Hi Curwen
What do you bias the 2C22 at? looking at the triode curves, I would be tempted to use a constant current source over resistive loads.
Regarding the 841 DHT, they are getting quite rare. But the biggest issue is solving the matter of the high rp...
Ian
841 plate resistance is not low, but not so high: 40k (for example 12AX7 has 63..80k).
The bigger problem is the limited driving capacity at HF (see 841 -5ma CCS loaded- sweep at 100k loading).
If you use it with large impedance second stage (CF or source follower), the bandwith will be wider, and driving capacity (for example at 20kHz) will be adequate.
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Hi euro21. What you post is but one of the symptoms of what I state about the high rp.
The 841 has an rp of 63k Ohm for a plate supply of 425V. If your plate supply is 1'000V then the spec sheet says it will lower to 40k. In my books, this is a significanly high plate resistance.
The result of poor loading (such as effective 100k in your simulated example) will of course be poor frequency response - but it is not the only sort of distortion to consider. 100k is not enough. I would go for well around 300k and use a 6SL7 in bootstrap follower topology instead
but you can't rightly do that with an 841 - so of course you are stuck with a follower. If you must use a follower, try a MOSFET follower. The MOSFET follower will work very well, but you must have a higher effective plate load than 100k.Maybe it would be worth looking at the posibility of filament bias too.
Filament bias on an 841? Anyone tried that yet? Too bad they are rare as hen's teeth...
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I use my 841 tubes with filament bias (through Rod Coleman regulator).
The first picture in my #22 post shows the -3R bias resistor- filament biased 841 stage (5mA CCS anode load, 220nF coupling cap, 100k load) transfer curve.
The second picture is my 841 (same topography as above) -10Y (direct drive to)-801a PSE amp driver curve.
p.s. 100k load is too low for 841, but if you compute, that how much pF represents 100k impedance at 20kHz... it has very few tube with such small Miller capacitance. :-(
Well, I used 841s in my Unity-Coupled amp and they can swing very large voltages (600V pk-pk) with very low distortion if driven by a mosfet into the positive grid region.
One solution for the high rp issue might be to CCS load the tube and take the output from the source of the mosfet in the CCS. I did this and got 120kHz out of my amp.
Oh, and if any of you are having trouble making your 841s work in your design, you are always welcome to sell them to me.
One solution for the high rp issue might be to CCS load the tube and take the output from the source of the mosfet in the CCS. I did this and got 120kHz out of my amp.
Oh, and if any of you are having trouble making your 841s work in your design, you are always welcome to sell them to me.
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