Cathode resistor 91 ohm on 1974x ?

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musicheals
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Cathode resistor 91 ohm on 1974x ?

Post by musicheals »

Hi all,

I've seen in the schematic of the Marshall 1974X a cathode resistor of only 91 ohms. How is it possible?

Thanks
musicheals
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Bieworm
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Re: Cathode resistor 91 ohm on 1974x ?

Post by Bieworm »

musicheals wrote:
Mon 05/31/21 4:01 am
Hi all,

I've seen in the schematic of the Marshall 1974X a cathode resistor of only 91 ohms. How is it possible?

Thanks
musicheals
The original Marshall power transformer had a different HV than the current ones that we use...
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Re: Cathode resistor 91 ohm on 1974x ?

Post by musicheals »

....so the voltage on the plates should measure way under 300 volts?
Im the old schematic there's a 125 ohm 8O
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Re: Cathode resistor 91 ohm on 1974x ?

Post by Bieworm »

musicheals wrote:
Mon 05/31/21 4:52 am
....so the voltage on the plates should measure way under 300 volts?
Im the old schematic there's a 125 ohm 8O
You should land on 180 Ohms value territory with the normal 18W type transformers. I think the guy that advises you on putting in less than 150R is afiliated with some tube merchant. Better keep them EL84's in a safer zone than max or more...
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Re: Cathode resistor 91 ohm on 1974x ?

Post by musicheals »

Yes, I agree.
The original 1974X with a 91 ohm will never put out 18 watt. I think about 15 watt or less is more realistic. But it will run in real class A I think...
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Re: Cathode resistor 91 ohm on 1974x ?

Post by JMPGuitars »

The value of the cathode resistor in a schematic is not really the question. The question is what your tube dissipation is with the cathode resistor in YOUR amp. You want to target 75% to 85% dissipation. Typically that's around 180Ω and 12V at the cathode with modern source voltages, but YMMV. You need to calculate the bias to know what your real values should be.

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Josh
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Re: Cathode resistor 91 ohm on 1974x ?

Post by geoff 1965 »

yeah that 1974X re-issue is a different beast! it has a HT of 275-0-275 with a higher current rating as well of 150mA probably related to that crazy 91R biasing.
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Re: Cathode resistor 91 ohm on 1974x ?

Post by musicheals »

Yes, it will burn out the tubes quickly
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Re: Cathode resistor 91 ohm on 1974x ?

Post by Bieworm »

Best case scenario they cr@p out at home...
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Re: Cathode resistor 91 ohm on 1974x ?

Post by musicheals »

....my believe is, Marshall uses the russian 6p14p. They got 14 watt. With a 275-0-275V PT it will be around 50 mA at class A
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Re: Cathode resistor 91 ohm on 1974x ?

Post by zaphod_phil »

musicheals wrote:
Tue 06/01/21 6:10 am
With a 275-0-275V PT it will be around 50 mA at class A
But the Marshall 18W is a warm-biased Class AB amp!
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