Plexi 6V6 Hoffman style

[tubenit]

I have noted over the years that Mark Huss's 6V6 Plexi has been a popular build per different amp building forums.

It occurred to me that there is no reason why one could not use the Hoffman Plexi 50 board to make a 6V6 version, IMO.  So, with very
minor changes,  Doug's Hoffman style Plexi 50w board should be a great turret board for this project!!

Using Hoffman's 18w PT and either the 18w OT or the Deluxe Reverb OT (#041318), this should be an easy build with all of Doug's documentation.  Layout, schematic & BOM (that would have slight changes) are all below in link:

http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf

Using solid state rectification with Doug's 18w PT, I am guessing that one would get about 406v on the 6V6 plates.  I would suggest perhaps trying the suggested "blue" values for dropping resistors on B+.

I am not sure what the dimensions are for the Hoffman Plexi 50 turret board is?   It would be cool if it would fit into Doug's new 18w chassis?   However, if it did not fit into Doug's new 18w chassis ......... then you could use Doug's chassis blank  17"x6.5"x2.5" (which is a great value buy, IMO)

Attached is Doug's Plexi 50w schematic showing Mark Huss's Plexi 6V6 values in red.  Suggested B+ resistors in blue.  Suggested PT and OT in maroon.

I personally would probably build this using Doug's Hoffman Plexi 50w values except using the suggested dropping resistor values (in blue) on the B+ rail.  I might consider 20uf filter caps on nodes C & D & E? I might consider a 100k for the negative feedback resistor?

THOUGHTS on this idea??   

With respect, Tubenit

[EL34]

Looks good
I could edit my 50 Watt Plexi documents if there is any interest in doing this

[tubenit]

Doug,

Any chance you know the length of your Plexi 50 turret board?  I am wondering if it would "just fit" within the 18w Stout chassis using the 18w PT? 

If not, your blank 17" chassis would definitely work!

This could be a very very cool build, I think!  The Plexi 6V6's seem to be a fairly popular build and I think a nice alternative to EL84's for those who are partial to 6V6's.

I think only editing the B+ rail would be needed?  Maybe change the filter caps for nodes C,D & E?  And the dropping resistors would need to be lowered to maintain good voltages on the 12AX7 plates.

I don't think any other changes would be necessary other then the B+ rail stuff ?

With respect, Tubenit

There are some very nice sound clips with Mark Huss's Plexi 6V6 here:  http://mhuss.com/Plexi6V6/page4.html

[Ken Moon]

According to the turret board order page, it's 11.25 inches long (by the usual 3.125 inches wide).

Ken

[tubenit]

According to the turret board order page, it's 11.25 inches long (by the usual 3.125 inches wide).

Thanks, Ken!  Staring at me the whole time and I didn't see it there. I clicked past that and looked in the wrong place for the info.

The 17" chassis would be needed and probably skip the 18w Stout chassis.

With respect, Tubenit

[MakerDP]

This is a nice sounding amp! Clips can be found at AX84.com down towards the bottom of this page...

http://ax84.com/index.php/oldprojects.html?project_id=november

[EL34]

I could make the board shorter by eliminating the power diodes and 100 ohm resistors from the m ain board
That stuff can go onto a terminal strip

The board would be 10 inches long if I removed that stuff

10 inches would still be too long for the Stout chassis
I have couple other chassis that would work

The blank 17 inch chassis and the 18 Watt chassis
http://hoffmanamps.com/MyStore/catalog/parts6.htm


The board could stay full length in these two chassis

[tubenit]

I could make the board shorter by eliminating the power diodes and 100 ohm resistors from the m ain board
That stuff can go onto a terminal strip  The board would be 10 inches long if I removed that stuff  10 inches would still be too long for the Stout chassis

You could eliminate 3 more rows of terminals by having a paralleled V1 tube which gives 30% more gain with NO increase in noise.

Then you'd use a spdt to parallel a 1.2k cathode resistor and a 100uf cathode cap which would give a 100.68uf/830ohm option.  I would also suggest using a .01 coupling cap after the V1-1&6 going to the volume pot.

One can simply by the Hoffman Plexi 50w and with a hacksaw, remove some length/turrets from each end.  I've hacksawed thru his board material is literally seconds and there is really no "dust" to speak of just sort of a gritty board "sawdust".  VERY easy to do!

