So the question every amp builder wants the answer to about now is probably "So did it fire up first time?"'

Well Yes and No.

"Yes" in so much as there were no explosions nor horrible noises.
"No" because I somehow used a 5k trim pot for the bias instead of a 25K so my bias was almost double what it should have been (around 80mA eek!)

I turned it off, found my mistake and tried again. Biased nicely, I gave it a bit of a detailed listen plugged into it's 4x12 cab.

First impressions were that it sounds surprisingly good for something I built out of a schematic and my imagination! No noise, very minimal hum (on par with my USA made Peavey anyway) and power to burn! I haven't wired in the MV yet as I want to perfect the layout of the original circuit first, so as you can imagine, it gets real loud, real fast!

Two things need attention though and I absolutely welcome feedback on this.

I have noticeable 50Hz hum, but only in standby!

This is almost certainly lead dress. I had to run heaters close to the speaker jacks due to the whole lowering of the power tubes, as this kept them as far as possible away from my preamp tubes. The hum is cancelled as soon as I switch power on so it's not the end of the world, but I'll try a few things to see if I can improve it. Those solid core heater wires are NOT real flexible though so the chopstick method wont work there.

I've also noticed a really slight "fizz" as notes ring out. I'm pretty sure it's parasitic oscillation. It scales with volume and is unaffected by tone and presence controls. You have to listen hard to hear it, but for a circuit famous for it's clean sound, I want to get it right. I've got plenty to try as I haven't even added grid stoppers as yet. but my plan is to loop some notes and go prodding with the chopsticks. As you can see in the pictures I do have most of the preamp and PI wires parallel, which looks nice, but is terrible for radiated fields.

All in all though, it sounds a million times better than my first horrible attempt! Once I sort out the "fizz" I'll wire in the Lar/Mar PPIV MV (you'll see the pot is already mounted) and see how that goes. I'm also really keen to get it in the rehearsal room as I need to test for full power issues, which is patently impractical in my apartment

Still on the to-do list is getting some brass screw cups and screws to fasten the rear cover, sourcing new rubber feet (they need to be a specific size as there are mounting cups on the speaker cab) and probably fitting a cap across the B+ switch to kill the power on "pop". The original had one, mine doesn't as yet, and as I'm using a 50 year old rotary switch, it tends to have a little "pop" to it.

I imagine this thread will only be viewed by the amp nerds (you know who you are) but feel free to ask away if something intrigues you.

Oh and here's my logo.


The 45 by Ch0jiN, on Flickr