SET amplifier Q
On Tuesday, 6 November 2012 20:35:05 UTC+11, Phil Allison wrote:
"patrick-turner" Well, with 10:1 probe, presumably you have 2.6Vrms at 6SN7 grid. ** No, the 10:1 probe was used merely so I did not load the signal at the 300B socket grids. If the 6SN7 gain is say 17, then its drive to output tubes is 44.2Vrms, ie, 62.5Vpk, or getting close to the dc bias voltage for 300B. ** Wrong again. The two halves of the 6SN7 are direct coupled, so 260mV input becomes 74 V rms ( 200V p-p) at the 300B grid pins. .... Phil
I've often mis-interpreted some of what youv'e typed. So there are two 6SN7 stages and gain of both is 284, or average of 16.8 each, about typical. If you'd described the circuit more fully, I may not have made my assumptions.. However, 74Vrms seems right because you have -105V bias, and peak grid drive max could be about 108pk volts (76Vrms) allowing for some few volts of positive grid drive before grid current clips the wave, or coupling cap begins to charge up to push the grid bias up and 300B into cut off, when Ia is then found to decrease. My prvious assumptions didn't make a nonsense of my recommendation you test the 300B with a linear drive voltage source to test just how much 2H and 3H is made without cacellings, and to see how 3H is increased due to IMD action because of the feeding in of 2H from the driver with 1H.
Have fun playing with 300B, but methinks Ea = 550V with Eg1 = -105V is using voltages that are too high, even though allows a high RLa, near maximum anode efficiency, high natural DF, even though Ra at Ia = 50mA is much higher than the data figure of 800 ohms, and low THD. I might be a lot happier if the OPT tubes were KT120 in triode, IMHO.
Patrick Turner.
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