Advanced questions on the MC-30's - tube variants, etc...

Audible Nectar

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OK, here comes another big sandwich to digest over the weekend.

This is where I should have read GE's General Characteristics - and I do plan to do so - but I am just getting back to finishing my 2 channel rig after an audio hiatus (lack of working computer, other pressing issues, etc). Still listening, of course, but progress halted for a time. Since my wife has OK'd room in the budget for tube investments, I need to ask these questions....and since the crossover capacitor replacements have allowed me (and her) to hear more audible differences between channels, I wish to do the best I can based on what info you folks can give me.

Now that I've lived with my MC-30 pair for about a year with the given supplied tubes still in the amplifiers, I have begun investigating and collecting some tubes for these for trial. My amplifiers have mismatched (not electrically, but different branded tubes) as I received them almost one year ago. Since there is some room in our budget here - typically the case this time of year - I wish to begin collecting/selecting lifetime tube stocks for these amplifiers. I have obtained some used/fully tested and matched tubes for trials (12AX7 Tele smooth plates, 12AU7 Mullard and Amperex, 1950's RCA blackplate 12BH7A's and 1940's RCA 12BH7 single wings) - and am looking to purchase others (mainly rectifiers and outputs). I do, however, have some confusion as to what variants are actually proper in certain slots.

For the rectifiers (5U4's): I have been given some opinion from Mcintosh people that it is not recommended to vary to other rectifiers than the 5U4GB - 5U4G for example. I am well aware that many on this and other forums have gravitated to the 5U4G coke bottle rectifiers in these amps (instead of 5U4GB), with apparent success. I have a feeling that the Mc people are recommending 5U4GB's for safety and the fact that I am relatively new to tubes - and want me to make "safe choices" - which is understandable.

I figure, however, that if so many are using 5U4G's without smoking thier MC-30's, that something must be OK with this. I know these rectifiers spec differently, but am unaware of how to interpret the data in front of me.....and if they are inappropriate, what in the data should be telling me so.

I also want to know about "SLOW RISE" rectifiers - if they are desireable to an MC-30 owner for any reason, and if there are specific rectifiers I should be looking at in this regard.

I have a similar question regarding using the 12BH7 (RCA single winged black plates, 1940's version, in this case) vs. the RCA 12BH7A blackplate. Is there any electrical reason I should be aware of in regards to using the non "A" version in these amps? I know the "A" version specs differently, but am unsure if the non "A" version is OK by the manufacturer's specs on the 30's.

On the 350B: I read somewhere that the 350B doesn't give as much power output in 30's, and that the operating characteristics are different from a 6L6GC. In what way, and what long term effects could arise from using a 350B?

Also, would a WE 350A be an option here? I have access to a quad of these for (I think) a good price.


I looked at this page, the Duncan amps page for tube specs, to see the ratings for the 5U4G and GB variants:

http://tdsl.duncanamps.com/show.php?des=5U4G - 5U4G ratings

http://tdsl.duncanamps.com/show.php?des=5U4GB 5U4GB ratings

And the 12BH7 ratings:

http://tdsl.duncanamps.com/show.php?des=12BH7 12BH7 ratings

http://tdsl.duncanamps.com/show.php?des=12BH7A 12BH7A ratings


6L6CG ratings: http://tdsl.duncanamps.com/show.php?des=6L6GC 6L6GC ratings

350A or B ratings: Having a bit of trouble finding that info at the moment.

And finally, the manual for the MC-30 is this PDF file: http://www.berners.ch/McIntosh/Downloads/MC30_own.pdf MC-30 manual in PDF

The voltage spec info is on page three. I know what I need to know is in the links above, I am just unsure of how to interpret the data.

This is where I need to order "General Characteristics" by GE, I suppose - and I will get to that - but since my wife has approved a bit of fundage for the tube upgrades, and my amps have mismatched tube brands in them currently, I wanted to clarify these issues before I proceed :dunno:


Thanks very much in advance.
 
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When I used to own a tube amp, I once went through a tube rolling excercise and tried a few rectifier tubes in the 5U4 slot. GE, Westinghouse, a Tun-Sol. Theese were all very similar, such that it was not really worth rolloing.

The ONLY one that made an appreciable difference was: GZ37/CV378 MULLARD UK

It is supposed to be a drop-in replacement to a 5u4, but you really need to check. This rectifier really tightened up the bass of my amp, added oomph, and made things quieter. Angela Instruments still sells them. It has a slow warm-up (slow-rise) time, which will extend the life of your other tubes. I liked the effect. Takes about 15-20 sec upon turn-on.

