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Thread: JTM cathode biased

  1. #1
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    JTM cathode biased

    Hi. Has anyone built a cathode biased version of the 30watt JTM45? I'd like to as a) it will reduce output power a bit and b) it makes implementing VVR much easier. I've tried Hall VVR on cathode biased amps and a fixed bias Selmer Treble 'n' Bass 50. I don't know whether it was the Selmer or just the added complexity of VVR having to track bias voltage in it that made it less successful, but it didn't seem to work as well. Besides, I like the sound of cathode biased amps I built an Ampmaker P1800 (Marshall 18watt with twiddly bits) and love the sound of it. But it's just not quite loud enough for some gigs.

    I've built a few kits now but fancy making something from scratch. And I have a load of bits, including a suitable chassis and output transformer.

    So, does anyone have any suggestions on a good place to start with a scratch-built JTM45? And any experience with cathode biasing one?

  2. #2
    Senior Member Malcolm Irving's Avatar
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    This company has the parts you need for a JTM45:

    MODULUS AMPLIFICATION

    Haven't tried a cathode-biased version myself. It should work OK and sound a bit different at full blast.

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    I find that cathode bias works best when the operating point (ie at max clean power) is fairly close to class A.
    Hence it can sound a bit naff if a highish HT voltage prevents that being feasible.
    Naff, in that excessive bias shift occurs (ie cathode voltage rises) at high signal levels, particularly overdriven, and the tone goes a bit thin and weedy.
    With HT VVR I suspect that the cathode bias operating point gets colder as the HT is lowered, so it may be that you are ok with the above. But it probably helps if the operating point at least starts off (ie at the design normal V HT) close to class A (as is the case with 18 watters).
    Last edited by pdf64; 03-30-2017 at 12:26 PM.
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    Quote Originally Posted by Willholmania View Post
    I built an Ampmaker P1800 (Marshall 18watt with twiddly bits) and love the sound of it. But it's just not quite loud enough for some gigs.
    As you like the 18watt why not build one with 2 EL34s? I have an 18watt and a similar 30watt with 2 EL34. They sound similar but the EL34 amp is louder and has a more solid 'thunk' to it. The data sheet says 375V, 3k5 OT and individual 260R cathode resistors. I used 375V, 4k OT and individual 300R cathode resistors (because that's what I had) but it still sounds fine.

    el34-cb.png
    Last edited by Dave H; 03-31-2017 at 12:28 AM.
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    Hi Dave

    It's funny you should say that. That was exactly my first thought. More 'thunk' and a little bit of extra volume over the 18 watter is exactly what I'm after. Do you happen to have a circuit diagram or a layout of your creation? Does it use the standard 18w front end?

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    Quote Originally Posted by Willholmania View Post
    Do you happen to have a circuit diagram or a layout of your creation? Does it use the standard 18w front end?
    Here it is. It doesn't have an 18w front end but I have tried it with the 18w Lite 2b front end and it was fine.

    J30.pdf

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    I did a mini-5E3 with 6AK5's where I biased the output by ear. Listened to the guitar sort of clean and overdriven. Checked the bias when I was happy and saw that it was just into Class A at full power and moved toward Class AB, distorted with some crossover distortion showing up when overdriven. Mind you this was without any NFB.

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    Thanks for the circuit diagram, Dave. That could well be just what I'm looking for. Do you have a layout diagram or any pictures?

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    Quote Originally Posted by Willholmania View Post
    Do you have a layout diagram or any pictures?
    I don't have a layout. It's the first tube I made years ago as a sort of test bed. It has four octal sockets and can run either 2 x EL34 or 4 x 6V6 but I think the EL34s sound better.

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    Hi Dave

    I'm going to have a go at drawing a layout for your amp as I'd like to build it.

  11. #11
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    I built a Trainwreck with a cathode biased output stage like Dave H's.
    Minor variation, an output tranny with 3K4 Raa but the same B+ (well +380V). Cathode bias resistors 270 Ohms rather than 300 Ohms.

    Started with lower value cathode bypass caps but ended up where Dave did at 220uF.
    Interesting that Dave, I and the Bad Cat Hot Cat 30 designer all ended up at the same place, independently
    http://www.ais-moers.de/gw/badcat_hotcat.jpg

    More "thunk"? Not sure that would be the way I'd describe it but it sounded effortless and more relaxed as if everything was just easier for it.
    Sounded really good with 6CA7 too.
    32 Watts at edge of clipping. Plenty Loud.
    Cheers,
    Ian

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