Bias problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by DownByLaw, Sep 7, 2008.

  1. DownByLaw

    DownByLaw Supporting Member

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    Hi all,

    Recently received my Compu-bias, quite happy with it. Has worked flawlessly.

    However, I took the opportunity to order some -c-'s for my vht clx and cannot get them within range.

    I'm getting readings of 455 plate voltage, 42ma, 19 watts. The bias adjustment will not allow lowering any more. (maxxed out) I've been having dialogue with Doug, (of Doug's tubes) he feels these should bias up fine with any amp under 500 plate volts.

    I threw these in my Carr El Moto, and they biased up fine.
    The previous tubes in the CLX, (Mullared RI) were also within range when I took them out. However, the adjustment was at the very low end of the range. I decided to change them as the amp was distorting too early/much, making untoward noises, and losing definition.

    Any ideas? I can keep the -c-'s for the Carr, but that doesn't solve my problem with the VHT.

    I believe the bias supply was repaired once by VHT some years ago. (I'm the original owner) Could this be faulty again? Does this sound like the symptoms?

    Thanks,
    Patrick
     
  2. WaltC

    WaltC Member

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    sounds like the bias circuit is the problem (again?). give VHT a call ...

    a competent tech should be able to fix it, especially with VHT support.
     
  3. Blue Strat

    Blue Strat Member

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    Changing the bias range resistor may be required.
     
  4. DownByLaw

    DownByLaw Supporting Member

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    Well, I popped the old RI Mullards back in.

    I get:
    466pv - 37 - 17.2 watts (1st probe)
    466pv - 35 - 16.3 watts (2nd probe)

    A few times a pop sound was heard through the speakers, even though the master was all the way down. Once again, bias pot was at the lowest end of it's range. These tubes are decently worn, how does that affect what they want to see bias-wise? Should I just go with another set of these, or should I have the amp checked out?

    As you can see, my plate voltage is higher today than yesterday. I've been thinking about a power regulator, would this be worth it for consistency?
    2 days ago it was reading 475pv, or a range of 25pv over a 3-4 day period.

    Thanks,
    Patrick
     
  5. Blue Strat

    Blue Strat Member

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    Any two pairs of tubes can bias dramatically differently. That's why you order matched pairs or quads, because 2 or 4 tubes at random could be as much as 20mA apart.
     
  6. phsyconoodler

    phsyconoodler Member

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    There are some winged C's that draw more current than others.Your bias range doesn't seem right with two sets of tubes now,so you will need to make a range resistor adjustment to get the bias you need closer to the middle of the pots' range.
    Some amps are set up with a 'factory' tube set and swapping tube brands can make you wonder why they don't bias up right away.It is common for different brand tubes to be quite a bit apart as Mike has stated.I usually set my bias supply up so I have a wider range so I don't have these problems when I want to swap tubes.
    This is not a fault with the amp,just a choice that was made when it was built to use a particular set of power tubes.
     
  7. DownByLaw

    DownByLaw Supporting Member

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    Well, both sets of tubes were matched, so not an issue there.

    So, of course after putting the Mullards back in for now, I play for a half hour or so. Sounding quite good I thought, still somewhat worn, but I figure maybe my cleaning of the sockets did a little good.

    So there I am, rocking out, (as usual) and I hear a high pitched squeal between my rock greatness. I figure a preamp tube, or such. I put it on standby for a moment, flip back on, squeal is gone, rock ensues.

    Until I noticed one of my Mullards glowing like a nuclear reactor. Quickly I shut it down. No blown fuses or such.

    Now these tubes biased correctly, and up until this point, other than getting somewhat noisy, I hadn't noticed anything untoward.

    I suppose a tech is in my future, eh.

    I must say, that old saying about the best tone ever before melt down seems to hold some truth.

    Patrick
     
  8. Groovey Records

    Groovey Records Member

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    Well you got out of there before serious damage was done

    I would say near future like before you put it on again
     
  9. DownByLaw

    DownByLaw Supporting Member

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    Well, the saga continues. Called up VHT and spoke with Dave and described the problem. His advice was to first throw a set of the RI Mullards in, as I had them in previously and they were JUST in spec.

    Received the tubes, dropped them in, no luck. Most recent readings are:

    465 plate voltage
    49 mA
    22.7 watts

    That's the lowest I can set the bias. Needless to say, out of spec, and of course the amp sounds like **** at this setting.

    Called up Dave again to explain and get a RA to send it for repair. Don't know what I'm missing, but Dave seems to think the amp is within spec with no need for repair. In addition, he questioned the Compu Bias and saw it as a way to "save money" for folks who don't know what they're doing. Needless to say, I'm pretty disgusted. This is my favorite amp. Currently I own a 2500.00 paper weight.

    Is this out of the realm of someone local (say, Jimmy Somma) to diagnose and fix? Should I just give up on VHT's (lack of) service?

    Patrick
     
  10. slider313

    slider313 Silver Supporting Member

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    Make sure the battery in your bias meter is fresh. As a few have stated before, each set of tubes will draw different amounts of current. Even though they're matched they may not fall within the range of your amp. A resistor change on your bias pot was the best idea.
     
  11. rockon1

    rockon1 Member

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    IF they are 6L6's they are well within range. 42 ma at 455 volts is 65% idle dissapation. If they are 25 watt tubes disregard this post..... Bob
     
  12. Ronsonic

    Ronsonic Member

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    First, a tech is probably in order. There may be a problem like losing a cap in the bias supply or some such.

    Your plate voltage will vary with the amount of current you draw through the tubes. Power supplies are not all that well regulated. Your lower voltages yesterday were with tubes pulling 49 mils today you're drawing less current and not pulling down the supply as much. That's normal and something you expect to see on a healthy amp.

    Squealing noises, popping and glowing plates is not normal. A local tech should be able to handle this. If you're in Florida I can definitely get it sorted.
     
  13. DownByLaw

    DownByLaw Supporting Member

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    Thanks all.

    I guess I'll give Jimmy Somma a call and see if he can take a look at it.
    I'll bring my Compu Bias as well so he can verify it's accuracy. Very frustrating, and the attitude of the VHT tech didn't help. (not the first time I've run into that)

    Patrick
     

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