[quote=Patrick Turner;976270]John Stewart proposes a low cost SE amp using 6LU8.
http://www.audiobanter.com/attachmen...tachmentid=379
It is many years since that tube has been made. Maybe not everyone could obtain one. Pentode Pda rating = 14Watts, so say 12W max in practice for SE class A1. This means max Po = 5 Watts with low OPT losses, at 42% anode efficiency.
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I had a quick look the other day. The 6LU8 is available for 6.70 USD while the 6EM7 used in the original project at 8.90 USD. And the 6BQ7 at a princely 1.70 USD, all at
www.tubesandmore.com. So it looks like there is no availability problem at all.
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I suggest it would be easier to use say 6BQ5 driven by 12AX7 with its 2 triodes in parallel. Both are made now, and old stock is available.
The above schematic has the V1 triode cathode with 1k8 connected to whole OPT sec winding and for GNFB network. I humbly suggest there's a resistor missing between cathode and 0V; the whole GNFB network needs to be changed.
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All well & good. If one looks at what is out there the number of permutations & combinations should work out to be about the third root of infinity! As I had commented in the original article, the objective was to simply illustrate a starting point a newbie could try for the SET type amp. At a reasonable cost.
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Of course if one does want a nice cheap 5 Watts, it is even better to use a single EL34 in triode, idling with Pda = 16W, and same paralleled 12AX7 as input / driver, and with say 12dB GFB. Then Ea might be higher, and RLa higher, but then any cheap amp with low Po should be extremely well thought about to compensate for its basic shortcomings of having not much Po. UL with EL34 is good - and for reasons Stephie mentioned; although they'd go right over most ppl's heads. UL Po could be 7Watts with Pda at same 16W at idle, same Ea and Ia. But RLa changes, so thought must be used if any benefit is to be heard.
If EL34 is considered, then minor bias change will allow 6L6GC, and the world is full of 6L6, EL34, 12AX7. I'm not sure how much goodness resides in the probably small cheap Hammond OPT.
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For the cost it ain't bad. But as always, one can throw as much money at a project as they wish. I did suggest an improvement to a more costly Hammond. Since the original article was published Hammond have introduced an improved 125SE Series that would help. But it don't have a UL tap. So SEUL not possible with that one.
Cheers to all, John L Stewart