EF86 internal shield

[Andy Le Blanc]

I guess the preamp is the point, you got that ef86 driving that inverter,
and thats it. You can get voltage gains out of a pentode like that on the
order of 300-400. More than enough to drive that style power side to any
expectation for guitar, get that pentode coloring, and do it with one gain stage.

[M Fowler]

Ok I used the JTM45 tranny set so no ultralinear output for this amp.
What do I do about the 470R screen grid resistors wire them to the 40uf cap?

Also I do not have the 62vac bias supply shown on layout on my PT so what do I do there? Tap from one side of the rectifier ac side and lower the resistor connected to the diode on bias circuit?

Mark

[Sonny ReVerb]

The Z28 is basically a 6V6 version of the Rt. 66, but it's not UL. The Z28 uses a choke and has the screens connected to the far side of the choke from the PT (a la Fender AB763). You could probably use a resistor instead of the choke, if you like.

For the Bias supply, I would copy something like a Marshall 1987 which has a similar setup. You might have to play with the value of the 220k resistor to get the range you want.

[ampdoc1]

That first schematic was one I drew from a R66 on my bench in 2005. I questioned my own drawing, but it is pretty clear on the attached pic that pins 3,2,7 and 8 are all connected together, via jumpers from pins 2 to 3, 7 to 8, and the yellow wire from the preamp board joining pins 3 and 8.

I am pretty certain that the values of all components on the schematic are correct. At least for that particular amplifier.

ampdoc1

[ampdoc1]

I forgot,...I don't know where that layout drawing came from,...I didn't make it. As said before, it may be another version of the 666. Not exactly the same circuitry, but good enough for physical layout purposes.

ampdoc1

[Alexo]

That first schematic was one I drew from a R66 on my bench in 2005. I questioned my own drawing, but it is pretty clear on the attached pic that pins 3,2,7 and 8 are all connected together, via jumpers from pins 2 to 3, 7 to 8, and the yellow wire from the preamp board joining pins 3 and 8.

I am pretty certain that the values of all components on the schematic are correct. At least for that particular amplifier.

ampdoc1

So tying the shields to the cathode does two things:

It sends the noise to ground via the cathode cap.

and

It DC-references the shield to the cathode.

Maybe being at the same DC ref. prevents the cathode from drawing electrons off the shield? Or maybe it lets the shield first absorb any noise that would be drawn to the cathode as a result of its voltage?

Maybe it's just more convenient to tie them to the cathode than all the way to ground?

FWIW, I just built an amp where I tried this and it's at least as quiet or quieter than any other pentode preamps I've built.

[FunkyE9th]

I tried it and could not tell the difference. I just used a test clip and clipped the shields onto the cathode, chassis ground, and preamp ground.

[M Fowler]

FunkyE9th

What did you use for tanny set? Do you mind posting a gut shot of your amp?

I wired mine like the picture posted by ampdoc1's his schematic as well as the layout to get an idea of what to do.

Thanks Sonny ReVerb for your answers.

Mark

[FunkyE9th]

I'm using toneslut PT and OT. I'm not building a Rt 66. I'm just building an amp with an EF86 preamp and was using the the Rt 66 and the Vox as a reference to see how they use an EF86. The Z and the Vox are actually pretty much like what's on the datasheets, except the data sheet does not show how to hook up the shields. Also, I'm not using KT66 or a tube rectifier, or the Z tone stack. I might not even have a tone stack when all is done.

I don't really wanna post pics of the amp because it's a proto and it looks like crap. Got holes all over, switches all over, wires and mini clips hanging all over, toothpicks holding parts down. Basically doing a lot of experiments. When it's done and if it sounds good maybe I might post pics.

Your build is looking good. How does it sound?

[M Fowler]

Thanks for your replies and sorry I interrupted your thread, I thought we were both building the same amp.

Funk that makes sense to me now, that your building a prototype, I wasn't sure. This is my first time using EF86 and when I wired the heaters, you bet, I wired them for 12ax7 then I looked at the layout and schematic and slapped myself in the forehead.

I am considering pulling these trannies and use a dynaco OT with ultralinear leeds and another PT with the bias voltage. I want to build a JTM45 anyway and that is what these trannies are for. If I am going to build a RT66 might as well do it right. I didn't even realize that it was ultralinear until I got this far, dumb arse. Rushing doesn't get one anywhere real fast.

So your using toothpicks in your proto, colored, flat or round?

Mark

[Wayne]

The round ones impart more even-order harmonics...



W

[FunkyE9th]

You are so close. Why not just go ahead and finish the build. You might end up with something that's different, sounding really good and learn a lot in the process.

Is the hang up how to get the screen supply and bias supply? What's the spec on the 2ndary of your PT? You can generate -50V by tapping the 2ndary just like the Express. You have to play around with the 220K resistor. Lowering will get you higher voltages. Actually it's the same design as the Z but the Z is using 4.7k and using a bias tap.

I looked at mojo's web site and it looks like they have two versions of the JTM45 PT, a 300-0-300 and and a 350-0-350. If you are using 350-0-350 replacing the 220K with 150K should give enough range to get -50V bias. If you are using 300-0-300, using 120K should work.

For the screen supply, it's little harder cuz you kinda have to decide what screen supply voltage you want. The big question is what voltage? Maybe just drop it a few volts and then add a filter cap. This will then affect the PI and Preamp supply assuming you tap off of the screen supply to get the PI and preamp supply. So you may have to change the dropping resistors on the PI and Preamp supply. You can also just use a choke and the choke will determine your voltage drop which should be very small.

Oh yeah...mint flavored toothpicks have the best tone.

-FunkyE9th

[M Fowler]

FunkyE9th,

Yes I am very close just need to finish up the control wiring and figure out the bias supply as well as the screen resisitors.

The tranny is 350-0-350, so I will follow your suggestions and see how I come out. The choke would probably be a good idea as well.

Thanks Mark

[M Fowler]

I had this attachment all along but forgot I did.

EDITED I had to delete sorry, I got it from a web page but saw copy write info so deleted it. We don't need more problems.

Mark

[M Fowler]

I has been awhile since I finished up the root 666 but thought I had better post a picture of the final wiring incase some one was interested in building one of these. I had the cap wired wrong before and changed the bias resistor it works very well now.

Mark