'68 JMP50, screen grid resistors, grid stoppers ???

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ivan H
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'68 JMP50, screen grid resistors, grid stoppers ???

Post by ivan H » Fri Jan 22, 2021 7:51 pm

So, after much looking & wishing, I finally scored my "grail" amplifier, a '67/'68 JMP50 (1987, serial No.11225) in decent condition, all original iskras, mustards & RS SM's, red lacquer/dye still on 90% of the solder joins, original iron.
The previous owner had it outfitted with NOS Mullard (60's era) CV4004 12AX7's & a matched duet of '65 Rohen Telefunken winged plate "00" getters EL34's.
I also have a NOS closely matched duet of matching date code (B2J3) 1962 Blackburn Mullard "00" getters xf2 EL34's & a few matched duets of 70's era Blackburn Mullard "0" getter xf2 EL34's.
Now I know that old production valves of this calibre can cope with the stresses of being run without screen grid resistors (or grid stoppers), as was done in the early EL34 Marshalls. My question to the good, knowledgeable folk here is,
Would it be prudent to fit suitable (Ohmite or Welwyn) screen grid resistors in the interest of safe & prolonged service life of the output valves, also possibly (Iskra) grid stopper resistors for stability, or simply leave "as is" in the interest of originality? What say ye?
Any opinions greatly appreciated. Cheers

Tazin
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Re: '68 JMP50, screen grid resistors, grid stoppers ???

Post by Tazin » Wed Jan 27, 2021 3:33 pm

You shouldn't need screen grid resistor or grid stoppers for the 50 watt amps since they only have roughly 425 vdc on the plates which modern output valves can handle.

ivan H
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Re: '68 JMP50, screen grid resistors, grid stoppers ???

Post by ivan H » Wed Feb 03, 2021 8:06 pm

Thanks Tazin, very much appreciated.
Rather than plate voltage, I was thinking more of screen grid current draw during heavy plate conduction, when the plate's positive potential falls below that of the screen grid. I do know that the old production EL34's have no real problem handling this though & was just seeking opinions.
That said, I won't be fitting new production EL34's any time soon. Cheers

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Re: '68 JMP50, screen grid resistors, grid stoppers ???

Post by Roe » Sun Feb 21, 2021 10:45 am

I recommend at least 470r screen resistors with a 420v screen voltage, and even more resistance with higher voltages
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ivan H
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Re: '68 JMP50, screen grid resistors, grid stoppers ???

Post by ivan H » Mon Apr 05, 2021 8:27 pm

Thanks for the reply @Roe. I just noticed it as I haven't dropped in for a while.
The amp now has the Mullard double halo xf2 EL34's in it, biased at just a smidge over 60%, so yes, screen voltage is in the vicinity of 420V. I don't ever "dime" amps, volume is usually around 6~7, at most 8.
With these NOS output tubes, do you still recommend screen grid resistors be fitted??? Thanks for your input. Cheers

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Re: '68 JMP50, screen grid resistors, grid stoppers ???

Post by danman » Tue Apr 06, 2021 6:50 pm

By the time you have the volume knob up around 7-8, your power tubes and power amp are likely pushing full power at that point. Adding screen resistors will only serve to extend the life of those valuable tubes. Mullards are tough but excessive screen current can cause damage to any tube. Tone change should be negligible with a low value like 470r.

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