Univox output section. I don't understand.

[Axtman]

I really don't understand this Univox amp output section. I am used to seeing Fender type output sections where the control grids and plates each have their own resistors instead of sharing resistors.

[Helmholtz]

What is the question?

[g1]

The resistors you are wondering about are not shared. The 2 power tubes on the left do not have grid stoppers, the 2 on the right have them.
The 2 power tubes on the left have plate resistors, the 2 on the right do not.
I have sometimes also seen amps that have screen resistors on some of the power tubes and not on the others.

[Helmholtz]

All these resistors: grid stoppers, screen stoppers, series plate resistors have one common purpose. That is to avoid instability/HF-oscillation,
Paralleling tubes increases the risk.
Obviously is was sufficient in this amp to not implement all these measures to each of the tubes.

[Axtman]

I'm considering making the output section more like a Fender amp where the grids and plates have their own individual resistors. Can you see any disadvantage to this procedure?

[Enzo]

Well we know it will work, because Fender has been doing it for decades. But I have to ask the other question: What benefit do you expect to get by rewiring it?

[g1]

I could be wrong but I think more plate resistors will increase the 'sag' of the power amp.
100R's on power tube plates are not just simple 'fusing' resistors like seen in some designs.

[trobbins]

If each 6L6GC could be driven close to Vgk=0V, such that anode current approached the nominal datasheet saturation curve then each anode would see a different loadline. Each anode would see the reflected speaker loading, but the anode with 100 ohm in series would be at a point 30-40V higher on the pentode curve (depending on where operation was on the curve, and given all 6L6 have the same screen and cathode voltages). Probably not a noticeable effect given that aging valves could have significantly different max current capability - but yeh, many would wonder why would you go to that effort to change it for starters.

As far as differences go, at that time it may have been about how long it took to wire up each amp on a construction line, and the schematic may not tell all about practical locations of wires and parts - it wasn't all about reducing part count.

[melvin]

The amp needs to be stable. They tend to blow all kinds of stuff otherwise, and that may be why the plate resistors and grid stoppers are there. Perhaps on a smaller model they weren't needed, so when they added two more 6L6's and changed the OT, they had to solve problems. Layout is critical. Because I love to change things and see what it sounds like, I would add the additional grid stoppers (at the pins) and remove the plate resistors. But be safe and keep in mind it could become unstable and you might have to replace something that was original to the amp (if it burns up). So I would test it thoroughly.