.

Don't know about the pentode stuff.

[frequencycentral]

Thanks for the links Ian, a lot of good stuff there, particularly this from Merlin, which I will be trying:

"The slave triode of the 12AU7 (V3b) has to get all its drive voltage across the shared cathode resistor R13. Because this is only 440 ohms, that basically means almost no drive at all- that slave triode is just sitting there doing almost nothing.

The existing FF schem http://ax84.com/media/ax84_m276.gif shows V2b having an unbypassed cathode, which means there's a signal there.
Instead of directly grounding the grid of V3b, add a grid leak (100k or more will do) and connect the cathode of V2b to the grid of V3b via a 22nF coupling cap.

This allows V2b to act as a crude cathodyne splitter and will roughly double the drive to that extra triode."

[frequencycentral]

"The slave triode of the 12AU7 (V3b) has to get all its drive voltage across the shared cathode resistor R13. Because this is only 440 ohms, that basically means almost no drive at all- that slave triode is just sitting there doing almost nothing.

The existing FF schem http://ax84.com/media/ax84_m276.gif shows V2b having an unbypassed cathode, which means there's a signal there.
Instead of directly grounding the grid of V3b, add a grid leak (100k or more will do) and connect the cathode of V2b to the grid of V3b via a 22nF coupling cap.

This allows V2b to act as a crude cathodyne splitter and will roughly double the drive to that extra triode."

Or maybe not..........

[doitle]

Hey Rick, speaking of different output configurations have you looked into output transformerless designs at all? A few weeks ago I was working on figuring out some kind of work a like for the... 6080? Tubes that are normally used in output transformerless designs. My hope was to use SMT parts and a SMPS coupled with output transformerless topography to make an insanely tiny all tube amp. I didn't get very far with research on the OTL designs though as I couldn't find much information bout them. Have you thought about them or know anything about them?

[frequencycentral]

Never really looked into them, but have come across the term when browsing. I kind of feel that an output transformer is an essential part of the 'sound' of a tube amp. As I understand it you'd need very (VERY) high impedance speakers. But I'd be fascinated to see your project come to fruition.

http://qtron.org/aboutotl.html

http://www.tubebuilder.com/schematic3.html

[doitle]

They do tend to require a higher impedance speaker but my thought was that if you use 8 ohm speakers and made a stack with them, say 4 8 ohm speakers in series gives you 32 ohms. I saw many OTL designs that were targeting 32 Ohms for headphone usage.

Also I just got a message from one of my professors that he was wondering if I wanted some old Tube books and texts. OF COURSE! Apparently one of the other professors is throwing them away.

[maarten]

The EL86 was used in OTL circuits, Philips made special 600 Ohms speakers for this tube. These speakers were discontinued and are now are hard to find. I recently saw an EL86 amp (push-pull, 14 watts) in which an output transformer was added in order to be able to use 8 Ohm speakers....

I remember having read somewhere that self split configurations are only feasible for low power amps, which might explain why you won't see this very more often.  I''l let you know if I rediscover this source and find any information of interest.

Maarten

[Taylor]

Don't know if this is really any help to you guys, but along the lines of OTL amps, this guy's stuff is nuts.

http://milbert.com/guitar

You can use any tube, any number of tubes, any speaker or number of speakers, it has unique power stepping, auto-bias, auto-standby, 50 watts all tube at 5 lbs/2.3 kilos. Want.

[frequencycentral]

- self split but using pentodes? But how would the 'lower' tubes' grid 2 be configured? A mirror image of the top tube? And what about negative grid 1 biasing - presumably not possible with the lower tube as grid 1 is tied to ground?

I think I've answered my own question here, from the Superfly thread:

I read one of DougH's posts over at AX84 yesterday, which i think refers to the same thing(y):

"Maybe there's a way to make self-split more efficient. Personally, I like the idea of screen self-split using pentodes."

From this thread: http://www.ax84.com/bbs/index.php?id=333694

A bit of googling turns up these:

http://ax84.com/media/ax84_m141.pdf

http://geckoamps.com/magnatone109/

......which I haven't got time to study right now, but I think demonstrate screen self-split using pentodes.

Look at the self split AX84 High Octane, the screens are connected to HT via 1K resistors, other than that the topology looks like the Firefly.

[DougH]

There was another type of self-split discussed at ax84 at one time involving using the screen to pass signal to the other tube. I don't remember the details now. I got away from self-split circuits because I like class a/b better. 

[kurtlives]

So Doug the self split with pentodes never really went anywhere?

[Johan]

it might be possible to do a para-phase type selfspliting output stage, but I have never seen it and someone has to try to find out... ( look at GibsonGA-40 for example of para-phase)
j

[frequencycentral]

Gibson GA40, very poor schematic though: http://www.freeinfosociety.com/electronics/schematics/audio/gibsonga40.pdf

A better one: http://www.harpamps.com/gibson/pg_0181.jpg

[Johan]

same as your but better still.. http://www.schematicheaven.com/gibsonamps/ga40.pdf  .. and this fenter too http://www.schematicheaven.com/fenderamps/deluxe_5a3_schem.pdf  and this page.. http://www.freewebs.com/valvewizard/paraphase.html
j

[DougH]

So Doug the self split with pentodes never really went anywhere?

Chris had a self-split Hi Octane with EL34's but I never did anything with it.

[frequencycentral]

So Doug the self split with pentodes never really went anywhere?

Chris had a self-split Hi Octane with EL34's but I never did anything with it.

Yeah, I think it must be this one: http://ax84.com/media/ax84_m141.pdf

[Johan]

..thinking a little more about it, the paraphase might not work for a selfsplit...the tubes suck current through the OT and it might not show on the anodes as voltageswing...but I dont know...hmmm.
j