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Thread: Switchable Class A/AB

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    Junior Member Johno's Avatar
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    Switchable Class A/AB

    I'd like to build an amp similar to the Traynor YCS50 or Jackson Britian. El34 output tube configuaration with switchable Class A/AB, going from 15W to 50W. I have the Traynor schematic but can't follow it as far as what and where is switched for the output tubes. Any links or help would be appreciated.

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    Old Timer defaced's Avatar
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    Could you post the schematic you have? That will make communicating easier.

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    -Mike

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    Junior Member Johno's Avatar
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    I can't post the schematic. It's in multiple parts, but here's the link to the Traynor service manual that has the schematics (it's a 3.5MB PDF).
    http://www.traynoramps.com/downloads...an/smycs50.pdf

    I'm not so interseted in the Traynor amp but how to implement such a switching arrangement.

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    Last edited by Johno; 03-10-2010 at 10:46 PM.

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    Junior Member Johno's Avatar
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    Epiphone does a similar thing in its Blues Custom 30. There I see the screens of the 5881s are switched via a 500R between the plate and a lower voltage from the power supply.

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    I may be blind, but I can't seem to find the A/AB switch on those schematics

    But here's a though: why not a Fixed bias/Cathode bias switch?

    In fixed bias, you can afford to bias hotter and you're also dropping precious volts across the cathode resistor. I also feel that cathode biased amp tend to sound like a cranked amp much earlier because of the inefficiency of the biasing.

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    Junior Member Johno's Avatar
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    Thanks for the suggestion, Hardtailed. I'll look into that option for this project.

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    Senior Member kldguitar's Avatar
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    i don't think it is good idea

    I don't think sw Class A/AB good ideas. how about Opt? different amp have different opt, how to deal with, the important thing of amp is it's sound, not it can play in different Class. Do you think so?

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  8. #8
    Junior Member Johno's Avatar
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    I was mainly interested in it as a reduced output power scheme. I guess there are other ways to accomplish that objective, but it was curiousity thing.

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    A cathode-bias/fixed-bias switch may affect the class of operation, but won't make a real difference volume wise.

    Volume reducing depends on the amp configuration too. If you're using power pentodes for example, you can switch to triode mode but this changes the sound (I personally don't like it).

    Personally, whenever I've had an amp with various power options, I always ended up using it in the maximum power setting using the Master Volume instead. The amp is designed around the maximum power, when you try to reduce it, the power supply and output transformers become oversized and the sound is stiff and dull.

    That said, I have yet to try VVR (variable voltage reduction).

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