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Post subject: '65 DRRI Power Tube Biasing Question...
Posted: Sun Apr 16, 2017 10:13 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: Wed Apr 26, 2017 9:04 am
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If they are factory tubes, its likely that the amp is biased correctly.. Blue GT from the factory at 21 sounds about right. They run em cool for warranty.
If you are measuring from the power tube socket with the tube removed, the B+ voltage will read artificially high. Removing the tube removes some of load on the power supply and B+ goes up.
I'm not sure of the math you're using, but mathematical bias is seldom chosen out of blind trust that that is the best sound. The factories seldom bias hot enough for proper tube operation. They are interested in long lasting tubes for few warranty returns. Musicians like hi bias for better tone. You can pump em up to the ragged edge of red-plating for the best sound. But keep spares around.
The Ebay search probes that come up are voltage probes, not current probes. Lets assume you have a 1 ohm resistor in the voltage probe. Then voltage read equals milliamps.
I usually bias 6V6 to 25-27mA. You would read .027 volts DC. Its a good starting point. Keep it under 35. Let your ears tell you what sounds best. Let the bias probe numbers tell you if its safe. Have fun


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Post subject: Re: '65 DRRI Power Tube Biasing Question...
Posted: Wed Apr 26, 2017 11:19 am
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TimsAudio wrote:
Let your ears tell you what sounds best. Let the bias probe numbers tell you if its safe. Have fun


+1

I usually bias 6V6GTA's at 22 mV to 23.5 mV but I'm always overcautious when it comes to output tubes.

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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Post subject: Re: '65 DRRI Power Tube Biasing Question...
Posted: Thu Apr 27, 2017 8:53 am
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My 1973 RCA tube manual describes the 6V6 with a maximum design capability of 350 volts.
The typical class A push pull operation at 285 volts plate and 70mA for two tubes. This is a hot enough bias to push it into class A operation. We'll call that the hottest bias that is rated safe.
But Leo Fender was building amps that routinely went to 415 volts on the plate. And he warrantied them.
This is about 150% of typical operation and 120% of maximum rating.
Applying those percentages to current rating, we get .050 for typical operation and .042 for maximum.
The tube manual also lists Class A operation for one tube at .045mA. So it can take a higher current than ratings would suggest.
Now why would you want to go this high? My studies into output transformers, inductance and permeability of laminations suggests that high bias improves the articulation in the OT for better tone.
A DC current passed through an inductor with a ferrite core will align the molecular structure of the iron in the laminations to raise the permeability of the iron. This is what an OT is.
Permeability is the ability of a material to hold a magnetic field. By raising the permeability with high bias, the OT couples its primary and secondary closer to get better coupling and better tone.
Pushing the specs was Leo's key to success. What are you gonna do?


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