I thought I'd build/hear/taste a 6V6 PP amp in Triode mode.

The idea is not original. There already is a "direct input" 6V6 amp out there somewhere on the web using 6v6's in triode mode.
Mine is different in that I use a CCS (CC Sink actually) in the long tailed pair.
The input is a 5760.

This all original design was inspired by The Junk Box Amp as seen in the link.
I will use a long tail pair to directly drive the triode connected outputs.
We depart ways on several counts, so please read on.

I decided to stick with using tube rectification for this one. This is because the tranny was designed for that service and it's not good to tempt fate with large solid state startup currents. The power transformer gets normally warm under full load + 10% (about 180mA continuous).

That big 250mA choke you see was replaced by something a little more fitting.
The big choke and its twin brother will appear later in a dual supply 6L6 power amp.

Here is a shot of the amp on breadboard (power supply is behind).
Those big green "c-battery" caps you see didn't cut it. Better ones were used.

Higher Math??

You would not believe the path I followed trying to get it all just right. Quadratic equations? - whew! In the end, it turned out far simpler. Isn't that often the case? Overlook the obvious and get buried in the detail. Oh well; lesson's learned.
I thought a pix of the OPTs before cleaning would be a good idea.

Orig. trannys with my testing ref papers still on them.

The nice cloth leads are still very pliable.

The top layout.

You can see the many pencil markings. That front tube socket is special. A USA made type with a serated underside held in by a circle clip. You see these in test equipment, they are high quality.

Completed cutting the chassis today. Here are some shots:

Makes an interesting pattern, does it not?

Those ugly 1/0 marks on switch are erased.
Stainless steel #8 bolts secure all the trannys.

I decided to paint the power tranny (as seen in these pix). It looked OK, but there were some factory printing on it. One concession tho: I will paint the choke core black. I don't normally like painting this part of a tranny, but I'll make an exception in this case.

Done!
Got the side panels made and mounted. Changed the coupling caps to some nice paper foils I got from eBay. Love those vintage parts. These were NOS and sound waaay better than the big green ones. I also tried some Sprague original orange drops, and while these sounded OK, the paper ones are better. I added additional filtering and bypass to the screen in Pentode mode. Smoothed out the freq. resp. I then added my pentode mod to the plate/screen circuit. This mod is really terrific, smoothing out the harshness of the pentode mode by reducing the higher order harmonics. Now I think the Pentode mode sounds as good as the triode.

How does it sound?
Sweet. I love this tube in PP triode. Nice even tight bass and silky highs. Those OPT's are very nice and I look forward to making more of these with the ones I have left. The shocker was how good this sounded in Pentode mode with my tweak installed. Whew! I still prefer the bass of the triode mode, but other than that perference, I use the pentode mode the most. This amp sounds great and really rocks!

Specs:
Power: 3.5W triode, 12W pentode into 8 ohms
Bandwidth: 20Hz to over 20KHz Pentode, 20Hz to 25KHz triode
THD: 1% pentode 1KHz and 10KHz at 10W; 0.2% triode 1KHz and 10KHz at 3W triode
S/N: better than 60 db below 1W
Input Sens: 300mV pentode, 600mV triode (preamp recommended, but a CD player will drive it to full output).
Global Feedback: None

Picture Gallery:

Those ugly power switch markings have been erased (no pix yet).

Note the deluxe 5-way binding posts.
Excellent quality vintage parts.

The power supply is shielded from the amp.
Sprague "black cat" style coupling caps.
0.33uf at 1000V ... sweet

Vent holes for the 5U4 and tranny.
BIAS and Balance adjustments and test points.