As for the exotic output tubes, I have not tried the 6L6 variants but looked at these at one time. I agree with a comment someone once made on Audio Asylum: sure the 350B may be a somewhat better tube, but not a $1000 premium better!

It seems that many people really like the Valve Art KT-66 replicas of the GEC original lately, and believe this represents the best value out there of all the 6L6, 350B and EL37 options. Supposedly made on the same equipment in China/Russia imported from England. Valve Art then matches them and grades them. Reasonably cheap ... and you get very close to what the legendary GEC KT-66 can offer for much less. I think Ron-C here has some GEC KT-66s because the 350Bs are really ridiculously priced. I hear conflicting stories in regards to the Grove Tubes version of this tube.

As for the 12AX7 ... the Teles have a rep, but are expensive too. Supposely 1950s vintage Rayethons are just as good and considerably cheaper. Some of my Teles tubes in my MR-67 tuner, tested good, but were dying. I don't know if this same affliction exists with the 12AX7.
 
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OK, here comes another big sandwich to digest over the weekend....

I looked at this page, the Duncan amps page for tube specs, to see the ratings for the 5U4G and GB variants:

http://tdsl.duncanamps.com/show.php?des=5U4G - 5U4G ratings

http://tdsl.duncanamps.com/show.php?des=5U4GB 5U4GB ratings

Thanks very much in advance.

Re the rectifier types of MC30, I've been wondering about the choice between 5U4G (or 5U4GA) and 5U4GB. I've been strongly tempted to use the coke bottle types of RCA, GE, Sylvania, and other brands. When I examined the schematics, however, I now know the early model of MC30 used 5U4GA, and the later model used 5U4GB, given their production year respectively. The S/N of my MC30 is 8Exxx, it's a later model, and that's why I stick to 5U4GB. You had better check yours. Please see the pics.
 

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Resurrection of 17 year old thread!
Yep, I've been reading all the threads on MC30 here, including yours, to glean some useful tips! How about your MC30 these days? No issue at all since you had them back from AC? To the best of my knowledge, Mr. George Melnyk is credited with repairing all the vintage Mac gears including MC30. If you look at the service report of your MC30, you will notice his typical handwriting on it.
 
Yep, I've been reading all the threads on MC30 here, including yours, to glean some useful tips! How about your MC30 these days? No issue at all since you had them back from AC? To the best of my knowledge, Mr. George Melnyk is credited with repairing all the vintage Mac gears including MC30. If you look at the service report of your MC30, you will notice his typical handwriting on it.

That's cool. I constantly am reading old threads that would contain any info relevant to my current situation. Lot of good stuff in the archives.

My 30's have been dandy since getting them back. Had a problem with a couple of 1614 tubes that I got from a separate vendor, but have been using a quad of GE 6L6's now for a few a months and all is well. Great sounding they are, I presently have them hooked up to an all original set of early KLH Model Six's. Probably from early 60's latest, so the pairing probably much like many had back in the day. I switch between them and an MC225 that I have connected to a pair of Klipsch La Scala's, and both amps are connected to a MX110Z. So I'm living the high life for sure.

Checked my performance report from AC and other then some basic bench numbers(both MC30's check out at 40 watts!) no names to be found, but the handwriting looks to be written by an older individual, carefully printed, so maybe that's George?
 
That's cool. I constantly am reading old threads that would contain any info relevant to my current situation. Lot of good stuff in the archives...
Checked my performance report from AC and other then some basic bench numbers(both MC30's check out at 40 watts!) no names to be found, but the handwriting looks to be written by an older individual, carefully printed, so maybe that's George?
Nice. I've one such service report on C20 from AC, which Ryan confirmed was Geroge's work. I can tell if it's George's if you upload the picture of the service report here. I've C20 in original but great working condition, and another one restored by AC.
 
Nice. I've one such service report on C20 from AC, which Ryan confirmed was Geroge's work. I can tell if it's George's if you upload the picture of the service report here. I've C20 in original but great working condition, and another one restored by AC.
377799BE-6FEE-483D-9AF2-9CD811012911.jpeg
 
Is that all? It looks like his handwriting, but I mean the service paper showing all the replaced caps, etc, that he wrote down by himself. AC usually includes it in your package, like the one in the picture.

