Question about biasing tube amp

Discussion in 'Beginners/FAQ' started by mnemonic, Apr 26, 2018.

  1. mnemonic

    mnemonic Custom User Title

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    Hey guys, pretty noob question about biasing tube amps, I’ve never done this before, I figure I should make sure I’m understanding this right.

    So I bought a little bias probe off eBay that plugs into my multimeter. I tested it with my multimeter and it interrupts pin 3, so when it’s plugged into my multimeter it inserts my multimeter in series with pin 3 (plate).

    So if I understand this right I just set my multimeter to millivolts and boom, there’s my plate current draw, right? I could then just flip to voltage on my multimeter and measure plate voltage right?

    I tested it earlier and after letting the amp warm up a bit (it’s a Jet City JCA50h), it read -5mV (!).

    I turned up the bias control to bring it up to around 25mv which added a lot of volume, made it squishier feeling and also removed a lot of high end and fizz. I admit I didn’t like it as much, it lost some of that sharp attack that I like (and I guess I’ve gotten used to over the last few weeks). I compromised by turning it to 10ma, and just turning up Treble and Presence to compensate.

    My question is... does the above assumption about measuring current on pin 3 sound right? Do 6L6’s even work at 5mA or was I doing something horribly wrong?

    Given my preference for tight articulate modern metal tones, I knew I wasn’t going to be running them anywhere near 70% plate dissipation but 5mA to 10mA sounds laughably low.

    The plate voltage is around 500v I think, I forgot to write it down.
     
  2. Jacksonluvr636

    Jacksonluvr636 SS.org Regular

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    I would consult an amp doctor.

    JK, buy one of the eurotubes probes that reads both plate voltage and Ma. Their website also explains how to do the math. It is not very difficult really but as you probably know can be very dangerous if you are just tinkering around in there.
     
  3. Shask

    Shask SS.org Regular

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    I was trying to remember..... I think on my 2203 clone I put 1W resistors between pins 1 and 8 on the powertubes. That gives an easy place to measure where ma=mv, so a setting of 35mv is equivalent to 35ma. I think bias probes do something similar. I know I have one from Weber, but I haven't used it in probably 10 years, so I dont remember the details at the moment. Usually that is the goal though.... to convert ma (which normally has to be tested in series) to mv.

    They usually say 70% of B+, whatever that is in your amp (400v, 500v, etc....) However, on amps like the 5150 and my 2203 clone, I always preferred closer to 60%. As the bias goes up, it gets warmer, mushier, fatter, etc... and you lose the aggressive edge in the treble. I want to say mine are set about 30ma-32ma or so. I know many people would do 35ma to 40ma, but it got too mushy for me.

    However, if 5mv is really the correct reading, that does sound way too low. I think even a stock 5150 is at 10mv to 15mv or so. I know those JCA amps are known to be set way too low, so I would probably raise it, but running a little on the cool side I think it OK for keeping it more aggressive.
     
  4. mnemonic

    mnemonic Custom User Title

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    I played around with the bias some more today, plenty of safe range in that bias pot. After setting it to 20ma which is still pretty cold (plate voltage about 504v) and re-eqing it to suit that bias setting it sounded really good.

    Then I backed it down to about 10ma and set my eq as I had it before, and to be honest I prefer it biased real low.

    I do have the depth mod, and I’m boosting it and using a seven string tuned to A though, so that extra tightness and aggressiveness is preferred. Also the tubes are really old, not sure if that matters.

    Oh well, as long as it sounds good I guess!

    The eurotubes probe looks pretty nice, but I’m in the UK and shipping would be a bit expensive.

    It looks like probe I have is basically the same as their cheapo probe though.
     
  5. mnemonic

    mnemonic Custom User Title

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    Okay I tried the original tubes in the amp again (some Chinese made ‘6L6WGB’) and with the bias in the stock position (5mA for the old JJ’s) these bias at 13mA.

    Also these tubes sound a lot better. Same as before, I kinda prefer the low bias setting. But these do sound a lot better than the JJ’s when I turn it hotter (yet still cold, around 25mA). These don’t go as mushy and retain more top end clarity. I figure at these low bias settings, despite the tubes probably being 5 to 8 years old, they’re probably not that worn.

    Also worth noting, the JJ’s I think are pretty beat. With the JJ’s I get kind of a static sound (like lightly rubbing the top of a mic with your finger) for the first 10 minutes or so when the amp is first turned on. They did exactly the same when I had them in my recto.

    I will rebias and try this all again when I do buy new tubes, probably next week. Planning on some EH 6L6GC’s, as they are apparently the exact same as the Sovtek 6L6WXT+, which seem impossible to find in the uk.
     
    Jacksonluvr636 likes this.

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