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Help on McIntosh MC30 rebuild

Correction to an error I made above, which you all probably realized: filament voltages on both amps have been consistently at 6.7vac at my typical mains voltage of ~122.5vac.

But at the 131vac voltage required from the variac to get the 6L6 plates up to 440vdc, filament voltages went to ~7.15vac across the board.

I corrected my previous post and the voltage measurement attachment as well for folks in a similar predicament down the road.
 
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CL-80s will be here tomorrow. Couple quick questions:

Do I install these one per amp, in series on the hot (fused) side, before the fuse, i.e., on the wall side of the fuse?

Am I aiming for 6.3vac on the heaters when the plate voltage is brought to 440vdc using the variac? Or do I want to see 6.3vac at idle on my mains ac?

Same question for current - do I want to hit the idle current draw target at idle on my wall ac mains voltage, or at whatever 13xvac I need to get 440 on the finals?

Thanks again
 
A thermistor on the mains will lower all of the voltages, not just the heater voltage.

Electrically it doesn't matter a ton if its before or after the fuse, but if I have a choice in the matter I prefer the fuse to be the first thing the cord connects to. It gets hot, so keep it away from anything that won't appreciate it.

Get the heater voltage and the idle current correct, let the rest fall where it may. Did anything change when you replaced the rectifier tube?
 
gadget73 said:
Get the heater voltage and the idle current correct, let the rest fall where it may. Did anything change when you replaced the rectifier tube?
Click to expand...

Hopefully the thermistors will get me where I need to be tomorrow. No significant changes using a new pair of matched NOS 5U4GB rectifiers.

Am I correct that I should be looking for the right idle current and heater voltages using my mains ac?

gadget73 said:
A thermistor on the mains will lower all of the voltages, not just the heater voltage.
Click to expand...

I'd like to install the thermistors wherever's best...I probably shouldn't have, but I assumed @dcgillespie and others were referring to installation of the thermistor on the mains.

So should I install the thermistors at the mains or the heaters?
 
Please stop fixating on the B+
Get the filament right. The other voltages will fall slightly but not to the detriment of anything.
I think the collective is, there are no underlying issues. Macs in generally met and often beat specs. if your Mc 30s make 30 watts rather than 32 your won't hear any difference whatsoever.
Oh as stated before stay with the 125v tap. It'll be a much easier starting point to work from.
 
nj pheonix said:
Please stop fixating on the B+
Get the filament right. The other voltages will fall slightly but not to the detriment of anything.
I think the collective is, there are no underlying issues. Macs in generally met and often beat specs. if your Mc 30s make 30 watts rather than 32 your won't hear any difference whatsoever.
Oh as stated before stay with the 125v tap. It'll be a much easier starting point to work from.
Click to expand...

Thanks for this. You’re giving me too much credit - I think I’ve been fixating on all of it without having much idea of what I’m doing. Appreciate the help. I’m not so much concerned with wattage (I’m running 99db Altecs), as I am with having it as right as I can within reason and calling it ‘done’ - but I’m def focusing on bringing the fils down with the thermistor as you all have suggested.

Unless you recommend otherwise, I’ll be putting one thermistor in series on the mains/hot per amp. I got four of them and can do both hot and neutral if that’s preferable.

I’ll report filament voltages and current draw at idle once I get that done.

I will say that as is, the amps are sounding phenomenal.
 
Thermistor update: installed a CL-80 in series with the hot/fused side of the mains, immediately after the fuse. Did amp #1 first.

Filament voltage came down from 6.7vac to 6.53vac

Current draw stayed the same, or if anything went up slightly: I measured .75vdc on v5 and .94vdc on v6, resulting in 56.8ma current draw on v5 and 59.4ma on v6.

B+ voltages went down as expected.

Two questions:

- 6.53vac is right at the +4% mark @dcgillespie mentioned for acceptable heater voltage range. Am I good here, or can/should I drop voltages further (I have two CL-80s per amp to use if necessary)?

- if heater fils are ok, should I drop a value down on the bias resistor to lower the current draw?

Getting close!
 
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