My advice is not to use WD-40 to clean the amps. It's an oil, and will leave a residue.
they don't look "dirty".
A regular cleaner like 409 or Fantastik is more than strong enough to remove residual goo and dust.
A run through the dishwasher is possible, but doesn't seem necessary - and depending on how the chassis labeling was done, it might wash off.
This is clearly and industrial application incorporating the McIntosh amps as a power stage...
so one problem may be that they are not "amplifiers" as they now sit, and you will have to re-modify them back from their present application...
You should rid yourself of this terrible and onerous problem and project and send them to me, so that they may not darken your door again!
_-_-bear
they don't look "dirty".
A regular cleaner like 409 or Fantastik is more than strong enough to remove residual goo and dust.
A run through the dishwasher is possible, but doesn't seem necessary - and depending on how the chassis labeling was done, it might wash off.
This is clearly and industrial application incorporating the McIntosh amps as a power stage...
so one problem may be that they are not "amplifiers" as they now sit, and you will have to re-modify them back from their present application...
You should rid yourself of this terrible and onerous problem and project and send them to me, so that they may not darken your door again!
_-_-bear
Ah, and I think I know what the amps where used for. The late owner used to work as an engineer and part of his work seems to have involved aviation. I remember his wife telling me about some resonance-testing of wings. So there you go, a shaker amplifier indeed.
The outlet next to the output tubes has four pins, two for heater connection (parallel) and two for B+ and ground. The tube making up the oscillator-circuit was fed from one amp this way. The other amp's outlet simply fed a power-indicator lamp.
I very much hope that I won't get shook by the amp as I'm a toob-noob, at least practically.
And bear, thanks for the kind offer
But I feel ready to take the burden on me and fix 'em. And I'm still not convinced that WD-40 is a bad idea for chrome...
The outlet next to the output tubes has four pins, two for heater connection (parallel) and two for B+ and ground. The tube making up the oscillator-circuit was fed from one amp this way. The other amp's outlet simply fed a power-indicator lamp.
I very much hope that I won't get shook by the amp as I'm a toob-noob, at least practically.
And bear, thanks for the kind offer
But I feel ready to take the burden on me and fix 'em. And I'm still not convinced that WD-40 is a bad idea for chrome...Attachments
What a good find....
You asked about a date of mfg,
the sandstone resistors in one of your pictures seems to
be made in the 38th week of 1963 so I would suspect
somewhere around 1963.
Good luck in your rebuild these are very sought after amps
and you will enjoy your good deal.
Steve @ Apex Jr.
You asked about a date of mfg,
the sandstone resistors in one of your pictures seems to
be made in the 38th week of 1963 so I would suspect
somewhere around 1963.
Good luck in your rebuild these are very sought after amps
and you will enjoy your good deal.
Steve @ Apex Jr.
Oh - my bad, they are just two amps in the bigger chassis, I didn't look at all three pix...
For the chrome? I guess it will "work"... I'd of course wipe it clean with a dry soft cloth...
As far as parts substitution - if you want maximum resale to collectors, then keep it stock. Otherwise, better parts including polypropylene coupling and decoupling caps will likely sound better.
The circuit is very good as is. Attempting to change it to DC coupled will make it into another amp. You can look at the schematic of the McIntosh 50-W for some ideas...
_-_-bear
For the chrome? I guess it will "work"... I'd of course wipe it clean with a dry soft cloth...
As far as parts substitution - if you want maximum resale to collectors, then keep it stock. Otherwise, better parts including polypropylene coupling and decoupling caps will likely sound better.
The circuit is very good as is. Attempting to change it to DC coupled will make it into another amp. You can look at the schematic of the McIntosh 50-W for some ideas...
_-_-bear
Rodeodave... you are fortunate beyond words. Nice find, to say the least.
Wanted to offer a general cleaning tip, and it can be applied to so many other things...
"Start gently, and work your way up"
In many cases, chassis labels, date markings, etc, can be obliterated forever with a simple dry wipe. Many "protective coatings" will wipe away like dirt on contact with the most mild solvents.
