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**Twist** · 03-11-2009, 02:22 AM

Take a look at the schematic for a JCM 900. Marshall just uses a full wave bridge rectifier off of the filament circuit to run a few tubes with a DC filament voltage. This just helps with the hum issue due to high gain. What I discover on some of these are cold solder joints on the bridge rect.

**anthillrich** · 03-11-2009, 02:45 AM

which jcm900 schem should i check?
seems like they(marshall) revised the ckt a little in 1992(the year)from two years prior(1990). i have seen it wired a couple of different ways.
i have a 900 4501( circa 1992), a friend owns a jcm900 4100 head that was built 2 years earlier, and his does not have the same filament layout. his is missing the two large "rectifier" diodes 1n5408.
as far as full wave bridge rectifiers go....in my amp anyway, i can only find three.
one from the mains, another for the IC's, and another in the pre amp (that i am guessing they use for some sort of voltage doubler?). i cant find a fullwave bridge rect that is connected to the filaments anywhere on the schem for this particular amp.
i am probably way off, and im missing something obvious, but i am not seeing how they do it.

**anthillrich** · 03-11-2009, 03:02 AM

feeling real dumb, are these heaters dc? i thought i remembered measuring dc on them, but i could be(probably am) wrong. er...i think they are ac after all. i looked inside the chassis and the heaters seem to be hooked up straight out of the PT. NOT EVEN NEATLY TWISTED, by the way. in any case, what are the 1n5408 diodes doing in this ckt?

**Enzo** · 03-11-2009, 05:07 AM

First, is the schematic you linked the exact schematic for your model amp? If not, let's ignore it and look at the right one.

But if so, here is a link to the 4100 in a pdf so you can work with the image better.

http://www.schematicheaven.com/marsh..._100w_4100.pdf

The two 1N5408 in parallel cross wired with additional C1 and 22 ohm R36 are not a rectifier. Note above them it says "star point"? That point is connected to the heater center tap, yes, but also to the circuit common or "ground." The lower end of those parts connects to chassis and the earth or ground lead to the mains. They reference the amp circuit ground to the chassis. They are not there to make DC or anything else. They maintain a bit of isolation from the earth at the same time not letting the circuit ground get more than a fraction of a volt away from it. They are not a power supply.

The tubes in this amp all run on AC heater current.

The bridge for the solid state parts is a separate bridge from the heater bridge in amp models that have a heater DC supply.

Now look at a JCM2000 DSL100

http://www.schematicheaven.com/marsh...00w_dsl100.pdf

Page 2 of the file center of left edge. There is BR1, the heater bridge. Note the power tubes and PI run on AC heater current, then that bridge provides DC heater current for the first two preamp tubes only. That is the bridge we often discuss. Personally I don;t think it is under rated, it only has to source .6 amps for the pair of tubes. But i do find that it often has solder not fusing to the legs of it well. they were not clean when soldered. The resulting poor connection is unreliable and gets resistive, so it heats. But replacing it is quicker and insures no heat damage compromised the part.

Note also page 1 of the file, lower left corner, there is your ground reference network - the diodes cap and resistor.

**anthillrich** · 03-11-2009, 01:21 PM

thanx for taking the time to explain that, i learned something new.
much appreciated, rich.

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