McIntosh MC240 cap job

[lowell]

I'm recapping this incredible stereo power amp. It's pretty neat to have one on my bench. I am confused by the bridge rectifier however.

As seen in the schematic... the bridge rectifier, instead of the usual 4 diodes, has 2 250uf 250v caps. Now I'd think this would blow a fuse if I wired it up... but clearly that isn't the result with this design. Anyone care to explain this one? Thanks!

https://www.google.com/url?sa=t&sour...gZ3Z-AHrethKA6

[Leo_Gnardo]

Not a bridge, it's a voltage doubler power supply. Also seen in some Bogen PA gear and the occasional guitar/bass amp. It's possible to up the value of the filter caps & beef up the power supply that way. There's a recommended Panasonic 820 uF cap.

Jim McShane Audio in Chicago sells kits for refurbing & upgrading mostly Harmon Kardon Citation amps but he also has parts & advice pertaining to Macs. Scan part way down this page for a look see. You can take your rebuild a bit at a time to appreciate the improvements each step yields. Disclaimer: I'm not an employee nor friend of his. I just know he has a terrific reputation when it comes to caring for these amps.

http://www.mcshanedesign.net/citpage.htm#parts.htm

To be clear, it's not even necessary to buy parts there. But you can load up on ideas, that costs nothing. The big filter caps can be had from DigiKey and perhaps other sources. Interstage caps, well some of those audiophiles are impressed with old Russian teflon film caps. I've never bothered with them myself. I've used Mallory 150's and they work well enough for me. Of course you can choose anything you like. The bumblebees and dried up electrolytics in an old Mac are getting leaky by now.

Good luck on your project!

[The Dude]

^^^^^^ What he said. I'll add: If you replace the original can caps with similar can caps, BE CAREFUL to note that not all of the cans are attached to ground! Some will have chassis insulators. Be sure to salvage and re-use them.

[lowell]

Thanks For the feedback guys. I'll report back on the results. I'll be going with JJ can caps. I respect the brand and believe the caps are fairly new production.

[lowell]

Nevermind those JJ can caps are pcb mount. Probably go with CE brand from Tubes and More. And yes Mallory 150s for coupling.

[lowell]

Will a JJ 385v 350uf work in the 2 voltage doubler positions? Not sure if the extra uf is an issue since I don't have much if any experience in doubler theory and design. I'll need to add cap clamps as well ofcourse.

[Jazz P Bass]

For a voltage doubler circuit, the voltage rating of the capacitor is more important.

Than uf rating is tailored to the circuit load.

Increasing the value will not hurt.

How to Build a Voltage Doubler Circuit

[lowell]

Thanks Jazz!

[cerrem]

I restore MC amps and wind those Unity Coupled transformers..
I would not increase the uF that much from stock value...
For one you wont get that much improvement since the caps is already sized large in mJ...
If the caps get to big you will reduce the charging conduction angle during rectification, which will produce very large current spikes.....
This will most likely cause the diodes to fail..and increase heating of the power transformer windings...
The 240 will sound 10 times better with 6550's on that OT.... more dynamics and bit more extension.....
Bypass the cap right at the input with a wire ...this will make a difference in better transparency...

[Rhodesplyr]

And yes Mallory 150s for coupling.

For guitar amps, yes. For HiFi, you might want to look at Mojotone's Dijon line. They are film & foil polypropylene and, IMO, very neutral and un-colored, not to mention very inexpensive relative to the quality.

[lowell]

Alright fellas.. and ladies? So what in the world is going on in the PA in this thing? What is the 6L6 cathode circuitry all about? And how does one check bias current in this thing? Does the transformer shunt method work? That's my go to method for checking bias.

[Leo_Gnardo]

Alright fellas.. and ladies? So what in the world is going on in the PA in this thing? What is the 6L6 cathode circuitry all about? And how does one check bias current in this thing? Does the transformer shunt method work? That's my go to method for checking bias.

First, you can use the safer alternative to shunt method. I learned about it here on MEF (thanks guys!) and gave up the slip & spark shunt. First measure resistance of each leg of the OT primary. Then with your prize powered up & warmed up, measure the voltage across each leg. Ohm's law gives you the bias current in amps. Multiply by 1000 for milliamps.

Mac's tricky transformers, that's a story too long for me. Suffice it to say they work very well, the technique applies an extra feedback loop via the cathodes, that lets output tubes of varying quality work to satisfaction. I think our member cerrem recently recommended the use of 6550 (!) in 240's, and that makes me wonder whether KT88 and/or KT77 would also work. cerrem may pop up here & give an explanation, or you can search some Mac fan websites or Audio Asylum for detailed answers.

[The Dude]

If memory serves, those have a selenium rectifier for the bias supply. Did you change it? If not, you should. You'll likely need to add a series resistor to get a proper voltage drop. I usually size the series resistor for proper bias measured with a bias probe.

This may help.

http://w3hwj.com/index_files/RBSelenium2.pdf

[cerrem]

I would concur with "The Dude"... that it would be good idea to replace the selenium rectifier with with a silicon one... and you would need to put a resistor in series with it to keep things all the same... I use to know the value to use for the McIntosh circuit, since I measured it... I will check in my notebooks.... To adjust BIAS, there is a voltage divider at the bottom of the follower bootstrap circuit... Either R60 or R61 is the resistor to adjust for BIAS.... the 240 however biases "both" amplifiers with one bias circuit..
I have found it best to just use good strong matched tubes in the Mc circuit and not bother with the bias adjusting...sine the circuit is very forgiving and tubes are biased roughly at 30mA each in a well running MC circuit... I would not advice "Shunt Method" of biasing in this amp since the error would be big and possibility of going into oscillation........ I use a home made octal socket and octal plug probe, I interrupt PIN# 3 and connect to to my DMM mA inputs with a banana plug.. ....this way the meter is in SERIES with PIN #3 ...so it is true PLATE current... Problem with this probe is it is HOT !!!!and if your not cautious you can get hit with High Voltage...
The Unity Coupled OT's are unique....basically equal plate and cathode windings makes for 100% feedback...gives you roughly .96 GAIN for the power stage...the driver stage needs to then drive a large signal voltage to the grids of the output stage...
The very last stage is a follower with a bootstrap feedback from the transformer...to both lower the driver impedance and help linearize it....

[lowell]

Thanks for the help!

Got some JJ 6L6 in there and replaced the boas diode. Ended up using a 200ohm in series to get at 30ma per tube. Found an old TAD bias probe someone gifted me years ago that Ive never used. Compared its results to the shunt method and it works well. Ofcourse gotta subtract 2-3ma for the screen current.