Marshall grid resistors? There, not there

[drewl]

I think I asked this before, BUT-. Working on a killer '67 ish Marshall Superlead Tremolo 1959T.

Two tubes have 1.5K input grid resistors, the other two tubes don't.
Anyone know why?

My '69 Plexi has none. No screen grids either
My '73 also has no input grids, but does have the 1K screens.

[Enzo]

Stability. Keeps the two tubes from interacting.

[Helmholtz]

I think I asked this before, BUT-. Working on a killer '67 ish Marshall Superlead Tremolo 1959T.

Two tubes have 1.5K input grid resistors, the other two tubes don't.
Anyone know why?

My '69 Plexi has none. No screen grids either
My '73 also has no input grids, but does have the 1K screens.

The same with my '67 1959 SLT and my '68 Super Lead: Grid stoppers only for one pair.

My explanation: Grid stoppers are mainly used to avoid HF oscillation, as they produce a low-pass filter effect in conjunction with the tube's input capacitance.
Stability is influenced by the components' arrangement and lead dress. It is known that paralleling tubes increases the risk of instability, mainly because the additional tubes and wiring cause more unwanted (parasitic) coupling.

They probably found that while a single EL 34 pair in their 50W amp didn't require grid stoppers, adding another pair did. So they just used grid stoppers for the additional outer pair as a minimum cure.

[Chuck H]

They probably found that while a single EL 34 pair in their 50W amp doesn't require grid stoppers, adding another pair does. So they just used grid stoppers for the additional outer pair as a minimum cure.

That's the best hypothesis I've read on the matter. I've wondered "why only on one pair" myself. This makes sense.

[Enzo]

We see something similar in screen resistors.

[Helmholtz]

We see something similar in screen resistors.

In vintage Marshalls?

[nsubulysses]

I think he's talking about Traynors and Garnets. Maybe some old ampegs too

[Enzo]

I had other brands in mind. Peavey for example had screen resistors on two of four power tubes in various models. Music Man in some models. Others. The OP seemed to be asking about power tubes that were not all four wired the same.