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Post subject: Bias stting and how to mid 90's Twin
Posted: Thu Nov 08, 2012 7:56 am
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Hi Folks,
Looking for the bias setting and best way to bias this mid 90's Twin. 4-6L6's.
Any info out there?
Thanks
Tube Dude


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Post subject: Re: Bias stting and how to mid 90's Twin
Posted: Thu Nov 08, 2012 9:47 am
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Joined: Tue Jun 29, 2010 1:20 pm
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Twin or Twin Reverb?

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Post subject: Re: Bias stting and how to mid 90's Twin
Posted: Thu Nov 08, 2012 6:28 pm
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Yes, I was serious, it matters which one that you have. Some people say "Twin" when they mean "Twin Reverb", which of course are two totally different amps. Glad that you figured it out. :)

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Post subject: Re: Bias stting and how to mid 90's Twin
Posted: Thu Nov 08, 2012 9:06 pm
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The TRRI has no output tube test points for bias measurement, use bias probes to measure current (Plate or Cathode). The voltage from the bias pot is really irrelevant for setting the idle bias.

Measure Plate volts DC at red or blue wire of OT primary (connects to pin 3 of output tube), negative meter lead on amp chassis. Calculate desired idle percentage of the tubes max Plate wattage dissipation rating. Typically between 50% and 70% of the tubes max Plate wattage dissipation rating is the target. Most 6L6 are 30 watt, some 5881 are 23 watt, some are 25 watt, etc. Check the manufacturers rating. 50% of a 30 watt tube would be 15 watts, etc. Generally speaking, 50% = more headroom and longer tube life, 70% = earlier breakup and shorter tube life. Sound desired is probably most important.

Plate volts x Plate current = Plate watts, or Plate current = Plate watts/Plate voltage.

Choose the % that you want to use and calculate the desired Plate wattage dissipated at idle, measure the Plate VDC, and plug the numbers into the formula above to calculate the necessary Plate current. For example, 50% of a 30 watt tube with 440 VDC Plate voltage would give; Plate current = 15 watts/440 VDC, so the Plate current for one output tube would need to be .034 amps or 34 mA of current. Which means that you want to adjust R71 to achieve 34 mA of current for one output tube. For 70% in the same scenario, 47 mA would be the target current. Note that when adjusting the current, the Plate voltage will also change, so remeasure and recalculate after any adjustment.

After initial adjustment, recheck Plate voltage, recalculate and adjust if necessary to achieve the desired idle wattage. After playing the amp for some time, recheck the setting before buttoning up the amp, as some drift might occur. The same procedure can be used for any amp, and some amps have test points to measure Cathode current.

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Post subject: Re: Bias stting and how to mid 90's Twin
Posted: Thu Nov 08, 2012 10:36 pm
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Hey shimmilou, who you talking to man? Are we off our meds again? :lol: :lol: :lol: :lol:

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Post subject: Re: Bias stting and how to mid 90's Twin
Posted: Fri Nov 09, 2012 7:18 pm
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Well,.....no one else will talk to me.... :o

:lol: JK

The OP started sending email instead of posting in the thread, and I was answering on the thread, the last post so that others could see the same thing that I sent in email. I finally gave up and started answering via email. We're working on a completely different amp now. It's OK, I guess. :)

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Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Bias stting and how to mid 90's Twin
Posted: Fri Nov 09, 2012 7:23 pm
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:lol: :lol: :lol: :lol: :lol:

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