Everything We Know So Far

[Ralle]

That's true, but I would lift the mixer bright cap even so, cause the lead 470k with the cap is connected to ground in this scenario... but other than that this would work fine. The buty of the real PA head is the tonecontroll on the gain... that is VERY usefull... not to mention the location of the master volume...

[jnew]

Good point. I have 3 plexi heads and am thinking maybe I could just convert one over to the full PA Head spec. I'm pretty sure finding an original PA Head will not be easy. Or cheap.

[jnew]

Hold the phone. Is this it?

https://www.ebay.com/itm/Marshall-Maste ... m570.l1313

[Ralle]

Close, but not the exact model Ed had... Ed's was a 4 channel with a tonecorntroll per channel. If you convert one of your head, you will only need one channel, really... that's what I did. Do you have the schems for it? It's on Marshalls Archive site... 2003 Master PA. There's a number of different circuts for that model, but it's that 4 channel circut you should be going for

[Ralle]

Here's a link;
file:///D:/Pictures/Master%20PA%20head.htm
This is the one I belive Ed had

[jnew]

Hmm, no link there. What's the name of the Marshall Archive site?

[Ralle]

If you copy/paste you'll get there... I just did that, and it works... you'll have to copy the whole link inc. "file..."

[Tazin]

Ralle, how exactly do you have your line out (0.68 ohm) setup?

[jnew]

I still don't understand the link. I get nowhere with any of it. Help me out here.

www.???

[Tazin]

I still don't understand the link. I get nowhere with any of it. Help me out here.

www.???

That link can't be correct... The D:/ refers to a local hard drive. Amparchives site is setup with folders "/folder" for each amp entry.

[Ralle]

Sorry about that... here's the correct link:
http://www.marshallforum.com/threads/20 ... date.6855/
The marshall archives is fro the schems only, I think...

[jnew]

That's more like it. I saw and read it all but I couldn't find a schematic. Do you have a schematic for that PA Head?

[Tazin]

That's more like it. I saw and read it all but I couldn't find a schematic. Do you have a schematic for that PA Head?

Just do a search for Marshall Master PA 2002 and you should be able to find the 1970 schematic for the 50w version. The pre-amp and phase inverter section should be the same for the 50w & 100w versions. The power amp sections are "typical" for the PA amps of that time frame.
The only mistake; on the schematic, I believe is the way the input(s) jacks are shown...The high sensitivity jacks should not be present.

[Ralle]

That is correct... If you look at the pics in the link, the inputs only have one 68k ( instead of two 68k )... in other words; no low sensitive inputs, but other than that, it's just the same as any other input. The powerstage is the same as any other 100w marshall bass head with the big cans. The only thing that I could find different there, is the feedback resistor of 100k... I have 47k in there... I guess that's very opptional.
It's very easy to see that you you don't have to do much to convert a bass head into this PA head ( alltough with only one channel, but that's all it takes ). I use the lead channel volumepot for the master volume, since there's the most logic place to put it, fysically. The gain and tone pots goes into the normall channel and into one of the remaining holes were the normal input was. I guess one can do it differently but that's the way I did it, most important is to get it right circutwise.
DON'T FOTGET the tonecontroll... can't stress enough how important that one is...

[jnew]

I'm following you on how you converted your plexi. Makes sense to use input jacks for gain and tone pots, so as not to have to drill more holes and butcher a stock amp/chassis. That's what I'll do as well. Only question left for me, because I cannot see the input jacks from the dark photo's in the link, is whether or not there are 1Meg resistors like the high sensitivity inputs would have. I know there is only on 68K like low sensitivity would be but you and Tazin are saying opposite things. He says no high sensitivity inputs, which to me would mean no 1Megs, and you say no low sensitivity, which would suggest the 1Meg's would be there.

On another note, I have a pretty convincing RWTD clip I did with my 50 running flat out into a variac between the head and cab, and into a 4X12 loaded with 55hz 25W greenbacks. Probably my best tone yet but I'm having trouble getting it from my iTunes to Soundclick. Haven't had trouble in the past with this. Has something changed about uploading lately?