I think that would shorten the length of the board enough to fit inside the Stout chassis! 

Thoughts on this??

with respect, Tubenit

[tubenit]

And if someone was willing to have a cathode biased  Plexi 25w (which I actually prefer), then one could definitely fit this into a Stout
chassis using a paralleled V1.  Just hacksaw past the bias circuit & then mount a 250R/10w & 220uf on to a terminal strip. Easy to do.

With respect, Tubenit

[DummyLoad]

tubenit, if you eliminate one of the channels, IMO it isn't a plexi anymore. a large part of the allure of that circuit is being able to mix and set the levels of each channel to get unique tones you can't get with just more gain.


my 2cents...



--pete

[HotBluePlates]

... a large part of the allure of that circuit is being able to mix and set the levels of each channel to get unique tones you can't get with just more gain.

Right. For better or worse, one channel is mud while the other is ice-pick; jumpering channels and using the volumes to balance the sound is key in my opinion to getting a great sound from the amp (at least that was my experience when I had a '73 Model 1987 head).

[tubenit]

tubenit, if you eliminate one of the channels, IMO it isn't a plexi anymore. a large part of the allure of that circuit is being able to mix and set the levels of each channel to get unique tones you can't get with just more gain.

Pete, I think that is a very fair comment. 

However, about 7-8 yrs ago, I build a 6V6 cathode biased version of Hoffman's Plexi 50.  I paralleled the V1 and I added reverb.  It was a GREAT sounding amp and I loved it! 

I have a friend who was in a band that was the opening act for Derek Trucks & another time for Dave Matthews,  he had me convert his Marshall JCM900 into a similar amp with the reverb and paralleled V1 after playing mine.  His bandmates loved the tone that he got from it. The original conversion was fixed bias. But some months later, he had me change it to cathode biased which he decided he liked even more. It had 6L6's.

So yeah, it's different ............ but not necessarily in a bad way. 

With respect, Tubenit

[DummyLoad]

tubenit thanks for the info. i'll experiment with that concept shortly, since the plexi preamp is still on the breadboard.

...to use the stout blank chassis stout PT and stout OT:  with stout chassis and stout PT we can assume that the circuit board has a maximum length of about 7.75". if we move the preamp caps to the top and make them dual section, e.g. 1 x 16uF/16uF paralleled to make a single 32uF; with the second can is a 32uF/32uF, then we have room for nearly all the plexi circuits. 

the cap that's 16uF/16uF is paralleled for LTPI power, the 32/uF/32uF cap uses one section for the summing amp and CF, the second section for the two input stages. i'd use a 100uF/100UF in the octal rectifier hole, and the inside mounted cap use i'd use a 32uF/32uF paralleled for the screens. the circuit board would be mounted 0.75" above the chassis floor. 

in the drawing the choke is a fender large 4H and the OT is the large pattern that fits chassis: hole ctrs. are 3 9/16" mojo770 with 8 & 16 ohm load. it is the mojotone tremoluxe replacement.   

all the PS diodes and the bias supply are on the board. the grid leaks and grid stoppers mount on terminal strips next to the power tubes as do screen resistors. 

this was taken from the boilermaker drawings and changed in haste, so please check for errors.

--pete

[DummyLoad]

tubenit thanks for the info. i'll experiment with that concept shortly, since the plexi preamp is still on the breadboard.


...to use the stout blank chassis stout PT and stout OT:  with stout chassis and stout PT we can assume that the circuit board has a maximum length of about 7.75". if we move the preamp caps to the top and make them dual section, e.g. 1 x 16uF/16uF paralleled to make a single 32uF; with the second can is a 32uF/32uF, then we have room for nearly all the plexi circuits. 


the cap that's 16uF/16uF is paralleled for LTPI power, the 32/uF/32uF cap uses one section for the summing amp and CF, the second section for the two input stages. i'd use a 100uF/100UF in the octal rectifier hole, and the inside mounted cap use i'd use a 32uF/32uF paralleled for the screens. the circuit board would be mounted 0.75" above the chassis floor. 


in the drawing the choke is a fender large 4H and the OT is the large pattern that fits chassis: hole ctrs. are 3 9/16" mojo770 with 8 & 16 ohm load. it is the mojotone tremoluxe replacement.   


all the PS diodes and the bias supply are on the board. the grid leaks and grid stoppers mount on terminal strips next to the power tubes as do screen resistors. 


this was taken from the boilermaker drawings and changed in haste, so please check for errors.


