60 Cycle Hum in '74 50watt JMP

[neikeel]

I'm going to put a pair of standard turrets across the v2a cathode resistor so I can bypass it with a .68uf too.

My favourite 50w other than the JTM is 68 plexi spec which has the 0.68 over V2a but you may find that with 100k NFB and the 0.68uF on the presence pot things get a bit messy. Try it and see what you think but I suggest you either keep it stock or go 68/69 spec and use the 0.68 from the presence for V2a and a 0.1 on the presence pot with a 47k/8ohm NFB combo

Do you remember the value of the V2a cathode resistor in your 68? Mine is 1k and I see 820 being used too for more gain I guess.

Yes, it uses 820R on both V1 cathodes and V2a.
The 1k is more typical mid 70s and unbypassed.

[maxwedge572]

Thanks Neil.
I have a left over 820 from my last build, guess I put it in.

Scott

[maxwedge572]

The board is all done and just waiting for the correct Phillips 330uf cap and all of the wires on the pot side were too short to go through the holes so that side had to be rewired. We've now got a 47k feedback resistor, 820 V2a cathode resistor and the 100k V3 plate resistor had drifted to 130k so the old 100k feedback resistor took it's place. Also indestructible 3A diodes, the presence is done and the preamp filtering was reduced to 32/32. The V2a .68uf will go on later as long as the amp turns out okay!






Oh and her name plate she had back in the 80's, after I got her with no name, went back on!

[maxwedge572]

Getting closer and I need to get this one back together before any further work on the Super Lead!
Getting the socket leads soldered back in was kind of a pain!

[neikeel]

Getting closer and I need to get this one back together before any further work on the Super Lead!
Getting the socket leads soldered back in was kin of a pain!

[maxwedge572]

Thanks.

Only the AC to wire up...Firing it up tomorrow!
The little black .68uf electrolytic is only temp.

[maxwedge572]

Stayed up to midnight last night and finished it and had some hum using my 12" speaker for bench testing but traced that to the V2a grid lead being too close to the heater wires. After moving that the hum dropped a bunch. Biased it last night and put it away.
Trying it out this morning and this one is a winner!!! This amp was almost unplayable before.
So now I'll go put a better .68uf in it.
One other thing I had changed, that I didn't mention, was to replace the preamp supply to a 32/32 can. All of the changes puts it at 68 spec.

[maxwedge572]

Finished!



Found some NOS Amphenol AC sockets and mounted the new one from the outside. Looks much better!

[neikeel]

That's much better!

Might be tempted to shorten those heater wires a little myself!

[maxwedge572]

Yeah, that's probably a good idea. The V2 area is very sensitive.

It appears that I've lost my pilot light. I was trying to straighten the chassis flange near the switches and I guess the banging busted the bulb inside. The chassis has been in and out of the cabinet way too many times and over tightened has bent the flange some. Best just leave it alone I guess!

[maxwedge572]

Made a short sound clip of the amp today.

https://soundcloud.com/scotty-power/marshall-1987-mkii

[neikeel]

Look familiar?

http://www.ebay.co.uk/itm/VINTAGE-MARSH ... 6016.l4276

[maxwedge572]

Another! That is about the 6th I've seen so far and all 74 as I remember but this one is 250v ruling out USA only.
You need to fix your link, you missed the h in the copy/paste.

[maxwedge572]

One more and this one is a Super Lead. 74 again.
http://www7b.biglobe.ne.jp/~showzi666/a ... 59-74.html