Parallel output tubes/valves in push pull. Advantages/disadvantages?

[Matt.B.H.]

If this has been covered before I cant find it all in one place in the archives.

What are everyones thoughts on parallel outputs? I intend to parallel some 807/6L6 type valves in AB2 triode. Although discussion on all types in general would be appreciated.

Apart from the advantages of possibly better output transformer and averaging of characteristics. What are the other possible advantages and most importantly disadvantages?

Cheers Matt.

[jfitz57]

Disadvantages: more tubes = more sockets. Advantages: many.
Including, if you put enough tubes in parallel you don't need an output transformer.

Jim

[ray_moth]

Another disadvantage is the risk of current hogging but this can be minimized to some extent by adding a resistor of 100 ohms in each plate circuit.

[jfitz57]

Yup. But not as bad as SS. Anyway, I wish people would say "This is for a home stereo or a guitar anp or a pirate radio transmitter modulator".
Matt B.H. ?

Jim

[Matt.B.H.]

Sorry, yes its for home use. I know 80-100W is lots but I already have the PS transformers and valves/sockets etc. I just need to get some output transformers.
I have read that some people think parallel outputs degrade the sound. I can only see advantages though.

Cheers Matt.

[Irakli]

Input capacitance of the stage increases when you parallel the tubes. So, You need to make sure that driver stage can provide enough current to drive output stage.

[kenpeter]

Well, as Don Smoking Amp once explained it to me:
A triode is any number of parallel triodes internally.
Any one path by itself would have perfect Mu and
parallel curves that never "lean over to the right".

Its the average of those many slightly different
triode paths, each with slightly different cutoff
thats makes for the leaning curve set you would
typically see. And more so, the less truly parallel
and well controlled the internal pathing.

Now your beam power tube dumbed down to a
triode probably has really good internal parallel
sameness, like a high quality triode. But start
pluggin in multiple tubes, you can expect more
and more the leaning over curves of the triode
with sloppier internal construction.

I would look into Triodlington, where a single
high quality triode provides 1/(Mu-1) plate to
cathode feedback path for a power transistor's
collector to base.

Thus loaning the sand: high input impedance,
low output impedance, and proper triode rule
of Mu. Without having to parallel a bunch of
real tubes to get required transconductance.
And avoiding degradation of the triode curve
in the blender.

Like magically duplicating single malt Scotch in
a current mirror. Would it be better than a huge
but blended barrel of the real thing? I don't know
when sandy mirror magic might be the better or
worse than a blended reality?

If you are going into AB2, you can only use
the PNP (Sziklai CFP) version of Triodlington.
Else forward grid current gets multipled too.
But transconductance of such a pairing is so
high, no need or reason to operate into AB2...

[jfitz57]

Originally Posted by kenpeter

no need or reason to operate into AB2...

Oh yeah, that too. AB2 and complex waveforms (music) don't go together.
Go class A (it's been done forever) or class D (it's almost never been done with tubes).

Jim

[kavermei]

As Irakli said... you will need one hell of a driver to drive two parallelled grids in an AB2 configuration. Even in AB1, many PPP amps sound bad because the driver is inadequate (i.e. the designer took a PP design and simply added output tubes, disregarding the driver stage. Don't do that.).

Kenneth

[kenpeter]

I was not suggesting any problem with AB2, except that NPN Darlington arrangement
would multiply grid forward currents. PNP Sziklai would have no such troubles.
Was the only point about AB2 I was trying to make earlier... but I can expand on it...

I'm not against AB2 with music, except it usually takes sand to drive it properly. And
then you might just as well have thrown your sand into a Triodlington, and have more
transconductance than you could ever dream. You can do both of course, but its sort
of pointless. Once you have nearly unlimited current of sand under the firm voltage
control of one really good triode, what reason did we need the added hassle of tryin
to drive AB2 on both sides of the forward current knee?

If your talkin' one of those transmitter triodes thats in AB2 all the time, the knee is
maybe not a problem. But you still need big current to drive it. I think you still end
up throwing sand at the problem of driving a triode act like sand with Mu, so why?
Triodlington operated below the grid current knee is a better deal all ways round...