Nope never got anything like that, which still pisses me off. I asked more then once for a report indicating what was changed out, restored, etc., and even after all that, and the repairs, nothing. I like Ryan, and the amps are fine(as is a C11 and MC240 which I also got from AC) but the issues with the MC30, and the avoidance of supplying me with more then just a cursory report left a very bad taste in my mouth. But I've moved on because life is too short.

EDIT: Checked in with Ryan and asked again for this report on the items above and he assured me that the guy who completes them(George?) is currently out of the office ill but once he returns he will get them to me.
 
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Nope never got anything like that, which still pisses me off. I asked more then once for a report indicating what was changed out, restored, etc., and even after all that, and the repairs, nothing. I like Ryan, and the amps are fine(as is a C11 and MC240 which I also got from AC) but the issues with the MC30, and the avoidance of supplying me with more then just a cursory report left a very bad taste in my mouth. But I've moved on because life is too short.

EDIT: Checked in with Ryan and asked again for this report on the items above and he assured me that the guy who completes them(George?) is currently out of the office ill but once he returns he will get them to me.
If you don't mind pulling the bottom off of one (or both) MC-30 and take pictures of both sides of the phenolic turret board, we can probably give you a really good idea of what has and, hasn't, been replaced. I know that is easier said than done; these amps are heavy. I open up my MC-30's and MC-60's every year just to make sure the big external electrolytics aren't leaking goop below the chassis and, the MC-60's in particular, are a real pain to carefully pick up and flip over to remove the bottom. But, if you do that, we'll be able to answer your curiousity. PS - the pic in my avatar is the inside of one of my MC-60's that I rebuilt.
 
If you don't mind pulling the bottom off of one (or both) MC-30 and take pictures of both sides of the phenolic turret board, we can probably give you a really good idea of what has and, hasn't, been replaced. I know that is easier said than done; these amps are heavy. I open up my MC-30's and MC-60's every year just to make sure the big external electrolytics aren't leaking goop below the chassis and, the MC-60's in particular, are a real pain to carefully pick up and flip over to remove the bottom. But, if you do that, we'll be able to answer your curiousity. PS - the pic in my avatar is the inside of one of my MC-60's that I rebuilt.

Yeah not going to do that. I did do that when the amps had issues previously, and I have some pictures of one of the amps before it got sent back to AC for repair somewhere, but I can't find them right now. Suffice to say that a few folks who looked at them agreed that all caps were changed out, and that a bad resistor caused the previous issues. As I've indicated there's been no issues since receiving the amps back from AC, and when I receive the aforementioned reports I'll let everyone have a look.
 
Resurrection of 17 year old thread!

Right, and I long ago settled on 5U4GBs as they are the proper spec tube for those amplifiers.....

Yeah not going to do that. I did do that when the amps had issues previously, and I have some pictures of one of the amps before it got sent back to AC for repair somewhere, but I can't find them right now. Suffice to say that a few folks who looked at them agreed that all caps were changed out, and that a bad resistor caused the previous issues. As I've indicated there's been no issues since receiving the amps back from AC, and when I receive the aforementioned reports I'll let everyone have a look.


Flipping the amps and opening them is actually NOT that difficult. I could do this without even removing the tubes if necessary (although I usually would pull the tubes every 12-18 months and clean the chassis and inspect the units). You FIRST must allow the amps to "discharge" after power down. The caps inside can store juice and give you a "kick". I would do this following an overnight of a power-down, leaving them connected to speakers overnight so I KNOW the caps have discharged. Then I would place the amps on a solid bench or table with a Terry cloth covering the bench; you can literally "roll" the amp over by using the flanges as "grips" and the spacer that raises the amp slightly above the table as a lever such that you don't lose control of the amp. I lift the amp from the non-trans side and let the amp rest on it's side, then flip it over using the end flanges. So now the amp is resting trannies down on a Terry cloth topped bench, then you can remove the hex screws and slowly lift the cover off, pulling evenly from both ends. This exposes all of the goodies, and makes sure the electrolytic didn't go "SPLORT", as these amplifiers DO on occasion have a can/electrolytic issue from the power coming on all at once at start. Point of fact I prefer to use a good variac on mine as that makes for an easy life on those multisection cans and electrolytics......but that's just from being super anal about that and wanting to avoid the shop bench for harder work (as I have to pay others to do that). This is because I should NOT be soldering on that stuff and playing around inside these, but I can SURELY do some basic look/see on an occasional interval as well as check a few things voltage wise with a tech on the phone walking me through it if I really have to do it.

