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Project MC-30s

Wayniac

Wayniac

Active Member
Several years ago, a good friend gave me these MC-30s. This will be my first experience with tube audio amplifiers. Initially, I plan on restoring functionality with cosmetics a secondary concern. Using the search function, I have read every post on this forum concerning MC-30s. Yes, every post.

Regarding my skills, I currently service Linear Accelerators used for cancer treatment. Prior to this, I performed international field service on various flavors of military radar systems. Prior to that, I spent 8 years in the USAF servicing radar systems which began with a solid six week basic electronics class followed by about 10 months of radar maintenance courses, primarily tube type systems.

Fortunately, I have good test equipment at my disposal, including a variac, a signal generator, an oscilloscope and DVMs.

Any and all comments are welcome. :smoke:
 

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Sounds like you are ready to go except for that 8 years in USAF (says the guy who spent 8 in the USN doing about the same thing :thmbsp:). If you've read all the posts then you know that you can pretty much ask any questions you want and someone will help out. For the really tough questions, we always have Terry DeWick to fall back on.

Worked in the linear accelerator lab when was in college. We upgraded the system to a pellaton charging system, which really jumped the energy. Great fun. How much energy do your accelerators use for medical applications? Am guessing you use turbo rather than 24" diff pumps. Am also guessing you don't actually climb into your target chamber to clean it, either.

When you get to the cosmetics, there are a couple of folks who make after-market stainless chassis should yours not be worthy of redoing. Keep an eye on eBay, as they list there from time-to-time. Once I finish with my hip replacement (Monday) am going to see what it will take to get chassis' rechromed and relettered for those who want to stay with the original. Good luck and have fun with your MC30s.
 
Looks like a fun project. And it sounds like you have the skills to get through it.

There is just something the makes a old tube amp with point to point wiring a thing of beauty.

Congrats.

Take pics along the way so we can watch your progress.
 
Your amps look like they're in pretty good shape.

Of course, that one with the gold colored cap on top has had some work done to it and I assume it's the same one that has the can cap tacked on underneath.

A fun project, for sure.

Murray
 
Sounds like you are ready to go except for that 8 years in USAF (says the guy who spent 8 in the USN doing about the same thing ).
Click to expand...

I thought I smelled squid. :)

How much energy do your accelerators use for medical applications? Am guessing you use turbo rather than 24" diff pumps. Am also guessing you don't actually climb into your target chamber to clean it, either.
Click to expand...

The energy output is variable with a maximum of ~20MV. For drawing the accelerator down, we use a standard displacement pump followed by a turbo pump. The final vacuum level is achieved and maintained using ion pumps. Our target is liquid cooled and under vacuum.

Looks like a fun project. And it sounds like you have the skills to get through it.

There is just something the makes a old tube amp with point to point wiring a thing of beauty.

Congrats.

Take pics along the way so we can watch your progress.
Click to expand...

Thanks. I will document the progress.

Your amps look like they're in pretty good shape.

Of course, that one with the gold colored cap on top has had some work done to it and I assume it's the same one that has the can cap tacked on underneath.

A fun project, for sure.
Click to expand...

Both amps have been worked on. I am currently researching and sourcing the required parts.

What is your plan for restoration? Do they work unrestored as they are now?
Click to expand...

The amplifier with the gold capacitor is not functional at this time. According to the previous owner, the gold capacitor failed and puked its guts out. The other amplifier has one tube which glows brightly. I have not applied power to either amplifier yet.

My plan is to replace the multi-section can capacitors, replace the selenium rectifier and set the bias according to Terry DeWick's instructions. Resistors will be checked and replaced as required. A wholesale replacement of capacitors will also be performed but I would like to hear the amplifiers first, more out of curiosity than anything else.
 
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Wayniac said:
The energy output is variable with a maximum of ~20MV. For drawing the accelerator down, we use a standard displacement pump followed by a turbo pump. The final vacuum level is achieved and maintained using ion pumps. Our target is liquid cooled and under vacuum.
Click to expand...
Pretty much what I figured. "Back in the day" turbo pumps where highly unreliable. You can imagine the volumes of LN2 we used to front the pumps and cool the targets and detectors. Good customers of Air Products. Good luck with the MC30s.
 
Wayniac said:
....The amplifier with the gold capacitor is not functional at this time. According to the previous owner, the gold capacitor failed and puked its guts out. The other amplifier has one tube which glows brightly. I have not applied power to either amplifier yet.

My plan is to replace the multi-section can capacitors, replace the selenium rectifier and set the bias according to Terry DeWick's instructions. Resistors will be checked and replaced as required. A wholesale replacement of capacitors will also be performed but I would like to hear the amplifiers first, more out of curiosity than anything else.
Click to expand...

