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A triode-connected
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 The 50L6GT Smaller than it looks, only 12" wide and 10" deep. Here is what the new owner has to say about this amp: Thanks Bill!
Don't turn away! Yes, with apologies, the humble 50L6 makes one hell of an amp! Deborah and I listen critically to EVERY amp we make especially during the development, making comments about the sound, the amps plus and minus points, etc. and making changes etc.
But with this amp, we just played it. We're hard pressed to find any faults, except it should have more power and be this good! It is very much like a 2A3 push-pull amp, maybe better. Yes, BETTER, that's what I said. So OK, it only makes about 1.5W to 2W, not 10+W per channel. I find it interesting that another amp I really loved was the 6Y6 SE amp I made sometime back. The 6Y6 is another low voltage, high current output tube that was uncanny in musical reproduction. So is there a message here? Yes! We all should be building these things! I'll not be posting the schematic since all the special voltages were derived from an unusual power transformer making duplication of this amp unlikely. That is not to say it could not be built using other trannys (I will support builders and share my schematic as needed). The thing is, using the 50L6 (or the 25L6) no heater windings are needed. This actually complicates things, because now you have to deal with the AC line issues, but it also simplifies finding a power tranny suitable for the high output stage current you will need (about 180-200mA at idle). My net plate voltage was about 150V at 46-50mA depending on the tubes at about 11.5-12.5 bias via the cathode (250 ohms each cathode, bypassed). You can run it higher net plate (about 170V or so) and up the resistor to 270 or 300. Each cathode has its own resistor and bypass cap (best overall, even in class A). Estimated rp of this tube is about 900-ohms triode connected.  A mix of high quality wire including some silvered teflon used throughout. Hand wound coils for noise filtering. Three types of solder used. Input signal run wires pulled from an old HP instrument amp - nice stuff. PIO caps for the coupling, and output cathode caps bypassed with foils - subtle difference. NOS tube sockets avoid the junky Chinese ones. For output trannys I recommend using good ones. The amp is worth it. I used some pulls from an old HK receiver. 5K works fine or 4K depending on your final plate voltage, but I use it at 6.5K and I can recommend using a 6K6 PP tranny which are easy to get and seem to sound best in this circuit. Anything in the range will work and sound good. Even a 10K PP tranny will work. The HK trannys let me experiment with a wide variety of load conditions (many secondary taps), and the amp sounds good under any of them, changing only the power output, distortion and overload character as expected - as well as a more subtle change of sonics. I also tested this configuration using a pair of simple open-frame trannys such as found in old consoles and such. The ones I had were 5K PP, but a 6BQ5 type (about 8K normally) would work, and if it has a 16 ohm connection you can connect 8 ohms there and net about 4K reflected, so there is room to play. Some 6BQ5 amps used 10K so you could net 5K which would be better IMHO. Same applies to 6V6 type output trannys where you're more likely to find 10K PP units.  The heater switch lets me chose either 25L6 or 50L6 quads. No 25L6's were available for testing. There are three output impedance connections. I found I could easily drive my 8-phm speakers from any of them, with each connection providing a subtle change in the sound. While designed as a 5K PP amp (the green connection) I found myself preferring the 6.5K connection (yellow). The blue connection sets up about 11-12K into 8 ohms, and would be suitable for a 4 ohm speaker. A 16 ohm speaker would be limited to the green connection. I am listening to this amp as I write this, and again I have say I find no fault. It is entirely musical, with solid, detailed, extended bass, delicate sweet highs and a midrange to die for. I keep saying "no way" as I listen, and the hairs on my arms raise - get the picture? You haven't built one yet?? jef "Research has shown that repeated action, learning, and memory can actually change the nervous system physically, altering both synaptic strength and connections. Such changes may be brought about by cultivated change in emotion and action; they will, in turn, change subsequent experience." This tells me three things: Variety is the spice of life, you can change your behaviour, and you can alter the meaning of what you experience. Tube Notes: We tried various "quads" of 50L6 in this amp. We have a quad of Tung-Sol, RCA and Sylvania. The Tung-Sol are the "black glass" types with deeply located bottom getters. The RCA's are the clear glass types with dual horseshoe bottom getters. The Sylvania were the smoked glass type, and the only quad of NOS we had. We also had a pair of GE clear glass with top getters. Except for the Sylvania, all tubes were used, matched sets. Sound character wise, the RCA's and the Tung's sounded the best and about the same. The Tung's has slightly better midrange, but the RCA's sounded smotther overall. The Sylvania's sounded dull next to these and generally not as clear throughout the range. This was a great disappointment as these were NOS VERY old tubes in the green boxes. The pair of GE's we had sounded excellent, but a quad was not available for a definative signature sound opinion. They seemed to mate well the with RCA's, and I suspect a quad would be excellent.
The 12AT7's we used were the Tung-Sol's and I can avidly recommend these tubes.
 An amp it not about how it looks, but how it sounds. Here's a zip file of the schematics. |