Help with biasing '57 Tweed Twin RI?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mickey64, Feb 22, 2009.

  1. Mickey64

    Mickey64 Member

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    [FONT=Verdana, Arial]I decided that it's about time I learnt a bit more about biasing my amps, [/FONT][FONT=Verdana, Arial]instead of just adjusting for a "correct" cathode current. Can anyone who [/FONT][FONT=Verdana, Arial]knows about these things just verify that what I have done is correct?[/FONT]

    [FONT=Verdana, Arial]I found the GT6L6GE's in my new '57 Tweed Twin RI a bit muddy sounding [/FONT][FONT=Verdana, Arial]and one dimensional, so decided to install a spare pair of Winged C Svet [/FONT][FONT=Verdana, Arial]6L6GC's. I fitted the tubes, and used my Weber Bias Rite to measure the [/FONT][FONT=Verdana, Arial]cathode current, then adjusted the bias pot until it read around 33Ma. I then [/FONT][FONT=Verdana, Arial]measured the plate voltage by using my digital multimeter with the black [/FONT][FONT=Verdana, Arial]probe touching the chassis, and the red on pin 3 of one of the 6L6's. That [/FONT][FONT=Verdana, Arial]read around 480vdc. As the Svets are 30 watt tubes, and 70% of 30 is 21 [/FONT][FONT=Verdana, Arial]watts, I did the calculation 21/480 x 1000 which gave 43.75ma. This [/FONT][FONT=Verdana, Arial]s[/FONT][FONT=Verdana, Arial]eemed a bit high when most people seem to bias for 6L6's at around 30 to [/FONT][FONT=Verdana, Arial]35ma. So, I did the calculation for 60% of 30, ie 18/480 x 1000, as this gave [/FONT][FONT=Verdana, Arial]me 37.5ma, so I set the bias [/FONT][FONT=Verdana, Arial]pot to give an average of this; in fact it was [/FONT][FONT=Verdana, Arial]a bit higher on one tube and a [/FONT][FONT=Verdana, Arial]bit lower on the other, but there's only about [/FONT][FONT=Verdana, Arial]2ma between them.[/FONT]

    [FONT=Verdana, Arial]I want the amp to break up more, rather than go for clean headroom, so I'd [/FONT][FONT=Verdana, Arial]prefer to bias hotter rather than colder. In fact, although the Svets sound [/FONT][FONT=Verdana, Arial]nicer than the stock tubes, I'm planning on trying 5881's in the near future. [/FONT][FONT=Verdana, Arial]However, can the Tungsol 5881, which seem to be a favourite for this amp, [/FONT][FONT=Verdana, Arial]handle what seems to be quite a high plate voltage?[/FONT]

    [FONT=Verdana, Arial]Many thanks for any replies, just trying to get the best out of my new amp! [/FONT]
     
  2. Mickey64

    Mickey64 Member

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    Oh well, I'm either doing it totally right, or so wrong that nobody wants to
    contribute to me electrocuting myself:eek:

    In case anyone is interested, I've gone into this in a bit more detail.

    I went back to the amp today, and tried four different pairs of tubes. The Svets, the stock GT6L6GE's, and two pairs of the JAN Phillips 6L6WGB's. The latter measured more or less the same, so I'll give the readings for the slightly "better" pair. With the Svets and GT's I took some advice and lowered the cathode current. With the Phillips I turned the bias pot to its minimum setting, as this was the only way I could bias them close to what they "should" be. I checked the plate voltage much more often this time. Readings were as follows, and I have given the current on each tube, as they were slightly apart from each other:

    Svets: 32.9ma/34.8ma, plate 470v.; 0.038x470=16.356, ie 65% of 25watts. (Apparently these should be regarded as 25 watt tubes, rather than 30)

    GT's: 32.5ma/34.8ma, plate 460v.; 0.0348x460=16.008, ie 64% of 25 watts, or 53% of 30 watts. (I assume these should also be regarded as 25 watt tubes?)

    Phillips: 35ma/37ma, plate 453v.; 0.037x453=16.761, ie 67%of 25 watts. or 73% of 23 watts. What wattage would you suggest these Phillips tubes are? There is conflicting information on the net!

    So, overall, does all this ring true, ie have I biased these appropriately, and do you think the Phillips can be used safely with these readings?
    I have left the Svets installed for the time being, as they do sound good.

    Thanks for any responses:)
     
  3. phsyconoodler

    phsyconoodler Member

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    Ok,to start with,you are just fine with your bias adjustments.There is no science to it and other than running them too hot,there is no 'right' bias setting.With 480v on the plates you don't want it too hot,especially with 5881's.The voltage is not the problem,it's the higher current that the 5881's can't handle.
    Set your bias to where it sounds the best to your ears.It could be colder too,so don't automatically bias them on the hot side.
     
  4. slider313

    slider313 Silver Supporting Member

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    I think your numbers look good. As for the Winged C I would consider these a 30 watt tube as they carry the 6L6GC designation. The Phillips 6L6WGB is a 23 watt tube and at 70% mpd with the 453 plate volts you would be at 35.5ma, so your not that far off. I like to run them closer to 65% at 32ma. You could change the resistor on the bias pot to give you more range.
     
  5. Rosewood

    Rosewood Member

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    Seems like you have the math correct and doing well, as far as the phillips I don't know for sure but when in doubt I treat them as 25 watters and check for crossover distortion.
     
  6. Mickey64

    Mickey64 Member

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    Thanks very much for responding, it's reassuring to know I'm on the right track here. I didn't leave the Phillips tubes in long enough to make a true assessment, so I'm going to try them again, as I do like those tubes in my Vibroking, so they should sound good in the Twin too. Someone else has suggested they are 26 watters, but it's probably best to go on the side of caution, and treat them as 23 watts, and either change the bias resistor or get another pair of these tubes which will bias at 32ma.

    In the meantime the Svets sound good, and I may end up sticking with them. Thanks again for your help, much appreciated!
     

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