Senior Member
So my 2A3s were supposed to show up today, but they're delayed in transit. So I spent a bit of time double checking all the connections, double double checked the wiring of the umbilical's as that's an easy place to make a mistake. I fired up the power supply with a 5k load across the hv, and voltages are pretty near where I would expect. Heater voltages look good as well.

I was laughing at myself for all the iron and huge oil caps in the power supply as I was moving it around on the bench, then I picked up the amp and realized the opts pretty damn heavy as well...ha! So much metal, Nathan Explosion would approve I think.....

Hopefully I'll have a much more exciting post tomorrow night....


Staff member
Cool trip down memory lane - thanks for that. I went to Daytona a couple times in the late 90s for the 24, and still remember the sounds, especially the 12-cyl normally aspirated Ferrari 333 SP at full-scream up on the banking or ripping through the gears on the infield - pure audio pleasure of an entirely different kind. Also saw that car raced at Watkins Glen and Mosport - nothing sounded liked that (OK, maybe the 12-cyl NA F1 cars of the time).


Senior Member
I should have ordered them sooner, stupid holidays had me focused on other things!
So the package arrived from the vendor I ordered the 2a3's from.... but they sent me 300b's. I emailed them and they sent me the correct tubes, with a shipping label inside so I could send the 300b's back. The package arrived at my door in under 2 days, I was very impressed and pleased, because I am excited to get the amp rocking and rolling.

So, naturally I cleaned the bench up, setup my gear to measure voltages, etc. popped all the tubes in and got to it!

PSU voltages look close, 2a3 was where I would expect it, same for the 6sn7's,,,, the 76's though were off.... turns out one of them was bad, heaters weren't lighting up. The problem followed the tube when I swapped it into another socket. Well, stupid me thought I had two pairs of 76s, but I only had one...

So now I'm waiting again for tubes to show up.....this is turning into a theme for me with this amp..

The good news is this gave me some time to build a new dummy load, organize the lab a bit, and even get started on another project (which I'll start a thread about soon, new tube line stage).

@Redboy don't worry, the bench is already messy again.... it happens so damn quickly!


Senior Member
The 76's got delivered, I plugged them in, then setup some gear to take voltages. Voltages were all close, so I plugged in some music. No output. Some hum, nulled that out with the pots on the 2A3's... still no sound.

So, I traced each connection, and that's when I noticed the bonehead mistake of the year....... I forgot to connect the 6SN7 grids to the plates of the 76's........ :/

Wired that up, and no have sound, but it's quiet and a bit distorted.

All filament voltages are correct, ~2.5v for the 2a3s, and ~6.3v for the 6SN7s and 76s.
All voltages are with inputs shorted, outputs connected to dummy load.

76: Plates ~58v, ~2v cathodes
6SN7: Plates ~275v, ~68v cathodes
2A3: Plates ~343v, ~60v center of hum pot

I think the 6sn7's should have ~250v on their plates, but I can't tell if the schematic is 250 or 280.

@je2a3 Can you confirm?

I'll hook it to the signal gen and scope tonight, which I guess will prove that I see low output and distortion, heh.

Any insights on what to look for? The low output and distortion is in both channels.



Senior Member
I got the 6SN7GTB plates down to ~258v, same problem persists. I'm evaluating all the connections currently. Measuring and double checking resistors where I can, etc.


Junior Member
With those voltages, the amp should work. Check if RCA jacks are connected to 76 grid, .22uf coupling cap from 6SN7 plate to 2A3 grid, 220K grid resistors shunted to ground in the 2A3s?
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Senior Member
Those all check correctly.

The 2A3 grid resistor (in series with 0.22uF coupling cap) is 1k per schematic, and as built. 220k to ground from the cap/1k junction.


Senior Member
A couple pics of the scope.

~1v ac from the signal generator at ~1000hZ.

Scope of signal generator output:


Scope connected to amp output:


Signal gen set to sine.

I know I'm missing something obvious......