((ST35+SCA35)+PAS3)=?????

[Pyre]

I am not sure what it equals but here is what it looks like. I needed a small amp for my office which could sit on a shelf and have a minimal amount of wires going to it. I had a seriously beat up and rusty SCA-35 with good transformers as a base to start with. It had sat for years, who knows where, so was pretty nasty inside. I gave the chassis a light sanding as well as the transformers. I painted the inside a graphite grey and the transformers with textured graphite automotive bumper paint. I am not a fan of the SCA-35 preamp section with the 7199’s and tone controls so opted for a pair of ST-35 boards which I stuffed with Takman resistors and Russian PIO caps. For the power supply I used a JJ section cap, Mills resistors and a Hammond choke. Instead of the SCA preamp board I shoehorned a PAS3 line stage board stuffed with Russian PIO’s, Solen caps , PRP and Arcol resistors. It’s pretty tight in there but everything fits. I wish there was enough room for a regulated filament supply board but the amp is surprisingly quiet even using AC on the 12AX7’s.

The amp sounds fantastic and has enough juice to drive any of the bookshelf speakers I own. I redid the RCA inputs on the back but left the speaker connectors original. It’s not a big deal but I may change them to something better. I need to add a power indicator light as the bulb was burnt out. In all of the PAS3’s I have built I always change it to a LED but I use DC on the heaters so it’s easy to hook up. When I get home tonight I will make up a small board with diodes, resistor and a LED and see how that works.

[EWBrown]

Nicely done! I built up something similar several years ago, but I took out the phono stage, and made up a cap board to fit in its place, and replaced the 50 ohm 5W resistor with a C354 choke. The original multisection can cap was shot, so out it went, and I replaced it with a J/J 32/32 uF 525V cap, and used a separate 470 uF 35V cap for the EL84s' cathode bypass. I damaged the 95 ohm 5W WW resistor in the process, and replaced it with 100 ohms 5W, and added a 1K 1/2 watt resistor and red panel mount LED in parallel, which brought it back to behaving as the 95 ohms would, and it also gave me a "downstream" power on indicator.

This was before I obtained the DIY35 rev B boards. Since I didn't have the phono stage, I also took out the selector switch - as I really needed only one input, from a CD player. ANd I added two of the common insulated RCA jacks on the back, using the existing 3.8 inch hole in the back panel, and drilling another next to it.

I used the "Triman" ST35 boards, and without that nasty rats' nest of unshielded wiring and torturous signal path, tone stack and filters, slide switches, the extra voltage gain of the 7199 isn'tneeded, and the signal will be a LOT cleaner, with or without the phono stage.

I also replaced the original ST35 volume and balance controls with a 100K stereo step attenuator (the volume and balance pots were kinda scratchy).

For a first shot at this kind of design, it worked and sounded really good!

/ed B in NC

[Pyre]

Thanks Ed

Sounds pretty similar to what I did to the power supply but i used a 100uf/100uf and a 47uf underneath. The choke and updated diodes makes a big difference in sound.

When I originally started I was going to leave out the preamp section and put in a small DAC board but when I tried the amp there was not enough gain to drive my speakers with my ipod as an input.

I like your idea with the LED, very simple and effective. When I got home I made a little board with a 4 diodes, 220uf cap, 390R resistor and a red LED. I cut it to fit where the original light goes and it runs off the 6.3v tap. Had to glue some cardboard on the back to prevent any potential shorts. Seems to work fine and provide enough light so I remember to shut it off when I am done listening.

[EWBrown]

LEDs make perfectly nice pilot lights, plus you can choose from more colors than simply an anemic yellow-white, and the current consumption is a LOT less, and they should just about never burn out.

With mine, I "stole" the LED voltage off the 95 ohm resistor, and it lights up only after the tubes are all warmed up. I still have to original puilot light, too, and filled the empty tone and balance control holes with additional panel mount LEDs, which are not yet connected to anything. THey just serve as hole fillers, and they were salvaged off some long forgotten electronic carcass which has long ago rusted away.

/ed B