Parallel SE vs. Single SE Output Question

[valveitude]

I’ve searched the forum, and haven't found an adequate answer to this question.

What are the downsides of running parallel output tubes in a SE amp?

I have ran across some comments here, and elsewhere that suggest it isn’t a good idea, but I haven’t seen a good explanation as to why. Is it the complexity of getting two tubes to operate at the same point (matching), or is there more to it than that?

Thanx,
Casey

[analog_sa]

or is there more to it than that?

Who knows? I have tried matching to within a percent or so but only at the operating point. Didn't help. The sound loses focus and clarity. Sounds even worse than paralleling input tubes. I don't think a convincing explanation exists.

[dave slagle]

I also fail to see how the logic most people give holds. When you look at tubes like a 45, you have two plates and two grids on either side of a VV filament.

Since the tube characteristics have to do with the various element spacings a slight shift of any one of these structures will give you two different tubes in parallel within the same envelope, one could argue they could never be identical yet they never seem to blur

if you look at the way the current is drawn form the plate, you will actually see a "shadow" of the filament with diffuse edges on the plate structure. The fact that the plate pattern is not a line that mirrors the filament suggests that different distances are traveled by various electrons which again nets you varying tube characteristics (ie multiple tubes in parallel) within the same envelope.

Whether you have multiple tubes in parallel in a single envelope, or in multiple envelops, you still end up with a single set of average chatacteristics, but seeing the distince multiple structures sure might give the brain a nudge that blurring of the sound is happening :-)

dave

dave

[dhaen]

Yes, there are many different views and not many facts.
I have, and will, argue that if parallel SE causes loss of "definition" then so should class A push-pull - or any other class of push pull (within the region where both valves are conducting).

[dave slagle]

Originally posted by dhaen
Yes, there are many different views and not many facts.
I have, and will, argue that if parallel SE causes loss of "definition" then so should class A push-pull - or any other class of push pull (within the region where both valves are conducting).

I'll go one step more and suggest that if parallel tubes do indeed blur due to their differences, the PP should be the worst offender, since you are not only dealing with mismatched devices, but you have to add the mismatched driving circuits in parallel, and the mismatched iron that "adds" the parallel signals together :-)

dave

[ThorstenL]

Konnichiwa,

Originally posted by valveitude
I’ve searched the forum, and haven't found an adequate answer to this question.

What are the downsides of running parallel output tubes in a SE amp?

I have ran across some comments here, and elsewhere that suggest it isn’t a good idea, but I haven’t seen a good explanation as to why. Is it the complexity of getting two tubes to operate at the same point (matching), or is there more to it than that?

I am stomped for any specific explanation, but here is some experience.

I once has a 300B PSE Amplifer. It used seperate cathode resistors and capacitors for biasing (maybe a "wrong" approach) and had also seperate grid coupling capacitors and grid leak & grid stopper resistors (actually, the whole thing was switchable PP & PSE, but that as they sayis another story, I preferred PSE over PP).

When I replaced my 90ish db/W/m speakers with some 104db/W/m speakers I decided to see what would happen if I pulled one output valve and merely re-connected the 8 Ohm Speaker to the 4 Ohm Tap, the AMp was cruising at a few 100mW into that load.

This pretty much keeps all variables equal to a resonable degree.

What I found was that with some music the difference between SE and PSE was small enough to be counted under "uncertain if there was any". BUT with some music and some recordings SE had a much greater degree of resolution, a much greater degree of "see-troughness" and "thereness".

That said, I did NOT find the same effect to anywhere near the same degree with a Kondo (ANJ) Kegon, which in comparison to my own Amp uses fixed bias common in value to both valves with individual cathode followers per valve, but we did not test as much. I still personally felt however that overall the Single SE Shinri sounded a little purer.

Equally, the Arthur Loesch preamp circuit that circulated on the Joe-List. This offered two options of line-stage, one with a PSE 6922/6DJ8 and the other with a single section of 5687. Here the variables become very divergent, but having build both versions more than once again I felt the PSE version to sound a little more opaque in terms of resolution, compared to the "single SE".

I cannot offer any reasons why this SHOULD be so, but that is what I have observed (BTW, paralleling power devices, including IC Poweramp's also seems to cause audible problems, but they are more various).

Draw your own conclusions and best do your own testing and listen to what happens in PSE vs SE (and perhaps also PP), it is IMHO a quite instructive and illuminating thing to do.

Sayonara

[valveitude]

Yes, there are many different views and not many facts.
I have, and will, argue that if parallel SE causes loss of "definition" then so should class A push-pull - or any other class of push pull (within the region where both valves are conducting).

Differing views can be a very good thing, but after a while , if no body of evidence shows up, I have to wonder about there validity.

The SE/PP debate that still rages on is a good example. At first the SE crowd had nothing to go on but their ears, and were being told that it was nothing more than the "euphonic" effect of adding distortion. At first blush, this SEEMED to hold water, then the "hard" evidence started to mount. The best treatment I have read to date as to why our ears seems to contradict what our test equipment tells us regarding SE is….

http://www.next-power.net/next-tube/...er/cheever.pdf


…it’s a real eye (ear) opener. I HIGHLY recommend it.

Since the parallel/ single debate has been going on for about as long, and nothing comparable in the way of evidence has shown up, I tend to believe it is an article of faith in the “Simpler is ALWAYS Better” doctrine.

Casey

[dave slagle]

i was just visiting a friend and he had a 4 pillar 45 sitting right there. Upon close inspection it was clear that the grid/filament structure was not even close to being centered inside the plate stucture.

talk about two dissimilar tubes in parallel.

dave

[jlsem]

talk about two dissimilar tubes in parallel.

That's why I prefer planar triodes and also single plate tubes such as EC88 and EC8010 at low levels...

Honestly, though, if you parallel two tubes, you end up with a significantly different animal, so of course it will not sound the same.

On the other hand, I've read stuff where the author dislikes parallel tube sections but recommends parallelling plate resistors for audible improvement!

John

[dave slagle]

Originally posted by jlsem
[B]

That's why I prefer planar triodes and also single plate tubes such as EC88 and EC8010 at low levels...

the argument can also be made for the circular plate/grid/filament structures like many of the eimac tubes. Rather than two distinct tubes, there becomes an infinite number of gradually varying tubes.

Honestly, though, if you parallel two tubes, you end up with a significantly different animal, so of course it will not sound the same.

but the fact that the "new tube" which has 2X the Gm always has the problems seems a bit fishy. plus, why does addign another plate to the EC 8010 more than quadruple its value :-)

dave