Refurbishing two MC75 Monoblock Amplifiers

[Rodeodave]

Hi,

I recently got a pair of McIntosh MC75 monoblock amplifiers that I want to give an overhaul. The two units were used in a shaker table amplifier which was stored in a garage for some decades. The tubes were dusty (still are) but seem to be stock and in good shape. I even found the second shielding can that's missing in the pictures.
Most of the info I got from the helpful people over at diyaudio, where I started this thread: http://www.diyaudio.com/forums/tubes-valves/163204-pleas-help-identify-mcintosh-valve-amplifier.html .
So why another thread here? Looking for anything related to the MC75 or similar units I stumbled upon this forum and it's McIntosh subsection. And after reading a lot invaluable info I hope to get some input on my specific project. I'll post step-by-step pictures (like http://audiokarma.org/forums/showthread.php?t=69835&highlight=mc240 ) if there's interest.

In the meantime I've figured out how to proceed with the amplifiers. The part selection is going to be reasonably modern, but conserving the looks of the amp. Not being used to carbon resistors (solid state background with matched 1% metal film resistors and high power MOX...) I initially wanted to toss them all but got convinced to keep a few in places where they might actually be the better choice.
After reading http://www.audiokarma.org/forums/showpost.php?p=1171856&postcount=93 I also decided not to keep any of the original carbon resistors but to replace them with new ones.
Replacing all electrolytic capacitors seems logic, but after reading
http://www.audiokarma.org/forums/showpost.php?p=1752684&postcount=130
http://www.audiokarma.org/forums/showpost.php?p=3442681&postcount=361
http://www.audiokarma.org/forums/showpost.php?p=1509868&postcount=119
I decided to also replace all of the other caps, except for the Black Beauty caps of the 2nd stage.
Other parts will also be replaced, like the thermistor and the diodes. More on that later.

So here's the plan:

Electrolytic capacitors:

250uF250V PSU with minus ground (C13)
250uF250V PSU with minus hot (C14, black isolation)
Replacement: Auhenticap KTL3, isolated can. Direct replacement.
Source: http://www.die-wuestens.de/k8.htm

100uF500V/30uF450V PSU, twice (C15AB, C16AB)
Replacement: Auhenticap KTL9. This cap has four 50uF500V sections. I intend to use one single 50uF500V and the three others in parallel to form a 150uF500V.
Source: http://www.die-wuestens.de/k8.htm

12uF250V bias, axial (C12)
12uF250V cathode KT88, axial (C9, C10)
Replacement: Sprague Atom. Maybe 20uF500V but I'm not sure if the actual voltage that will be presented to the cap won't be too low to keep up the oxide layer. Or maybe I'm just paranoid.
Source: Recommend me one.

100uF3V cathode 12AX7, axial (C3)
Replacement: Not sure yet, but will be axial part. Maybe a Sprague Atom 100uF50V.
Source: ?


Film caps:

0.47uF200V input (C1)
0.047uF600V 1st coupling (C5, C6)
0.22uF400V 2nd coupling (C7, C8)
0.22uF400V grid 12AU7 (C4)
0.01uF600V PSU (C11, C12)
0.1uF200V output socket (C19)
Replacement: All bumble bees (all caps except C7 and C8) will be replaced. I want to use the original values even though it seems a bit weird taking the tolerances into account...for electrolytic caps I don't mind tweaking the numbers.
I'd prefer Black Beauty 160p for C1, C5, C6 and C4. C11, C12 and C19 I don't know yet. But they should blend in with the other choices.
Source: eBay, ?

Then there's two additional caps:

100pF?V at the anode of the 12AU7 (C18). I can't figure out what type that is. Voltage rating should be around 400V methinks.
330pF?V at the cathode of the 12AX7. I'm not even sure this cap is installed...supposed to be “tubular ceramic”.
Replacement: Not even sure if necessary.
Source: ?


Resistors:
(Label, Value, Location; Replacement)