I always start with plain warm soapy water, and light pressure.
When I feel I'm rubbing too hard, I go up to something stronger, like 409
Solvents are a last resort, well, second to last, right behind abrasives.
WD40 is an oil suspended in a solvent, is it not?
That, to me, is akin to "Coming on a little strong..."
Buy her/them a drink first. Hot twins. You don't want to blow this one!
regards
art
PS... congratulations.
Wanted to offer a general cleaning tip, and it can be applied to so many other things...
"Start gently, and work your way up"
In many cases, chassis labels, date markings, etc, can be obliterated forever with a simple dry wipe. Many "protective coatings" will wipe away like dirt on contact with the most mild solvents.
I always start with plain warm soapy water, and light pressure.
When I feel I'm rubbing too hard, I go up to something stronger, like 409
Solvents are a last resort, well, second to last, right behind abrasives.
WD40 is an oil suspended in a solvent, is it not?
That, to me, is akin to "Coming on a little strong..."
Buy her/them a drink first. Hot twins. You don't want to blow this one!
regards
art
PS... congratulations.
Cool, makes perfect sense, nice thing to look out for. Thanks! btw, the serials are 320B3 and 320B4. Is there a way to determine who's the evil twin?
I don't plan to sell the amplifiers anytime soon. I have always wanted to have a tubeamp, let alone monoblocks, and it seems that as far as pentodes go this might be my first and last
But seeing how nicely it is built I for now hesitate to alter anything inside. I'm in no hurry so I can make a thorough plan. The thermistor and the electrolytic capacitors objectively are a safety hazard. Reforming and/or NOS electrolytics aren't an option (for the same reasons) so I'll have to replace them. With what I will see.
The film capacitors...hm, I assume the sound and safety of operation won't be infringed by modern technology. Me, I like Mundorf MCaps.
The resistors...safety-wise I think they could stay, but as the service info states they are 10% tolerance (5% for some critical ones). Noise figures are also likely to improve with MOX resistors. Hm.
I'll keep that in mind.
I cleaned them with a cloth dampened with slightly soapy water, wiped with a cloth dampened with plain water and dried it with some soft cloth. The labels stayed on, just like the speckled rust film. For now I've set them aside. We're in no hurry.
Do you have any advice on how to clean the tubes? Some have their labels made of that white stuff that already disappears by looking at it, some (like the KT88) seem to have their logos etched. And what about the pins?
And finally I'll quote myself and answer my own question:
On page 2 the manual states that the thermistors cold value is "about 79 ohms" and that the cold value is "less than .7 ohms". That should narrow it down.
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Remember, a GNFB loop, with its homogenizing effect, is present. If you want to buy something upscale for the interstage coupling positions, consider Soviet surplus PTFE dielectric parts. Run in for PTFE is a "bear", but it will maximize transparency.
Metal oxide resistors are nasty. Grid stoppers should remain Carbon composition, as that's the only construction both non-metallic and non-inductive. Carbon comp. parts in other positions, which have drifted or gone noisy, can be replaced by Carbon film stuff. Find a "local" distributor for KOA-Speer. If worst comes to worst, buy from Mouser.
The Selenium bias rectifier (SR3) is a ticking toxic time bomb. Install a UF4007 and add a dropping resistor to account for the reduced forward drop.
Replace the B+ doubler diodes (SR1/SR2) with UF5408s, as the OEM parts are very noisy by contemporary standards.
I recommend if you have to replace resistors in the amplifier due to aging related value drift that you use good quality metal films as opposed to metal oxide (MOX) as some of these have measurable linearity issues. (I have a long established bias against the colorations IMHO they add in 0 feedback circuits.)
This design uses a lot of negative feedback so the critical resistors in terms of sound quality would be the input and feedback resistors (including the resistors in the cathode circuit of the first stage.
Replacing coupling capacitors is probably advisable as it is hard to know what McIntosh was using at any given time. I don't think you will hear a lot of difference because of the high feedback levels regardless of the coupling caps you put in there, so I would not get carried away with extremely expensive caps - mid range Mundorf ought to be fine.
