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In Reply to: Speaking of 5687s... posted by frank on January 23, 2007 at 19:45:04:
It is feasible to get more gain by increasing the transconductance (Gm) of the bottom tube in the input dual-differential cascode. Perhaps that's why you mention the 5687. Problem is that higher Gm tubes like the 5687 (and related ones) need a lot of current to achieve optimal Gm and low distortion. In the MA1, I think you are limited to a B+ of 300V, so if you increase current you are likely to have to reduce the value of the plate resistors. This will defeat or at least be counterproductive with respect to the purpose of increasing gain, since the gain of a cascode is approx Gm*Rp, where Gm is the transconductance of the bottom tube and Rp is the plate resistor. For example, I am using 6900s at the input of my MA240s. The 6900 is a milspec relative of the 5687. The MA240 uses MA2 transformers and so develops +/-400V for the input and driver stages, so this gave me a little more leeway to use a large-ish Rp and at the same time provide enough current for the 6900 to sing. Nevertheless, the net result was only a little better than the now standard 6SN7-based input stage in terms of gain. I suggest you play around with the 5687 or ECC99s, and see what you can do. Good news is that in the dual-differential topology, you can get away with a little less current (and therefore a slightly higher value Rp) under DC conditions and still get good results, because of the way in which a dual-differential amp works.
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Follow Ups
- Re: Speaking of 5687s... - Lew 08:43:11 01/24/07 (0)
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