Grid Load resistor / Phase inverter Resistors sizes.

[pleasantbullet]

I've got a bunch of old Carbon Comp resistors that are in good condition.

Values like 4.7M, 2.2M, 1.5M, and so on. For instance, i have a pack of 50 4.7M 1/2 watts.

How much difference would a value from 4.7M to 1.5M make if you used them for the grid load resistor at the input stage. instead of 1M

Similarily, the 1M resistors typically used at the grids of the Phase inverter, is anything above 1M safe to use?

[potatofarmer]

Sounds like you're going to be the king of grid-leak biasing!

[pleasantbullet]

wtf,

[tony hunt]

wtf,

5C1

[pleasantbullet]

thanks tony, what about the phase inverter?

[tony hunt]

Never seen a 4m7 Rg on a phase inverter. I guess it is going to mess with the input impedance. Would like to know myself.

[martin manning]

An overly large grid leak resistor will upset the bias point. These old CC resistors may have some uses, but they are few and far between. Have you measured them to see if they have drifted in value?

[matt h]

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[potatofarmer]

IIRC grid-leak bias starts around Rg=2M for 12AX7s. I believe it's lower for other types. Something to look into; maybe you can use those 1M5s after all. You might have to adjust coupling cap sizes though.

Resistors are so cheap, and the grid-leak position doesn't require much in the way of voltage rating or power dissipation, I would even just go to Rat Shack and buy their lousy carbon films if need be... but only if I absolutely couldn't think of anything else to order from a supplier to make the shipping cost worthwhile.

Also if you really really want to, you can always use a couple (or a few) in parallel to get close to 1M. 4M7||1M5 = 1M14, not too bad.

[Firestorm]

Ampeg used large grid leaks (5M6) even though the triodes were cathode biased. No idea what sonic difference they were trying to achieve.

[PaisleyTube]

4M7 works fine but you'll get slightly less loading (more highs and lows) on the pu's

4M7 is also nice in 5ExA circuits as 2nd stage neg.feedback-resistor

etc.

[Leo_Gnardo]

Ampeg used large grid leaks (5M6) even though the triodes were cathode biased. No idea what sonic difference they were trying to achieve.

Less loading of guitar than Fender's 1 Meg, results in a tad more bandwidth, mostly heard as brightness/brilliance. IIRC the very handy MXR MicroAmp also had a 5M6 input impedance. Some complained it made their guitars sound too bright. Substitute a 1 meg & everything's "magically" back to normal.

Couple of days ago I was repairing a 1959 Silvertone, model 1391 I think. Had a pentode input, cap coupled, and 15 meg resistor at input grid. Even with a new film cap at the input, it ran nearly out of control in terms of noises and probable parasitic oscillation. Swapped in a 1 meg and everything settled down nicely, sounds great with guitar or harmonica. Sometimes the reach for bandwidth backfires. Old sound engineer's saying "the wider you open the window, the more s#!t flies in."

[martin manning]

Check this out: https://tubeamparchive.com/viewtopic.ph ... 331#329331
It does not look like you can upset the bias point too much with a large grid leak resistor.

[matt h]

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