86 Special Duet

[tubenit]

This is only a DRAFT idea ..................  

I am simply posting an editable version so Mat and other's can interact to come up with a final version.

CHECK for ERRORS  &  compare schematic with layout.  PLEASE read posts below also.


Tubenit

[NEjoe]

On pin 6 of the EF86 the .015 cap isn't in the same place on the layout as the schematic.  The wire from the front panel needs to go directly to turret 13 and not in between the 82K and 56K.  I think it's off in the single tube 86 special also. 

The layout does not match the schematic values on turret 8, the schematic has 4.7 uF, the layout value is 5 uF.  There is also two V1 designations on the layout. 

Also, when I pull up the schematic in ExpressSCH, the component properties are wrong, I get values (like 22k) where parts ID information (like R1, R2, etc) should be and it's blank where the component value should be.  Is my version of ExpressSCH broken or is that happening to other people too?  Is there a reason I don't know about why it should be this way?  It just makes it hard to say that the random C component value is wrong in that section over near.......instead of just calling it Cx.  I'd like to run the auto assign part ID but I think that would wipe out the values if the info is in the wrong place.

And, did the HoSo56 run better with Sovteks or EH or JJs or doesn't matter what brand EL84s?

Thanks

[tubenit]

Good catch on the .015 hookup on the turret layout board. Thanks!

4.7uf or 5uf .......... doesn't matter. Take your pick and use that. They'll sound the same.

I have never used the R1, C1 type assignments.  Feel free to use the editable feature of the program and reassign them. That's the plus of the program being editable. You can print of the paper versions and use those to guide redoing them.

Ask the guys on the Tweaks forum about the brand EL84's.  I first built the 56T with EL84's and changed them after a week or two to 6BM8 tubes. 

With respect, Tubenit

[NEjoe]

The 70pF cap on the master volume needs to move up top onto the other side of the pot so it's not directly touching ground.  Both the schematic and the layout have this.

Also, on the OD tone pot, is the 1 MB value a typo? And is the NFB switch a SPDT ?

This isn't considered a HRM any more is it? .......Thanks 

[tubenit]

The 70pF cap on the master volume needs to move up top onto the other side of the pot so it's not directly touching ground.  Both the schematic and the layout have this.

Also, on the OD tone pot, is the 1 MB value a typo? And is the NFB switch a SPDT ?

This isn't considered a HRM any more is it? .......Thanks  

EDIT:  the hookup on the plate coupling cap on the EF86 is corrected per NEjoe's catch. This is on the layout. Thanks!
Mat directed me on each of those concerns. For example, he used the 70pf to drain treble not add. He and I communicated off line.
It doesn't matter on the spdt or dpdt, they both will do the same. Take your pick.  

with respect, Tubenit

[NEjoe]

Oh, OK then the 70pF  stays where it is.  Here's an update of the schematic.  I named a few things, moved a few wires, and assigned numbers (so the BOM is a bit easier) but the schematic is still the same.

[tubenit]

OK,  Mat posted some voltages for the 86 Special single ended on the TAG.

In light of that,  I think the 86 Special Duet probably should have a 5Y3 rectifier to get about 302v on the EL84 plates.

Then, I would use larger dropping resistors on the B+ rail to get the preamp tube and EF86 pentode to match his.

With respect, Tubenit

[NEjoe]

I'm thinking of skipping the tube rectifier route, and using 4 individual MOSFETS to drop and vary the voltages for the plates, and screens, and the EF86, and the 12ax7s. Or perhaps the two 12ax7s should be able to have independent voltage adjustment, hmmmm......Anyway, the MOSFETS only cost $0.72 so it's not like 4 or 5 of them is breaking the bank, the trimmers to control them actually cost more, but even with the caps everything is probably under $25.

Attached is what I'm thinking. Anyone who has any comments why this will or will not work please sing out. The part numbers are filled in and are from mouser, 12 turn trim pots are listed, but the 250K is not in stock so I used another very similar one that was in stock.

And about the reverb (you know, that one that isn't there......yet).......does anyone reading this have opinions about the single tube 12ax7 versus the two tube 12at7 and 12ax7 design? Do you think there will be enough front panel space for a few more knobs.  

[tubenit]

Man, that looks very interesting and is also waaay above my knowledge level!  Try PM-ing Sluckey and ask him to weigh in on this. I think he would know. Some of the other forum guys might understand it also.

The one tube has always worked just fine for me personally, but I don't use much reverb at all.  In the ARCHIVES, there is a 6BM8 one tube reverb design that I hope to try someday also.

With respect, Tubenit

[NEjoe]

Well Sluckey said it would probably work, but thought it would be better on a prototype board than an actual amp.

Still, I'll build it, (moving forward at glacial speed).  It really looks more complicated than it is.  I suppose I could just do one or two MOSFETS since the EF86 and the 12ax7s are pretty close in their voltage requirements on this ....but what fun is that..LOL

The more I look at the relatively low volts on the SE 86 special, the more I'm thinking a EF95 (6AK5, 6J1P russian)  just might fit in there (180V plate max on the EF95, versus 300V on the EF86).  Of course, the EF95 is a 7 pin tube instead of a 9 pin so a different socket and wiring would be required (on the EF95 you don't have to tie the cathode to grid 3, it's done internally). Significantly less $$$$$$ for EL95 than a EL86........which makes me think it might not be possible, or someone would have done it already.......but then again, it doesn't cost much to try. 
 
With the relatively low volts perhaps some of the cheapo EL84/6P14P (non -EV version) would fit too.....maybe go for some 6n2p instead of 12ax7 and make it an all russian tube amp.......or........perhaps run in the other direction with a 300-0-300 transformer to put the plates at 400V and drop the screens to about 320V (like the amp manufacturers), and then if the tubes survive I might do about 345 plate/335 screens.  I'd have to go use some EL84's that could handle that though.......... 

[NEjoe]

Over at ampgarage a member named layupe pointed out the PAB switch in the layout doesn't quite match the schematics.

I made a few wire changes in the layout so now the PAB switch hopefully matches the schematic. I also corrected the second V1 designation to V3.  It's posted below.

BTW the PAB switch mistake also exists on the original 86 Special layout (SE).

EDIT  I am adding a schematic and layout from 11-'13 that Mat says he used. Another TAG member built this and stated the 270k going into the LTPI was a problem and he removed it.

Tubenit