Hopefully those KT88 are in good shape and will give you many more years of good service before they need to be replaced.
Replacing the electrolytics in the power supplies is probably necessary at this point. Antique Electronic Supply manufactures a line of FP (twist lock) style caps - whether or not they make the appropriate values is another question. You could use the appropriate values with brackets from JJ or FT instead which affects the look, but since they are inside you may be ok with this.
This design uses a lot of negative feedback so the critical resistors in terms of sound quality would be the input and feedback resistors (including the resistors in the cathode circuit of the first stage.
Replacing coupling capacitors is probably advisable as it is hard to know what McIntosh was using at any given time. I don't think you will hear a lot of difference because of the high feedback levels regardless of the coupling caps you put in there, so I would not get carried away with extremely expensive caps - mid range Mundorf ought to be fine.
Hopefully those KT88 are in good shape and will give you many more years of good service before they need to be replaced.
Replacing the electrolytics in the power supplies is probably necessary at this point. Antique Electronic Supply manufactures a line of FP (twist lock) style caps - whether or not they make the appropriate values is another question. You could use the appropriate values with brackets from JJ or FT instead which affects the look, but since they are inside you may be ok with this.
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I meanwhile identified all electrolytic capacitors in the circuit and found a source for replacements. Authenticap Elektrolytkondensatoren sells Authenticap twistlock FP-electrolytics.
Here's a list of the electrolytic caps and their replacements:
250uF250V PSU with minus ground (C13)
250uF250V PSU with minus hot (C14, black isolation)
Replacement: Auhenticap KTL3, isolated can. Direct replacement.
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. Or is it a bad idea to increase the capacitance?
12uF250V bias, axial (C12)
12uF250V cathode KT88, axial (C9, C10)
100uF3V cathode 12AX7, axial (C3)
Replacement: It shouldn't be too hard to find capacitors for these values (askjanfirst stocks a variety of axial elkos, and also Farnell). C12 will probably stay electrolytic, C9 and C10 I'd like to replace with foil caps. For C3 I had an OSCON in mind but read a lot of bad things about them. Are they really that bad/dangerous (failing in short circuits, no voltage headroom,...)?
And here's a list of the 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: C11 and C12 will be replaced, not sure about the others yet. But should be fairly easy to find replacements.
Then there's two caps I'm not so familiar with:
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...
As for a replacement of the rectifier diodes I'll go with Eli's advice and use UF5408. For the selenium rectifier I'll use UF4007 and a potentiometer to set the bias. Once the voltage is set I'll put in a resistor.
Speaking of which, Frag Jan zuerst- Ask Jan first: Roehren und mehr also stocks quite some carbon resistors. I'm used to metal film resistors and MOX or WW for higher dissipation, but I'll give the carbons a try. Morgan Jones seems to really dislike them "as their tolerance and noise specifications are so very poor" (M.J. Valve Amlifiers 2nd Ed. p124).
I think I'll start with a slight makeover, keeping the looks but with modern safety standards (electrolytes, fuses, thermistor, diodes...).
Thanks for reading and also for your replies,
Dave
Here's a list of the electrolytic caps and their replacements:
250uF250V PSU with minus ground (C13)
250uF250V PSU with minus hot (C14, black isolation)
Replacement: Auhenticap KTL3, isolated can. Direct replacement.
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. Or is it a bad idea to increase the capacitance?
12uF250V bias, axial (C12)
12uF250V cathode KT88, axial (C9, C10)
100uF3V cathode 12AX7, axial (C3)
Replacement: It shouldn't be too hard to find capacitors for these values (askjanfirst stocks a variety of axial elkos, and also Farnell). C12 will probably stay electrolytic, C9 and C10 I'd like to replace with foil caps. For C3 I had an OSCON in mind but read a lot of bad things about them. Are they really that bad/dangerous (failing in short circuits, no voltage headroom,...)?
And here's a list of the 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: C11 and C12 will be replaced, not sure about the others yet. But should be fairly easy to find replacements.