Yes, care and respect of these things are certainly in order but too it is the case that my ownership experience is much better being able to keep an eye on these issues. Once a set of MC30 have been properly rebuilt (and especially with the power supply sections redone, plate resistors, as well as the voltages all measuring out with some consistency) they are pretty stable over time if used regularly. Main issue will be electrolytics and multisection cans having an occasional "issue" from power ups, as apparently they weren't all made like they used to be, and will give out from the "instant on". Aside from that they are pretty stable, well running amplifiers.
 
Right, and I long ago settled on 5U4GBs as they are the proper spec tube for those amplifiers.....




Flipping the amps and opening them is actually NOT that difficult. I could do this without even removing the tubes if necessary (although I usually would pull the tubes every 12-18 months and clean the chassis and inspect the units). You FIRST must allow the amps to "discharge" after power down. The caps inside can store juice and give you a "kick". I would do this following an overnight of a power-down, leaving them connected to speakers overnight so I KNOW the caps have discharged. Then I would place the amps on a solid bench or table with a Terry cloth covering the bench; you can literally "roll" the amp over by using the flanges as "grips" and the spacer that raises the amp slightly above the table as a lever such that you don't lose control of the amp. I lift the amp from the non-trans side and let the amp rest on it's side, then flip it over using the end flanges. So now the amp is resting trannies down on a Terry cloth topped bench, then you can remove the hex screws and slowly lift the cover off, pulling evenly from both ends. This exposes all of the goodies, and makes sure the electrolytic didn't go "SPLORT", as these amplifiers DO on occasion have a can/electrolytic issue from the power coming on all at once at start. Point of fact I prefer to use a good variac on mine as that makes for an easy life on those multisection cans and electrolytics......but that's just from being super anal about that and wanting to avoid the shop bench for harder work (as I have to pay others to do that). This is because I should NOT be soldering on that stuff and playing around inside these, but I can SURELY do some basic look/see on an occasional interval as well as check a few things voltage wise with a tech on the phone walking me through it if I really have to do it.
Yes, care and respect of these things are certainly in order but too it is the case that my ownership experience is much better being able to keep an eye on these issues. Once a set of MC30 have been properly rebuilt (and especially with the power supply sections redone, plate resistors, as well as the voltages all measuring out with some consistency) they are pretty stable over time if used regularly. Main issue will be electrolytics and multisection cans having an occasional "issue" from power ups, as apparently they weren't all made like they used to be, and will give out from the "instant on". Aside from that they are pretty stable, well running amplifiers.

Thanks for your thoughts.

As I've indicated, I've already opened them up when things went wrong and had some folks from this forum who are extremely qualified explain to me what had been done on the amps and what might need more attention. . That was good enough for me. When I sent them back to AC I let them know what I had learned, what should be addressed, and repaired in addition to what they found. Since receiving them they have worked flawlessly and until something goes wrong with them again they are set and forget. Which is exactly what I want. I bought these from a reputable shop, and paid a premium, so that if something goes wrong I can rely on them to fix it. Other then that, it's hands off for me. I look at the tubes occasionally to make sure no red plating is going on, and I feel the transformer to make sure that it's not running unusually hot. If all looks and sounds good, and I know that they have been updated/restored within the last 10 years or so, I'm content to just sit back and enjoy them.
 
Thanks for your thoughts.

As I've indicated, I've already opened them up when things went wrong and had some folks from this forum who are extremely qualified explain to me what had been done on the amps and what might need more attention. . That was good enough for me. When I sent them back to AC I let them know what I had learned, what should be addressed, and repaired in addition to what they found. Since receiving them they have worked flawlessly and until something goes wrong with them again they are set and forget. Which is exactly what I want. I bought these from a reputable shop, and paid a premium, so that if something goes wrong I can rely on them to fix it. Other then that, it's hands off for me. I look at the tubes occasionally to make sure no red plating is going on, and I feel the transformer to make sure that it's not running unusually hot. If all looks and sounds good, and I know that they have been updated/restored within the last 10 years or so, I'm content to just sit back and enjoy them.

Re the temperature of the transformers, I see this useful tip from Terry in an old thread on this, as attached in the pic.
 

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