Mixing power tubes...

[greiswig]

In a cathode biased circuit (not sure that is germaine to this question), is there any issue with mixing an EL-34 and a KT-77 as a pair? Not sure I'll like it, but I won't even try if there is likely to be some catastrophe associated with it...

[lastwinj]

it is going to be dependent on your OT. usually need to keep the halves current roughly the same, but it can be done.

germ

[David Root]

Probably not, but you could always bias each tube separately to balance the OT primary currents.

[rfgordon]

Here's a link to an interesting TubeCad article on mixed topologies, which is similar to what you're talking about. The two tubes could be biased independently, or even differently.

I'm intrigued by the diagram in this article of the triode connected EL34s (shown with UL pentode EL34s). Has anyone here done the triode EL34 output? The article states that the pair (at 400 VDC B+) would produce about 15 watts. That's a nice low power, but would it still yield the EL34 character?

Thoughts?

http://www.tubecad.com/articles_2002/Mi ... ifiers.pdf

[David Root]

I built one amp with a pentode-triode switch on a single ended EL34, and preferred the pentode mode. Bigger, more dynamic. Probably has something to do with the change in Gm and rp between the two modes.

Haven't tried it in push-pull, but there are others that don't care for triode mode on this forum and elsewhere, for example the editor of ToneQuest magazine, who seems to be a pretty knowledgeable guy.

Having said that, I have in the back of my mind a jazz amp using four PP 2A3s, or maybe 6BH4s, as these old triodes do sound very good if you set them up right.

Maybe it's just the "pseudo-triode" operation of a pentode or beam tetrode that doesn't sound like a true power triode?

[dBe]

Maybe it's just the "pseudo-triode" operation of a pentode or beam tetrode that doesn't sound like a true power triode?[/quote]

***

FWIW, I have been using both pentodes and triodes for years and what you said about a triode strapped pentode/triode is quite true: they don't 'really' sound like true triode. Then again, a triode run in a push/pull circuit is a bit counter intuitive if you are going for the magic of a triode. Triodes are inherent producers of second harmonic distortion. It is what makes them sound so 'round'. Push/pull amps cancel this type of distortion and leave you with a very lean sounding triode. I have SE and P/P amps (for stereo) that use the same output tubes and they sound completely different. These are 2A3's and the P/P amps have very potent bass but do not have the harmonic richness that the SE amps have.

What would be really cool for a jazz amp would be a high output triode, like a Svetlana 572 in SE mode: harmonic color to die for. Even a bunch of paralleled 2A3's would be low power and need enough heater current to run a blast furnace...

YMMV

Dave

[David Root]

Thanx for sharing your experience. My problem with 2A3s in SE is they don't put out much power. I hadn't thought of a 572 in SE, I must admit. Don't they need like 0.8 to 1.0 kV plate voltage though? Safety/liability etc.

[Johnhenry]

David, Check out Carr amp's, his amp's have the triode switch's I think and there may be some MP3's on his site, i know there use to be, I've been checking around on S/E'ed amp's running an EL 34 in the triode mode, I'm building 2 small S/E amp's for myself and I hav'nt found much info on the subject.
Johnhenry

[dBe]

David,

True story about the 2A3's, but that is the richest sounding 2-1/2 watts on the planet

The 572-3 is the one in the family that is really optimized for SE use and is a huge sounding 20 watts at 595V on the anode with about 100-110 mA bias current. A Hammond 282X is a perfect power TX for it. Be prepared to supply a couple of amps to the cathode. DHTs are very demanding in having adequate current to sound good.

The coolest part, besides their sound, is the unearthly glow from their thoriated tungsten cathodes. They lite up like 20W lightbulbs with a very white light. When I'm in my listening room with my Cary 572's making music I use no additional lighting. People would certainly remember a jazz guitarist with a lighthouse for an amp. There you go, I just named your jazz amp line: LightHouse Amps for HeavyWeight sound.

Too slow here at work and I tend to let my mind wander. If you see it, let me know.

Dave

[David Root]

Verrry interestingk, as that little fellow in the German helmet used to say. I shall have to look into this further. I have also been thinking about a tube stereo amp, preamp + monoblocs, but God knows where I'd put them, never mind the folded horn speakers.

We seem to have strayed way off the original topic. Can you also mix (related) tubes in parallel SE? I can't see why not, and that might increase harmonic complexity too.

[dBe]

Absolutely. I haven't done it, but there are others that have and from what I understand the results were very good.

For a small amp, an EL84 and a 6V6 would be a great combo - bite from the EL84 and that murky growl from the 6V6. Subbing a 6L6 for the 6V6 would be interesting.

For a little more hit an EL34 (or derivitive like a KT77) and a KT88 would be very cool: ballsey with a big midrange crunch. A 7591 could be killer with a KT66, too. Sweet and creamy.

Hmmmmmmmmm........................ it boggles my mind, the potential combos!

Dave, too

[greiswig]

it is going to be dependent on your OT. usually need to keep the halves current roughly the same, but it can be done.

germ

germ, I'm not sure I understand this statement...which current needs to be the same?

Also, let me ask another, further stupid question. If you've tried YellowJacket adapters that switch from 6L6's to EL84 tubes, you've heard the gigantic switch in tone. Something in between might be nice. Could you mix one of these with a 6L6 or EL34? The relative output power would be pretty drastic, but what harm might come from that?

[mooreamps]

it is going to be dependent on your OT. usually need to keep the halves current roughly the same, but it can be done.

germ

germ, I'm not sure I understand this statement...which current needs to be the same?

Also, let me ask another, further stupid question. If you've tried YellowJacket adapters that switch from 6L6's to EL84 tubes, you've heard the gigantic switch in tone. Something in between might be nice. Could you mix one of these with a 6L6 or EL34? The relative output power would be pretty drastic, but what harm might come from that?

I have a prototype amplifer that mixes one 6L6 with one EL34 via twin master volume controls. I do something called "PA" voicing. I use the 6L6 for body, and the EL34 for sparkle. Apparently, there is no harm doing it this way, provided you maintain a static current balance between both tubes.

-g

[lastwinj]

germ, I'm not sure I understand this statement...which current needs to be the same?

the current in the OT. each side if the OT needs to have the same amount of current running thru it. some imbalance is ok, and actually is preferred by many, but there is a limit.

just be mindful.

germ