Cathode resistor 91 ohm on 1974x ?
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Cathode resistor 91 ohm on 1974x ?
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
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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Re: Cathode resistor 91 ohm on 1974x ?
The original Marshall power transformer had a different HV than the current ones that we use...musicheals wrote: ↑Mon 05/31/21 4:01 amHi 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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Re: Cathode resistor 91 ohm on 1974x ?
....so the voltage on the plates should measure way under 300 volts?
Im the old schematic there's a 125 ohm
Im the old schematic there's a 125 ohm
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Re: Cathode resistor 91 ohm on 1974x ?
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...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
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Re: Cathode resistor 91 ohm on 1974x ?
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...
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 ?
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.
Thanks,
Josh
Thanks,
Josh
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Re: Cathode resistor 91 ohm on 1974x ?
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 ?
Best case scenario they cr@p out at home...
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Re: Cathode resistor 91 ohm on 1974x ?
....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 ?
But the Marshall 18W is a warm-biased Class AB amp!
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