PT test

[Tommy_D]

That's right, the PT itself is buzzing

May be wrong but it seems that an unloaded PT would generate a significant magnetic field

Anyway the build is completed. I am planning to start testing in the next couple of days and will report back to the group!

[Tommy_D]

Good news, the amp runs!

Just try running it from my phone with an old set of headphones as load. Can't work well with a guitar amp but just wanted to see if some sound came out of it. Yes it does!

Zero noise with the volume pot turned down. With this particular set up there is some noise with the vol pot turned to the max. I'll have to see how it is in real life, with a guitar and through a cab.

Loaded PT has an almost inaudible buzz which feels totally normal.

Voltages:
- EL84: up to 392V at plates
- 12AX7: up 270V at plates (my plate resistors are closer to 115k)
Slightly hot but I have a PA&PI VVR which I don't expect to run at the max settings much at all. I suppose if I do need a little extra push over the cliff it's available
- EF86: 196-204V plate, 133-134V screen.

As predicted by Josh the 250k plate resistor for the EF86. put a 220k in parallel to solve it at least temporarily).

Keen to get your views on voltages.. I mostly have question marks around the EF86. It's higher than what most people here seem to get but still seems within specs.

245V mains where I am in the UK so overall the amp runs on the hotter side.

thanks for all the help and advice so far!

[Tommy_D]

Missing part of the sense on EF86 plate resistor.. 250k was too high and screen voltage was above plate

[Tommy_D]

Finished the testing. Other than the EF86 with the VVR I can dial voltages to exactly the base line + push a little harder if I really want to. All good.

It sounds like a million bucks!!!

Running with a very quiet NOS Sylvania EF86, JAN Philips 5751, JJ EL84s

A/B’ed against my 65 Amps London, it sounds at least as good

Pics to follow

Will order some resistors to drop the EF86 voltages and see how it sounds at ~150V and 100V at the plate and keep whatever I prefer

Very happy with how the build turned out! Thanks for all the tips

[JMPGuitars]

Careful with JJ EL84s. That's their worst tube, and typically aren't reliable.

[Tommy_D]

Did some faulty JJs blow up one of your amps?

Josh, what is your view on EF86 plate voltage? Noticed the Lite tends to run plate around 100V while your Xtra is closer to 150V.

I know you've wrote screen voltage should be 60-70% of plate. I'm at 67% currently so bang on

[JMPGuitars]

Did some faulty JJs blow up one of your amps?

Josh, what is your view on EF86 plate voltage? Noticed the Lite tends to run plate around 100V while your Xtra is closer to 150V.

I know you've wrote screen voltage should be 60-70% of plate. I'm at 67% currently so bang on

The Lite version is based on old docs, but with improved grounds.

The Xtra is my version for high gain, and better values.

If your voltages are good, and you like the way it sounds, then enjoy it. If you still feel like tweaking, you can completely copy the V1 from the Xtra and replace V1 in the Lite.

Regarding the tubes: https://youtu.be/21G6TkukwY8

I posted that video in 2013. That defect was present in at least 10 sets of tubes, across different batches. And in the years following, a number of other people have also expressed that issue, as well as other reliability and tone issues. I liked JJ tubes until that happened. Now, even though I haven't had issues with other tubes, I tend to avoid them in general (though I do still have a few of their preamp tubes I liked). Almost everything I build is for sale. I can't take the risk of something potentially exploding or starting a fire.

Thanks,
Josh