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I guess I'll post this to my Blog!

Posted 2nd June 2011 at 08:01 AM by DigitalJunkie
Updated 2nd June 2011 at 08:08 AM by DigitalJunkie

Inspired by This Thread I decided to build something similar,I've though about it before,and had some parts sitting here,so what the heck!

Amp Schematic

PSU schematic

Here's what I've got at the moment.
25BQ6GTB tubes,(re-)branded DuMont. These are the skinny 6V6-sized bottles.(11W anode rating,and ~200V screen rating)

A pair of those are driving a 3.5K PP output transformer,rated for ~100W. (came from a scrapped Bogen M120 4x PPP 8417 amp.)

Bogen Schematic

Vk (33ohm Rk's) ~1.3V ,About 40ma/tube.
(I saw cathode current peaks of ~80ma per tube,perhaps more? hard to tell with my DMM)

That gives a dissipation of ~18W per tube. Much more current and the plates start to 'blush'.
I did *not* see any screen glow,but the mica's on these particular tubes make it kind of hard to see much inside.

Input to the 25BQ6 tubes is supplied by a 6.4K:8ohm ~20W output transformer used 'backwards' as an input/splitter transformer.
I loaded each half of the input transformers 'secondaries' with an 8K 3W resistor,just on a whim,and because that was a handy value. (This was made with stuff just sitting around,and a half-finished amp chassis)

I have seen peak power outputs of 100-150W, while running 1-second sweeps from ~30hz to 30khz,output does drop off a bit as the frequency increases,but not too much.
Sine waves look pretty good.
Triangle/Sawtooth waves are 'ok',but a bit 'soft' on the points.
Square waves are a bit of a mess.
But,it's still running open loop,with no feedback. (Well,maybe a bit from the cathode resistors.)

More pics and stuff here!

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