current sources

Using tertiary windings to offset DC.
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dave slagle
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Joined: Sat Jan 22, 2005 3:54 am
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current sources

Post by dave slagle »

hey all,

yesterday I came to a startling realization wrt my previous attempts at using a CCS to feed the filaments and an offset winding. My previous attempt had a simple LM338 wired as a CCS feeding an offset winding and the results showed lots of noise and oscillations and my conclusion was that the CCS didn't like driving the inductive nature of the offset winding (60mhy)

Essentially i had built a nice 15V oscillator at 70Khz and when stepped by the 1:50 ratio the 750V across the primary leads gave me quite a wakeup call.

What I failed to remember was that both the Rp of the tube and the speaker impedances would be reflected in parallel with the 60mhy of inductance and in the case of the GM70 amp, this placed about 1/2 an ohm in parallel with the 68mhy which should effectively swamp the inductance and keep the CCS happy (so goes the theory)

I tried to replicate my earlier oscillation and was unable to get anywhere near the oscillations i had before driving the 60mhy, but i still had a ton of hf noise across the open primary and speaker leads. simply connecting a speaker across the output leads killed the noise and everything seems to operate as expected.

On the GM70 OT I measured 110hy's with no DC and placing a 3A CCS across the offset winding netted me a null at 90hy's @ 85ma. I think it might be time to try guido's boards again.

dave



dave
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