ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Marshall preamp schematic.

:

5/21/1998 12:51 PM
Glenn Lasiuta	Marshall preamp schematic. HI, I was wondering if anybody could help me out on this one: For the schematic of the Marshall preamp (found on this site), what rating are all the unmarked resitors (wattage)? Also, what is the power supply? Thank you very much to anyone who may be able to help me. Glenn.

5/21/1998 1:49 PM

I looked at the master volume preamp schematic. Only the 10k power supply voltage dropping/ decoupling resistors are shown with a wattage rating. It's a fairly universal convention that unless otherwise stated, all reistors are 1/2 watt. Usually a statement like this is printed on factory schematics, but escaped the informal redraw presented here.

So, all the resistors, including the pots, are 1/2 watt except for those two 1-watt 10k's.

6/1/1998 7:00 AM

I have just finished building this preamp and the Marshall 50W power amp. I used 0.6W resistors throughout (except where marked otherwise). My power transformer kicks out 280V AC (no load) which translates to about 350V on idle. I didn't have a center tap so I had to use a full-wave rectifier and do a bit of fiddling for the bias circuit (with the help of Bruce on this forum).

Works very nicely too!

- Paul
Leeds, UK

6/1/1998 8:49 AM
art	the power supply voltage is about 450 volts, it's hard to find a definitive answer to that question cos of all the different models and whatnot. Of course it will also work on lower voltages. art

6/2/1998 10:05 AM
Ted	Most of the early 50 watt Marshalls had a 380 volt power supply, which I personally think sounded better than the 450 volts of the later Marshalls. I've tried both on my 71 50 watt head and there is a noticable difference. The lower voltage seems to make the amp warmer sounding.

6/2/1998 10:51 AM
Bruce	THANK-YOU Ted!! I couldn't agree more! I try to build all my stuff at lower voltages for exactly the same reason. I think the extra power at higher voltages is completely unneeded and the tone suffers. Don't buy it? Check out a stock Sovtek MIG50 at over 520V plate voltage compared to the same amp with a smaller power transformer at 430V on the plates! What a diff!! Bruce

6/6/1998 5:12 AM
wiley	All unmarked resistors are 1/2 watt.

Page 1 of 2