Need some JCM 800 100 watt PT wiring info please.

[newbie]

I'm building a jcm 800 100 watter 2203 schematic but running EL34's(220K bias splitters). I'm using CE distribution/AES power transformer P-TM-P100 which only has a 350vac B+ secondary winding(thats 350 total, 175 each leg).

My confusion comes from the many different online schematics that show different PT wiring. Some show a bridge style rectifier, have the CT going to two 100uf caps between ground and rectified B+, and utilize the bias winding for bias. Others show four non bridge diodes, a grounded CT, and pull the bias from one leg of B+ ac. Others have variations of both.

My question is, what is the correct way to wire my PT to get correct B+? I would assume a non grounded CT, standard 4 diode bridge rectifier, and use the bias winding for the bias?

Thanks ahead of time on my first post ever.

[axeman]

look here http://forum.metroamp.com/viewtopic.php?f=28&t=25151" onclick="window.open(this.href);return false; Don't let Larry find out you didn't introduce your self.

[newbie]

Thanks for the link, but while I can see the diode layout I can't see how it is actually wired in reference to the two 100uf's on the CT and the bias hookup. Is there a schematic that was followed that I could see?

As for introducing myself, I was so anxious to get an answer I forgot too!

[axeman]

The white wire is your bias wires, the two black wires are heater wires, the two green wires are ground, the gray and blue wires are going to your stand by switch (bottom) the yellow wire is going to the 100 uf (the mains filters) the primaries are obvious I hope.

"As for introducing myself, I was so anxious to get an answer I forgot too"!
You better do it because Larry haves no mercy.

[newbie]

So you are saying that it's wired like this then?


Bias from PT winding, B+ CT to two 100uf caps, and full wave bridge recto?

Why wouldn't it be better to ditch the two 100uf's on the CT and just leave the CT ungrounded like the 50 watter? I mean PS filtering from the remaining three 50/50 caps cans is more than adequate isn't it? Why is the CT neither ungrounded or grounded in that schemo?

[axeman]

What are you trying to do, and what schematic are you using. Show us some pictures, more people will help. So is it a 2203?

[newbie]

I'm now asking if the two 100uf caps are necessary as most bridge rectified amps simply don't hook up the CT and leave it disconnected. If I'm using the three 5r0/50 cap cans in the power supply rail, wouldn't leaving the CT disconnected do the same thing? Just trying to educate myself here.

[axeman]

If you are building a 2203 you need 6- 50x50 uf caps, and you have to connect the ct.

[newbie]

Some of the schematics would indicate that...........So what is this then?

http://www.schematicheaven.com/marshall ... w_2203.pdf" onclick="window.open(this.href);return false;

I thought at first it's the difference between the 1959 2203 & later 100 watt lead 2203............until I saw this one.

http://www.schematicheaven.com/marshall ... w_1959.pdf" onclick="window.open(this.href);return false;

So my question is, if these are in fact original marshall 2203 schematics as it states at the lower corner, and they do not utilize the two 100uf CT connection(ct not used at all like a normal bridge ss rectified amp)

What makes the ones showing it the other way more correct?

[axeman]

The 1959 2203 had 3 caps. But thats not a 1959 The second schem is. Someone else will follow up on your question.

[newbie]

Thanks for your help High Decible.........I'm just going to build it with the 6 caps. I'm not sure I see the need, but it'll make life easier I suppose!

[paulster]

With the -V side of the bridge rectifier connected to ground and the +V side being B+, the centre-tap of the transformer is then at 1/2 B+ voltage.

This is tied to the junction of the two sets of caps to ensure that they both have equal voltage across them, rather than one jump over its voltage rating by having a different internal resistance.

You don't need to connect the centre-tap, but if you don't you should use a pair of resistors to balance the voltage on the caps, as in the screens supply node.

[newbie]

Thanks Paulster, thats exactly what I had intended to do. Not connect the CT and add the lost PS capacitance in the power rail downstream of the stby switch, which mine is after the bridge and not like the original. My main reason was to save a few $ on 50/50's. I just finished wiring it with the 5 cans(used one 100/100 can and 4 50/50's) and all is right with the world.

Thanks for the help guys.

[uiovbged332]

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