EF86 internal shield

[FunkyE9th]

So I'm looking at the a picture of the guts of a Dr Z Route 66. It looks like he's got the the internal shields (pins 2 and 7) tied to the cathode instead of ground? Why is that? What would be the advantage of doing that instead of connecting it directly to ground?

Thanks,

-FunkyE9th

[Structo]

The schematic I have shows that pin 2 goes to the cathode and 7 & 8 jumpered.

[Alexo]

There's a bypass cap on the cathode, so the cathode is at AC ground.

[FunkyE9th]

Tom,

We have the same schematic. I think the schematic shows that 2 and 7 are connected.The schematic I have also came with a gut shot of the amp. From what I can tell from the pic, pins 2,7, and 8 connect to the cathode, matching the schematic. Also it seems the layout that came with the schematic does not match the schematic.

[FunkyE9th]

There's a bypass cap on the cathode, so the cathode is at AC ground.

Just saw your post after I responded to Tom....that makes sense.

[Structo]

Tom,

We have the same schematic. I think the schematic shows that 2 and 7 are connected.The schematic I have also came with a gut shot of the amp. From what I can tell from the pic, pins 2,7, and 8 connect to the cathode, matching the schematic. Also it seems the layout that came with the schematic does not match the schematic.

You are right, I didn't zoom it in far enough to see that connection looping around the top of the tube!.

[Structo]

Do you have this layout as well?

[Andy Le Blanc]

Pin 2 is an internal shield, pin 7 is a guard ring around the control grid input
to provide isolation from the rest of the tube elements. Pin 8 is the suppressor.
The data sheets from different makers vary in their descriptions how to obtain minimum hum.
Mazda states that the shields should be earthed but Mullard states they may be connected
to the cathode as desired for minimum hum. They agree that a large cathode
bypass should be used (50-100uf) and that the heater should be earthed at pin 4 or a hum pot should be used,
but give different statements concerning socket type and additional external shielding.
Wouldn't mind seeing the layout.

[Andy Le Blanc]

gotcha..... thanks

[FunkyE9th]

Yup that's the layout I have. On the layout, 2 and 7 goes to ground and not the cathode.

Thanks for the info Andy. I guess I should go read a real ef86 spec instead of the 1-page jj specs.

[M Fowler]

There is also this schematic too.

Mark

[FunkyE9th]

Hmmm.... a 12ax7 version of the rt66. Interesting...

[M Fowler]

Also the amp varies between guts shots as well see this one. This one uses a cap can with the 40/20/20/20 instead of two caps on the end of the board.

[dobbhill]

Curious: those 2 posted schematics are completely different animals. One has an ultralinear-non-feedback output, the other has a choke and feedback, not to mention the preamp differences... Are there multiple versions of the Rt66?
D

[M Fowler]

The RT 66 has been around for many years and I would suspect has gone through many changes; however, the layouts and schematics are not original someone's interpitation of the amp. So I am going to improvise and follow the layout, schematics, pictures and the Marshall JTM 45 for my build. I don't care if there are differences to the exact RT66.

I am going to use the JTM 45 mojotone tranny set, hammond 16x8x2 chassis, which fits the Z head cab well, even though the actual RT66 is 16x8x2.5

Its all fun anyway.

Mark