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 Post subject: Question On A Cathode Resistor Used To Measure/Set Tube Bias
PostPosted: Aug Wed 18, 2021 5:16 pm 
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One way to measure accurately the current through the output tube of an amplifier is to break the connection between the cathode and ground. It is often advised to put a 10 Ω resistor between the cathode and ground and measure the voltage, use Ohm's Law and figure the current.

My question is, should that resistor be shunted with a capacitor to keep the cathode at AC ground as they do in a lot of small radios? Or is that value so low that it doesn't matter.

I hope that question makes sense.

Thanks.

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 Post subject: Re: Question On A Cathode Resistor Used To Measure/Set Tube
PostPosted: Aug Wed 18, 2021 7:21 pm 
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10 Ohms won't make much difference - it's tiny compared with the plate resistance of the tube. Yes, there's a little bit of negative feedback from it, reducing gain a tiny bit, but it's probably inside another feedback loop which will reduce gain by a tiny bit less than it would have without the resistor...

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 Post subject: Re: Question On A Cathode Resistor Used To Measure/Set Tube
PostPosted: Aug Wed 18, 2021 7:44 pm 
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Tom Bavis wrote:
10 Ohms won't make much difference - it's tiny compared with the plate resistance of the tube. Yes, there's a little bit of negative feedback from it, reducing gain a tiny bit, but it's probably inside another feedback loop which will reduce gain by a tiny bit less than it would have without the resistor...

Thanks Tom, I figured as much. I appreciate your input.

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 Post subject: Re: Question On A Cathode Resistor Used To Measure/Set Tube
PostPosted: Aug Wed 18, 2021 10:37 pm 
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A lot of the guitar amp guys insert 1 ohm resistors from the cathode to ground. It makes no discernible difference, and allows for pretty simple calculations...!

73...Jordan VE6ZT


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 Post subject: Re: Question On A Cathode Resistor Used To Measure/Set Tube
PostPosted: Aug Wed 18, 2021 11:05 pm 
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A number of commercially designed amplifiers like the Eico ST-70 have unbypassed cathode resistors with jacks to do measurements while setting bias, and they work fine that way. I don't think you could possibly hear or measure any difference if they were bypassed.

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 Post subject: Re: Question On A Cathode Resistor Used To Measure/Set Tube
PostPosted: Aug Thu 19, 2021 12:23 am 
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Bias set with no signal? Amount of AC on cathode negligible?

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 Post subject: Re: Question On A Cathode Resistor Used To Measure/Set Tube
PostPosted: Aug Thu 19, 2021 1:16 am 
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arbilab wrote:
Bias set with no signal? Amount of AC on cathode negligible?

Yes, the bias would be set with no signal. I guess there is minimal AC loss (with a signal) across the bias checking resistor.

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 Post subject: Re: Question On A Cathode Resistor Used To Measure/Set Tube
PostPosted: Aug Fri 27, 2021 7:58 pm 
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Mr. Detrola wrote:
A number of commercially designed amplifiers like the Eico ST-70 have unbypassed cathode resistors with jacks to do measurements while setting bias, and they work fine that way. I don't think you could possibly hear or measure any difference if they were bypassed.
At least one Heathkit power amplifier (W4M) has jacks in the o/p tube cathodes to allow a mA meter to be inserted to set bias. It scared me that the circuit could go open for some reason, sending the cathode voltage sky-high! As insurance, I bypassed each jack socket under the chassis with a precision 10 ohm resistor. Now a simple volt meter allows current balance to be set with no risks.
Cheers,
Roger

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