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Post subject: '65 DRRI Power Tube Biasing Question...
Posted: Sun Apr 16, 2017 9:42 am
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I have just purchased a used 2015 '65 DRRI with very few hours on it judging by its condition. I bought the amp as a back up and to use as an acoustic amp with my D-18. The amp came with the Fender factory (I believe) pair of GT-6V6-S, BLUE #1 power tubes. Since the tubes are at the 'very low headroom' end of the spectrum for GT-6V6-S power tubes, I wanted to check the power tube bias to see where the amp bias was set. With my left hand tied behind my back, my Bulgarian eBay purchased octal bias probe and my cheapie multimeter I got the following readings:

Plate Voltage via digital multimeter (octal pin # 3 slot - red probe; octal pin # 8 slot - black probe): 465V

Bias Pot Range measured on Bias Probe: -29.4V min; -43.5V max

If i do the math (70% Plate Diss x 14 Watts) / 465V = 20.9mA

Obviously something is not right here... Does anyone have any insight as to what I need to do to correct my approach so the I get readings that make sense so that I can ensure a proper range setting for the amp?


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Post subject: Re: '65 DRRI Power Tube Biasing Question...
Posted: Mon Apr 17, 2017 1:37 am
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
If your probe are good and your meter too, they look ok to me , you reading are right.

The tube ( one or both same ? ) bias, is a bit hot bias at 29.4v
You write Volts, bias probe read milli- volts ( mv) not volts and it is same as milli- amp ( ma) here.

Bias should read 21 ma or less at plate voltage 465 V. Your calculation are right
Turn down bias negative voltage pot to read close to 21ma and check plate voltage ( 465V)
Do new reading at each new pot adjustment.

Use this ;
http://www.tedweber.com/webervst/tubes1/calcbias.htm

465 volts plate voltage look too high for a 6V6GT, schematic show 391 Vdc !
_______________________________________

You write;
" Since the tubes are at the 'very low headroom' end of the spectrum for GT-6V6-S power tubes ".

Do you mean amp lack of headroom ?
Amp may need some news tubes 12XX7s and 6V6GT ?
I don't like the high plate voltage 465 V


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Post subject: Re: '65 DRRI Power Tube Biasing Question...
Posted: Mon Apr 17, 2017 1:52 am
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Joined: Wed Mar 31, 2010 12:48 am
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stratele52 wrote:
465 volts plate voltage look too high for a 6V6GT, schematic show 391 Vdc !


+1

You should see 396 VDC at TP1 and 391 VDC at TP37. The original AB763 DR's plates don't even run that high (+/- 415 VDC).

Double-check your PV measurements.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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