What is the difference between the filter & bias sections in these two circuits?

[mort]

I'm working on building up a JTM45 copy from scratch and I'm comparing these two versions. What's the difference between the filter/bias sections? Is there any particular reason[s] that you would prefer one over the other?

https://taweber.powweb.com/store/6m45_layout.jpg

http://www.marstran.com/66JTM45Schematic.gif

[loudthud]

The Weber has effectively 50uF and 40uF on the plate and screen nodes. The Marshall has 32uF on each. The Weber uses a series connection for the plate node. That could be because their transformer is slightly higher voltage or an issue with voltage ratings of the caps. The bias supply of the Weber uses a bias tap on the transformer and has more filtering and a bias adjustment. The Marshall pulls the bias off of the HV winding and has no adjustment.

Your choise depends alot on what transformer and caps are available to you. The Marshall is closer to a true vintage JTM45, if you believe in mojo, go that way.

[mort]

HY022798 is the PT, made by Heyboer. Can't seem to find a spec sheet on it.

Right now I have a JJ 32/32 and an F&T 16/16 can caps, as following the marshall schem. But I could order parts against the Weber design if there was enough reason/benefit to following the more modern design. I'm more a believer in a good sturdy circuit than I am in 'mojo, on fresh built amps.

[loudthud]

One of the magic features of the JTM45 is attack-sustain-compression profile. The power supply has a big influence on that. Unless the transformer used is very very close to what's in the vintage JTM45, there is no way to predict the results. Actually, there is a way, but a vintage JTM45 would have to be reverse engineered and blue printed. I wouldn't expect the differencs to be huge, on the order of some people couldn't tell the difference, and some that say they can wouldn't do much better than random chance in a blind test.

[mort]

The bias supply of the Weber uses a bias tap ....... The Marshall pulls the bias off of the HV winding.

So the bias voltage can be pulled from either place? How does that work? I thought bias voltage was ~40v, and HV would be ~330v ... what am I not getting?

[Justin Thomas]

Mort,

The Weber has an extra wire on the transformer - the red/blue one in the layout. It's a tap. Say for easy math the high voltage secondary has 1000 turns. They get the bias tap by winding, say, 100 turns, sticking a wire to it, then continue with the other 900 windings. Maybe my way of seeing it is a little off, but I kind of see it as another "mini-secondary winding," but without having to go through all the extra insulation/layering/etc.

The Marshall takes it straight off the high voltage leg. No separate tap usually means easier to produce means cheaper tranny. But they've got that 180K resistor to knock down the high voltage. Probably 1¢ for that resistor and maybe 10¢ extra for the tranny tap. Multiply by a million, the savings add up. The current in the bias supply is so low that it doesn't affect the high voltage winding. The HV current is still way lower than what's needed for tube heaters, that's why the heaters have their own windings. But it mostly comes down to efficiency/cost for why they do one over the other. One reason why bridge rectifiers came in, I think - no HV CT needed on the PT.

I'm still learning, too, so correct me or delete if it's wrong, or tell me if I'm in over my head... and I picked the numbers just for convenience. I doubt there's any tranny that matches that!

Justin

[mort]

they've got that 180K resistor to knock down the high voltage.

So if I have a bias tap, I can just pull voltage from it, and omit the 180k resistor from the Marshall diagram?

[Justin Thomas]

Mort,

Use it if it's there, so long as it gets you the amount you need. Should be no problem at all. Less parts to worry about later. Fender got it off the HV too, on a lot of the smaller amps. And even if the tap burns out, at least now you know how to get it from anothe spot!

Justin

[mort]

Cool, thanks.