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Post subject: Another Bias Question
Posted: Tue Oct 18, 2016 10:58 am
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Just over a year ago, I had a local amp technician replace the power tubes (matched pair of Tung Sol 6V6GT) in my DRRI. He told me that he had set the bias to 60% maximum dissipation. When I measured the current using my simple bias probe it read 22.

This gives a plate voltage of 327v (unless my calculations are wrong!). This seems very low compared to what I have read about DRRI plate voltages and I was expecting above 400v. Does 327v seem reasonable for a DRRI or could the tech have made a mistake in his calculations?

I do not want to venture inside the amp to measure the plate voltage so was wondering if there is another way to determine what it is. We know that V=I/R so could I measure the resistance of the tube and knowing the current (22mA) calculate the voltage? If so, what pins on the tube do I use to measure the resistance with my multimeter?


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Post subject: Re: Another Bias Question
Posted: Tue Oct 18, 2016 11:56 am
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Location: Province de Québec, Canada
Deluxe Reverb RI schematic said 391 VDC for plate voltage.

Weber bias calculator said 25 ma for 70% power dissipation.
22 ma look a good bias.

http://www.tedweber.com/webervst/tubes1/calcbias.htm


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Post subject: Re: Another Bias Question
Posted: Tue Oct 18, 2016 12:08 pm
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fifty4, use Power Equation for biasing. You need two measurements. The anode plate voltage and the idle bias current. Idle bias current is equal to the voltage drop across 1-ohm resistor tied from cathode-to-ground.

P (watts) = I (amps) times V (volts DC on plate)

To be accurate wrt your amp, you need to measure the plate voltage. Pin 3 (red lead of DVM) to ground (black lead). Every amp is slightly difference. Fender schematic places the nominal plate voltage around 391VDC. Expect your plate voltage to be around +/- 20% of that value.

EG: say your plate voltage measures at 400VDC and the bias mV = 22mV = 22mA. Then, the idle power = (0.022) times (400) = 8.8 watts per 6V6GT. This is about [8.8] divided by 14 = 0.628 or 62.8% of suggested max.

There are bias probe tools that not only measure the cathode-to-ground mV, but also plate voltage. These are nice, as you don't need to remove the chassis to do the measurements.

HTH. Good luck!


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Post subject: Re: Another Bias Question
Posted: Tue Oct 18, 2016 1:03 pm
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Thank you for the replies.

Do you know the name of the bias/voltage tool and where I might be able to get one. I am in the
UK.


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Post subject: Re: Another Bias Question
Posted: Tue Oct 18, 2016 1:07 pm
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fifty4 wrote:
Thank you for the replies.

Do you know the name of the bias/voltage tool and where I might be able to get one. I am in the
UK.


I use Amp Head bias tester. I can read cathode current and plate voltage.
The Dual bias Tester- MPD , I can't see it anymore on the web :(
I bought it in 2010

http://www.amp-head.com/index.php?cPath=21


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Post subject: Re: Another Bias Question
Posted: Tue Oct 18, 2016 6:00 pm
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I use the Euro-Tube Pro 1.

https://www.eurotubes.com/store/pc/bias%20probes.htm


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Post subject: Re: Another Bias Question
Posted: Wed Oct 19, 2016 1:01 am
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BMW2002Ti wrote:


One thing I don't like about these kit is if digit module go bad you must trow away all the kit .
I like the ones we can use with a separate voltmeter.


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