18W EF86 Lite - VVR PA&PI

[Tommy_D]

Hi again everyone - the build of my EF86 Lite is progressing slowly but steadily..

Now trying to get my head around how a PI&PA VVR would work for this build.

Majority seems to think that PI&PA arguably works better for 18 watters although not sure about EF86 specifically.

I've marked up Josh's EF86 schematic. Feedback would be appreciated please!

What would be the values of R1 and R2 in the area connected to the EF86 screen?

Are any other changes be necessary?

thank you in advance!!

[Tommy_D]

Also not entirely clear why 2 additional filtering caps are required.
Could something simpler like the attached work?
The additional 22uF cap would be in parallel with the first 32uF cap before the VVR. Maybe the additional zener throws my logic off?

Anyway, your views would be highly appreciated please - thanks

[colossal]

Also not entirely clear why 2 additional filtering caps are required.
Could something simpler like the attached work?
The additional 22uF cap would be in parallel with the first 32uF cap before the VVR. Maybe the additional zener throws my logic off?

Anyway, your views would be highly appreciated please - thanks

The way you have drawn your power supply looks good. The diodes allow for better isolation. The 32uF cap in front of the VVR is needed or you risk MOSFET oscillation and tracking problems.

Your comment about the plate resistor for the EF86 being closer to 250k than 220k is fine. You'll just have a wee bit more gain.

[Tommy_D]

Thanks!

Just to confirm, you think it would work with the simpler drawing i.e. only 1 additional 22uF filtering cap?

How do you recommend calculating R1 and R2 in the drawing to get to the right voltages on the EF86?

Thanks again

[Tommy_D]

Having thought about it further..

Check RC filter cutoff value and I will probably be fine with 1x22uF

Looks like one has to experiment with resistor value to land back to pre-VVR voltage. Feels like something a little higher than 10k should do.

thanks again

[JMPGuitars]

I've marked up Josh's EF86 schematic. Feedback would be appreciated please!

I don't like the 250K resistor. If I was going to change that, I would lower it, not raise it.

Those EF86 Lite docs are based on the old docs with some general improvements. BUT I may put out a second version of the EF86 Lite with the EF86 portion based on my EF86 Xtra design.

The issue is that the plate voltage of a pentode should be higher than the screen voltage. For best tone/performance, the screen voltage should be between 60 to 70% of the plate voltage. My EF86 Xtra is set at 65%. You might want to look at the EF86 on that doc instead of this one.

The difference is this: when I changed to my new design, microphonics were reduced, and the tone / distortion was much improved. I believe by unbalancing them further, you'll likely make it sound even more microphonic and the tone will potentially be worse as well.

Thanks,
Josh

[Tommy_D]

Thanks Josh

You’re going to hate the story..

I thought I’d experiment putting a NOS carbon comp resistor on the EF86 plate. It’s a 220k but the value is close to upper tolerance (which seems to be frequent, at least in the batch of carbon comps I have).

Figured most Voxes would have been running on marginally higher resistors (admittedly the nearby 1M may also have been a little higher). Also looked at the AC10 schematic it seemed to be running the EF86 even hotter.

I’ll clip a 1M resistor in parallel to the 250k (~200k total) and see if it improves the tone. If it does then it’s an easy fix.

The resistors connected to the 12ax7 are also closer to 110k than 100k, which would presumably result in a similar effect.

I did use high quality metal film resistors around the inputs

Thanks for all the help

Will report back when the VVR is in and I start testing

[JMPGuitars]

Thanks Josh

You’re going to hate the story..

I thought I’d experiment putting a NOS carbon comp resistor on the EF86 plate. It’s a 220k but the value is close to upper tolerance (which seems to be frequent, at least in the batch of carbon comps I have).

It doesn't bother me.

Keep a few things in mind with your experiments:

1. The only spot that really benefits from the CC resistors is the PI plates.

2. The CC resistors may introduce extra noise here (V1), especially if they're NOS.

3. The value of the resistor is less important than the voltage comparison between the plate and screen. 60-70% is ideal, but as long as you get the screen lower than the plate, you're okay.

Thanks,
Josh