Converting the Hoffman Plexi 6V6 to Single-Ended EL-34.

[Big_Mike]

Hi guys -

I am working on trying to build a single-ended plexi for about 5 watts of power using a single EL34.  I would like to use the Hoffman Plexi 6V6 layout as the starting point, as it has the bright and normal channels and is a real solid design.

In looking at most single-ended plexi designs, the master volume is directly in front of the EL34.  It seems to me that I could use the Hoffman 6V6 turret board and modify it slightly for a single-ended EL34 instead of the push-pull 6V6s.  I need a bit of help, though, figuring out splicing the preamp from the Plexi 6V6 into a single-ended EL34.  I was wondering if anyone could help me out on this.  I think this would be a great project to work on, and I have most of the parts on hand.  Thanks!

[kagliostro]

Draw on paper your schematic idea (also by pencil if you don't use a CAD program)

it will be easier to argue about


Something like this ?




Franco

p.s.: Did you give a look here ?

http://chasingtone.com/dvnator/the-seplex/

[Big_Mike]

Yep, I have looked at the SE Plex as well, I am in contact with the guy who designed that.  His version has a pseudo PI stage for presence control.  I'm not sure I want to do that on this build.  I think I would rather have the channel blending capability.

[Big_Mike]

Attached is the Allen Class Act schematic...you can see the master volume directly to the power tube.

[shooter]

fwiw I've used this "template" on a couple builds, 1 SE, 1 PP.  I added V3 to get more drive to Big bottle tubes.  Most of my SE builds want somewhere between 18 - 25VAC on the PA grids.  Also works modifying to CF into the PA tube if you don't need/want the extra signal.

[sluckey]

I think having a gain stage between the tone stack and power tube will be necessary to produce enough drive signal for the power tube. This also provides an opportunity to provide a NFB circuit with presence control. The NFB is a big part of the plexi sound.

[Big_Mike]

What do you think of this SEPlex schematic?  I could go with that:

[sluckey]

What do you think of this SEPlex schematic?  I could go with that:

That takes care of the needed gain between the tone stack and the power tube but you lose the two channel blending. I would use three small tubes and have it all.
 

[Big_Mike]

I like that idea, seems I could splice the Hoffman 6v6 preamp section onto the SEPlex schematic with regard to negative feedback and add a third preamp tube.

[JB]

I think having a gain stage between the tone stack and power tube will be necessary to produce enough drive signal for the power tube. This also provides an opportunity to provide a NFB circuit with presence control. The NFB is a big part of the plexi sound.

I think the PI is part of the sound too, when driven hard. I think the PI starts to distort soon after the output stage and the blend of the two is what does the business.  So having a gain stage post stack pretending to be the first half of the PI is a good thing, for NFB, Presence and distortion characteristic.

[DummyLoad]

i built the attached schema on my breadboard. sounds really good. i have a layout for it, but never got around to building it. as shown it's parallel SE: dump one EL34 and use a 6-8k 15W OT. V3 replaces the PI and as shown has nearly same gain.


--pete

[Big_Mike]

Interesting, I think I am going to try and splice the plexi 6v6 schematic onto the pseudo phase inverter/output section of the SEplex design.

[DummyLoad]

a big part of the sound of a plexi is the bright and normal channels - you bond the two and dial in each for unique overtones not possible with just one channel. try it with two, if you don't like it, ditch/disconnect the second.


--pete

[kagliostro]

if you don't want to use a further tube and want to preserve the second channel

may be an IRF820 as Source Follower in place of V2a CF will be of some help

Franco

[Ed_Chambley]

i built the attached schema on my breadboard. sounds really good. i have a layout for it, but never got around to building it. as shown it's parallel SE: dump one EL34 and use a 6-8k 15W OT. V3 replaces the PI and as shown has nearly same gain.


--pete

Pete, when you had this breadboarded did you try higher votages like in the 450 to 460 range? Just courious because Imknow Se is different, but I prefer the tone of El34 with higher volts and current in PP designs.


I have built this amp  in a three different ways. The current one is a single el34 with a smaller edcor which is 3.5k at 4 and running it at 7000 at 8. Has a ul tap I made foot switching and cleans up nicly. It iis their 15 watt ot.


All 3 versions sound unique and I thoughtt it may be the 10uf bypass, but increasing it limits the mixing of channels and going smaller thins the clean too much.


I ask because I just  got the perfect pt to put it at 470 with ss recrifier. Want to know if it is worth trying.


One thing I will say, the dual tube 15 watt version as you have it is plenty volume.

[DummyLoad]

did you try higher votages like in the 450 to 460 range?

ed, i did. i can't recall if it was a WOW difference from 420V to 450V. that was too long ago and i don't have any notes, other than i used a one-electron UBT-2 and a bassman PT. i did do something similar with 440V B+ with the all pentode dual SE amp i built, but that's not a valid comparison.

--pete

[Big_Mike]

Here is the schematic with the spliced Hoffman preamp section with the SEplex output section.  I am not sure if the filter cap and resistor values on the power section should carry over to the Hoffman preamp, any help in that area would be appreciated.

[shooter]

The R's between taps might need to be tweaked, but the starting values look ok.  take a look at the schematic, I like doing stacked caps - bleeders in the beginning, 1st node.  I would do your pre-amp taps near the preamp, do away with the CT config on your filaments and just ground it.  you may need to elevate it if you have some noise issues, so moving it from a ground to cathode will be easier.

[DummyLoad]

I am not sure if the filter cap and resistor values on the power section should carry over to the Hoffman preamp, any help in that area would be appreciated.

they'll work fine.


--pete

[Big_Mike]

Thanks for the input.  If anyone could help me out in modifying the Hoffman 6v6 turret board for the single-ended plexi design, I'd appreciate it.  Thanks!

[shooter]

modifying the Hoffman 6v6 turret board

since you have a "working schematic", print out the existing layout, use a pencil to "work-in" the new/changed parts.  Your power tube can be wired with a couple 5 lug terminal strips if you have room.

[sluckey]

What ever became of your mini AC-30 project?

[Big_Mike]

The mini AC30 is the next project after this mini Plexi on my workbench.  I had a little delay in getting started due to work.