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MC-30 and MC-40: choke background

arcsound

arcsound

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Both the MC-30 and MC-40 use uH value chokes in the output stage, in series with the 6L6GC plates.
On the MC-30, they are two different values; 1.5uH and 2.7uH, which on the MC-40, they are both 2.2uH.
Couple of questions:
1. What is the purpose of these chokes?
2. Why are they different values on the MC-30, but not on the MC-40 (since the circuit topology is so similar and the output transformers are likely the same or nearly so...)?
I'm trying to source replacements for my MC-40, in anticipation of breaking them while replacing the tube sockets, and have found some 1.5uH NOS, so trying to get a sense of how critical is the actual value, and if I can use them instead of 2.2uH, in a pinch. Thanks in advance for any insight and/or opinions..
 
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1. To control parasitic oscillations in the output stage.
2. Different values due to different output transformers specifications and possibly/probably results from testing at the factory driving different speaker loads. Also, over time standard (stocked) values of components have changed over time.
New 2.2 uH axial leaded inductors are available from a number of sources.
 
arcsound said:
. . . 2. Why are they different values on the MC-30, but not on the MC-40 (since the circuit topology is so similar and the output transformers are likely the same or nearly so...)?
Click to expand...

I don't know but that's an interesting question. To clarify a bit, the schematic you (and I) saw shows different values for the coils for the two push-pull output tubes when everything else is identical / symmetric. So it's not like they substituted because they ran out of parts that day or someone has changed one over the years. That's the design. So that's an interesting question.
 
Interestingly, this pattern of different chokes, one amp with equal value chokes, and the other amp with different values, repeats with the MC-60 and MC-70. The MC-60, which I believe was contemporaneous with the MC-30, uses the same 1.5uH and 2.7uH chokes, while the MC-75, which I am assuming was later, and more aligned in time with the MC-40, uses the same 2 x 2.2uH chokes. So, it does almost seems like they were using whatever they got a deal on! (I'm sure this is not the case, just seems oddly coincident....).
 
Glad to see this discussion. I thought these Mc 30 (1614 tube) chokes were resisters .Watthour informed me they were chokes. Having thought they were resisters I took ohm readings. One amp showed 2.1 ohms and the other amp showed 7.1 ohms on the chokes marked Red-Violet-silver. These amps had not been powered up in many years when I took the readings. Not sure if higher ohms reading is hinting a problem or not. If I have to replace them can anyone suggest a good replacement? Thanks Rick
 

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johnny_fever said:
Glad to see this discussion. I thought these Mc 30 (1614 tube) chokes were resisters. Watthour informed me they were chokes. Having thought they were resisters I took ohm readings. One amp showed 2.1 ohms and the other amp showed 7.1 ohms on the chokes marked Red-Violet-silver. These amps had not been powered up in many years when I took the readings. Not sure if higher ohms reading is hinting a problem or not. If I have to replace them can anyone suggest a good replacement? Thanks Rick
Click to expand...
They are available from a number of electronics suppliers (Mouser, DigiKey, etc).
 
Thanks. Any idea why the ohms reading is higher than the other one marked with the same color bands? Rick
 
Those chokes are two different values, I think the MC60 may be similar w/ 2 different values. Although they're only a few uH different I'd expect you to measure at least a little different with the Ohm meter. But 2 vs. 7 is a pretty big difference. Still, it's a choke, so l likely no issue as long as there is continuity. The newer amps like versions of the MC275 seem to have standardized on use of 2.2uH for all locations and you can get them from McIntosh. But there are chokes in many values, including these two original values, at other suppliers if you want to replace w/ what's in there.
 
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