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    Valve replacement on 30th TM36 Anniversary edition

    chet
    chet


    Posts : 2
    Join date : 2018-04-09

    Valve replacement on 30th TM36 Anniversary edition Empty Valve replacement on 30th TM36 Anniversary edition

    Post by chet Mon Apr 09, 2018 9:32 am

    Hi guys,

    Would appreciate your advise on valve replacement on my TM36.
    Last weekend when I was playing all of a sudden I lost volume. I found it was very hot and closer inspection showed 2 outer valves are somewhat discolored. I guess I need to replace these now. I would like to know the below.

    [1] Can I replace all 4 valves with 4 matching valves with same PC and TC ?
         Now its configured with EL84 PC:26 / TC:6350 and EL84 PC26 / TC:6025

    [2] Do I need to match PC & TC to original ?

    [3] Can I change to EL34 ? ( It appears EL34 is more warmer )

    [4] I would like a warmer / creamier  tone on clean. What valves would you suggest.

    thanks  Smile ,

    Chet
    bordonbert
    bordonbert


    Posts : 1772
    Join date : 2015-01-28
    Age : 72
    Location : Southern England

    Valve replacement on 30th TM36 Anniversary edition Empty Re: Valve replacement on 30th TM36 Anniversary edition

    Post by bordonbert Mon Apr 09, 2018 10:47 pm

    No, no, no, no, no! affraid Just read the thread above or below this one by Lateralus - "Anode Fuse Blown". He has just gone down this very route, believing someone's cockeyed idea of "equivalent" valve replacement to make the amp sound better and he is now looking at repairs.

    Assuming we are talking about only your EL84s here, just replace the outer pair with a matched pair of EL84s from any reputable manufacturer, NO NOS things of unknown age and dubious condition even though sold "tested", no exotic types which you have been told are "equivalent" because it is "common knowledge" (it may be common but it sure as Hell isn't knowledge), no following the insane recommendations of someone who did the valve equivalent of swapping his Honda Civic injection system for a twin barrel Holley carburettor and achieved a 0-60 time of 4.7secs.

    As a high level electronic engineer for my entire working life I have to admit, I have no idea of what you mean by "PC" and "TC". And whatever they stand for it isn't relevant to this amp. There is no requirement for any parameter in the valves you put in other than they are match pairs of EL84s. A quad is great but isn't even necessary as long as the outer pair and inner pair are bought as matched within that pair.

    And where you got the idea to change for an EL34 from I can't imagine. It is a totally different valve in a totally different holder. You couldn't fit one if you tried. Start up your learning curve by reading that other thread and use your own common sense with what you read there!


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    chet
    chet


    Posts : 2
    Join date : 2018-04-09

    Valve replacement on 30th TM36 Anniversary edition Empty Re: Valve replacement on 30th TM36 Anniversary edition

    Post by chet Tue Apr 10, 2018 8:45 am

    Bordonbert, thanks for your reply. I will stick with EL84s and without any experiments. PC & TC is written on the tube. PC stands for plate current and TC has some other term. Not sure these parameters matter. I thought EL34 is 9 pins but found out its not.
    bordonbert
    bordonbert


    Posts : 1772
    Join date : 2015-01-28
    Age : 72
    Location : Southern England

    Valve replacement on 30th TM36 Anniversary edition Empty Re: Valve replacement on 30th TM36 Anniversary edition

    Post by bordonbert Tue Apr 10, 2018 2:48 pm

    I've got you.  They must be specifying the conditions during the test.  The other will be Transconductance then.  It's a parameter which is wrapped in with Anode Resistance and Amplification Factor.

    I'll point out here that, contrary to "common knowledge", a valve doesn't have a "Gain".  It does have a parameter called "Amplification Factor", (you will sometimes see Gain Factor), which is tied in with the two other parameters through a formula.  You can't simply consider one of those parameters without bringing in the others.  If a valve has an amplification or gain factor of 100 it does not mean that you get a gain of 100x when you put it in a circuit.  Other components determine the gain of a stage, the valve's parameters do put limitations around how the circuit can behave but they aren't usually in overall control, unless the circuit is badly designed!  You design circuitry to remove the influence of things like valve parameters as much as possible otherwise you end up with a weak circuit that needs a selected component to work predictably.  Witness the state of re-valving amps from "a certain manufacturer" which, due to their chosen bias setup, have to have a specially selected range of valves supplied for the most part by the manufacturer themselves.  Poor design according to any engineer, good design according to the marketing men of the parent company.

    PC and TC are irrelevant for your needs.  As long as the valves are matched in pairs then the TSC takes care of everything else.


    And the attached pic shows the EL84/EL34 comparison.  You can see why they are not equivalent.  That is to scale.  Try getting an EL34 in an EL84 socket!!!
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    Newcastle Brown, can sure smack you down
    You take a greasy wh*re, and a rollin' dance floor
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    Rock On Humble Pie

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