The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    A caution about individual power tube-biasing



    Posts : 200
    Join date : 2011-08-05

    Re: A caution about individual power tube-biasing

    Post by zx on Mon Dec 21, 2015 9:56 pm

    I use Bob carvers...... tube DC restorer circuit.......
    Bob was glad to help me setup my Dynaco M3s....I use SS diodes....not the 6AL5...that work an sound a little better......Bob states.....but this is one of the best thing I have ever done to any tube amps....
    I got this setup in about 10 amps now......neat mod to do too any tube amp.....

    Bob Caver well respond to be a little time....never let me down.....great tube lover....I sold his amps in the 80-90s.....Hell..... Bob winds his own tranfourmers..... an thay sound great...........well my old ears......
    Mr Pass.. like Mr carver an Jim McShane ... are glad to help Diyers....

    Bob uses only one bias pot on 6ea....KT150s.........output has helpt with my bias some info on how Bob setup his amps...but theres a lot more info on the web.........have fun with tubes

    One of the nifty things about the Cherry 180 is that it retains a retro hobbyist flavor by allowing the end user to tweak the bias current. The manual specifies a range from 60 to 150 mA with 100 mA being the starting reference point.  Bob Carver on his speakers prefers a setting between 80 and 100 mA. I clearly preferred 100 mA to 80 mA because of increased soundstage dimensionality. I performed most of my listening at 120 mA, but noted minimal sonic differences between settings of 100 and 120 mA. As far as the feedback switch position, I preferred the classical setting when driving difficult loads such as the MartinLogan Summit X ESL. The higher feedback resulted in a much cleaner and sweeter sounding midrange.

    The DC Restorer  
    Carver says he copied the DC restorer from old TV circuits with tube video amplifiers. "Those sets needed to deliver the DC component of the video signal to the CRT all the way down to DC. As used in my amp, this circuit reduces distortion by a factor of three as well as the tube dissipation by approximately the same amount. It works by keeping the DC component on the control grids the exact correct value over the whole signal swing, getting rid of the need to operate the tubes at very high currents (even at low level operating conditions just to get them to work right). The main thing that causes non-linearity in tube amps is that the screen voltage drops when the amp is driven, not so much because of low idle current to begin with (though higher idle current does make for lower distortion, but the trade is a bad one). It takes lots of idle current to drop the distortion a substantial amount, so the DC restorer is a better choice than lots of idle current. I get lower distortion at eight watts idle per tube than at 28 watts idle per tube."

    The restorer is simple yet ingenious. It uses a 6AL5 dual diode. Each diode section services one bank of power tubes. The 6AL5 cathodes are connected to the control grids of the push-pull power tubes which are held at a nominal fixed bias of -47VDC. The diode plates are at an acquisition threshold of -56VDC. Bob admits that some aspects of the DC restorer operation are somewhat mysterious to him, as they are to me as well. My take is that the circuit aids significantly in recovery from hard cutoff conditions. Under those conditions the KT88 grids act as rectifiers and shift the effective DC level below -56V, to the point of causing the 6AL5 to conduct momentarily and pull the DC bias back to its nominal value. Bob estimates the lifetime of the 6AL5 as about 50 years. "I know that seems wrong, but 6AL5's are ubiquitous in tuners, and 50-year old tuners always have these tubes and they still check as new. There are so many of them in this world that a guy on eBay sells a string of them for three cents (each tube) to be used as Christmas tree lights."

    Thanks for the site Bob....

    Last edited by zx on Tue Dec 22, 2015 6:26 am; edited 2 times in total

    Posts : 942
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: A caution about individual power tube-biasing

    Post by peterh on Tue Dec 22, 2015 4:18 am

    GP49 wrote:Biasing individually allows you to set quiescent current exactly the same for both (or both pairs of) output tubes, even if they require different bias.  However this does not guarantee that under SIGNAL conditions, the drive required by the tubes will match.
    Correct. That's done with the use of matched tubes where not only DC BIAS but
    also Gm AT WORKING VOLTAGE is equal.

      Current date/time is Tue May 22, 2018 4:06 am