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McIntosh MC40 lights DBT (almost home)

BTW, did anyone notice the bias rectifier`s & bias cap`s tie point to the terminal strip has been completely broken off from that end of the terminal strip next to the terminal strip`s chassis attachment bracket ?
Might wish to address that attachment point to prevent any future potential short circuit problems in that area.
 
Back again with another. :rolleyes:

I have been enjoying my amps with my Tannoy project since my last post. These are really nice. The nicer amp looks to be (mostly) untouched so I took some measurements that were within reason and have been playing the heck out of them. Someone answered an old ad, so I lucked out and got another nice one. It had a blown fuse and faulty tubes, so I replaced them and brought it up over a few hours. Took some measurements that were low, so I restuffed the cans. It's a miracle I didn't have to order anything. Checked the bumblees, 12 uF and resistors in that area. They all check out unloaded. Thought I had it when the first measurement was in the 460s. SR3 had nearly a MOhm resistance, so I thought I had it again. Replaced with 1N4007 and no resistor. I put a 500R in there and I don't recall it changing anything, which was weird, so I took it out. My other amp doesn't have a resistor there. I was hoping we could get this going so I would have a pair of nice ones. I think I'm close. Thanks for reading.

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I’m confused.
What are the red numbers you show? Your measurements?
SR3 appears to be the negative bias supply.

Ron
 
Yes. The red numbers are my measurements. With regards to the capacitors, it's what I replaced the OE values with. When I say, "I think I'm close". I mean I think I'm close to getting there. Some of the numbers are a good bit off. Maybe a component or two to bring things in line. I don't know what to check. In what I gather, substituting the selenium rectifier with a diode and adding a 470R is to bring the negative bias in spec. Thanks

Getting things to post on the forum is a bit challenging lately. I hope this isn't the beginning of the end

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Are those numbers with all tubes installed? Looks like zero current draw on everything which is either no tubes or no heater supply
 
Putting the tubes in helped. I knew I was a couple components away from getting there.:rflmao:I put the 500R in series with the diode. I'll get a 470R 5W, just don't have one. My rusty patina amp didn't need a resistor, this one does. The voltages aren't perfect, but it makes good power and the square waves are pretty good. The other thing I need is a 2.2 microhenry inductor. I had to solder one of those tiny wires and luckily it is making contact. I looked on Mouser, but didn't see a matching component. Are they readily available? Thanks

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Making the resistance in series with the diode smaller will make the negative supply even more negative. Its very slightly too negative now. Not enough that I'd mess with it, but I wouldn't want it even more negative

Your output tubes may be tired, cathode voltage is low and uneven despite other voltage being very close.
 
There ya go! I'd call those spot on as you are subject to line changes etc.
I'd also say you are fine with that coil fix. I highly doubt you will find a replacement.
You have an 8 ohm load on the output right? Square wave looks a little ringy. Maybe compare it to the rust belt amp?
Good job man.
Ron
 
MissionX said:
Putting the tubes in helped. I knew I was a couple components away from getting there.:rflmao:I put the 500R in series with the diode. I'll get a 470R 5W, just don't have one. My rusty patina amp didn't need a resistor, this one does. The voltages aren't perfect, but it makes good power and the square waves are pretty good. The other thing I need is a 2.2 microhenry inductor. I had to solder one of those tiny wires and luckily it is making contact. I looked on Mouser, but didn't see a matching component. Are they readily available? Thanks

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Mouser, among others, has them in stock.
 
gadget73 said:
Your output tubes may be tired,
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My output tubes are new JJs matched quad from Eurotubes. Less than a hundred hours. Maybe not so matched?

On the inductor, is there some kind of electrical spec I need to follow like ampere rating, resistance and wattage? Thanks



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MissionX said:
My output tubes are new JJs matched quad from Eurotubes. Less than a hundred hours. Maybe not so matched?

On the inductor, is there some kind of electrical spec I need to follow like ampere rating, resistance and wattage? Thanks



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For reference, in the SAMS manual for the MC30, the 2.7 uH choke was listed as a Miller 4606. This choke is rated at 1500 ma. If I was replacing the chokes in an MC 40 I would use 1500ma or above (probably overkill) and let physical size be the other deciding factor. On the other hand, these devices rarely fail unless their current rating is exceeded in service (think red plating tube).
 
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