abraxasaudio
An 807 in Triode Mode
by Jef Larson

Pair of 807's are RCA NOS JAN's perfectly matched.
The Sylvania 6SL7 round plate in the front.
Vintage RCA 5V4G provides the power.

Finished 9-28-2004

Here's a sweet little amp commissioned by a gentleman named Dean. We started out to build a 45 amp, but ended up here when my 45 "pair" turned out to be one of a 45 and one of a VT52. The output trannies were once part of nice stereo amp from Japan. They are 5K to 4 or 8 ohms and I pulled them when the power tranny gave up since the condition of the amp was not worth restoring. So not only had I heard these trannies already, I had also measured them on my impedance sweep. So I knew I had a good one.

I've always wanted to build a 6L6 family triode connected SE amp just to hear one. While the 807 is a member of that family, it is different in how the screen is implemented. So what we get is a 6L6 triode connected amp without the 6L6 "leaky screen" syndrome, so we have much better control over the performance.

As it turned out, I was right about the screen ... almost too right. It took a while to come up with the right R for the screen to the plate. All the design work suggested certain values, but ultimately I had to go to trial and error. The perfect size for the voltages I was using turned out to be 4K9. I tried several standard values around this (5K, 4K7) but no matter what tube I selected (I had several brands) the 4K9 (or 4K85) worked the best. So naturally, I hand selected some 4K7's.

Nice perspective of the 807's vs. the 6SL7.

The power tranny is another vintage piece I bought from Walter. I re-wired it since the original cloth leads had deteriorated too much. This hunk of iron supplies the 100mA idle current of the amp and easily supplies all the heaters.

I keeping with the 807 vintage, I used a 6SL7 as the input/driver tube. I am running this well up the curve in the higher current linear region so the gain is not all I wanted. Nevertheless, this tube did not disappoint in the final sound quality of the amp. The bias position allows direct plug in of the 6SN7 which also sounds fine in this amp. The 6SN7 is then biased in the low current range near 125V plate which is fine.

Also in keeping with the spirit of the amp, I am using a 5V4G rectifier. This tube was selected since it could easily handle the idle and peak currents, but also for its voltage drop characteristic. It's running into a 47uF cap; a tad high for this tube, but I have plenty of protection from the R of the vintage power tranny. The main voltage is then buffered by R and filtered by 240uF. Additional filtering is provided for the preamp section. Output hum measures 5-7mV.

The obligatory gold plated connectors.
It's sure hard to find good ones at fair prices.

We managed to achieve 1.7W. The 807 handles a bit over voltage without breaking a sweat. It sounds MUCH larger than the watts would indicate. I've built quite a few amps in this size range, and I have to say, this one has more muscle than any so far. The response is ruler flat from 30Hz to 30KHz, dipping to -3db at 20Hz and over 40KHz. I measured the distortion, but I can't find the figures. I think it was about 1-2% or so. I remember noticing how nice the second harmonic residual looked, indicating a pretty low 3rd.

It sounds great. It handles about everything I toss at it with ease. I used the word "listenable" to describe it and not at all tiring. Even my wife got more involved with this amp than usual, and we played all her stuff, too. Rarely do we play that much stuff during our evaluation and tuning. We had a lot of fun.

And of course, a peek inside.

Thanks for looking!