Dual Bias For JTM

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Dual Bias For JTM

Postby 908ssp » Wed Sep 17, 2008 5:13 pm

OK I already have the board stacked out as in the drawing and the top photos below. Everything in the below photos is pretty clear except that there are some wires that go underneath the board. Now that the Bias resistors have two separate turrets do you wire one turret to one bias capacitor and the other bias resistor to the other bias capacitor? On either side of the 15k resistor above the bias capacitors? And the wire that runs under the board at the bottom right from turret 24 to turret 26 that now touches turret 25 as well? Since they are all grounded right?

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Re: Dual Bias For JTM

Postby Brian Wallace » Wed Sep 17, 2008 10:30 pm

Here is how the 220K and bias pots are wired. You will need to raise the value of the bias pots up to 100K for this set up to work. Not sure what turret is 24, 25 and 26 but the ground scheme is the same as a normal board.
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Re: Dual Bias For JTM

Postby 908ssp » Fri Sep 19, 2008 2:38 pm

Hi Brian
Can you tell me the values of these resistors I can't accurately judge the colors.
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Re: Dual Bias For JTM

Postby Brian Wallace » Fri Sep 19, 2008 5:26 pm

Two 56K resistors a 15K and the bias feed resistor is 150K. Pretty much a stock setup except the added 56K.

The PI section is identical to the JTM45.
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Re: Dual Bias For JTM

Postby 908ssp » Fri Sep 19, 2008 6:09 pm

Thanks ...that is easy for you to say to me it is still greek.
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Re: Dual Bias For JTM

Postby Brian Wallace » Fri Sep 19, 2008 6:36 pm

I'll mark what is what a little later tonight. :)
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Re: Dual Bias For JTM

Postby Brian Wallace » Wed Sep 24, 2008 9:00 pm

How's this?

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Re: Dual Bias For JTM

Postby 908ssp » Fri Sep 26, 2008 11:57 pm

That's great thanks so much. I realized with a little help from Steve That I'd have to up the voltage capacity of the 8/10uf caps from the 160 I have to 200 or higher. I have some 250 volt caps on order. I suspect 8uf or 10uf won't matter?
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Re: Dual Bias For JTM

Postby Brian Wallace » Sat Sep 27, 2008 12:06 am

I don't see why you would need to raise the caps voltage rating as it's still a normal JTM45 bias circuit.
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Re: Dual Bias For JTM

Postby chubs » Tue Oct 14, 2008 12:08 pm

Here is a Steve (SDM) version that is 100% independant... Works like a charm in my JTM45/KT66
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Re: Dual Bias For JTM

Postby Brian Wallace » Tue Oct 14, 2008 7:09 pm

Yes, two separate circuits would be best but we are working with what we have for this amp. :)
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Re: Dual Bias For JTM

Postby SDM » Wed Oct 15, 2008 4:29 pm

Brian Wallace wrote:I don't see why you would need to raise the caps voltage rating as it's still a normal JTM45 bias circuit.


Regarding that, there is a wrong value on that dual bias drawing Alex posted above. The 150K resistor should be lower (56K actually used), and that will result in the - DC on the filters going up a good deal from a stock setup.

The two bias supply outputs are at the junction of each of the 56K/100K bias pot connections. So the -bias voltage is always attenuated by an extra 56K/100K divider (at the lowest attenuation case with pots set maxed), not directly fed from the junction of the 56K/bias filter like in a stock setup. So max available - bias voltage for the power tube grids is knocked down to 64% of what would be available in stock form. That's too low, lower - voltage than you could even get out of a stock single bias set up, and tubes will burn up/ be biased too hot. The higher value bias pots don't compensate for this attenuation as 156K parallel to 156K = 78K, which is less than a stock single bias supply resistance (56K+25K bias pot = 81K) would be. So with the dual setup you have the 15K there forming a divider of 15K/78K vs the stock divider of 15K/81K and actually you end up having even less than that already reduced 64% - bias voltage to work here at this point. Can only adjust down from there, making things worse, tubes hotter.

So using values in the drawing above, the extra attenuation of the dual circuit will just give you too little - voltage to bias the tubes properly. To remedy that, just need to increase the overall - voltage supply available to work here, and can do that by just lowering the 150K value at the start of the bias circuit. So the input resistor of the whole circuit ought to be around 56K, not 150K, and that change results in higher - voltage on those bias filter caps than you'd have in a stock set up.

In Alex's case he's duplicating the set up from another amp if I remember any of this correctly lol, and the bias input resistor is 56K (not the drawn 150K value) and it works well for him I believe. Could argue for a bit higher input resistor value, but 56K should help account for interaction of that circuit and for strongly mismatched tubes where say one 100K pot is set pretty low, thus dragging down the whole - supply a bit.

Man I meant that to be a simple response, I know you don't need the explanation Brian, but maybe others could benefit. Any way that's why I recommended higher voltage bias filter caps to Alex :lol:.
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Re: Dual Bias For JTM

Postby 908ssp » Sun Oct 19, 2008 1:18 am

I got the Offset running to today. It needs a little tuning but it runs fine. I used these resistor values and both the chinese and GEC KT66 biased up great .
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Re: Dual Bias For JTM

Postby Roe » Sun Mar 15, 2009 1:35 pm

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Re: Dual Bias For JTM

Postby Roe » Tue Jun 02, 2009 6:10 pm

the SDM version worked great in my jtm50. I managed to fit it to the standard board, partially on top and partially on the side of the first bias circuit
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