Marshall JMP Transformer and Rectifier Circuit

[Blind Lemon]

Why is there a center tapped transformer and a bridge rectifier with the center tap ground elevated between the series-ed cap. Transformer is 186-0-186, does the center tap have to be used to get a 450DC working voltage?

Don't quite understand all I know about it. Got a good answer or can you point me to a good read?

BL

[sluckey]

The center tap connected like that forces the voltage to divide equally between the two caps. Could have done the same with a couple resistors across those caps, but if you got the CT, use it. It's free.

[Blind Lemon]

 That makes since. Still don't quite understand the bridge rectifier with a center tapped transformer. It must have something to do with the low output of the transformer and the need for a much higher B+. But I guess I am looking for the "why" do it this way with so many higher output transformers available. Probably came down to economics.


BL

[sluckey]

Still don't quite understand the bridge rectifier with a center tapped transformer.

The CT plays no part in the operation of the bridge. It's only purpose in that Marshall circuit is to force the B+ voltage to divide equally across the two series connected caps. I don't know if there are any advantages between the FWB versus the conventional (2 diode) full wave rectifier. Plenty of each circuit out there. Just a little trivia info... Fender's famous Twin Reverb used a conventional rectifier with high voltage plate secondary. But with the Reissue Twin Reverb, Fender changed over to a FWB with lower voltage plate transformer. And they used a separate bias winding. I like the idea of a separate bias winding but really don't know why Fender made that change.

There is a fundamental difference between the two types of rectifiers... The FWB uses the entire winding all the time, but the conventional full wave rectifier with center tap only uses 1/2 the winding at a time. One diode conducts on the positive half cycle of the AC and the other diode blocks during that time. Then the roles of the diodes reverse for the next half cycle. Only half of the winding voltage is being rectified. This is the reason that a FWB will put out twice the B+ voltage that a conventional rectifier puts out (assuming using the same PT)

Using a center tapped winding and a FWB has big applications in solid state circuits. Many SS circuits need a dual polarity power supply. By grounding the center tap the bridge can supply a positive voltage and also an equal negative voltage. If you think about this dual polarity rectifier you'll soon see that it is really just two conventional rectifiers, one set up for positive output, the other set up for a negative output.

Maybe some of the heavy theory guys will join in and point out advantages (if any) of the different circuits.

[PRR]

This amp runs over 450V (peak 520V) so it needs stacked caps.

Then you need a way to force the stack-caps to share near-equal. This can be two fairly large resistors, or a part-voltage tap on the supply.

(An option is to just buy 600V caps. However the AlOx film in electrolytics breaks-down slightly over 450V, 500V at a premium. "600V electrolytics" were two 350V in one sleeve, so you pay for two just already assembled. 600V film caps are a LOT more expensive, especially before modern plastics.)

There's two (three) "full wave" rectifiers. Two diodes with a high volt CT winding, or one winding (optionally CT) and four diodes in a bridge. (There's also 1 winding and 2 diodes to 2 caps, two half-waves making full-wave.) The voltage and current specs are different. The 2 diode and CT form will require >900V diodes, which used to cost more. (The 2-half-waves has very low diode V specs, and was seen a lot when Silicon diodes first appeared.)

> Probably came down to economics.

EXactly!

So you do *complete* plans for the several possible implementations. (I count at least four.) It comes down to the relative costs of more windings, added CT, several-Watt resistors, and how diode price scales with voltage. Abstractly the cost difference may not be large. However talks with part suppliers may turn-up "special opportunities" like over-stocks. Or they already have a suitable part in production for another customer and would rather keep running that product at marginal cost than stop and idle the line while re-jigging for a different specification.

For YOUR builds, the economics of 1971 are different today, and 7 pence loss/profit either way makes no real difference in DIY work. You do NOT want custom-winding for a one-off. You buy whatever clone/replacement PT suits the build and assume the original rectifier and cap arrangement still "makes sense" (it does).

[shooter]

This can be two fairly large resistors

IIRC I read in a resent thread that I can't find the center of the stacked caps n balance R's is a "good cheat for fixing G2 to a lower voltage".  was I just delusional

[PRR]

> center of the stacked caps n balance R's is a "good cheat for fixing G2 to a lower voltage".  was I just delusional

Or incomplete.

Cap leakage is usually a lot less than screen current. Also two caps leak both ways, while screens only suck to ground. Resistor-string is usually not a great feed for screens.

There is a case where a DC CT is a great operating point. When you increase plate voltage you must decrease current to stay inside Pdiss. Low current strongly suggests low screen voltage. For several tubes, you can go 550V-600V on plates and now 300V is a happy place for G2. See 6550 and 8417 sheets. Also probably the BIG sheet for EL34 which shows 800V operation. And Bogen's 150W and 2x150W 8417 PA amps.

[shooter]

Or incomplete.

There is a case where a DC CT is a great operating point

I'll go chew on this for awhile, sorry for the hijack