Then there's two caps I'm not so familiar with:
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...
As for a replacement of the rectifier diodes I'll go with Eli's advice and use UF5408. For the selenium rectifier I'll use UF4007 and a potentiometer to set the bias. Once the voltage is set I'll put in a resistor.
Speaking of which, Frag Jan zuerst- Ask Jan first: Roehren und mehr also stocks quite some carbon resistors. I'm used to metal film resistors and MOX or WW for higher dissipation, but I'll give the carbons a try. Morgan Jones seems to really dislike them "as their tolerance and noise specifications are so very poor" (M.J. Valve Amlifiers 2nd Ed. p124).
I think I'll start with a slight makeover, keeping the looks but with modern safety standards (electrolytes, fuses, thermistor, diodes...).
Thanks for reading and also for your replies,
Dave
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Dave,
Over use of metal film resistors can lead to an ugly, "etched", sound. Use MF parts where their low noise is of benefit. Look at the schematic. R1, R2, R5, R7, R9, R15, R16, R19, and R20 are places where 1% tolerance RN65 metal film parts look good to me.
R4 should remain Carbon composition. R29 through R33 can be MOX. 12.1 KOhm non-inductive WW Mills MRA-12 parts should be satisfactory for R18 and R21. KOA-Speer Carbon film parts, of appropriate power rating, can go everywhere else.
R13 and R14 can contribute to noise, but the high potential difference across those parts is a warning against the use of MF construction.
Over use of metal film resistors can lead to an ugly, "etched", sound. Use MF parts where their low noise is of benefit. Look at the schematic. R1, R2, R5, R7, R9, R15, R16, R19, and R20 are places where 1% tolerance RN65 metal film parts look good to me.
R4 should remain Carbon composition. R29 through R33 can be MOX. 12.1 KOhm non-inductive WW Mills MRA-12 parts should be satisfactory for R18 and R21. KOA-Speer Carbon film parts, of appropriate power rating, can go everywhere else.
R13 and R14 can contribute to noise, but the high potential difference across those parts is a warning against the use of MF construction.
Thanks Eli, very good input.
Now all I have left to do is raise funds to finance the overhaul
Here's a link to a review of the 60th anniversary edition of the MC75 with the first pics it's insides I could find: http://www.mcintoshlabs.com/downloads/product_downloads/60_anniversary_system_French_Review.pdf
And here's a nice tube database I stumbled across: Nostalgia Air Tube Substitution & Characteristics Guide Very helpful!
Now all I have left to do is raise funds to finance the overhaul
Here's a link to a review of the 60th anniversary edition of the MC75 with the first pics it's insides I could find: http://www.mcintoshlabs.com/downloads/product_downloads/60_anniversary_system_French_Review.pdf
And here's a nice tube database I stumbled across: Nostalgia Air Tube Substitution & Characteristics Guide Very helpful!
McIntosh MC-240 Restoration - AudioKarma.org Home Audio Stereo Discussion Forums features a rebuild of a MC240, with step-by-step pictures! Awesome.
The "12uF250V bias, axial (C12)" is C17 in the schematic, not C12. I think I'll put a 22uF in it's place.
Edit: Is it wise to put a 450V electrolytic in a place where it only sees 150V? Wouldn't the oxide layer grow thinner due to the initially lower leakage current?
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Temporarily continued here: Refurbishing two MC75 Monoblock Amplifiers - AudioKarma.org Home Audio Stereo Discussion Forums
Here are some pictures of the isolation transformer. It's probably the maximum of what I can safely lift and carry. 4000VA with a 28A 115V secondary. That should do it. Didn't fire it up yet, hope it's quiet.
The company "Getra" is located in Bavaria, Germany and still exists. I haven't mailed them yet, so I don't know for sure what the SI, SII and SIII taps are for. Are these screens?
The company "Getra" is located in Bavaria, Germany and still exists. I haven't mailed them yet, so I don't know for sure what the SI, SII and SIII taps are for. Are these screens?
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