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In Reply to: RE: 20B SET: look ma, no driver tubes posted by Frihed89 on February 11, 2012 at 14:02:28
I've considered building a single stage amp with their 20B. With its 3.3k plate impedance, it needs a 10k primary impedance output transformer. 10k:8 ohm yields 35:1 step down ratio. The tube has a mu of 20. So you need some form of input gain, be it another tube or input step-up transformer to achieve unity gain, let alone positive gain.
How would one hope to drive the Miller capacitance through a step up transformer?
It's got to be high in a DHT with a mu of 20.
Have Fun and Enjoy the Music
"Still Working the Problem"
Unfortunately the capacitances are not specified. The description talks of the greater tube element spacings, which would on the face of it reduce capacitance, however in order to maintain the transconductance the grid and cathode area must increase (relative to miniature tubes).
A Type 10 is a similarly large tube with a relatively high mu (8) and has a grid to plate capacitance of 7pF; scaling by the mu ratio (greater plate spacing increases mu) gives 3pF. A 6SN7 is 4pF. I conclude this is about the right general area, so the Miller capacitance is going to be not far from 60-80pF, which is pretty much the same as a 300B or 2A3. Those tubes are easily driven by an interstage with a 10K winding. The only real limitation is the low primary impedance, which few tube preamps are able to drive well.
The step up ratio does not need to be that large. The 20B at factory match (medium bias) conditions is biased at -7.5v, which is 5.3vRMS. Many preamps can supply this voltage directly without stepup. Almost all preamps can supply over 2 volts RMA, so a step-up of 2:1 is adequate - that would be 2500:10K.
Just running some numbers for fun.
Paul ... I just assembled a 801A spud amp. just that tube and an input tranny 1:8 (to be installed)
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