--pete


EDIT: fixed bais PS - attached schema.

[tubenit]

Pete,

I like your idea!  Schematic looks good to me.  Thanks for sharing it.  Still allows you to blend those two channels for a variety of tones.
Good idea.

With respect, Tubenit

[DummyLoad]

tubenit, that was the intent. a 1987 preamp and lower power power amp stuffed into a 12" x 6.5" chassis. only thing that changes is the power output stage tubes, NFB loop, and the power supply as needed to accommodate the lower power and filtering requirements. i took some liberties with the cap values, but that should not affect overall tone as much as the power tube change does.


IF you run with the same blank chassis and use the mojotone GA/30 goldtone X mount PT instead of the 18W Z mount, then the circuit card can be extended to 11.5" x 3.125" with that PT yielding around 375V with a FWB config. use the 24V sec. for the pilot lamp. :-)


http://www.mojotone.com/transformers/Goldtone/Goldtone-30W-PT.PDF



--pete

[EL34]

I read through the post

So the mini marsh is the best way to go to wedge the plexi into the Stout chassis?

[sluckey]

Here's a layout I've been playing with for a Plexi 6V6...

[EL34]

That looks good Steve
Is that based on that mini marsh schematic posted above?

[sluckey]

It's based on Mark Huss's schematic. I didn't compare to see if any changes have been made in this thread. That PT is sized the same as the PT used in the Hammond AO-43. Looks about the same size as a Deluxe Reverb PT.

I can send you the in-progress Visio file if you want to play around with it. The board is only 6 7/8" long. Iron and cap can placement is just an experiment right now. It should all fit your 12" blank chassis but will require some careful planning. I'm probably gonna build it during January.

About the only difference between this and my November amp is 6V7s versus EL84s.

[EL34]

Thanks,
I'll wait and see how your build goes first


keep us posted

[bkvance]

Looks good
I could edit my 50 Watt Plexi documents if there is any interest in doing this

Doug,
If you do I would really be interested in building this. It will be my 3rd build so I may still have a few questions for the experts here.


Keith

[sluckey]

I'm about 75% done building a 6V6 Plexi. I have my own layout and am using iron from a Hammond AO-43 amp. It all fits in a 12.5" x 6.5" x 2.5" chassis. Here's a link to my plans. May give you some ideas...

http://home.comcast.net/~seluckey/amps/6v6plexi/6v6plexi.pdf

[EL34]

Doug,
If you do I would really be interested in building this. It will be my 3rd build so I may still have a few questions for the experts here.
Keith

I'll wait until Steve is done with his build

[tubenit]

I'm about 75% done building a 6V6 Plexi. I have my own layout and am using iron from a Hammond AO-43 amp. It all fits in a 12.5" x 6.5" x 2.5" chassis. Here's a link to my plans. May give you some ideas...

http://home.comcast.net/~seluckey/amps/6v6plexi/6v6plexi.pdf

Sluckey,  your documentation looks incredible!  Excellent job!

It should all fit your 12" blank chassis but will require some careful planning.

Given Doug's chassis is 12" instead of the 12.5" ................. will the layout work with his chassis also or will the 1/2" length difference be a problem?

With respect, Tubenit

[bkvance]

You Guys are Phenomenal 


I can't wait to get started

[sluckey]

Given Doug's chassis is 12" instead of the 12.5" ................. will the layout work with his chassis also or will the 1/2" length difference be a problem?

It will fit. Just shave a quarter inch off each end of the control panel dimensions, ie, power switch 1" from left end, input jack 1" from right end. Tube lineup remains the same except last tube on the right will only be 3/4" from end of chassis.

I already had this 12.5" chassis, otherwise, I would have bought Doug's 12" chassis. It is a snug fit so careful layout is essential. The hard part is done though, if you stick to my dimensions. And using a standup PT would almost make it look spacious inside!

I'll post a couple pics of the unfinished amp in this thread just so you can get a feel for the layout. When the project is completed, I'll start a new thread so this one does not wander off too far.

