Grid Stopper Values on 1940s Masco Guitar Amp?

[misterc57]

I am finishing up my refurb of a 40s Masco Map-15 guitar amp. 2 of the inputs each have a 220K grid stopper resistor. Preamp tubes are 7F7.

Wondering if I should lower the grid stopper to 68K and what difference that would make (if any) to the input signal.

The attached schematic has been edited by me to show the 7F7 preamp tubes. Other schematics out there have 6SL7 preamp tubes.

Thank you, MC

[Helmholtz]

Lower value grid stoppers will result in extended HF response and lower noise (hiss).

I would use 33ks.

[Jon Snell]

What is shown are not grid stoppers. Grid stoppers are there to stop instability and would be fitted between the grid pin and the grid leak resistor 21.

[Helmholtz]

What is shown are not grid stoppers. Grid stoppers are there to stop instability and would be fitted between the grid pin and the grid leak resistor 21.

That depends on definition.

These resistors act as grid stoppers in the sense that they form a low pass filter together with the Miller capacitance.
Ideally they would be directly connected to the grid socket pins, but that only makes a difference at MHz frequencies.

It is insignificant if the grid leak connects before or after the grid stopper.

[Enzo]

I consider them mixing resistors. Allows both inputs to that channel to be used at once. A lot more common in those days than it is now.

[Leo_Gnardo]

I consider them mixing resistors. Allows both inputs to that channel to be used at once. A lot more common in those days than it is now.

Misterc57, what Enzo said ^^^ makes most sense. Now, you can decide if your modded Masco "really" needs 2 inputs. Does it? If not you can bust the circuit down to a single input and do away with both 220K resistors. But it may be wise to include a stopper - as Jon Snell suggested - between the grid leak resistor (which also acts as a load on the output of your guitar/cable input) and grid. "What value stopper?" I've used as little as 8K2, other times 10K, 22K, 33K, and for those who must have their Fender favorites 68K. Best answer is "the minimum value it takes to suppress radio interference." I have to add, having no stopper can result in the guitar volume pot getting scratchy - and adding a stopper seems to put a stop to it.

[Enzo]

Unless you used the low level jack on a fender, the two 68k were in parallel, so it was really a 34k stopper.

[Helmholtz]

I consider them mixing resistors. Allows both inputs to that channel to be used at once. A lot more common in those days than it is now.

Yes, these resistors decouple the two sources, avoiding that one source loads the other.
E.g. inputs 1 and 2 could be left and right channels of a stereo source.

But they also roll off the highs above 5kHz assuming a typical input capacitance of 150pF.

[Chuck H]

Since inputs 1 and 2 are feeding the same grid I vote for the typical Fender Hi/Lo arrangement. 68k resistors. For a net parallel resistance of 34k as Enzo noted. That adds a little versatility and no dead inputs on the front panel. If, for whatever reason, it must be stock for the restoration (and questioning the value of these resistors is just for educational purposes) then the subject is moot. Just wire it as shown in the schematic.

[misterc57]

Since inputs 1 and 2 are feeding the same grid I vote for the typical Fender Hi/Lo arrangement. 68k resistors. For a net parallel resistance of 34k as Enzo noted. That adds a little versatility and no dead inputs on the front panel. If, for whatever reason, it must be stock for the restoration (and questioning the value of these resistors is just for educational purposes) then the subject is moot. Just wire it as shown in the schematic.

This is what I ended up doing. 68K resistors. Net 34K.

The 220K that were there sounded pretty good. Not sure I am hearing any difference with the 68K. However my hearing is not all that good.

Thank you all for the input!

MC

[Helmholtz]

As/if the amp doesn't have a switching input jack wiring like a Fender or a Marshall, two 68k resistors won't result in 34k net, but rather 68k on each input.
That's what I had in mind when I suggested two 33k resistors in post #2.

But there shouldn't be an audible difference.

[misterc57]

As/if the amp doesn't have a switching input jack wiring like a Fender or a Marshall, two 68k resistors won't result in 34k net.
That's what I had in mind when I suggested two 33k resistors in post #2.

But there shouldn't be an audiible difference.

Yes I should have mentioned that all 3 jacks are switching.

[Helmholtz]

Yes I should have mentioned that all 3 jacks are switching.

They might be grounding but are they wired as with a Fender amp?

If they are only grounding, the 68k resistors will mutually load the other input.

[misterc57]

They might be grounding but are they wired as with a Fender amp?

If they are only grounding, the 68k resistors will mutually load the other input.

Hmmm. Not sure and the amp is buttoned up and put away.

I know that they are grounding and I read 34K across each 68K resistor. I did not change the design, just the value of those 2 resistors.

[Helmholtz]

I know that they are grounding and I read 34K across each 68K resistor. I did not change the design, just the value of those 2 resistors.

That will be so with both wirings.

But with a typical Fender/Marshall wiring the inputs have different sensitivity and input impedance.

As this is a receiver, the 2 inputs may be equal sensitivity, but that would mean mutual loading with shorting jacks.
With such wiring, lowering the 220k resistors without changing the jacks's wiring actually isn't a good idea.