R1 220k Input, 5%; RN65 Metal Film
R2 100k Input, 5%; RN65 Metal Film
R3 250k gain pot, switch, stays
R4 22k grid 12AX7
R5 2.7k cathode 12AX7, 5%; RN65 Metal Film, match other channel
R6 3.3k cathode 12AX7
R7 68R cathode 12AX7, 5%; RN65 Metal Film, match other channel
R8 100k anode 12AX7
R9 2.2M grid 12AU7; RN65 Metal Film
R10 18k 1W cathode 12AU7, 1W
R11 27k anode 12AU7, 5%
R12 30k anode 12AU7, 5%
R13 2.2M grid 12BH7, 5% high voltage drop!
R14 2.2M grid 12BH7, 5% high voltage drop!
R15 220k grid 12BH7, 5%; RN65 Metal Film
R16 220k grid 12BH7, 5%; RN65 Metal Film
R17 2.7k 1W cathode 12BH7, 1W
R18 12k 2W anode 12BH7, 2W, matched 1% to R21; non-inductive WW
R19 1M grid 12BH7; RN65 Metal Film
R20 1M grid 12BH7; RN65 Metal Film
R21 12k 2W anode 12BH7, 2W, matched 1% to R18; non-inductive WW
R22 18k cathode 12BH7
R23 18k 1W cathode 12BH7, 1W
R24 18k 1W cathode 12BH7, 1W
R25 18k cathode 12BH7
R26 220R 5W grid KT88, wirewound 220R, 5W; act as fuse, must fail open, 5W maximum! See link below.
R27 220R 5W grid KT89, wirewound 220R, 5W; http://www.audiokarma.org/forums/showpost.php?p=2900298&postcount=8
R28 Thermistor PSU, 79R cold, <0.7R hot; CL80 http://www.audiokarma.org/forums/showpost.php?p=432963&postcount=3
R29 10k 2W PSU; MOX 2W
R30 100k 1W PSU; MOX 1W
R31 100k 1W PSU; MOX 1W
R32 100k PSU, 5%; MOX 1W
R33 62k PSU, 5%; MOX 1W

Where no replacement is specified I will use carbon resistors. KOA-Speer Carbon Film have been recommended to me. About the higher power ones I'm not sure yet. Any opinions?
The WW resistors could be WW Mills MRA-12. Opinions?
The thermistor will most likely be replaced by a CL80 thermistor. It has a cold (77°F or 25°C) value of 47Ohm and the maximum steady state RMS current is 3A. As the MC75 will draw 240W at 75W output, the current through the thermistor will be around 2A. The CL80 will have a residual resistance of 0.71Ohm at 75% of it's rated maximum current so it should be a good replacement.


Diodes:

SR1, SR2, 600V500mA, PSU
Replacement: UF5408

SR3, 400V65mA, PSU, Selenium
Replacement: UF4007 plus a dropping resistor to restore the -150V across C17. Anyone knows a good starting value for the MC75? I have no idea, 5k for starters?

I also want to connect te amplifier's chassis/circuit ground to earth-ground (or safey earth). The solder lug next to the output strip semms to be a good place, don't it?

Well, that'd be it, that's all I know for now. What do you think? What parts would you choose? And why?
Btw, I'm from Austria and we have 240VAC50Hz mains. But that is taken care of since I have a 4000VA isolation transformer with a 115V28A secondary...lucky me.

 

[dshoaf]

Excellent analysis, Rodeodave. Here's how I'd suggest you proceed:

- Bring both amps up slowly on a variac first. Confirm that they do, in fact, operate properly.

- Next rebuild the power supply caps and the bias circuitry. Standard rail voltages are likely below spec and will drop considerably under load. This is normal for old tube amps and results in flabby bass (AKA: bloom and warmth) , blurred sonic images and a wandering soundstage.

- Reset the bias operating voltage per the specs in the Hints and Kinks section.

- Verify all operating voltages are in spec. Do what's needed to get them there. If you have a leaking coupling cap, it will upset the voltages around it, for sure.

- Next, swap out the interstage coupling caps mainly for safe operation and again verify operating voltages.

- Operate the amps for a few months, depending on how much you'll run them.

- Periodically, check the operating voltages that all tubes stages are still in spec.
- Don't consider resistor swaps until there's clear evidence they're needed.
- The operating voltages tell you what you need to know.

Boutique cap brands are your choice. Plenty of discussion here about what folks prefer. I like the Illinois Cap and the JJ's for the voltage doubler cans. See my posts for pics of the set of MC-75s that are now in Analog Addict's capable hands.

Make sense?

BTW, that's a nice way of packaging the amps together.....congratulations!


Cheers,

David

 

[macyjrm]

That's probably the best first post I have seen on this or any other forum. Welcome to our group. I look forward to seeing your future posts

 

[Rodeodave]

Thanks for your input and the flattering welcome! Here's a quick update:

I finally found the time and courage to clean the tubes' envelopes. I was using a soft cloth dampened with water and a mild detergent (pH neutral). For cleaning around the more delicate labels I used wetted Q-tips.
As expected, the Telefunken labels are very easily damaged so I couldn't clean these tubes very thoroughly. But they look good enough.
The "Made for McIntosh by RCA" 12BH7A labels stayed on perfectly and I'd say they can take one or two wipes easily.
The Genalex Gold Lion KT88 were quite dirty and it took two or three wipes to get them clean. This is likely also the maximum the labels can take. The were slightly fainted in some places but stayed intact generally. I guess with some time between the wipes to let them fully dry up again, cleaning shouldn't be a problem in general.
The pins all seem fine and I might use some Deoxit, but probably only if I run into problems with the contacts.

