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Thread: KT88 conversion

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    KT88 conversion

    Hello.
    I replaced old EL34 quartet with brand new JJ KT88 in my Sound City 100 amp. Over the common rule max 70% idling dissipation, maybe someone can help me to determine optimal anode bias current for 1750 ohm primary impedance at 500v for KT88 fixed bias,in tetrode connection,please ? The screens are feed it also at 500 v around .Thanks

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    Last edited by catalin gramada; 11-16-2018 at 01:34 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    In lack of informations I started to bias with 30w per tube.It was not such big difference from 20 to 30w the transformer sag a little bit, 4-5 v around so I.m thinking is robust enough and still capable (it.s a Partridge so should be...). I leave it so for a while to see how warm will come and try to push it up. 485 v on the plate at nominal 230v mains.
    I wonder how much can I put into output for 1750 ohm OT load ?

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    Last edited by catalin gramada; 11-19-2018 at 01:49 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    If you're wondering if the transformer is okay, I'd check the heater voltages to make sure they're still up at 6.3VAC...

    Justin

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    The heaters voltage is all right, just 0,1v dropped. As I read it around those Partriges for 100w models are willing to manage 400 mA arround in HT side. 60 ma for each tube meant 240. At this current the tubes will be biased at 28W if substract the screen current. That means 66% from its nominal 42w power, still under max 70%. So may I consider it is proper biased? ... except we didn.t consider a hot power transformer as hot biasing by means...I need a little help, please , the transformer run pretty hot. Thanks

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    Last edited by catalin gramada; 11-19-2018 at 08:52 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    It's not clear how many KT88 have been swapped in place of the EL34 quartet;eg 2 or 4?

    I think that the 70% thing is a guideline for the max limit, not a rule or target. And I don't see how it can be appropriate for what you are doing, ie swapping in a tube type with a higher plate dissipation limit than the amp was intended for.

    What is the amp's low power sine output waveform like (negative feedback loop disabled)? In your situation, the avoiding of excessive crossover distortion seems a far more appropriate indication of appropriate bias than using 70% as a target.

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    Hey, thanks
    All four tubes was replaced. Thinking is not a problem as both works well with the same plate load. But have to set a working point. I asked in a previous post which one should be optimal for actual configuration, cause have no ideea. Found 70% to be most recommended over all internet. Maybe is not, who knows... But to bias a 42w tube for 17,5 at iddle just to keep plate disipation into a target seems very cold. Should be an optimal to be well balanced in respect with the load and voltage applied to the plate and screens
    Think is no matter if you get 20 or 40w with the same tube , it should be settled in respect with I write above. Am I wrong ?

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    Last edited by catalin gramada; 11-19-2018 at 12:21 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Thinking of it in terms of an absolute correct value isn't so useful. Calculate safe limits and then twiddle the bias until it sounds good and is within those limits. Hot isn't always good either. 10 years ago it was all the rage to mod 5150s with bias trimmers as they are biased pretty cold. The rock players found some utility in it and some preferred a warmer bias but a lot of the metal guys preferred is cold as the 5150 sound needs that cold bias it seems. You can argue that it's good to have the option to set the bias wherever you want it but in this case Peavey got it right out of the box, and if people were given the option (and I dare say most lazy techs) then they'd bias it up to 70% as that is the conventional internet wisdom and then wonder why it sounded like crap.

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    I have found that both KT66 and KT88 sound better, even with clean tones, when they are biased a good bit hotter than what "I" typically consider "normal range." The KT88 being a 42 watt tube, I would set my UPPER LIMIT RANGE somewhere between 30-35 watts without too much worry. The KT type is quite rugged by design and seems to benefit from a hotter bias.

    Start out with about 25 watts and assess the tone and the amp's "feel." Too much fuss goes into obtaining some magical dissipation number when in reality a precise bias setting is unnecessary. Not to mention the idle current setting you just made goes right out the window when the first note is struck As stated above, calculate the upper limits and bias until it sounds and feels the best to the player.

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