Klausmobile Tube Tester Files

Reflector (Saratov), 12/1972 - Closed box plate type. Referred to as ECC88 clone. There are two designs of this tube that I know: one has box-shaped plate, consisting of two Pi-shaped halves, closed on all four sides. These tubes were marked with Reflector logo, dated 1970. The other type (Voskhod Kaluga logo, dated 1980-s) has a Pi-profile plate, missing one side so the grid/cathode assemblies are visible. Today, I checked 8 unused tubes of both varieties, dated 1971 to 1985, and 7 of them measured surprisingly close to each other and the tubes' halves were also close. Unfortunately, one was emitting weakly, just opening at zero grid level. So what, at 1/4 dollar a piece it's not a great loss.

However, none of the tubes ever came close to the operating point's Ug, that is, specified Ua/Ia is achieved at -4 .. -3V grid level not ar -9V specified. Even more confusing are Vade Mecum numbers for ECC88 (Ug=-1.2V for 90V/15mA through plate).

Standard specifications per each triode
Parameter Maximum Operating Point Measured - 12/1972 tube
Ua, V +300 open -470 closed 100 100
Ug, V -200 (-9.0) -1.0
Ia, mA 20 continuous 200 pulse 15 (10..20) 15
Pa, W 1.8 (1.5) 1.5
Gm, mA/V - 10 .. 12.7 10.6
Rp, kOhm - (2.0 .. 4.6) 3.3
Mu, V/V - 34 (25..43) 33

Looking at the curves, you see that standard 15mA/100V (Ug=1V) is too close to where grid begins conducting - why not shift it to lower Ug?

1. Curve at normal Uheater, full operating power

2. Same at Uheater=5V. S,Mu,Rp are identical, only grid levels shift by 1/4V.

3. Closer look at specified operating point (normal Uheater)

4. 8mA closeup

5. As the hybrid amp first published Here (+/-35V powering ECC88 diff amp and a class A FET current stage) is copied, let's look at 6N23P performance in this circuit. Quite dissappointing... Too close to grid current zone (yellow line) where driving stages will be overloaded. 6N30P will be much better in this role (3mA-30V).

6. Gm/Mu/Rp measurements. Very consistent for all 7 tubes I checked, despite different plate designs.

Legend: White curves: Ig<5uA Yellow: 5<Ig<20uA Red: 20<Ig<300uA. Measurements terminate at Ig>300uA

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