Patrick Turner wrote:
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.
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.
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.
Patrick Turner.
Decided to check out RAT after years absence.. not much left, but at least the arguments appear to be gone. I'll add my two cents to this topic, since it looks interesting (and maybe to enjoy contributing to usenet while it still exists!).
I think the amplifier looks like a great idea for a project... Sure someone may have a little trouble getting the 6LU8, but on the other hand old TV tubes are a great way to get NOS tubes on the super cheap. Supply is fairly minimal, but demand is even smaller still, so unless it becomes a national pastime to assemble cheap as dirt tube amplifiers, someone will be able to secure a pair of RCA or GE or Sylvania 6LU8 for well under the cost of a single quality 12AX7 (Perhaps living in North America is an advantage here). eBay is flooded with the 6LU8, and other similar tubes, and it avoids being forced to go
with either inferior modern production tubes, or high dollar desirable types of NOS which have likely been picked over. I understand it may be somewhat controversial to call modern tubes inferior to NOS, but a quality NOS tube is quite a safe bet from a reliability standpoint, considering they were made in the days tube tech was to be relied upon for communications between humans, rather than a pastime enjoyed mainly by eccentrics, people overcome by nostalgia, "connoisseurs" and obsessive musicians.
Also, it's kind of elegant to have a two tube stereo amp.
A salvaged output transformer from an old TV chassis would make this even cheaper, and it would be possible to figure out how much NFB it can take before instability sets in. Those Hammonds are the right ticket for this sort of cheap and cheerful effort, but I think they're priced about double what their real value is.
This amplifier would be right at home driving a paper cone speaker, for example, as a small scale computer hi-fi setup. Naturally the limited power output and small output transformer core would severely limit applications in a serious hi-fi context, but nobody would expect a $100 amplifier built of junk-bin parts to perform in the league of serious dollar equipment.
I would submit that the problem with a low cost SE 6L6 or EL34 amplifier, is to do it properly you would exceed the cost of a push-pull amplifier and at the same time suffer worse performance. I don't really see any value in it. A push pull transformer won't be gapped, so you avoid this cost plus get free cancellation of second harmonic distortion, and increased hum rejection all in one stroke by moving to push-pull. The special single ended sound (if it indeed exists) that some people rave about won't be captured at this price either, so all you really end up with, when you make a cheap SE 6L6 amplifier is an
amplifier that costs more than a push pull amp and performs worse.