PA774 wire confusion

[JimDaBomb]

I'm putting my GSG together and I'm confused about the three red wires on my PA774.. I see where the yellow marked one goes but are the other two red wires different? Does it matter which sides they go to? I'm also a little confused about how to wire in the on/off switch and IEC, and do I have to ground to the chassis somewhere? Just a clearer picture of the IEC and switch wiring would help.. I'm mostly going off of pictures of Tom's builds.

This is where I'm at now... I'll probably clean things up at some point.

[Shannon Parks]

I see where the yellow marked one goes but are the other two red wires different? Does it matter which sides they go to?

Doesn't matter. One red goes to J6-1 and the other red to J6-4. The red/yellow wire goes to J6-2 or J6-3 (PCB ground).

I'm also a little confused about how to wire in the on/off switch and IEC, and do I have to ground to the chassis somewhere?

ON/OFF switch: Connect pin 1 to the fuse, pin 2 to one side of the primary and pin 3 to the other side of the primary (the neon bulb is parallel to the primary winding).

Connect IEC safety ground to chassis. Per IEC standards this should be a dedicated chassis bolt and the wire should terminated with a crimped ring terminal. Then also run a wire from J6-2 or 3 (the spare) to the chassis, too. This should be the single chassis reference from the analog ground. Without it, a DMM from the PCB ground to chassis should be high impedance (eliminates ground loops).

Hope this helps! This is the method I use for all my amps these days. I do plan to eventually add better hookup diagrams for the GSG, and you've addressed some good questions.

Shannon

[TomMcNally]

hi Jim -

Amp looks great so far !

Since you're not using a lit power switch, disregard the pin 1 2 3 instructions.

Just follow the schematic Shannon posted. Ground the center pin on the IEC directly to chassis and then to the empty terminal on the board.

Hook the L (hot) lead of the IEC through the fuse, then the toggle switch, hook the black wire from the transformer to the
other lead of the toggle switch and you're done !

Let us know how it sounds.

... tom

[JimDaBomb]

Does this look right at all? Sorry, I'm not good with schematics and such and this is my first time ever building an amp of any sorts.

[JimDaBomb]

Well, I plugged it in, first time no sound.. had the little tubes mixed up, switched them around and I have sound but no left channel... all the tubes glow. I'll have to see if I went wrong somewhere..

[TomMcNally]

Are the hot and ground reversed coming from the RCA jack to J-1 or J-2 ?

The schematic has two right inputs, just noticed that.

[JimDaBomb]

Okay, I fixed it.. I had one of the inputs wired wrong. It's making some beautiful music right now warming up with Pandora. Unfortunately I sold my Klipsch La Scala's yesterday so I never got to hear them with the amp and I'm running some inefficient Pioneers right now, the Andrew Jones ones but I ordered some Tekton Lores so hopefully those will be here in a few days. I can tell this is a great amp and it was really pretty simple to build. Thank you so much for this design and the help and allowing me to finally hear and own an SET amp, otherwise I could never afford an amp of this quality.

[Shannon Parks]

The schematic has two right inputs, just noticed that.

Ooops! Better than two wrong inputs.

Okay, I fixed it.. I had one of the inputs wired wrong. It's making some beautiful music right now warming up with Pandora. Unfortunately I sold my Klipsch La Scala's yesterday so I never got to hear them with the amp and I'm running some inefficient Pioneers right now, the Andrew Jones ones but I ordered some Tekton Lores so hopefully those will be here in a few days. I can tell this is a great amp and it was really pretty simple to build. Thank you so much for this design and the help and allowing me to finally hear and have an SET amp, otherwise I could never afford an amp of this quality.

Too bad the La Scalas are gone! No prob, and glad you are enjoying it.

Shannon

[JimDaBomb]

I do have a question.. I know it's only 3 watts and I've used a chip amp with only 8 watts or so, but I'm guessing the gain is pretty low since I have to turn my volume up much more than usual, which I know is also due to the inefficient speakers I'm using at the moment.. but, being tube amp.. how do I tell if I'm overdriving it? Clipping and overdriving are pretty apparent in solid state designs but I'm not sure how to tell in a tube amp. Does it just run out of steam or are there any negative consequences?

[Shannon Parks]

The GSG is around 20dB gain, while a lot of SS amps average 27dB. But this lower gain is pretty common among SET amps as you don't need much input voltage to go to clipping (i.e. max output power). As far as how do you tell when you are really into clipping? Distortion will skyrocket and you'll hear the muddiness, particularly if you back off the volume and it clears up.

Shannon

[JimDaBomb]

Okay, that's good to know.. I'm pretty sure I can almost max out my volume before it clips but it's too loud at that point anyway. And one more question, does this amp have to be biased? I don't know anything about biasing. The amp sounds amazing though, so silky smooth and transparent.. so nice to not have any electronic glare or digital grain or artifacts, such a clean sound and amazing soundstage.. so glad I found this before I bought some cheapo SET amp on eBay.

Oh, and should the 6AX5GT run the hottest of the four tubes?