New Arrival: MC240

Hi guys,

My new toy, a MC240 has arrived. It is a relatively good shape, rust and paint peeling, etc but otherwise untouched, and not molested in anyway.

I wanted to get my hands dirty on the MC240 and compare it to my beloved MC225.

So far the comparison is unfair as my MC225 is on top of it's shape after the work I have done on it, and the MC240 is all original. But I have to say, the MC240 raw like that sound damn good. It's muddier than the MC225 but it put that to the coupling caps, power supply etc.. Maybe the tubes also, I don't know yet.

I plan to change:
- all power supply cap with Authenticaps (OTW from Germany)
- Selenium recifier with a UF4007 + 470R
- Silicon rectifier with UF5408-E3/54 diodes.
- All coupling caps C5, C6, C15, C16 with Russian K40s.
- R24, R27, R47, R50 12K 2W with Vishay 1% Metal Oxyde CPF212K000FKE14.
- R57,R58, R59 100k 1W also metal oxyde 261-100K-RC.
- Bias cap C27 with a 22uf 450V Vishay MAL204281229E3, or with a 33uf or 22uf 250V film capacitor if size fits.

Questions for the experts here:
- Do I need to change the other coupling caps C7, C8, C17, C18 0.22uF 600V, and/or C9,C10,C19,C20 0.47uF 200V? Are those failing like the classic bumblebees?
- What about C4 and C14, 0.22uF 400V?

One last thing: This morning I blew the internal fuse when I by mistake switched off the MC240 and back on immediate after (within the same second).

The only explanation I see it when I did power on the amp again, the thermistor was hot so it did not slow start the amp and the inrush current blew the fuse.

Anyway I replaced it with a sloblo 3A and the amp is back online.

Here is a picture of this nice piece of history.

Thank you for reading.

Brice,
Do you have new chassis coming for that too?

No but I am considering it.

I will restore it to the XXI century first.

It's a big job to change a chassis, so I have to get into the MC240 mood first

I sent an email to Joe regarding a Stainless steel replacement, let's see if he's willing to try.

I'll keep you guys posted.

IMHO, every Mc guy should have an MC225, MC240, and MC275 at one point or another. These are amplifiers that simply need to be experienced in YOUR listening room driving YOUR speakers. All are great in their own regards. I've had a half dozen MC240s over the years and I liked them best with Amperex Bugle Boys and stock 6L6s (if you can find them!).

Enjoy it!

Agreed with Tony - there's something special about any of the Mc tube amplifiers made before 1973.

Looks like a nice one. Good luck with the restoration and enjoy.

Looking forward to seeing the results of the project!

Congrat's on the 240 I will be following this thread can't wait.

Hi guys,

Got all the caps straight from Germany, resistors, diodes.

I re-builded the power supply from scratch and bias supply with diode + 470R.

My voltages are off now: here is what I got with same tubes on.

Specs Before New
435V 456V 430V
300V 356V 342V
125V 114V 108V
-150V -155V -138V

My B+ is good, then too high, too low and my bias too low.
I tested the caps, resistors before installing and they were all at exact value (thanks for new technology), and I did not expect to have such variation in results.

Any ideas as of why this is happening?

Thanks

Sorry I forgot: pin 5 power tube: -45V

If you really want to experience the glorious Mac tube sound, look for MC 60's or 30's. There's a warmth the other amps don't have. My favorite is the 3500 followed closely by the 21st century 2301. But I'm a SS guy and like tight bass with low distortion and very smooth top end. And I think the SS amps aRE A BETTER MATCH for my speakers and tastes.

I have read that replacing all of the carbon comp.resistors with metal film can make the amp.sound harsh.Keeping the MF out of the signal path was what was advised.

Yes, I ordered a resistor kit from Jim. It's a smart mix of:

CC = Carbon composition
CF = Carbon film
PCF = Premium carbon film
MF = Metal film
PMF = Premium or NOS metal film
WW = 1% tolerance wirewound

For the caps, it's even more pronounced with cheap film coupling caps versus PIO/high end films.

I will be using Russian military grade PIO.

Brice said:

No but I am considering it.

I will restore it to the XXI century first.

It's a big job to change a chassis, so I have to get into the MC240 mood first

I sent an email to Joe regarding a Stainless steel replacement, let's see if he's willing to try.

I'll keep you guys posted.

Click to expand...

Hi Brice, Sorry for the delay in getting back to you, we've been traveling and working some other projects. No luck with the stainless, as no one seems to have any remnants that we can use and a new roll is way over my budget. Regular pre-chromed is all I can get for the foreseeable future, unless I win the lottery... :tears:

Hi Guys,

A quick update.

I changed all resistors in the amp provided by Jim. It's quiet a job but the result is worth it.

I checked each resistor I replaced and indeed, some were out of specs. The worse were the 12k 2W (R24,27,47,50) where 2 of them were at 19.4k. I replaced them with 12k 3.75W instead of 2W.

Of course, while I was there I replace all the caps with hand picked Russian K40 caps.

I was amazed to see some old solder literally falling off like sand. And the amp was still running fine ! Thanks to the quality of the mechanical contact between components. Solder should not be used as glue !

I did one channel first then the other channel so I could compare the sound as I go on test speakers. And indeed, I could hear the sound getting clearer and better as I replaced components. Definitively a huge change from the stock component.

I checked old caps and all bumblebees were pretty bad, values all over the place.
But what you guys call the black beauty, the Sprage .22 600V were all their spec. amazing for 40+ years of service. I don't have the equipment to check their leakage factor but I would assume they were still good.
Anyway I replaced them.

Here are 2 pics of the amp.

I also added a very nice picture of the caps location on the MC240 made by a fellow AK'er. It was very helpful and I want to thank that person. This can help others.


Now the bad news, my DC voltage is still off, and I don't understand why.
I thought all voltages would correct themselves with all the resistors replaced to specs.

Now I suspect the tubes. I will replace them all and see.

Thanks for reading guys,

Brice.

Forgot the nice pic. Sorry.

Brice,
How far are the voltages off from spec?

Hi nj_pheonix,

Quite a bit.
spec 435V, me 431V
spec 300V, me 344V
spec 125V, me 110V
spec -150V, me -138V
spec -48V, me -44V

All with stock tubes.

:scratch2:+/- 10% isn't bad
Those second 2 #s seem odd not absurdly so though. I'll glance at schematic and see if anything strikes me (i'm doubting it). Someone smarter than me will likely be along shortly.!

i went to digital docs and printed out pages 2&3 the voltage and the resistance chart and the schematic. i'm not connecting your spec #s to the chart. So i can try and wrap my head around this. can you show your test points?

Yes it's strange. I haven't look at that problem yet as I thought it was an off resistor value somewhere and because I was going to change them all, I waited.

I just took the voltage out of the power supply as per schematics.
The first hight point is at 435V, then 300V, then 125V, then the 2 bias voltage -150V and -48V.

The power supply resistors are also new.

I will start by removing all tubes and see where I am at. Then going stage by stage.