Marshall Super Lead 100 MKII

[CJ]

ever seen one of these?

well of course you have, back when we had a pretty good music scene happening,

you will not find this amp on a New Kids on the Block CD,  ;D

even the Back Street Boys are afraid of this amp,  too much voltage, 

 

[CJ]

this guy uses the 6550 valve which was manufactured to compete in the power wars going on back then, like a 426 Hemi vs a 427 Chevy with dual quads,

you can get expert info on this tube from  tube god  Eric Barbour,  starts on page 21>

https://web.archive.org/web/20131122141457/http://www.jumpjet.info/Pioneering-Wireless/eMagazines/VTV/VTV06.pdf

this Marshall has been retubed a quad of JJ's,

a blown 1A. B+ fuse has been replaced and the amp works fine, so we are going to go over it and see if there are any other problems,

 

[CJ]

i guess this would be called the chassis,

 

[CJ]

looks like we had a flameout on the port engine,

no engine parts entered the cabin and hydraulic fluid pressure was restored,

all passengers are safe but there might have been some minor smoke inhalation,

wonder if that carbon on the board blew the fuse?

probably not but we are going to clean it up for looks only,

 

[CJ]

pot wiring, pink wire, have not seem that in a while,

was hoping that the output transformer was blown so we could rewind it but no such luck,

early Marshall amps used to pop a OPT about once a week,

 

[CJ]

just 3 preamp tubes, no bells and whistles, don't need em if you have tone,

schematics here>

http://schems.com/manu/marshall/jmp_superlead_100w_1959.pdf

 

[peterc]

Its a pity that the PCB is not through-hole-plated so as to allow component replacement without lifting the PCB. That PCB design is almost  'tag board masquerading as a PCB' which I really like.

So what is the HT voltage?

 

[CJ]

yes, there were some resistors and caps and those diodes that were soldered to the old leads because they did not want to lift the board,

so we un-soldered the preamp tube wires in order to restore this thing,

this allowed access to one of the filter caps and also revealed a bunch of crud on the bottom of the board, the whole chassis is coated with something, probably a lytic cut loose,

 

[CJ]

we soldered the filter ground directly to the chassis,

grounding can be a sore point on some of these Marshall amps,

 

[CJ]

cleaning the board with alcohol and the next door neighbor's tooth brush,

we also cleaned the tube sockets, Q Tips can be handy for this,

 

[CJ]

looks like these filter caps are new, pwr trans made in the USA?

looks original,

 

[CJ]

these old caps sound great, also a nice sized choke,

 

[man-bot]

Love me a superlead!  A bit loud for rocking the basement though  ;D - still every guitar player should have the chance to dime a full stack at some point.

I have a friend with several vintage amps and I prefer the 50 watt metal panels and of course JTM 45s (super clean tone with KT66s!), but a 100 watter firing is quite a thing.

That pink wire is a Marshall special haha.

Best of luck!

 

[walter]

I usually remove all the pots and jacks to get to the underside of the board to avoid unsoldering a bunch of wires. Probably because of having broken tube socket pins while aggressively de-soldering and the kick back from the solda-pult. I have replaced just one or two pins in tube sockets in Fender amps to avoid replacing the whole socket, but some tube sockets won't let you do that.

 

[CJ]

thanks for the tip!

i have to remove the pots anyway to clean everything,

new sockets would be good but they still do not make them as good as the old days so a re-tension job and we will be ready to go,

 

[CJ]

restored the main board and input jacks,

ready to solder the tube socket wires and rework the front panel controls,

soldered the input grounds and cathodes of V1 to the chassis,

before and after pics with a wiring diagram for the Cliff jacks, this amp uses the diagram on the right,

they still had plenty of spring to the contacts after all these years so we just cleaned them up,
there are many websites that you can find for determining the date of the chassis from the ser #, this one has an H at the end of the number which means it is a 1976 vintage, favorite year for those cats in AC/DC,

 

[sahib]

......was hoping that the output transformer was blown so we could rewind it but no such luck,

....

There is nothing worse than having itchy hands and not being able to do anything about it.  ;D

 

[CJ]

non destructive testing on the OPT-

Pri Inductance:

5Hz - 345 Henries
10 Hz - 293 H
15 - 240 H
20 - 205
40 - 138

Pri DCR: 30 Ohms

Turns Ratio - 16 Ohm Tap - 10.3:1  = Z Ratio 106:1 = 1700 Ohms Pri P-P

6550 P-P = 5,000 Ohms P-P  - 2 tubes = 2,500 -  4 Tubes

Response -  starts dropping off at 20 K Hz, no rez peaks - dead at 500 K Hz
Core-Sq Stack 150 EI  Lace 1x1

 

[PRR]

> 6550 P-P = 5,000 Ohms P-P  - 2 tubes = 2,500 -  4 Tubes

Any tube may be run over a w-i-d-e range of load impedances.

"Most" 6550 spec-sheets exploit the 6550's HIGH voltage rating and come up with high load impedances.

1700 (half of 3400) actually comes from EL34 practice. While '34 also has high voltage rating, many-many designs use less-insane voltages and high currents. 3,400/pair is THE way to load EL34 in guitar amp work.

6L6GC, EL34, and 6550 are fairly similar. EL34 overloads different, true, and bias may want a tweak. But when a designer has been a bit "bold", and a player is a lot LOUD, putting 6550 in 6L6GC/7027/EL34 holes is a way to stay together a little longer. (The only limit is that EL34/6550 higher heater current may stress some marginal PTs aimed for 6L6GC heater current.)

 

[CJ]

here are a few of the presence control circuits used by Marshall,

they seem to depend on the tube type, EL34 has the classic 0.1uf/5K  pot borrowed from the Fender Bassman circuit,

6550 tubes use the .68/5K with a 4.7K across it,

and one version has the feedback going further back into the signal chain,

going to use the .1/5K pot with the 6550 tubes, seems to sound a bit better,