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Old 31st May 2012, 6:22 pm   #111
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by Radio Wrangler View Post
Transformers for single-ended stages like this are more like chokes with an added secondary. Design effort has to go into making enough inductance, and for it to not saturate when the valve is at peak current... approaching 80mA
That's not quite right, Radio Wrangler. But if you designed according to this, you'd not have a bad transformer, just one rather bigger than necessary!

The transformer is indeed like a choke with a secondary, as you say. It needs to be away from saturation at the DC anode current (say 40mA). Now, when the valve is working at full output, the anode current goes from 0 to 80mA. But the secondary is then passing a cancelling current, as well. If the primary inductance is high, then the field in the core is practically constant throughout the cycle. At really low frequencies, the finite inductance of the thing gives rise to a very low output, and yes you could argue that the core's field would go from zero to twice the steady-state value as the cancelling current is peanuts. But under those conditions, there will be almost no output from the secondary anyway so who cares if it is distorted by saturation.

As a reasonable engineering approach, you might drive the valve at full output down to a frequency such that the output drops by 3db due to the finite transformer inductance. Under those conditions, with 40mA steady anode current, it turns out that the core gets the equivalent of 28mA of swing. Thus if you design such that it does not saturate with 68mA, you'll be fine - and at higher frequencies, everything will be even more OK. Good quality design would put not drive the valve anywhere near the transformer 3db frequency anyway.
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