Screen grid resistor help on old marshall

[aaronz28]

Hi guys,
got a question about screen grid resistors.

i have 3 vintage marshall amps, 2 of which do not have screen grids across pin 4-6

I just picked up another 68 Plexi and since my others sound so great without the resistors, I' removed them, and wired the OT directly to pin 4.

now i'm blowing main's fuses.

the 2 main power supply caps are original, and i've ordered replacements, but i want to be sure i haven't overlooked something else before replacing original parts.

on my other amp, i found that the choke had failed and my tech "compensated" by adding 1k1w resistors to prevent the tubes from arcing and blowing... i can hear a difference in my particular amps, so i'd like to run without them - as I use NOS mullards and they can handle the high voltages.

i guess my question - if the power supply caps are failing, would the addition of screen grid resistors prevent the mains fuse from blowing?

i suppose i could put the resistors back and see what happens, I just hate doing unnecessary soldering

thanks

Aaron

[aaronz28]

UPDATE -

i put the 1k1w screen grids back and the amp no longer blows fuses, but it really doesn't sound as good.

any ideas?

Thanks

Aaron

[Firestorm]

I just picked up another 68 Plexi and since my others sound so great without the resistors, I' removed them, and wired the OT directly to pin 4.

OT doesn't go to pin 4; choke (Vscreen) goes to pin 4.

Is that what you really did? Or did you mistype?

[Andy Le Blanc]

After you figue which pin to wire the plate to the tube socket...

measure the difference between the plate and the screen grid.

just about all tube sheets put the screens equal to or less than the plate.

going without screen grid resistors is ok if the screens are at a potential that
is less than the plate under all operating conditions, if the potential swings relative to the plate
you set up a current that as the condition worsens will
see the screens acting as the plate which will eventually cook them...

some very old designs have no screen grid resistors, its not unheard of..
but many recommended tube applications put 100r resistors on the screens and plates..

Id took a good look at the voltage conditions through the powerside at idle
before you make the assumption, I'm a little surprised it didn't bite you sooner

I wouldn't go without the resistors but you might find a low value might work towards what your listening for,
the relationship between the plate and screen will change with the bias condition of the amp so your gonna have to pay attention...

[aaronz28]

i misstyped.

its all wired correctly -


bottom line - it blows fuses w/o screen grid resistors, but does not blow Mains fuse with them..

problem is - it sounds better w/o the resistors

Thanks

A

[Firestorm]

If I had to guess, I'd say the main filter (likely two 50uF caps paralleled) is leaking enough current to take the total draw (at the AC input) above the 2A fuse rating when the amp is run without screen Rs. If you have to replace those, you really should replace all the electrolytics down the power rail. Just a matter of time before they all do something unpleasant.

[Roe]

usually you need 1ks with el34s. if the voltage is low you might try 470R or higher though

[Gaz]

Most Marshalls would sound subjectively better without the screen grid resistors, but removing them almost guarantees very shortened tube life, even with the Mullards. It was a unwise to take them out regardless of the fuse thing, and besides, I'm sure you'll be able to compensate somehow. Haha, and I'm sure them amp still sounds amazing.

[67plexi]

Use this 1k 7 watt resistor.

http://www.mouser.com/ProductDetail/Wel ... Pq5ujQc%3d

I have a room full of Plexi Marshalls and this resistor works well.


Good luck.

[Gaz]

Oooo, those are sexy lookin' ones to boot. You know, if you have no life and think resistors can look sexy.

Use this 1k 7 watt resistor.

http://www.mouser.com/ProductDetail/Wel ... Pq5ujQc%3d

I have a room full of Plexi Marshalls and this resistor works well.


Good luck.

[roberto]

I know that there is a well known italian guitarist that is often on tour with a 1987 Marshall without screen resistors, and only cheap chinese EL34 can survive in it. The sonic result is like removing a carpet from the cabinet, and an amp that is more dynamic. I don't remember if B+ is around 430 or 460V.

[xtian]

The 1987 schem, attached, shows no screen resistors, right? We're talking about pin 4 on the power tubes.

And the grid, pin 5, is the input. Why is the OP saying "screen grid resistor"?