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ECC 83 / 12AX7A Valve question

Discussion in 'audio' started by kjb, Jan 12, 2022.

  1. G T Audio

    G T Audio Trade: Manufacturer and Distributor

    I'm not sure you understand what has been said above about these different valves/tubes. It's not just about gain. It's about plate voltage, grid voltage, amplification factor, plate resistance, transconductance, plate current, load resistance, inter electrode capacitances to name a few. Each valve has different parameters (see their data sheets) and the designer will have taken those parameters into consideration when designing the circuit. You can't just swap these valves around as it will affect the performance of the circuit. Best to study the data sheets, or speak to an electronics expert to see what will be affected. It's no different to buying a high performance car optimised for 19" wheels and then trying to fit 21" wheels and tyres to it yourself and expecting it to perform as well. Of course it won't perform as well because the suspension and chassis have been optimised for 19" wheels. My advice is to keep the circuits and the choice of valves exactly as the manufacturer intended. Remember, just because the valve/tube can be taken out and replaced by the user doesn't mean it should be.

    As a manufacturer 95% of the problems we see are user inflicted...
  2. kjb

    kjb pfm Member

    Thanks Graham: you are 100% right! My knowledge and understanding of electronics could be written on the back of a stamp - hence the rookie questions.

    The top plate on the Leben says ECC33/5751 and reading more knowledgeable folks comments in the past its been suggested the amp can be "enhanced" with a few tube adjustments. I was also told this by John at Midlandaudio who sold me the amp and is the Leben distributor.

    I've been tapping into the wisdom of those here exactly to avoid being one of the 95% who wrecks their kit through ill advised tinkering.
  3. G T Audio

    G T Audio Trade: Manufacturer and Distributor

    I am sure the top plate of your Leben says: ECC83/5751... :)
    kjb likes this.
  4. Juancho

    Juancho pfm Member

    Another vote for Sovtek 12AX7LPS which I've never found microphonic in hifi usage. Also Watford Valves ECC83 Harma STR sound great and are low noise /microphonics. They're my go to tubes for new builds.GE ECC83/ 5751 is also excellent if you want the best, but at higher cost
  5. paulfromcamden

    paulfromcamden Baffled

    Tonbridge is brilliant. Very sad that the Feb show has been cancelled due to plague.
  6. The Bish

    The Bish pfm Member

    If your Leben does take 5751s I can lend you some different types to try out from my stash, drop me a PM if you're interested. I find if the circuit does support a 5751 in place of an ECC83 it can make a subtle and sometimes desirable change to the sound.
  7. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Well said Graham. I wonder, does it boil your piss as much as it does mine to be constantly helping people out with definitive technical advice based on a lifetimes professional experience only to see someone who doesn't know a capacitor from a carrot try to countermine your advice and based on "what Dave the bricky on the wam said" or "what Steve from my hi fi dealers (who got a second in modern history and politics) said":rolleyes:
  8. kjb

    kjb pfm Member

    Hence my further queries. The advice given by folks who have a lifetime's experience in electronics isn't always 100% clear to those of us with 1st class and post graduate degrees in Humanities subjects. I could see the key points but was trying to clarify the detail to avoid buggering up my kit or wasting my cash.

    "boil your piss" is a fabulous phrase btw!
    Cato and Arkless Electronics like this.
  9. gints

    gints pfm Member

    Ok, question from other side. Is there any amplifier which is designed for and sold with 5751 valve, or KT77 valve? I tried to look up, give it up after half hour. All what I find is that KT77, 6CA7 can be replacement for EL34, no amplifier models sold with those valves fitted originally.
  10. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    EL34, KT77 and 6CA7 are roughly equivalents to one another. Michaelson and Austin TVA10 and Grant Lumley GL50A used the KT77 as standard fitment and FWIW at least some Marshall guitar amps used it at one point. There will be plenty of American amps that used the 6CA7. EL34 is a pentode whereas the 6CA7 and KT77 are beam tetrodes.
    KT77 (GEC/MOV) was intended as an improved replacement for the EL34 and could handle a bit more power without self destructing. This is why Marshall used it for a short while when, IIRC, they were having reliability issues with some of the EL34's they could still get their hands on when the likes of Mullard etc stopped making them. They were much more expensive than the EL34.