Red flag on the glowing tube - indicates potential red-plating due to leaky coupling caps.

Also, be aware that the 3 MC-30s I have restored all needed resistors to be swapped out due to drifting of values. These took about 30-60 days to sort out _after_ I swapped out all the caps, rebuilt the bias circuits, and ran them in. I attributed the drifting to using the amps on a daily basis and the heat up/cool down cycles contributed to stress the old resistors. I also suspect that my stressing the boards while replacing the caps contributed to some of the cracking of the old resistors.

I ultimately just swapped out all the resistors with closely match values. There aren't that many to deal with.

Cheers,

David
 
drillher said:
You have a well thought out plan.Have you decided which flavor of coupling capacitors you will use?
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Therein lies the rub. Having never done this before, sorting through the myriad of choices and opinions is daunting to say the least. I thought I would see what turns up on this thread and then decide.

The amps will be built to the schematic for serial numbers 15329 and above.
 
As a guy who just did what you are doing, you will be very happy and proud when done. Sure you saw my posts and pictures. Good luck and have a blast. Take your time as well.
John
 
sloober said:
As a guy who just did what you are doing, you will be very happy and proud when done. Sure you saw my posts and pictures. Good luck and have a blast. Take your time as well.
John
Click to expand...

I am looking forward to the hands-on portion of this project. I did read your posts. I have also viewed your photo albums. Initially, I'll forgo the major cosmetic improvements you performed. Your amplifiers turned out very nice.

I am curious though, regarding the sensitivity of the speakers you use. I plan on using these amplifiers with my JBL L100Ts. They are rated at 91 dB sensitivity. I hope it's a good match. Currently, I drive them with a Yamaha M-80 and they really come alive when driven a bit hard ....~100watts. I will also try the amps with a pair of Kef C35s I have laying around.
 
dshoaf said:
Red flag on the glowing tube - indicates potential red-plating due to leaky coupling caps.

Also, be aware that the 3 MC-30s I have restored all needed resistors to be swapped out due to drifting of values. These took about 30-60 days to sort out _after_ I swapped out all the caps, rebuilt the bias circuits, and ran them in. I attributed the drifting to using the amps on a daily basis and the heat up/cool down cycles contributed to stress the old resistors. I also suspect that my stressing the boards while replacing the caps contributed to some of the cracking of the old resistors.

I ultimately just swapped out all the resistors with closely match values. There aren't that many to deal with.

Cheers,

David
Click to expand...

Thanks for the advice. This is the type of dialog I hoped this thread would garner. :)
 
Wayniac said:
I am curious though, regarding the sensitivity of the speakers you use. I plan on using these amplifiers with my JBL L100Ts. They are rated at 91 dB sensitivity. I hope it's a good match.
Click to expand...

You didn't say how large your room is, which is a factor in what you'll end up perceiving. What I can tell you is that they should do quite nicely with the 30wpc or so you'll be getting when you're done.

If, however, you are expecting head-pounding bass, enjoy the amps at lower volume.

Cheers,

David
 
dshoaf said:
You didn't say how large your room is, which is a factor in what you'll end up perceiving. What I can tell you is that they should do quite nicely with the 30wpc or so you'll be getting when you're done.

If, however, you are expecting head-pounding bass, enjoy the amps at lower volume.

Cheers,

David
Click to expand...

Sorry. The room is 15'x20', one side is halfway open. The ceiling is vaulted in the center and peaks around 15'. The room is carpeted. The speakers are located in front of a 20' wall with about 10' between them.

Frequency response on the JBLs is rated +/- 3dB, 35-35k. JBL recommends speaker placement as 3' from any wall or corner.
 
I have only rarely had these really cranked up. For the most part I run these in my workshop and they like most anything I throw their way speaker-wise. I usually have the amps at 50% volume and do the rest on my C28.
I would focus on getting the new can caps first, they run out of them from time to time. One thing you may want to try is a new binding post arrangement. At first I replaced the terminal blocks with new ones. I later went back and installed new WBT posts. It requires no chassis modifications, and just a bit of fabrication. If you decide to go so far as to strip the chassis of all parts, you will want to replace all wire as well. Let me know if you want to speak to me about this on the phone. It has been a year, but I remember most of what went into them and the problems I encountered.
John
 
Also, be aware that the 3 MC-30s I have restored all needed resistors to be swapped out due to drifting of values.
Click to expand...

I sampled random resistors in one of the amplifiers today. The values were all over the place. So, I am generating a parts list for complete replacement. :sigh:
 
I'd suggest you get some spares of the larger ones that go into the plate circuits - don't keep that level of detail in my head - as they need to be closely matched. This is the way I did it.

BTW, I tend to call a wholesale swap-out like you're doing a 'blueprint'.

Cheers,

David
 
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