MC 60 correct plate voltage

[JeffcoTech]

I have my old MC 60 on the bench for a checkup... it is serial #3144. I have an old Sam's Photofact schematic with no serial number reference that says the plate voltage should be 405 VDC. I also found a copy of a McIntosh manual for serial numbers 1858 and up that says that the plate should be 430 VDC.
My issue is that I modded my MC 60's around 40 years ago. Solid state rectifiers with some big Zeners to buck the voltage back down... now that it is open for a once every 40 year checkup It would be nice to nail this correctly.
I rack mounted them and fan cooled them. They passed the McIntosh clinic back then...

Any guidance?

Jeff Weissert

 

[damacman]

I had two pair years ago that were modded. Plate voltage was 575 VDC while the caps clearly labeled 450 VDC! The mad scientist I bought them from said they made 100W each. Man did they sound sweet but I never left the room with them plugged in and powered up.

 

[JeffcoTech]

I had two pair years ago that were modded. Plate voltage was 575 VDC while the caps clearly labeled 450 VDC! The mad scientist I bought them from said they made 100W each. Man did they sound sweet but I never left the room with them plugged in and powered up.

That sure is crazy... I have two 450 volt caps in series just to be sure and not running no where near 575... trying to keep it tame... Do you still run them?

 

[damacman]

That sure is crazy... I have two 450 volt caps in series just to be sure and not running no where near 575... trying to keep it tame... Do you still run them?

Stupidly, I sold both pair long ago. I had three pair in all at the time, one pair totally stock - sold those as well. The modded pairs also used 5AR4 rectifiers in place of the 5U4s.

IMHO, the MC60 is where it's at for McIntosh vintage tube sound. Of all the McIntosh that I sold over the years, only the MC500 and MC60s are on my why in the hell did I sell those list.

 

[nj pheonix]

I have a pair of stock 60s that has been the mainstay .
So far nothing tickles me like it.
I am curious what the original 75s and the 3500 sound like though it seems I'm not likely to hear them.
I did want to try something though.
There are (were) a pair of 75s on BT and while ogling I noticed a 6CG7 in the second position

I've come to learn its a pretty well known mod in mac tube aficionado circles.
From what I understand gives it better ability to drive outputs.
I have the tubes and being how adapters were available and cheap and they save me the effort of pulling bottom cover and rewiring seemed like a no brainer.
Probably try this week.
60s have been out of rotation for a while (just playing with other stuff) I probably should use them to get a base line for judgement.
Like i said, I'm happy with them stock so i really can't see them getting too much better.
I hope I'm wrong

 

[dshoaf]

My haunch is that 430V is closer to reality. This voltage is going to change, however, with AC input voltage so it is best to use a variac to get the plates on the output devices down to the spec before testing the other voltages.

One other thing, that plate voltage will also vary depending on the bias voltage applied to the grids. That is, plate voltage will vary as idling current through the output tubes changes. As a result, check that the bias voltage is also in spec with the plate voltage set for 430V. That will also tell you if you're close.

Cheers,

David

 

[JeffcoTech]

I guess my quandary is are the different specs due to the serial number group difference.... the Mac info that I have is for serial range 1858 and up... and the one on the bench is before that.
Probably only Terry DeWick would know the answer!

 

[nj pheonix]

It's really not that critical in that design.
I restored a pair of MC 40s several years ago.
I was reading 20-30v low on plates (compared to schematic).
I reached out to Terry and the time and he said his notes and my readings jibed.
The owner has been very pleased since.
I wouldn't get too caught up in the #s unless they seem absurd

 

[nj pheonix]

One other thing.
In case you haven't noticed, the PT on that has a 117 (maybe 115?) Tap and 125v tap.
IMO it should be run on the 125v tap.
It may lower voltages slightly but it runs your tube filaments in a much healthier zone

 

[JeffcoTech]

One other thing.
In case you haven't noticed, the PT on that has a 117 (maybe 115?) Tap and 125v tap.
IMO it should be run on the 125v tap.
It may lower voltages slightly but it runs your tube filaments in a much healthier zone

I have the primary tap on the 125 V. I measure 6.7 VAC on the filaments... and actually around 470 at the plates with 120 in but I have 30 volts of zeners in the center tap of the HV transformer... I would like to run without them but the HV will be higher. Not sure what the implications are. I have biased the outputs using the 435 volt/ -45 Volt bias...

 

[ajauwen]

and actually around 470 at the plates with 120 in but I have 30 volts of zeners in the center tap of the HV transformer... I would like to run without them but the HV will be higher. Not sure what the implications are. I have biased the outputs using the 435 volt/ -45 Volt bias...

470V at line sounds a bit higher than it should be... just curious, but what is the value of R34 & R35 (resistors coming off pin 4 of the 6550/KT88 sockets) in your amps..?

 

[JeffcoTech]

470V at line sounds a bit higher than it should be... just curious, but what is the value of R34 & R35 (resistors coming off pin 4 of the 6550/KT88 sockets) in your amps..?

Whoops... typo... here is a better description... the resistors are 220 ohms..
I have a McIntosh MC-60 question about the aftermath of installing solid state rectifiers instead of the 5u4’s. I did the solid state change in 1969! I added a quantity four T03 7.5 volt zeners in series in the center tap of the HV transformer to buck the plate voltage down. I would like to get them out of the circuit. With them in I have the following parameters:

Plate=430 VDC, Bias 45 VDC, 1.1 Amp AC Idle current

If I pull the zeners out I have:

Plate-450 VDC, Bias moves to 41.6 and 1.35 Amp AC Idle current.

The capacitor voltages are more than adequate to handle the extra voltage.

Can I just change the bias and run with the higher plate voltage without any other issue?

 

[ajauwen]

IMO, if you leave the solid state mods in, then it seems you would have to leave the diodes in.... if it were my amp, I would probably consider putting it back to stock with the 5U4GBs. I haven't heard one modded to solid state, but I have heard the opinion that tube rectification is part of what gives the MC60 it's good sound..

 

[JeffcoTech]

I hear you. The diodes are on a heatsink made to bolt up to a muffin fan... would be nice to loose the fan and the heatsink. They are rack mounted so only I who would go behind the rack would see them. I had them to the McIntosh clinic in 1970. I still remember the tech getting excited saying I have to test these... they tested great then... Its hard to get a real quiet 120 volt muffin fan too...