planning for Marshall Amp mod/conversion

[BrianGWN]

Today I was back at another of the electronics places, and I did manage to find a few different terminal strips that we could probably use. Maybe I'll have to take a look for those in a Radio Shack on Saturday.
By the way, it turns out that electronics place has a fair amount of stuff that is not on display, odd random inventory of components, probably someone else's surplused or liquidation inventory. They have a computer inventory system that is also online on the net, and in it I noticed what I recognized as the part number for some Sprague "Orange Drop" caps, so I grabbed a few samples. They have a few potentially useful values, .01uF 400V which might make for a subtle tone cap, and .022 and .033 at 800V... Hmmm, anyone else want to convert a stash of Super Bass heads to Super Lead and want a good pair of caps for the output section between the PI and the output tubes?

Now I guess we are down to looking for a knob for the master that matches the rest of the knobs. Hey anybody got some of those Marshall style knobs ? (for 1/4 inch shaft, set screw, diameter at the base about 3/4 inch, knurled body about 7/16 tall then a silvery top section that is about 1/8 tall, silvery top with a black line that indicates the setting of the control...)

By the way, I noticed this thread seems to be getting at least a little attention, 415 "views" at this point. Hee hee, maybe it's like when people are hanging around waiting for some baby to finally be born.

 

[Ed Rafalko]

We're waiting for the lights to dim and the explosion to hit.

 

[BrianGWN]

HEY!, I think that was a shot at me!
Geez Ed you are quick getting in a response, do you have some kind of "new post" alert mechanism in place?, and your computer makes that "thunder" sound like when you get new email?

I have never I repeat never been a threat to the well being of the public electricity system, never been the cause of "lights to dim" or any physical amp explosions. Now as to emotional or musical explosions, that may be another matter...

 

[Ed Rafalko]

Nah, you just caught me at the right time, Brian.
Now, I know you, I know your propensity for hotrdding. That 100-watt Marshall will probably end up with 1600 volts across the plates.

 

[BrianGWN]

>will probably end up with 1600 volts across the plates<
Hmmm, if we rig things up so the power transformer gets 240VAC input voltage or even higher instead of 120, and then cascade a pair of output trannys, maybe we could get close to that...

 

[Canuck Lefty]

So when is the delivery date?spin

 

[BrianGWN]

Progress has been made in the overhaul of that Rosemary. This evening I left without having heard the results, CanuckL's chief of household operations had turned in for the night and I understood he didn't want to risk waking her. However after I got home in Toronto I picked up a late email, it seems CanuckL could not resist giving it a very quiet test drive, hee hee...
So far so good, we still have some additional optional mods and improvements to consider, but in any case it sounds like the Canadian deathwall is coming along fine. :dude

 

[Sean]

Dammit, all this talk makes me want to get under the hood again. But I haven't got my Marshalls, it's going to be months before I even see them again :dead:

 

[Canuck Lefty]

No mushroom cloud, it just sounds great:dude Brian did a very through job on it, not just the MV but there were a few other problems with broken wires etc. Great distorted tones at very low volumes, this amp aounds incredible at any volume. Thanks to Brian for all his hard work!! Now I'm off to find a 4X12 to run it through

 

[RICH]

.........and so it begins.....

:lol2 :lolspin

 

[Marshallhead]

The sickness has started---wait till you tell him the slope resistor trick-heehee :lol

 

[RICH]

Brian may have already changed that anyway. It was probably 56K in a bass model, and would have to be changed to 33K.

 

[Canuck Lefty]

Yes, he did change it, any more suggestions

 

[Marshallhead]

Im sure RICH has a ton-you should hear that 1/2 stack of doom-I think BGWN did. What a monster.

 

[BrianGWN]

Hee hee, there are still a few other possibilities we haven't yet tried...
The feedback wire connection to the output trani taps has not been touched, we should compare moving it to the other two taps... I could maybe rig up a single-pole triple-throw switch so that temporarily we could switch on the fly between the three possibilities.
It looks like someone previously put in a "bypass cap" on the bright channel volume pot that is a value of only 100pf, the value from the early model 1959s, well atleast one schematic in the Doyle book page 204 shows the 100pf cap. If that volume control is flat out then that cap does not matter (the cap is bypassed when the volume is full on), but it might be worth it to try a few other values. It looks like that cap became a .005uF with later SL's, that's a big change 100 to 5000pf, and for some that may be a bit much (produce maybe too much upper mid to top end boost at mid-lower settings of the bright channel volume), think TedB previously mentioned that issue. Maybe something like .001uf or 500pf would be a better compromise. There's one way to find out...

What else... We could jump the channels internally and maybe reduce/bypass the 68K input "isolation" resistors.

I have one more new one up my sleeve that I am not going to mention yet, I should first try it out myself.

RICH, by the way it ended up that the MV installation even looked rather decent on the inside. I used a terminal strip with the middle terminal having a mounting tab or "foot", and that foot or lug did not need a new hole and bolt/nut to mount it rather we piggy backed it on the standoff bolt that was already there supporting the PCB near the two 220K resistors.

I wonder whether it would sound any better if the new MV had a matching knob? :lol :lolspin

 

[Sean]

Love it, love it

Hey BG, I'd appreciate a photo/s of work to date, I'd like to see how you wired the MV. Cheers!

 

[RICH]

""RICH, by the way it ended up that the MV installation even looked rather decent on the inside. I used a terminal strip with the middle terminal having a mounting tab or "foot", and that foot or lug did not need a new hole and bolt/nut to mount it rather we piggy backed it on the standoff bolt that was already there supporting the PCB near the two 220K resistors."""


Yup, thats the way I do them, its the way we did Fred58's head in texas last time. Nice and clean, the standoff bolt couldn't be in a better spot.
spin

 

[BrianGWN]

>Nice and clean, the standoff bolt couldn't be in a better spot. <
Yeah it's really in a great location. Ya think maybe the Marshall gods may have had a hand in that? :lol
(rumbling from the beyond: ..."In the future thou shalt receive enlightenment about heaven and hell and that which lies inbetween, and part of that knowledge will be that thou shalt get maximum tonal possibilities from installing a good dual master, so let there be a well positioned standoff bolt for the chosen ones to benefit from...") :lol

Regarding getting a matching knob... Today I checked with one of the local bigger stores that is a Marshall dealer, for a number of years I have dealt with their repair department (when trying to get parts). I asked about getting some of the set screw knobs for the 70s tube amps, he discovered to his surprise that they appeared to be almost completely out of Marshall amp knobs, had no set screw types, so due to my request he has decided to order in a few dozen of the different styles, set screw versus push on. He hopes to see them maybe in a few weeks.

 

[BrianGWN]

>Marshallhead: you should hear that 1/2 stack of doom<
1/2 stack? What half stack? There were the four full stacks along the wall, do you mean some of the other stuff sort of off to the side? :lol :lolspin
...Half stack?... He don't need no stinking half stack!!!