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Old 11th Jul 2020, 10:19 am   #22
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 15,924
Default Re: Hope to start an audio amp with CV415/CV4046/TT15

For audio, the EF91 wasn't designed for low microphony, and you'll find the available gain somewhat less than an EF86.

They were designed as an RF/IF amplifier as a later generation replacement for the EF50. But in the 7-pin bottle it lacked multiple cathode pins to keep the inductance down for the best high frequency performance, so the EF80 took over and ran away with the TV IF market. The EF91 can be a bit unstable with the bandwidth of its gain and the barely adequate cathode connection. If one was in use at audio it would not be too hard to have RF oscillation going on and that could be noticed by the effects of moving things near it. As a youngster I bought a very big box full of ex-equipment EF91/6AM6/Z77 and used them for a few years playing with building oscilloscopes. I learned the hard way how to keep them stable. I also learned that lower impedance circuits were needed to go fast, and that If I still needed the same voltage swing, the current became higher. I wound up with 5763s driving deflection plates with lowish value big fat hot anode resistors. I hadn't made the break through to understanding active loads, still less distributed amplifiers at that time.

Anyway, the input stage of a power amplifier really shouldn't need anything as sensitive as an EF86. It's good practice to not have very small signals in places they don't have to be.

Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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