Bias Resistor Increase = lower gain?

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geoff 1965
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Re: Bias Resistor Increase = lower gain?

Post by geoff 1965 »

Make yourself a dummy load with a large 100W wirewound resistor the same impedance as your speaker i.e.8ohm,16ohm.
90546B83-C4D5-4BEC-AFB2-9CC725E9911F.jpeg
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junior marbles
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Re: Bias Resistor Increase = lower gain?

Post by junior marbles »

I already have one of these. Perfect.
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Re: Bias Resistor Increase = lower gain?

Post by junior marbles »

geoff 1965 wrote:
Wed 12/01/21 7:25 pm
if you don’t mind give us some reference voltages with the stock 91R bias resistor i.e. B+ at first capacitor,plate,screen & cathode voltage of the el 84’s.
With line voltage at 117.5VAC I measure:
* 329VDC @ CT
* 314VDC @ plates
* 302VDC @ pins 9
(Forgot to measure @ cathode)
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geoff 1965
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Re: Bias Resistor Increase = lower gain?

Post by geoff 1965 »

Colin’s 1966 amp in the downloads shows 325V at first cap so that is very close,thanks for posting those voltages.
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colossal
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Re: Bias Resistor Increase = lower gain?

Post by colossal »

geoff 1965 wrote:
Sat 12/04/21 10:14 am
Colin’s 1966 amp in the downloads shows 325V at first cap so that is very close,thanks for posting those voltages.
But we should also consider the Mains AC for the measurement, if trying to make some connection between historic reference amps and today's copies. Junior's mains is running a cool 117.5VAC. Most places in the U.S. run 122-125VAC and on a PT wound for 117 or 120VAC input, the secondary is going to run hot.
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geoff 1965
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Re: Bias Resistor Increase = lower gain?

Post by geoff 1965 »

That’s the reason I have 275-0-275 HT colossal,not to simulate colin’s amp or the 1974X but to get the sweet spot B+ with the higher wall voltage.
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colossal
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Re: Bias Resistor Increase = lower gain?

Post by colossal »

I just mention that because the 1974X diverges from what we consider the 18W Lite foundation. I will be interested to see what Junior Marbles reports for his cathode voltage in his 1974X with the stated conditions.
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Re: Bias Resistor Increase = lower gain?

Post by zaphod_phil »

Just going back to the question in the thread title,No, since the power tube cathode resistor is bypassed with a cap, the resistor's value will have virtually no effect on gain.
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