Re: Hmv 1125
I ran some tests on a good used ECH81 on the Avo VCM163 valve characteristics meter. Here's the detail:
Using test parameters from the Avo manual (Va=250v, Vg2=100v, Vaosc=100v), gave the results as follows (spec values in brackets):
Triode (Vg=-3v) Iaosc=4.4mA (5.0) gm=1.6mA/v (2.3)
Heptode (Vg=-2v) Ia=4.2mA (6.5) gm=1.1mA/v (2.4)
Using the supply and bias conditions as close as poss to the HMV1125 circuit (Va=175v, Vg2=60v, Vaosc=90v) gave the results as follows:
Triode (Vg=-3v) Iaosc=3.3mA (5.0) gm=1.5mA/v (2.3)
Heptode (Vg=-2v) Ia=1.3mA (6.5) gm=0.5mA/v (2.4)
This test indicates that although the triode oscillator should work OK, the gain of the Heptode mixer is likely to be considerably reduced and therefore the set might sound deaf. Things can be improved simply by increasing the screen (g2) voltage from 55v to 100v: using this value, still with Va=175v, gives for the Heptode:
(Vg=-2v) Ia=4.2mA (6.5) gm=1.1mA/v (2.4). Increasing the screen (g2) voltage can be achieved by replacing the grid2 potential divider resistors R3 (15k) with 8.2k and R4 (12k) with 39k. So if the set is deaf then I would give that a try. Cheers, Jerry
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