What else...the capacitors are on their way and I acquired an insulating step-down transformer with a 220VAC primary and a 115VAC 28A (sic!) secondary, so that should be taken care of (4kVA). I also found the second shielding can.

Now for some questions:

David, when you say "Bring both amps up slowly on a variac first"...how slowly? Slowly as in reforming capacitors? What about the filaments then?

Do I need a load for the output transformers? http://www.audiokarma.org/forums/showpost.php?p=3201967&postcount=316 suggests that I don't, but I'm not sure. I'm quite new to tubes, and in fact, I never even heard a tubeamp IRL. But I have some theoretical knowledge and some experience with HT (and appropriate test equipment).

Thanks for reading. And now some pictures.

 

[nsgarch]

That's the most serious-looking transformer I've seen on the ground (as opposed to on a telephone pole!) I can't help you as much as others with the refurb, but the very best stuff for cleaning old tubes without damaging/removing the labels (even the fragile old white Teles, and Bugle Boy labels, etc.) is naphtha -- commonly found as cigarette lighter or dry cleaning fluid. Never, NEVER, use anything with water in it (that includes alcohol, Windex, etc.) and no strong solvents like acetone, of course.

 

[dewickt]

Use the variac as a safety measure, if an amplifier is going to be used on a regular basis reforming electrolytic capacitors is a waste of time, either they are good or if actually in need of reforming they will fail in the future. I bring up most amplifiers in less than 15 minutes, monitor for excessive current draw and any hot can capacitors, if either happens your power supply rebuild is the first order of business.

 

[Rodeodave]

Hi there,

Time for the next update.
After the caps arrived I decided to replace the cathode bypass and all the caps in the bias-supply. Unfortunately the Sprague TVA1906 is too fat, so the bottom lid can't be closed anymore. Will have to replace it with a lower voltage type (250V or 350V) as they have a smaller diameter.
Following the procedure from the service manual (if anyone wants it I can send it to you) I shorted out the first two caps and checked all the pins' resistances to ground. All values were within 10% of the specs.
I had a friend come over (with some beers), set up a nice measuring rig and slowly fired up the first amp (serial no. 320B4) with a variac (rated 1150VA).
Everything seemed to behave nicely, no unreasonable current draw, no nasty smells, no weird noises. I checked the temperature of the caps twice and they were around room temperature. When I noticed a slight hum from the speaker that I previously attached to the 8 Ohm tap, I put my finger on the membrane to feel the vibration. At that point I might have touched the minus-pole of the speaker, I'm not sure, and suddenly there was a roaring sound and excessive cone-movement (the beast has awoken!). I quickly turned down the voltage and as there was no visible damage, I slowly turned it up again, closely watching the current draw. The unit appeared to have survived unharmed, so I turned the voltage to 125VAC. The amp drew 1.4A, therefore dissipating 175 Watts. There was audible noise and hum from the speaker, but nothing unexpected. I attribute this to leaky power supply caps.
The manual states that it should draw 140W with 0V input.
Hm, now that I think about it, I removed the GND-COM link before attaching the speaker, so touching either of the speaker's poles shouldn't have done much. Any idea what might have caused this spur?

Yesterday I repeated the same procedure with the other amp (320B3) and things went very smoothly. I fired it up in about 15 minutes with comparable results. It drew 1.2A at 120VAC, therefore dissipating 144W. There too was noise and hum from the speaker.

In total, I'm quite satisfied with the results. Next up is a complete power supply rebuild, with new caps and diodes, once they arrive. Looking forward to it!

 

[dshoaf]

First guess to the noise on power up: Dirty tube pins and/or sockets.

Sounds like you're good to go on the next steps. I used JJs for the voltage doublers, IIRC so they would fit into the same holes as the old caps. Remove the entire cap plate, use a small nail punch with a socket on the back side to drift out the rivets and use cap clamps to hold the new caps in place.

Cheers,

David

 

[Rodeodave]

Time for the next update.

Progress is slow, but I acquired some Sprague Black Beauty caps. And since the opinions on these widely disperse and I meanwhile have doubts myself...I have to ask: Should I use them or just go with Auricaps? I plan to test the Spragues with a variac and a resistor for any leakage but I'd hate to see them blow out a tube, or even worse, a transformer, should they ever fail. They're 40 years old after all...

