Question for Terry DeWick - MR67 Selenium Rectifiers

crooner

crooner

Tube Marantzed
Question for Terry DeWick And Other Mc Techs - MR67 Selenium Rectifiers

Is it safe to run a completely original MR67 with the selenium rectifiers?
The GE seleniums seem to be of better quality than those found in say, Dynaco gear. Haven't heard any stories of Mc gear catching fire or giving away toxic fumes.

So far so good, but I am thinking about putting some 1N4007s in it. Don't have the schematic. If I add modern silicons, what value of series resistor should I add?

Kind regards,
crooner
 
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Yes, have had no failures in the many that I have worked on, replacing them with a 1N4007 is a common practice but if the unit has original filter capacitors it could be fatal. The filters may not be able to take the turn on peak voltage caused by the 1N4007, a series resistor will not solve this surge since the circuits are unloaded until the tubes warm up.
Part of an update on units with seleniums is bridging or replacing them with 1N4007 diodes, but this also includes new filter capacitors as part of the update.
As long as the B+ is correct don't worry about them, the quick check for bad filter capacitors is the tuner on a clean carrier (no music) you should be able to run the gain all of the way up and not increase the hum level significantly. On the bench I can find bad filters or low B+ when I check sensitivity, separation, or THD, they all suffer.

Now that I have rambled on I hope I have answered your question.
 
I replaced the seleniums with silicon diodes and a drop resistor in my MR-67 because they were quite "weak" and the B+ was just too low. I added a current inrush limiter to help provide a slower startup on turn-on. I don't remember the value of the drop resistor, I think I did the math to get it into the ballpark, and then did trial and error to get the voltage where I wanted it. I erred on the lower side of the specified voltage range.
 
More thoughts

Just started thinking about the current inrush limiter. Seems that it would help reduce capacitor inrush current, but the over-voltage "hump" would likely still be present since the CIL is also delaying the filaments from heating and thus delaying tube conduction. In other words, the CIL is just delaying the inevitable regarding the over-voltage "hump" but might help with true capacitor inrush.
 
Thanks for your answers. I think I 'll stick with Terry's suggestion and leave the unit undisturbed. It is amazing how good it performs. I have detected no hum whatsoever. I guess it could benefit from an alignment, but as it is sounds very good to me. It has the lowest distortion of any tuner I've heard.

There is then, some rationale about the use of Seleniums in vintage tube gear. If McIntosh would have opted to use top hat early silicons in the MR67, for instance, it would have required some sort of thermistor to prevent a turn-on surge.


BTW, the selenium is exactly the same as found in the McIntosh MC275 amp I restored many moons ago. I did replace it with a 1N4007, even though it had worked fine since the amp was new.


Kind regards,
crooner
 
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Different ball game in the amplifiers, the bias supply is a critical item, loose it and bye bye a lot more than a fuse. I do replace them in the amps, makes me feel better about long life and safety, rewinding a transformer is expensive…..
 
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