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Another MC225 rebuild-voltage tolerance

G

Gjoslyn59

New Member
I am a new member who joined to because my old mc225 died and I decided to fix it myself. I am a genetic engineer so am comfortable with technology but have no electronics training.

The sudden death of my mc225 turned out to be a shorted output transformer--it is now being rewound by doc Hoyer. While the unit was open, I decided to perform a thorough restore as I learned reading posts: power supply caps, bias rectifier replace, bumblebee caps replace. Also checked resistors and about 25% were off spec so I decided to just replace them all.

My question is about the voltage specifications. Measurements at the tube pins are all within 10% except for the low voltages on what I think is the cathode for the 12ax7 and the 7591 tubes. They measure 20% high (1.2v, spec 1v) and 50% high (1.2v, spec 0.8v) respectively. While not stated in the mc225 service manual, other McIntosh manuals state a 10% voltage tolerance. So, are voltages within 10% good enough and are the high cathode voltages a worry?

Thanks in advance for any insights you can provide.
 
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All bumblebee, bias and power supply caps were replaced. Per advice from this forum, the brown capacitors are reliable and were left in place. All resistors were replaced with units I measured and are within 1% of spec. Caps were also verified by measuring before installing.
 
The voltages sound like they're pretty consistent, then. Have you checked for proper bias voltage on the 7591s? The procedure involves a variac and is discussed in the Hints and Kinks section. If you replaced the bias rectifier and didn't confirm this, that's your next step.

BTW, can I ask what the transformer rewind set you back? PM me, is appropriate.

Cheers,

David
 
Thanks for the interest.

Yes, the bias voltage has been corrected to -20.6V; within 2% of the specified -21V. All other voltages are within 10% or less with the exception of the low voltages on cathode of the 7591 and 12ax7 where they measure 0.5V high which is a big percent of a specified 1 volt. Does this need to be corrected or is it within tolerance?

The OT rewind is a $650 job.
 
I talked to the local electronics repair shop and they confirmed that voltages within 10% are within specification. Also said that my high voltages at the cathode pin, 1.3V for a 1V specification was acceptable.

Any more knowledge out there?
 
G - In "setting the bias" such as it is with McIntosh amplifiers, the specified voltage on the schematic is only accurate when the specified B+ is present. Typically however, with today's AC line voltages, all voltage levels will be somewhat greater, including the output stage B+. However if you set the bias to the specified level, yet the B+ voltage is at a higher than specified amount, then the voltage at the cathode terminal will be greater. Therefore, when setting the bias voltage, first make sure that the specified B+ is present, and with that level maintained, then make the necessary adjustments of achieve the correct bias voltage. This will invariably require use of a variac to achieve the correct B+ level so that the proper bias adjustment can be made, and ultimately then help to ensure that the resulting cathode voltage is in line as well.

Dave
 
Thanks Dave--fortunately I do have a variac and did set the bias as you described. The negative bias is set -20.6 (spec is -21) at the specified b+ (390V) which was achieved at the specified 117V AC input. I did not hit it exactly since I replaced the original resistor with one I had on hand. I assumed this 2% difference is within tolerance. Need I be more accurate with this voltage?
 
OK, good. The only other variable then is the output tubes themselves, which of course as a manufactured device will all display a tolerance relative to their bogey characteristic -- ergo, the bias controls that more common amplifier designs need. With modern tubes, that tolerance can even extend to the brand itself. Therefore, if you are using the new Tung Sol 7591 tube for example, it is almost a guarantee that your cathode voltage will be higher than specified, even with the grid bias voltage properly set as described, as these tubes are known to run hot for a given bias voltage.

Understand too however that in the McIntosh Unity Coupled output stage, unlike more common designs, the bias setting of the output stage is quite uncritical. That's the reason that no bias controls were offered in the first place. The factory bias setting operates the tubes cool enough that at least back in the day, normal tube to tube variations in their characteristics still allowed any of the tubes to operate well within their design specifications, even if the hottest tubes were installed. And, there is so much local NFB applied to the output stage that full factory specifications will be met even if the coldest tubes are installed. This is quite unlike typical pentode designs of the day, like that used in (say) a Fisher X-202B. In this model, the same tubes are operated so close to their maximum ratings that a full complement of bias controls are provided. Without them, the normal variation in tube to tube characteristics would then either have some tubes red plating, or others operating so cool that performance specifications could not be met.

Dave
 
Thanks for the great mini tutorial. It is appreciated. Your bias explanation reinforces and clarifies ideas I had gathered from Google research.

Until you brought it up, I was not considering tube performance as a variable. I did just install a new set of tubes, electroharmonix, while I am messing around with restoring the amp to protect my old stock Sylvania tubes from accident. I will be sure to repeat the measurements with my old tubes.
 
I hope this is the right place for this. I replaced all power supply components as well as the bias components. Bias is on the money, but the output of the doubler is 434 volts, and after the 100k/82k divider it's 273 volts. I've replaced all of the bumblebees except the noise suppressors after the power supply transformer. It shows a great square wave into a resistive load, but I'm afraid the outputs might crater, despite the fact that their max plate voltage is 550 volts. Any ideas?
 
Joe Reeder said:
I hope this is the right place for this. I replaced all power supply components as well as the bias components. Bias is on the money, but the output of the doubler is 434 volts, and after the 100k/82k divider it's 273 volts. I've replaced all of the bumblebees except the noise suppressors after the power supply transformer. It shows a great square wave into a resistive load, but I'm afraid the outputs might crater, despite the fact that their max plate voltage is 550 volts. Any ideas?
Click to expand...
As you can't yet start a new thread, yeah - this is a good place. Maybe @dcgillespie or @clinic-audio can be of hep.
 
Joe Reeder said:
I hope this is the right place for this. I replaced all power supply components as well as the bias components. Bias is on the money, but the output of the doubler is 434 volts, and after the 100k/82k divider it's 273 volts. I've replaced all of the bumblebees except the noise suppressors after the power supply transformer. It shows a great square wave into a resistive load, but I'm afraid the outputs might crater, despite the fact that their max plate voltage is 550 volts. Any ideas?
Click to expand...
It would help (me for sure) f you gave the voltages at test points specified in the service manual, that being from "Table 1. MC225 VOLTAGE AND RESISTANCE CHART".
 
Pio1980 said:
Voltages off the skiz should be ref'ed for 6.3 VAC across the heater bus lines, adjusted by variac.
Click to expand...
Translation:
"off the Skiz" = not reflecting normal expected values - slightly closer to expected value than 'off the chart'
"ref'ed" = referenced ( less four pesky keystrokes)
The whole sentence = Do not just set the variac to 117V - set it such that the filament (heater) voltage is at 6.3 after warmup and then adjust bias.

I would be curios to know how close the settings on the variac would be if set by the three ways I see mentioned in this post.
  1. set to 117V
  2. set to correct B+ Voltage
  3. Set to 6.3V on filament line
I use the filament method and have never really tested to see the difference between these methods. I would think they would yield a similar setting on the variac - but maybe not??
 
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