So, off to ebay or not? What about the Black Beauty coupling capacitors that are in the amplifiers (coupling 2nd stage)? Replace them?

I tested all tubes but the KT88 on my Eico 666 and they test marvelously, strong and reasonably matched.
I also found a matching Telefunken ECC83/12AX7 for the input stage.

In the first picture:
Sprague 6TM-S10 .01uF 600V (voltage doubler)
Sprague 160P .047uF 600V (coupling 1st stage)
Sprague 161P .47uF 600V (coupling input)
Sprague 160P .22uF 600V (grid)

Other picture:
The ECC83s for the first stage, on the right the tube that came with the amp.

As usual, thanks for reading and your encouraging comments!

Dave

 

[MX117.MC7150]

Newer is better. Brand new is best.

 

[ron-c]

Replace the thermistors with new ones. These fall apart when old.

Thanks,
Ron-C

 

[dshoaf]

Pass on any NOS black beauty caps, IMHO. They're just as old as the originals, most likely. If you're out for reliability and stability, don't go there.

New caps have far better tolerances and use far more stable materials than the old stuff - plus they're new and not 40+ years old at the get go.

You'll find a demand for black beauties with the old guitar amp crowd as they were used in various effects pedals. This crowd believes that the old caps in that application maintains the specific sonic characteristics of those old devices.

The reason why they're not recommended in old tube amps is that the pedals don't have 200-400vDC across the terminals of the cap. This is what causes the leakage in old caps and why they're ok in a pedal and not as your coupling caps on the output stage of an MC-30 or 60.

Cheers,

David

 

[Rodeodave]

Alright, it's been quite a while since last time...and I'm really exited since I just placed an order for JJ can caps, they should arrive next week. I started to decipher the power supply and did some more research on the amps.
I'll add some more details in the coming days, and plenty of pictures for those who are interested.

Prior to firing up the amps (once they're done) I'll have to figure out how to silence a humming secondary of my 4kVA isolation step down transformer (remember,240V country). I suspect that something became loose due some weakened varnish or some shrunken/worn out shims. It's probably not a good idea to let it hum along like this, the wires' insulation will likely break down and render the beast useless.
I slowly brought the big transformer up to 240V AC with a variac, first with nothing connected and it seemingly drew 80W. The hum set in at about 200V AC. Then with a 300W bulb connected to the 120V AC secondary it drew short of 400W, same hum.

Any input on that? Is there an easy way to varnish/silence it? Reading this here: http://www.audiokarma.org/forums/showthread.php?t=280008 I'm afraid not...

What else...ach ja, the attached pic shows how the HV PSU (460V, 380V, 170V) is actually implemented.
More to come.

 

[dshoaf]

Rodeodave, are you saying that you've tried to run the AC input voltage to the MC-75s up to 200Vac and heard some 'singing' in the power transformer of the separate isolation transformer?

Firstly, you do not want to exceed about 120Vac to the MC-75s as they do not have dual primaries to wire - at least the ones made for US consumption. If there are unique versions made for the European market, I've never seen one.

Secondarily, I'll assume that you will changing out the 2 filter caps you have in the pic you posted. I swapped those 2 with the JJ dual-segment caps and then wired in new resistors and then used a separate cap for the last segment you dont get with the JJs.

What are you planning for with the 2 voltage doubler caps on the other side of the cap bank?

Lovely amp, BTW, when you get them up and running.

Cheers,

David

 

[Rodeodave]

Rodeodave, are you saying that you've tried to run the AC input voltage to the MC-75s up to 200Vac and heard some 'singing' in the power transformer of the separate isolation transformer?

Firstly, you do not want to exceed about 120Vac to the MC-75s as they do not have dual primaries to wire - at least the ones made for US consumption. If there are unique versions made for the European market, I've never seen one.

Oh my, that came across wrong. No, I simply tested the step down isolation transformer with a variac to see if it is still working. The MC's weren't involved. Here in Austria we have 240V AC line voltage, and said transformer converts it to 125V AC, which is fine since the MC's have a voltage selector switch for 117/125V AC.
The humming part was the step down transformer's secondary.

Secondarily, I'll assume that you will changing out the 2 filter caps you have in the pic you posted. I swapped those 2 with the JJ dual-segment caps and then wired in new resistors and then used a separate cap for the last segment you dont get with the JJs.

What are you planning for with the 2 voltage doubler caps on the other side of the cap bank?

