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In Reply to: Re: Questionable posted by rickl on September 29, 2004 at 11:48:30:
Hello ,
Sorry , the 6SC7 is unsuitable for concertinas driving triodes , Ra too high . For a single valve , you need something like a dissimilar triode such as 6EM7 or 6GL7 that has mu 68 , Ra 45k and mu 7 Ra 1k respectively . I used a 6EM7 with NimH battery bias and a CCS top load for the input stage with 10k loads for the 6EM7 beefy section . I can't remember the gain and clean P-P swing I got from this setup but it was more than adequate . The valves I really like for the concertina stage are the 12B4A and 6BL7 (paralleled) and should be low Ra , medium mu for optimum performance . Triode strapped EL84 also work well . For the input stage 6072 may be a suitable candidate for a 'clean' sound but may not have enough gain if you don't use a pre-amp . The decisions regarding valve choce are mainly dictated by the amount of HT available . You need to have approx HT/3 on the anode of the input valve . The input valve needs to be biased in such a way as to meet this requirement and have enough headroom to drive the grids of the output valves to full power . Hope this helps :)cheers
Follow Ups:
Thanks. This does help. I don't have a 6em7 but I do have a TungSol 6sn7gt. I'll check the specs on this (gain of 20?, Ra 7.7k). I think this is more like a 8 pin 6cg7 and each triode is pretty close to each other. I may not work either.Thanks again,
rick
The popular 6SN7 and 6SL7 are not acceptable. The only octals I found acceptable for this circuit are dissimilar tubes, like the 6EM7 that was already suggested. The popular 6DN7 and 6EW7 are a "no go"...
You may expand the variety of tubes that will work, if your source (or preamp) puts out a signal much larger than 1V RMS. The considerations then are further complicated by the high bias required at the input stage to avoid clipping.
Zero feedback with a single dual-triode for the MkIII is an act of fine balance...
I'd recommend that you stick to the 12AT7 circuit. It works well - without a question!
Thanks for the note. Preamp will either be a modd'd Lafayette KT600 (12ax7) or a Curcio PAS3 (12at7) line stage (or a Hafler 101 or a Mac c26 or ...).
Either preamp will drive this amp with 100K input impedance.
I don't know what their gain spec is and whether they can deliver more than 1-2 V RMS, so I will not recommend that you look at the low gain alternative tubes at this point. It gets a bit more involved and practically is a new design, even though you keep the same basic circuit. You'll have to make sure you have enough bias to not clip at the higher input and then adjust the input stage so it biases to the point where the direct-coupling to the splitter does not "choke" the splitter tube.
Unless you go back and forth on this "exercise" to reach an acceptable final circuit, I'd say you better just duplicate the 12AT7 circuit.
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