4x jj kt77 in jcm800-type circuit

[thetragichero]

something like this?

[martin manning]

That's the idea, but with 435-0-435 you are feeding 870V into the FWB, which will give you 870 * 1.414 = 1230V out!

[sluckey]

Also, don't put those 220µF caps across the secondary!

[roberto]

This is the way Hiwatt solved the topic:

[sluckey]

That's a good solution..... for Hiwatt. Pretty much insures you will buy a replacement PT from Hiwatt.

[thetragichero]

220nf was the value Marshall used but fair enough
edit: so if i did a full wave rectifier, grounded the center tap, and took the screens from the middle of the totem pole that would have the same effect, right? am i cooking with fire now or just trying to light my house in fire?
thanks again all. i try to find folks smarter/more experienced than i to lean on

[sluckey]

Sorry, my eyes thought you had written 220µF. I didn't see that it was actually 220nF.

[Phil_S]

I want to suggest again that you look at the Hammond rectifier sheet that I posted. If you really want to work with 435-0-435 secondary, the choice that seems to be the best fit is a full wave cap input circuit. 870v * .71 = ~617. After you put a load on the circuit, I'd guess at a loss of 4-5%, which will put that a bit below 600V. It's close enough. Just use a voltage divider for the screen supply. I can't advise on the cap. I'm going to guess that anything 40u or more is going to work. It depends on how stiff you want the power supply to be. Pay attention to the cap placement location!

Are you expecting some advantage from using a FWB? I dunno...just a feeling...you want to use the FWB. There is no reason I can think of to do that. If you want to use what you have on-hand, it is very important to be flexible. That is what will allow you the freedom to do what's needed, provided it is otherwise within reach.

[thetragichero]

nah i initially was on the same train as you with the full wave. started toying with the bridge after the Marshall schemo was brought up but yeah, definitely flexible and appreciate the whys behind everything. been making my way through the NEETS electronics course but i know I'm the kind of guy who learns best when I've got my hands involved. breadboarding effects circuits helped make the reading i was doing make sense
this is meant for a jcm800 bass amp so that's why I've got large capacitors throughout. thank you again

[Phil_S]

To put a finer point on this, if you want four KT77, the requirement is 400mA for the plates and another 40mA for the screens. Throw in 3 or 4 12AX7's and you're at ~450mA. FW cap input won't diminish what the PT has to offer. It's a high current requirement, and, IMO, makes it a better choice.

[JD0x0]

What about a Mosfet based 'VVR' type circuit, only on the screen supply to knock it down to around ~300VDC?

[thetragichero]

not on this one. if this turns out like I'd like it it'll end up being my #1 bass amp so i want it as simple as possible. building in the husk of an early Marshall dsl. too many ribbon cables and components
at some point i will try vvr power scaling

[martin manning]

What about a Mosfet based 'VVR' type circuit, only on the screen supply to knock it down to around ~300VDC?

This would work, and produce a constant low-ripple voltage for the screen, but I would worry about the FET failing shorted and letting the screen voltage shoot up to the plate voltage.

[pdf64]

...look at the Hammond rectifier sheet...

Any thoughts what is meant by

V (Avg) D.C. = 0.45 x Sec. V A.C.

?

[Phil_S]

...look at the Hammond rectifier sheet...

Any thoughts what is meant by

V (Avg) D.C. = 0.45 x Sec. V A.C.

?

I may have taken a wrong turn? 45% of 870 is 392VDC in this instance. My limited experience with this data sheet tells me to use .71 Peak; really, [sqrt(2)]/2) * 870 = 619VDC. When loaded, about 95-96% of 619, so, ~590.

If you look at the data sheet for FWB, 1.41x for peak seems to conform to the conventional wisdom and my own experience.

I tend to ignore the "Avg" spec. Honestly, I don't know how to answer the question about Avg.I don't know enough engineering to speculate. I suppose I'd like to know, too!

I'd like to know from those more experienced than I am if I've misread this. Apologies to the OP if I was not correct.