Re-biasing my Cornford Amp

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Paul Skidmore, Oct 18, 2004.


  1. Hi Folks,

    You might have seen my thread in the Amps & Cabs section asking about replacement tubes for my Cornford Hurricane Amp. At the moment its got 4 12ax7wa preamp & 2 EL84's (all Sovtek). I spoke to Derek at Watford Valves who told me that the design of the Hurricane means that I will need to get the amp re-biased if I want to put in different tubes. This is due to the sovtek's drawing less power & the fact that the amp would need a 200k resistor?

    I'm not as technically minded about this as I should be (the Hurricane is my first valve amp). Can someone try to explain this to me? How do I get the Amp re-biased & what exactly is involved?

    Thanks a lot for your help!,

    Cheers,
    Paul
     
  2. loverocker

    loverocker Member

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    First I'd call up Cornford and ask (does the manual not mention anything?).

    I can't recall if the Hurricane is fixed or cathode bias. The method of rebiasing is different for each. If it's cathode biased, the cathode resisitor may have been chosen to avoid the need for rebiasing. So you'd just pop in two reasonably well-matched EL84s and go. If it's fixed bias, there's a pot somewhere inside that needs to be adjusted (the rebiasing) when the new valves are in place. It is a DIY thing if you're familiar with valve amps, otherwise it's a simple job for a tech.

    If it does need rebiasing, here's a tip: order a matched quad of valves now (not just a pair). Use two of them now and rebias the amp for them. Then, when you need to replace them in the future, just use the two you have left. Because they were all matched at the start, you won't need to rebias at all. Saves a trip to a tech :)
     
  3. Mike Hansen

    Mike Hansen Member

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    It is cathode biased. Never had a problem with any decent tubes in mine, be it Sovteks, JJ's, or NOS stuff.
     
  4. Thanks for the replies Guys,

    I believe Derek was saying that the cathode resistor was 100K & that if I put any decent valves in (his recommendation was Tungsram EL84) they would not work properly & get damaged? (Although Mike you've obviously not had this problem - what valves did you settle on)

    I'll give Paul Cornford a ring, the manual's a bit of a flimsy affair ;-)

    Cheers,
    Paul
     
  5. Mike Hansen

    Mike Hansen Member

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    I've got JJ's in there now and they seem fine. Please let us know what Paul says.
     
  6. loverocker

    loverocker Member

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    >100K

    The cathode resistor would be something like 120-150 ohms normally. Assuming he meant 100 ohms, that is quite low for a typical 2xEL84 amp and that's probably his concern (possibility of burning out valves pretty quickly).

    But it'd be a strange choice by Cornford to opt for a resistor value that was marginal. :confused:
     
  7. Laugh at my electrical knowledge 100K, 100 ohms they're different right?!?! ;). Anyway I'll get the detail from the man himself & let everyone know.

    Cheers,
    Paul
     
  8. loverocker

    loverocker Member

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    Just a smidgen: 100K = 100,000 ohms :)

    I'd be interested in what Cornford says. I'm using 120ohm and 150 ohm resistors in my homebrewed Marshall 18Wers (which are also cathode-biased 2xEL84 amps).
     
  9. ChrisB

    ChrisB Member

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    Hi, have a Hurricane myself and recently asked Martin Kidd a similar question.
    (top bloke btw for answering all my silly questions so quickly)

    The amp is cathode biased class AB.

    When asked about replacement valves he said

    TBH, I have no idea what quiescent current and 385volts of high tension voltage is and would have liked to try NOS valves but he designed it so I don't really fancy going against his recommendation.

    I have used JJ's in it since I bought it second-hand about 2 years ago. I can't even remember what the Sovteks sounded like.

    One thing I did do was change the speaker to a G12H-30 and I find it much more pleasing to the ears than the stock V30.

    Cheers
     
  10. Thanks for the response Chris, appreciated!

    Cheers,
    Paul
     

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