[EL34]

The tube layout on my chassis is spread out across the back of the chassis
Steve's tube layout will not work in my Stout chassis unless you are drilling all the chassis holes yourself.

I only have one more blank chassis in stock and they will be gone
There's way more people wanting the punched chassis and so I am not sure I will have the blank Stout chassis back in stock soon

See this post for more info on the dimensions on my punched Stout Chassis
http://el34world.com/Forum/index.php?topic=17858.0

[tubenit]

I only have one more blank chassis in stock and they will be gone     There's way more people wanting the punched chassis

For anyone who has not tried punching out their own chassis,  it is VERY easy to do with Doug's 12" x 6.5" x 2.5" chassis.  I just did this on my Acoustic PreAmp build.

I have been using this $25 Knockout Punch Kit for over 9 years on all my builds.  It works great on aluminum chassis. (I think I paid $20 fo mine?)

http://www.harborfreight.com/knockout-punch-kit-91201.html

I would prefer a pre punched chassis, but with the builds I've done .......... I've needed to punch out my own.

I wouldn't shy away from punching your own aluminum chassis  IF you can't find something that is already punched out for your needs.

with respect, Tubenit

[sluckey]

Here are a couple pics...

[EL34]

Looking good Steve

[tubenit]

Yeah,  that looks fantastic!   GREAT job!   Very well thought out.

with respect, Tubenit

[EL34]

I think this will be my next build


I'll be able to dial in the build in the Stout chassis

[bkvance]

When this is finished, I would really like to see a build sheet so as a Rookie I can follow suite and build this amp. I'm really excited about this being my 3rd build


Keith

[sluckey]

Progress has been slow. Too cold in the shop! Hope to be done this weekend. Couple more pics...

[EL34]

looks nice Steve

[Willabe]

looks nice Steve

Yeah it does!


              Brad   

[SILVERGUN]

It's never gonna work...
He didn't twist the heater wires. 

 

[sluckey]

Ha! I came very close to just running a buss across the top and bottom of the little tubes. May do it yet.

[SILVERGUN]

Ha! I came very close to just running a buss across the top and bottom of the little tubes. May do it yet.

You'll have to save that for the next last amp you're ever gonna build 

[Willabe]

It's never gonna work...
He didn't twist the heater wires. 

 

[Willabe]

You'll have to save that for the next last amp you're ever gonna build 

   

[Willabe]

Ha! I came very close to just running a buss across the top and bottom of the little tubes. May do it yet.

That'll make it sound like a Soldano. (How ever he spells it.)


               Brad   

[Ed_Chambley]

I have actually used the electric fence wire and run parallel heaters.  I them got concerned and replaced it with individual wires, but still in parallel.  I have found with 12A_7 if I cut the wires I can easily remove them from the tube.

[sluckey]

You'll have to save that for the next last amp you're ever gonna build 

I've already built the "last" amp I'm ever gonna build. I'm calling this one the "just one more" amp. It's a catchy naming scheme I came up with for my dear wife.   

[Willabe]

I've already built the "last" amp I'm ever gonna build. I'm calling this one the "just one more" amp. It's a catchy naming scheme I came up with for my dear wife.   

 

[sluckey]

I'm done! No surprises. Small size does not equal small sound. It's LOUD. But it sounds very plexish (new word?)

The layout works well. Amp is quiet when it should be. I think the challenge was not "can I build the 6V6" but rather "will it fit in this small chassis". Ancient Aliens theorist say ABSOLUTELY! The AO-43 iron I used exceeded my expectations.

I'll put together a web page for this project in the next few days. I'm very satisfied. Even better, my wife knows this was the "just one more" amp and she accepts that.

Now back to the shop to play.     Here are a couple more pics...

[EL34]

Looks great Steve
Do we get to hear a demo?

[tubenit]

That looks just incredibly well done!  Beautiful build.  Bravo.

With respect, Tubenit

[sluckey]

Wish I could do a demo but I'm just not set up for that. My heart mainly wants the electronics part of this stuff. Maybe I should just send all this stuff to y'all and let y'all do the demos? 

Maybe y'all could pack up your laptop and mics and come to Mobile. It is Mardi Gras season after all. Free lodging!