Yup, all electrolytics must go. The caps in the picture originally are 100uF500V/30uF450V each. I will replace them with 100+100uF/500V (JJ TC 529 series, bought from here: http://www.tube-town.net/ttstore/product_info.php/info/p407_JJ-100-100--F--500-V.html) and 50+50uF/500V (also JJ TC529, http://www.tube-town.net/ttstore/product_info.php/info/p406_JJ-50-50--F--500-V.html). Here's link to the datasheet: http://www.jj-electronic.sk/pdf/TC_TE.pdf.
I'm quite sure that the increased capacity of 50uF each for the second section will pose no threat to the power transformer since they are charged through resistors (CC originally), which I'll most likely replace with some nice WW types.

I've attached a schematic of how it'll look.

The two voltage doubler caps (250uF250V originally) will be replaced by 250uF500V (JJ TC 509 series, http://www.tube-town.net/ttstore/product_info.php/info/p678_JJ-250--F---500-V.html). I believe they are fully isolated cans, so they should work quite nicely.

I'll be using capacitor clamps to hold the caps in place. I don't really like them, but I don't have a choice. Authenticaps would have been nice, but two of them cost as much as all eight JJ caps for both monoblocks.

Lovely amp, BTW, when you get them up and running.

Cheers,

David

I agree, and I'm very much looking forward to listening to them. And the gloomy glow of the tubes of course.

 

[Rodeodave]

There is one more thing I keep pondering over. The MC75 uses a ECC83/12AX7 at the input stage. The tube contains is a double triode system, but with the amps being monoblocks (and single ended) only one of the tube's systems is used in each channel. Since all monoblocks are built identically (that is not mirrored), it's always the same system that's being used.

Now what if one rewired one monoblock so that it uses the other system? If the 12AX7 tubes show signs of wearing out one could just swap them from one amp to the other, now using the previously unused system.
Amps like the MC275 employ one 12AX7 for both channels, using both systems.

With dwindling supplies (don't deny it) and rising prices (last time I checked a pair of Telefunken smooth plates went for ~$150, the S&H E83CC triple mica went for even more), wouldn't it make perfect sense? It would mean altering the circuit, yes, but it's just a twist, not a change in topology. I'm kind of new to tubes, so I may be overlooking something, but I believe one could get a couple of hours more from a pair of tubes that way.

 

[MX117.MC7150]

While a clever idea, what you might be overlooking is how good some of the new Russian reissue tubes sound. Gold Lion and Tung-Sol gold pin are both quite nice and reasonably priced. And, to my ears, some of the JAN tubes are also very nice and they are still in good supply.

 

[drillher]

Nice well documented thread which will help future restoration projects.Did you keep the black beauty caps?I was told that they are PIO caps that usually are still good,unlike the bumblebee caps which tend to leak.
Dave

 

[Rodeodave]

Hm, yeah, there are some seemingly nice new production tubes, but still, it would be more economic to use both systems. I thought I'd bring it up for discussion and get some opinions.

And creating a thorough and well documented thread was the plan, so it seems to work out so far, thanks

About the Black Beauty caps...the only place where they are used in my amps is decoupling of the driver stage (0.22uF600V). All others are striped bumblebee caps. Hm, I tend to replacing them all with Sprague Orange Drops 716p (film and foil, tinned copper leads, non magnetic). The newest MC75s use Wima MKP10, but they're only good for pcb.
I collected some Black Beauty caps, but I ended up selling them to some guitar guy in Taiwan. For such high voltages as are present in the MC75 I want some new and reliable parts. Almost forgot, interesting article here: http://www.paintedpostcards.com/misc2/BlackBeauties.pdf
Other than the 716p I could get some Sprague 730p, any opinions on these?

I removed the first capacitor bank (C15 and C16, the multisection caps) and tagged the wires. Being a solid state guy the wires are surprisingly thin, I'm more used to like 12AWG and such. I also redid the annotated pic of how the PSU is implemented irl.

Now off to decipher the voltage doubler and the bias supply.

 

[Rodeodave]

Okay, here's what will happen. Just like with the other capacitor bank, I'll desolder everything and tag the wires. The colours in the brackets are also denoted in the schematic (BLU, RED).

The capacitor in the bias supply (12uF250V, C17) will be replaced by a F+T 22uF350V axial type electrolytic cap (http://www.tube-town.net/ttstore/product_info.php/info/p2257_F-T-22--F---350-Volt--axial.html). The thing is suspiciously cheap.
As mentioned in an earlier post I initially replaced it with a Sprague TVA 1906 (20uF500V) but it was too fat and I couldn't close the bottom lid. I believe the TVA 1608 (20uF350V) would fit.

But first I'll have some cupcakes and coffe, and some shopping for the weekend maybe...