Bias Question (Carr Hammerhead)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by lgehrig4, Feb 25, 2012.

  1. lgehrig4

    lgehrig4 Silver Supporting Member

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    I just picked up a Hammerhead combo and wanted to try different tubes that I had laying around.

    I know the amp is fixed biased but since I wasn't sure of how well my tubes matched I still plugged in my bias probe to check on this. When I checked the first pair (Mullard Reissue) I got 60mV / 65mV and the second pair (Groove Tube) I got 65mV / 69mV. The plates were about 360V.

    I've never biased EL34s that high and I was wondering if it just read that high because it is fixed biased and perhaps my probe wasn't measuring the same way it would with other amps....or.....the tubes just run too hot in this amp??

    It sounds great and there are no weird noises. What prompted me to post this is that I just popped in a new pair of GT EL34Ms and there is a burning smell coming form them. They aren't glowing abnormally, but the smell combined with that high bias had me concerned.
     
  2. Blue Strat

    Blue Strat Member

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    First, some terminology:
    Fixed bias amps MUST be adjusted when installing new tubes. Some fixed bias amps have adjustment pots, others don't, but they ALL must be adjusted (by changing resistors in the ones with no pots).

    Cathode Bias (aka self bias): Requires no adjustment though is often set incorrectly by the designer in order to achieve a particular tone. Usually, when incorrect, they're set to run too hot and will run through tubes more quickly than a correctly biased amp.

    I'm guessing that your amp is Cathode Biased.

    Since the plate voltage is relatively low (under 400V) you're probably ok. Here's how the calculations go:
    You want to be at or below 100% of maximum power dissipation for your EL34s. For EL34s this is 25 watts (Google "EL34 data").

    Power=P
    Voltage = E
    Current = I
    Power = ExI

    65ma = . 065 Amps

    .065 X 360 = 23.4 watts which is obviously under 25 watts.

    The 360 Volts is actually not the correct number to use because the cathode voltage (which is always > 0 in cathode biased amps) must be subtracted first. This is likely in the 20-30 volt range. This will give you an even lower power dissipation calculation.
     
  3. lgehrig4

    lgehrig4 Silver Supporting Member

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    Thanks. Yes, I meant self bias
     
  4. Davidg1974

    Davidg1974 Member

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    for what its worth and this is not meant to bash but from what I can tell GT is just selling chinese and russian tubes with their logo and charging a premium price.I highly recommend JJ tubes from the supplier of your choice but if those GT cant handle your amps bias system (too hot) you may wear them out really quick or worse.I have heard nothing but great reviews about Blue Strat (Kano Tubes) there too if u want to look at getting some NOS tubes-seems to be one of the good honest dealers out there.
     
  5. lgehrig4

    lgehrig4 Silver Supporting Member

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    David, all you said is true but missed the mark with me. I actually tried JJs, Mullard Reissue, GT EL34LS and now the EL34M's. The M's sound the best in this amp so far so thats all I care about. I have to many amps and I don't gig so tube life is not an issue with me. I agree with you about Blue Strat. I have purchased tubes from him on numerous occasions.

    I don't know if the GT's can't handle the amp. I posted because I noticed how high all the tubes I tried were biased and the M's started to give of a burning smell. That said, they were the only new tubes of the bunch so some stink is expected.

    My favorite new production EL34 are Winged Cs. I think I preferred them to any other tube I've tried except maybe one amp. They are a bit expensive though and can only be ordered.
     
  6. Davidg1974

    Davidg1974 Member

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    Sorry, I didnt read that close enough- you obviously know what your doing and didnt need my two cents. I concur with tube life,I just want the best tone I can get and definitely would like to try the winged-c's sometime.Please excuse me if I get preachey on JJs but they literally are what got me playing again because after going through tons of hard earned money my sound still sucked compared to some amps I used to have before they became "vintage" and could no longer afford them. I educated myself on amps and tubes with the intention of building or modding my way into tonal nirvana-only to discover that allthose different brands and types of tubes I had bought were actually the same tube with a different label.Then I tried JJs from eurotubes and the difference was unbelievable and I have played regularly with a smile ever since.:D
     
  7. Blue Strat

    Blue Strat Member

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    ..and the best new production tubes too. ;)
     
  8. Blue Strat

    Blue Strat Member

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    GT also sells JJ's as their own brand, for a higher price. Most guitar amp tube dealers sell at least some JJ's.
     
  9. lgehrig4

    lgehrig4 Silver Supporting Member

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    I didn't mean it that way. I just wanted to be clear so the thread wouldn't go in that direction.

    I like JJs too, but in this case they weren't my choice for tone. I like to buy these from Bob too. The only time I don't is when I need them same day, then I drive down to Sam Ash. I bought KT66s from Bob for my Bluesbreaker clone. Up to that point I had tried the Gold Lions, Shuguang and Ruby and the JJs were the best sounding IMO....and least expensive too!
     
  10. jamesrsmith

    jamesrsmith Supporting Member

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    kca is a great source for nos rectifier tubes,good available el34s like siemens etc, that are better lasting than most new stuff, nos 6L6s,, and especially phase inverter position nos GE, sylvania, rca ,ef86 nos tubes, tubes that you need in a noisefree slot. i use thetube depot for some stuff, and for new production i just use eurotubes, and play the tubes within the 90 days period and usually that time period will weed out a "bad" new production tube that made it through the checkpoints they have in testing. I WONDEER HOW JJ KT77S WOULD WORK IN THAT AMP? fairly low cost replacement. if i had to have an el34 tube ( i have a bunch closeted) i would buy a pair of kt77s , they have a nice hair around the note .
     
    Last edited: Feb 26, 2012

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