New Production Tubes: For MPX-65

[Freddie73]

Does anyone have good experience and recommendations for new production 12ax7’s in the MPX section of Fisher 400/500c/800c? Or if you’ve tried new and just had better luck with vintage tubes I would love to hear experiences and recommendations.

 

[Dave451]

I have used Sovtek, new production Tung Sol, and NP Mullard in the MPX-65 with good results.
Dave

 

[dcgillespie]

With early version 400 receivers, V100 on the MPX chassis needs to be an original production tube. New production 12AX7 tubes are quite good -- but at the extremes of their plate curves, they don't always act exactly like the original devices. The early version of the 400 receiver does not include an AVC circuit (Automatic Volume Control) to manage the output from the ratio detector, so the output from that stage that feeds the MPX sub-chassis can be very hot on strong stations. This can overload the input stage of the MPX section when new manufacture tubes are used at V100. Original manufacture devices however seem to take a hot signal in stride. Just something to be aware of.

Dave

 

[Dave451]

Good point. My experience has been with late model 400’s and 500/800C’s.

 

[Freddie73]

Thanks. Luckily my 400 is late production. I guess it’s time for a tube tester to make sure my old tubes have semi balanced sections. This hobby is fun and addicting.