This started off with an offhand, perhaps ill-advised offer to build my brother an amp out of whatever parts I happened to have left over around the shop ... 8^o
I had a 290-0-290 Edcor transformer (6.3V output as well, but no bias tap), a little 10k --> 16 ohm OT that was originally in my 2 X 6BM8 lunchbox amp, a pair of 6L6Gcs that I replaced with KT66s in my modified Super/pro Reverb, and the usual pails of crap that I swept up off the floor but never bothered to sort out ...
Also had a Hoffman Stout board that I'd bought with something in mind, but never used.
Initial thought was a little practice amp.
No way I expected these disparate parts to fit together in any kind of coherent manner.
Then I stumbled across the old 6L6 datasheet that listed a configuration for an 18W class-A, push-pull circuit. My 15W output trans was 16k-->16-ohm, and I had an 8W Altec Lansing alnico 416 12", so that seemed like it would fit my 5k output trans requirement.
Figured I'd go with a tube rectifier to get the voltage down, but with no 5V tap available, that kind of limited my choices. I ultimately went with an EZ81. Still not low enough. Ordered some Zeners, but got inpatient and just stuffed a big honking resister in the circuit -- when in doubt, use a bigger hammer! Also, my PT had no center tap, so I had to make a hybrid rectifier using a couple of extra diodes.
Tried the basic stout tone control and really didn't like it -- to interactive, but ... found a little push-pull pot in the pail with a modified Big Muff tone control on it.
First pass sounded pretty good, but too 'Marshall-y'. My brother wanted something with some clean capability, and in an 18W package, really couldn't make that happen with all that preamp gain, so I added a bypass to the gain stage, and a 'gain' pot, configured so you couldn't turn it down to zero.
Cleaned up the phase inverter by switching to a 12AT7, and changing the resistors accordingly. Also reduced the negative feedback a bit from the original Stout design.
With the gain stage bypassed, and the tone control bypassed, you can get some pretty decent clean volume -- just edging into crunch with the volume dimed.
I also reduced the gain of the gain stage just a tad -- substituted an 82K plate resistor for the original 100k. Still sounded a bit brittle to my ear, so I bypassed the gain pot with a capacitor to ground, and that took the edge off.
That Altec speaker is a gem! I've always loved Altecs, and bought what was supposed to be a pair of 418s (75W) on Ebay. They turned out to be 416s 25W), with one ripped cone, and the other a rubbing voice coil. The seller refunded the cost and just told me to keep them. I fixed the tear, and will eventually use the other one to practice reconing. Unlike the guitar-centric Altecs, which feature frequency responses up to 7KHz, these are originally stereo woofers, and only go up to 4k or so -- still plenty of bite for my tastes, and in this application, the sound it suberb.
Also had a solid maple (including floating sound board) I made years ago. Woodworking isn't exactly my forte', and this came okay (dovetail joints and all), but wasn't quite square... Just recovered it with heavy hammock-weight canvas, 2 coats of Shellac and 2 coats of marine spar varnish. Had JUST enough grill cloth to finish the job.
DAMN, but the sound is absolutely incredible! Only problem is, between final test and installing it in the cabinet ... it stopped working 8^O.
Will address that tomorrow ...
Topic: Question about 18 Watt Stout options (Read 5209 times)