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Rat rod Mc240

detroit47

detroit47

Member
I have a MC240 that has been hanging around for at least 25 years. Complete with rust cobwebs and no tubes. So I swept the output transformers they are ok.OK. No shorts in the power transformer. So I am going to do all the obvious stuff. I don't have many 6l6gc hanging around but I have a bunch of 6550 old stock. I realize the 6550 draws 1.777 times the filament current. So tell is this going to end badly.

Johnathan N8QPC
 
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detroit47 said:
I have a MC240 that has been hanging around for at least 25 years. Complete with rust cobwebs and no tubes. So I swept the output transformers they are ok.OK. No shorts in the power transformer. So I am going to do all the obvious stuff. I don't have many 6l6gc hanging around but I have a bunch of 6550 old stock. I realize the 6550 draws 1.777 times the filament current. So tell is this going to end badly.

Johnathan N8QPC
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With four 6550s your filament current draw is 6.4 amps vs 3.6 with 6L6GC, as you pointed out. You will have to find a way to get -57vdc of bias for the 6550s vs -46vdc for the 6L6GCs (or close to it), the feedback network will need to be modified (6550 has more than twice the transconductance of the 6L6GC) and this is off the top of my head. Oh, a rewound or replacement Mc240 power transformer (if it can be found) will set you back at least a kidney. So yea, I think it is going to end badly.

You did ask.....
 
Hey I wanted an honest opinion. I have other Amps to use I just want to get this one going. I spent the weekend going thru my C20 it's ready to go. So next is the Scott Lk150 then a pair of Dynaco mklll. Paired up with my Altecs a pair of double 12's with a horn and a pair of single 15's with a horn. Legato's and Crecendo's old but cool.

Johnathon N8QPC
 
Update on restore on the 240. I am replacing all the electrolytic caps. I checked out all the signal caps with my capacitor analyzer. They all looked good so I am leaving them. Any thoughts I am sure there will be a lot of different opinions. I bought all new tubes I went with JJ"s for everything, I was looking thru my tubes and found a set of Harmon Kardon branded 6L6GC. Thy are definitely used but test nicely on a TV7. I am waiting on the caps to show up from Antique Electronic Supply. The thermistor checks out good I don't think it's necessary to replace it but I ordered one anyhow. I was surprised that none of the carbon resistors was radically out of spec. There is a coupe that are getting replaced. The gain pots on the twin amp are broke off flush. I wont be using them so i just left them alone they are still good electrically. I can adjust them with a screw driver if i really wanted to. I have attached a pic of the ugly girl.
 

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Those pots are standard, inexpensive and off the shelf--I always used the twin amp inputs when connecting back when I had a 240.
 
detroit47 said:
Update on restore on the 240. I am replacing all the electrolytic caps. I checked out all the signal caps with my capacitor analyzer. They all looked good so I am leaving them. Any thoughts I am sure there will be a lot of different opinions. I bought all new tubes I went with JJ"s for everything, I was looking thru my tubes and found a set of Harmon Kardon branded 6L6GC. Thy are definitely used but test nicely on a TV7. I am waiting on the caps to show up from Antique Electronic Supply. The thermistor checks out good I don't think it's necessary to replace it but I ordered one anyhow. I was surprised that none of the carbon resistors was radically out of spec. There is a coupe that are getting replaced. The gain pots on the twin amp are broke off flush. I wont be using them so i just left them alone they are still good electrically. I can adjust them with a screw driver if i really wanted to. I have attached a pic of the ugly girl.
Click to expand...

Unlike all of their other 9-pin types, which are great, the JJ 12BH7 (or any of the russian offerings) usually do not work well or last long in the Macs.
I strongly recommend something NOS. Anything will else from back in the day will do; American, Canadian, Taiwan, Japan, Korea.
 
I can’t say much about the small tubes, as I ran all NOS on my MC240 but at one point I tried a set of JJ 6L6GC tubes on mine (came on a British amp I bought) and found them to sound quite lovely. I also put very few hours on the amp so can’t speak to their longevity and these tubes were manufactured at least 10 years ago.

OTOH, my son bought a JJ rectifier for his ST-70 about the same time and it died a spectacular death quite quickly, where a nice NOS tube has survived since.

I’m fine with trying pretty much any manufacturer’s tubes provided they present no risk to the amp. I do agree however that on older amplifiers, pre-1970 tubes tend to perform better.
 
It is a Cornell Doublier. It tests at several votages 450 being the highest. I have an esr machine and a Huntron. The old school stuff doesn't lie.
 
SaturationPt said:
I can’t say much about the small tubes, as I ran all NOS on my MC240 but at one point I tried a set of JJ 6L6GC tubes on mine (came on a British amp I bought) and found them to sound quite lovely. I also put very few hours on the amp so can’t speak to their longevity and these tubes were manufactured at least 10 years ago.

OTOH, my son bought a JJ rectifier for his ST-70 about the same time and it died a spectacular death quite quickly, where a nice NOS tube has survived since.

I’m fine with trying pretty much any manufacturer’s tubes provided they present no risk to the amp. I do agree however that on older amplifiers, pre-1970 tubes tend to perform better.
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I love the JJ 6L6GC, but even more their 7027A. The additional pin connections increase rigidity and power handling/stability, just as RCA intended with the original 7027.
One of the only tubes (other than NOS) that can survive in the high-powered Ampeg combo amps. Sometimes a company just gets it right:)
 
detroit47 said:
It is a Cornell Doublier. It tests at several votages 450 being the highest. I have an esr machine and a Huntron. The old school stuff doesn't lie.
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fair enough. A lot of people use those cheapie "all in one" component testers for this, and those just don't cut it for high voltage leak testing. I have one and its useful but not for this.
 
gadget73 said:
fair enough. A lot of people use those cheapie "all in one" component testers for this, and those just don't cut it for high voltage leak testing. I have one and its useful but not for this.
Click to expand...
This is the same unit I have I ripped off the image.
 

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Mine is an old Solar of similar design. Its so old it still rocks it's original cloth wrapped cord and test leads. I rarely use it though. If its a paper cap they just go in the trash. If its not already bad, it probably will be soon enough. I hate dragging things back to the bench for touch-up work.
 
Second for the 6p3s-e in that amp.
I rebuilt one several years ago with help from Mike Samra, a guru on the AA site. Ised a mix of orange drops and k40y’s. Sounds great. He also adds a pair of specific 35uf caps for filtering. Id have to open mine up to see where, but he swears by them. So I did the same. Nice amp. I found a repro chassis for it but never got around to swapping it out.


IMG_5909.jpeg
 
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