Dual tester for 4P1L in triode strapped

Dual tester for 4P1L in triode strapped

I build this tester few months ago. All started when I decide to burn in a large stack of 4P1ls. First I built a small burn in tester for a single tube. It worked well but it took  a long time to burn in so many tubes.

The final design is double 4P1L in a setup that allows one to determine if tubes are a good pair. This a way to just preselect the tubes with closest characteristics at a certain operating point. A real pairing of tubes can only be done by tracing their plate curves.

The schematics   description is; tubes cathodes are connected together, but anode are separate loaded with CCS  adjustable by 10 turns wirewound pots. A sensing 100ohms resistor is placed in the each anode circuit for Gm measurement. Each anode current is displayed for easy adjustment. Tester is provided with RCA signal input connected to each control grid G1 through a 100nF cap. A switch is provided to choose between measuring mu factor and Gm. Anode current of each CCS has to be precisely equal in order to give good measuring results.

Tester is good for the following purposes:

  • burn in tubes (24 or 48Hours)
  • measure mu
  • measure Gm
  • measure THD
  • preselect tubes for pairing

This tester is good only for 4P1L tubes, but it can be modified and redesigned to fit multiple tube testing. The reason this works great for me is that all my present designed amplifiers are using mostly this amazing DHP.

Here are some pictures:ImageImageImageImageImageImageImageImageImage

photo

Design shows the CLC and shunt regulator for B+ and Coleman regulator for filament power supply. I tried to make the power supply as clean as possible, so the measured results for THD, mu and Gm are not altered by a undesired ripple.

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