    No doubt there will be American amps that use the 5751 but it was never common here and either ECC81 or ECC83 would generally have been used in Europe and circuitry designed to suit.
  11. chartz

    chartz If it’s broke fix it!

    I find GE 6CA7 more rugged than the EL34 (the real one).
    Last edited: Jan 14, 2022
  12. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Not GE? GEC UK, GE USA.... Sorry, being pedantic:rolleyes::)
  13. gints

    gints pfm Member

    I see, they are older amps. Is there any new one with KT77? I have Unison Research Simply Italy and couple pairs of KT77. Read somewhere somebody got into problems with KT77 in this amp, so afraid to use them in my. I did not see myself, do other manufacturers write in manuals if you can use amp with substitute valves or you definitely cannot? Previous owner sold it with additional 6CA7, so looks that those are fine.
    I have used 5751 instead of ECC83 in other Unison Research amp, did not notice anything wrong. I had impression that 5751 no more different to ECC83 like, for example, EL34 and KT77.
  14. chartz

    chartz If it’s broke fix it!

    Yes GE USA. My mistake ;) and I stand corrected.
  15. paulfromcamden

    paulfromcamden Baffled

    In fairness to @kjb retailers frequently describe 5751 as equivalent to ecc83/12ax7. It was news to me that they weren't so I've learnt something :)

    edit: wikipedia lists the following variations:

    12AX7A, 12AD7, 6681, 7025, 7729, B339, B759, CV492, CV4004, CV8156, CV8222, ECC803, ECC803S, E2164, M8137.

    (plus 5751 (which we'll discount) and 6851 (presumably likewise?))

    Which of these are actually direct equivalents that could be used?
  16. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    It should be fine to use KT77's as they were intended as an improved replacement for EL34's. However.... the real thing was only made by GEC/MOV and they stopped making them a long time ago and NOS one's will be VERY expensive. I wouldn't trust anything being sold as KT77 these days at all and prob will be rebadged EL34's or 6CA7!

    Do check bias and adjust as necessary if it's a fixed bias amp.

    5751 is close enough to ECC83 to work in most instances but things will be not quite as intended by the designer in most cases. It gets technical as to what the issues are so I'll leave it at that:)
    kjb and gints like this.
  17. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Check the datasheets for them! Many of those are eg special quality or military versions of the ECC83 and certainly the majority of those are direct equivalents. It is not uncommon to find variants of many valve types which are electrically the same in the characteristics of the valve itself but have different pinouts or heater voltages.
    paulfromcamden likes this.
  18. Barrymagrec

    Barrymagrec pfm Member

    Equivalent listings can be a mixed bag, for instance the old Mullard Maintenance Manual gives EL37 as a direct replacement for a KT66 despite gm being nearly double. Probably usually works though.
    Arkless Electronics likes this.
  19. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Indeed yes. A popular series pass valve in regulated PSU's at one time. Obviously some valves have exact direct replacements, maybe a rival manufacturers version of a popular valve or a special quality version, whilst others are just somewhere near and back in the day it was commonly known which were "near enough" as an emergency field replacement, even if the equipment would sometimes not be performing to full spec with the wrong valve.

    Some applications for a valve (and indeed most devices!) are far more critical than others and just because you could get away with replacing "X" with "y" in one application does not mean it's a suitable replacement in all applications. Only a thorough understanding of which parameters really matter under the actual operating conditions can lead one to a sensible choice of non standard replacement and even then some adjustment of resistor values or rewiring of valve base can be required.
  20. Perart1

    Perart1 pfm Member

    This is so so true. Having spent half a lifetime with tube amps and tube stuffing with sometimes unpredictable results; I have learnt that there are too many variables involved for me to predict sonic outcomes. In simple terms; you would not let a butcher perform heart surgery would you? Arkless and you have summed it up well. The trouble is that people would rather listen to their mate on social media than take the trouble to understand basic electronics and then talk to someone who has relevant knowledge and experience.
    Arkless Electronics likes this.

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