As an aside, I was looking into replacing 6SN7 with a MOSFET...
So far I get this... IXTP08N100D2... Not bad for 3 resistors and a MOSFET (and the coupling caps). 600V B+, 47k drain resistor, 510R on the source, 1k gate stopper. mu=~60 AC load = 1µF into 100k. SA12CA strapped from gate to source.
Adding 2k7 between 510R and ground makes it ~ mu=6 - much like a 6SN7 in the same circuit.
So far I get this... IXTP08N100D2... Not bad for 3 resistors and a MOSFET (and the coupling caps). 600V B+, 47k drain resistor, 510R on the source, 1k gate stopper. mu=~60 AC load = 1µF into 100k. SA12CA strapped from gate to source.
Adding 2k7 between 510R and ground makes it ~ mu=6 - much like a 6SN7 in the same circuit.
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If I'm not mistaken the 6BQ6, 6CU6, and 6AV5 are the same electrically. Pin-out and top cap are what differ primarily and account for some of the ratings.
Isn't 2200R a little on the low side for linearity? I would think 3-4k or so to be a better fit.
As long as they have the current, the gNFB does the rest... I run 6P45S (*LW6 and the like) and 6P36S (not sure, EL500 is smaller) at 1k3.
If you used 5k, you'd get less max power. If you limit yourself to that same max power with the lower load, the Dtot is the same... You just get to make more power if you need it (great for dynamics...).
Besides, they are running as triodes... Rp is <1k. I'm sure running a pentode connected stage with this load would be crap though.
If you used 5k, you'd get less max power. If you limit yourself to that same max power with the lower load, the Dtot is the same... You just get to make more power if you need it (great for dynamics...).
Besides, they are running as triodes... Rp is <1k. I'm sure running a pentode connected stage with this load would be crap though.
Really? Go and tell this Mr. General Electric (see page 2)
.And as discussed above, the Hohners did even more with 'em.
Best regards!
Not sure what is being demonstrated here? Where did you get the quiescent current figure from?
35mA Ik is not possible I don't think, and triode mode is probably not possible either since g2 would be at too high a voltage. The experience of that Orgaphon amplifier example earlier is that they do seem to be able to operate at regulated g2 of 200V, and 18mA quiescent Ik, 350V on the plate. That is very different to the published tube data.
I have a SPP with PL84, currently with a power supply for 220V to the output stage, and 290V to the driver/splitter. Would be a fairly simple change to try and emulate that Orgaphon amplifier.
G2 voltage is a problem when the anode voltage goes below G2, causing G2 current to go up. Not usually an issue with a triode-connected tube, G2 current is very small.
It can also be a problem at quiescent, even though G2 dissipation is small. Some of the big sweep tubes will eventually run away just idling if G2 is run far above rating (Triode or pentode). It’s insidious - it can run w/o problems for months and just go south on you. Even Koda has reported that not all sweeps play nice in his circuits. I had 26DQ5’s up and running for a while before building a real amp with them, and they drifted a lot with Vg2 up at 200. At 125 they didn’t go anywhere.
Ya it's helpful to use an automatic grid bias system, for sure. Haven't had a runaway in years
Interesting. Do we know why they run away? G2 too close to the cathode?
I have been running triode-strapped 6P41s for a while at 340V, so far they are playing ball.
My only beef with the gate protection is that the fet inputs will see all of the capacitance of the TVS diodes, since they are essentially being operated at 0V. I have a scheme that biases up the protection zeners to reduce the capacitance they present to the diff amp gates, and keep them out of the way until they actually encounter an ESD event..