JM, thanks for taking the time and trouble to post this information, it is really very helpful to me.  It's great to have real data.  Do you know for sure that this material used by Marshall is Sil-Pad? 

From a manufacturing perspective, I would guess that using a rubberized product vs mica/lube was as much ease and convenience as cost.  From reading the Berguist site, that appears to have been the motivation for inventing the product.  In my case however it's moot since I haven't found a source for the original material.  Marshall referred me to US Music Corp. for parts, but they say that they're transitioning as the US parts supplier from Korg, and the transition is happening over the next few months.

Also, the MOSFETS I'm replacing use a TO-3P package, which looks like a TO-220 but is a bit larger.  It looks nothing like a TO-3.  I ordered an assortment of mica insulators yesterday from Mouser, but they didn't carry one for a TO-3P.  I could either go with a TO-220 mica and have some overhang, or use another size that may be large enough but have a cutout or two in the wrong place.  Any suggestions on that front?

OK here are photos.  What do they tell us?



The sticker - there's one on each speaker, facing each other


The only "hole" that I saw


Appears to be a not earlier than 1997 build


The power output board is Rev(E), that may help date it

Thanks Bry, I picked up the amp for $100 with the intention to repair and resell.  I don't think it has the nicer speakers, they are marked British Series, 12", 100 watt, 8 ohm, with no mention of Eminence anywhere.  I'll get to try them out tomorrow when I can make some noise.  I don't have an idea of the age of this amp, but I may get one when I pull the circuit board out tomorrow.

What type of music do you play/record with this amp?

OK I think I'm detecting some sarcasm.  But riddle me this Batman - since Mica + thermal compound pre-dates the rubber solutions, why did Marshall choose to use rubber on this amplifier when both solutions were available?  It's hard to get cheaper than the Mica+ solution so what would explain it?  I'd be happy if you do know, and would enlighten me, but if I still don't know, then I think it's foolish to make a change out of ignorance, or the assumption that manufacturers switch to a knowingly inferior product.  On top of that, these Valvestates have a reputation for failure, and for never being reliable once repaired.  Is that because people are not using the OEM parts/methods?  I don't know, but why introduce another unknown into the equation when I don't have to?

That said, thanks for an entertaining response.

I have an opportunity to buy a Carvin SX-200 amp for $100 with a non-working clean channel (owner reports noise and hum on it).  The distortion channel is reported to work fine, and the effects are fine.  I haven't worked on one of these before but from a look at the schematic I think the clean channel will be fixable.  Carvin sells these new for $299 and I see one one eBay for $269.  If you know this amp, what do you think of it?

Thanks Bry, that's right, the Kapton type of insulator has replaced Mica in many applications but I haven't seen it on Mouser or elsewhere for the TO-3P package used by the MOSFETS I'm replacing.  

As for keeping it OEM, I may be out of step with the crowd, but when I restore vintage tube amps I go for authenticity and I don't see why I'd treat my solid state ones any differently.  As a past circuit designer myself, I also know that if I deviate from the manufacturer's build method, I may unknowingly change something, and while it may work initially, it may not be as robust as the original design.  I'm particularly careful with thermal issues on power devices.  I don't want the amp to work on the bench, but then fail in a gig under load.

Thanks Joe, yes I agree that the mica insulator with thermal compound will work, but I do this as a hobby and like to restore everything to OEM condition whenever I can.  I just got my answer this morning.  I spoke with Marshall's Spares Department and found out that they used a rubberized thermal conductor/ electrical insulator strip, which after years of use may appear paper-like when peeled back from the heat sink. The replacement part number is THRM-00006. They also confirmed that a mica insulator with thermal compound would be a suitable replacement.

I'm replacing a damaged 2SK1058 MOSFET on a Marshall Valvestate 8200 but I'm not sure of what type of heatsink coating they use to keep the MOSFET electrically isolated.  Does anyone know?  It's a yellowish-tan in color but I can't tell for sure if it was a film of some kind, or what.  The area behind the removed MOSFET is damaged a little so I'm leery of reinstalling without also fixing the surface of the heatsink.

Quote from: bgruncorn on March 15, 2011, 06:39:06 AM
the lights keep flicking but no power

Is that the front panel power light that's flickering?  Do you have any electronics experience?

HoneyNut, I'm curious about why you are replacing those three potentiometers?

Quote from: MJL21193 on March 11, 2011, 09:25:52 AM
I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.

A bridge rectifier is four diodes in a pre-configured package, so it isn't inherently safer.  If you replace lower current diodes with a higher rated bridge rectifier, you might keep the diode/bridge from failing but you'll just be inviting some other problem, since something is still drawing too much current through the rectification circuit.  I just finished repairing a Valvestate 8200 amp that was blowing the PC mount fuses and some series resistors in both the + and - rails of the low voltage power supply that feeds all sub-sections of the amplifier (preamp, reverb, chorus, etc.).  I had to isolate the problem first by disconnecting sections of the amp from the power supply until I could find where the current overdraw was coming from, and then further isolating it to a component.  In my case it turned out to be an opamp in the reverb circuit that was pulling too much from both rails.  I suggest you try a similar approach in your Genz Benz amp.