Plexi-7868 build in Stout Chassis

[AZJimC]

Howdy Y'all!
I am building a Plexi-6v6, but with 7868 output tubes.
I finally have a few of the major components to get started ordering the rest of the parts I need. My question is, with 7868 tubes, how will the bias differ. I am trying to get a close guestimate so I can order resistors and I thought the first bias resistor, 220K/3w or the second, 15k, might be better different than what is used for a 6v6.


Just getting underway, and still have a few parts to spec and source, and then I'll upload a pic or two. I'm using the Electro Harmonix 7868, which I understand is the closest of the modern production to the original.

Thanks for your attention,

Jimmy

[TIMBO]

Hey mate, you wouldn't believe it, I just saw a pair on ebay in Australia and looked them up and had some build ideas of my own.
Having them in a plexi circuit sounds great. Good luck

[AZJimC]

Thanks Timbo, I have a 7591A build, and I like it so much I thought I'd do it right. I still need to source transformers to fit the Stout Chassis. The sockets I've got (Magnoval, or Novar) look like they will bolt on from inside flush with the surface. The littleish big tubes will look alright on it I think, and I love the sound.

[TIMBO]

WOW, That's spooky, I recently picked up a organ chassis that have those tubes in it. It also has a separate reverb circuit that have a pair of 6GW8s in it.
But it will be awhile before I get it mod it.
Hope you can do some sound clips. Thanks

[octal]

Tube data sheet shows in the range of -15 to -21V bias for typical guitar amp B+ voltages. I think you'd probably want nearer to the -21.  As a starting point, "double" the 15K to 33K and you'll probably get in the ballpark.


I'm basing this on the Huss 6V6 plexi schematic, which shows -45v as the bias, so you're roughly looking to half that. Of course, modern 7868s could be different, and you didn't mention your B+ so this is "off the cuff" advice.

[octal]

As you can see from the tube data, you're looking at a bias voltage in the range of around -15 to -21V depending on your B+ . My plexi schematic doesn't have stock voltages on it, but the Huss 6V6 plexi schematic shows -45V. I say roughly double the 15K dropper resistor to 33K, and you probably will have enough adjustablity to get in range.

[DummyLoad]

use the mojotone TW35  model #MOJO791 it's a 6600ohm:4/8/16ohm part - doug can order it for you.

http://www.mojotone.com/amp-parts/amp-transformers-output-mojotone/Mojotone-TW35-Project-Output-Transformer#.VSWOx_nF8Wp

use the hoffman plexi choke or the hoffman large fender choke. the large fender choke will probably be a better fit on the 12x6.5 chassis.

run at 450V B+ so use PT hammond 376X - 640VCT @ 173mA, 6.3V @ 5A, 5V @ 3A with 5AR4 or SS rectifier.

http://www.hammondmfg.com/pdf/EDB376X.pdf

with G2 at 440V you'll need to set bias at about -25V.  basically, what sets the idle at about 38-40mA / tube.

7591 are a high slope tube so use a 1K 3W WW screen grid resistor for for each 7591.

--pete

[DummyLoad]

Tube data sheet shows in the range of -15 to -21V bias for typical

that's with G2 @ 350 or 400 - bias will be deeper with 440-450V at G2.


--pete

[SoundmasterG]

The EH 7868's are GREAT tubes. Sound as good or better than vintage ones and last a long time. I put them in an already existing amp so not sure where one might find sockets for a new build, but they are very nice tubes....good choice!

Greg

[AZJimC]

The EH 7868's are GREAT tubes. Sound as good or better than vintage ones and last a long time. I put them in an already existing amp so not sure where one might find sockets for a new build, but they are very nice tubes....good choice!

Greg

Tubedepot has them, and they fit flush on the stout chassis mounted from inside. had to sand a thousandth or so off the inside of hole, and it is a perfect fit.

[sluckey]

@AZJimC, I'm anxiously waiting on the outcome of this project.

@TIMBO, I'm currently working on that AO-63 chassis. The main amp looks so similar to the old Gemini II Ampeg power amps that I'm replacing the organ preamp with the Gemini preamp. I've got lot's of info on that AO-63 that I can share when/if you ever want it.

[TIMBO]

Sluckey, That would be great, I haven't given it much thought, although I was thinking I would do an UPSTAIRS/DOWNSTRAIRS chassis in a combo, or a "PIGGYBACK" head and cab, as the reverb has its own amplifier. 

[DummyLoad]

@AZJimC, I'm anxiously waiting on the outcome of this project.

@TIMBO, I'm currently working on that AO-63 chassis. The main amp looks so similar to the old Gemini II Ampeg power amps that I'm replacing the organ preamp with the Gemini preamp. I've got lot's of info on that AO-63 that I can share when/if you ever want it.

i have one as well: would you kindly share?


--pete

[sluckey]

This should keep y'all busy for a while. I've played guitar thru the virgin AO-63 main amp. Just plugged into the Organ RCA jack. Even with no tone controls this thing reminded me so much of my old Ampeg amp that I decided to replace the organ preamp with a Gemini preamp ((same as B15 but 12AX7 tube). It's very clean and powerful sounding. I'm still a couple days away from finishing the conversion.

Enjoy...

[DummyLoad]

much thanks!


--pete

[TIMBO]

Thanks guys.

[AZJimC]

Thanks guys, for your input. Sluckey, I too am anxious to get this one going. It's been a hectic couple of months, mother has had Alzheimer's for several years now, and at 95, we laid her to rest earlier this past month, back in my hometown, and I'm still traveling at this point. I have the turret board built, most of the components are there, except transformers. After financially recovering a bit, I'll be getting on this build again. Meanwhile I have a super reverb with crackling. I also picked up a Carvin Vintage 50, with 4xEL84, and I intend to get it rocking and sell it to finance future builds.

I have my 7591 build with me, and it receives accolades every time I fire it up. This 7868 will be a head, and much more compact than the 15" combo that has the 7591A's. Hoping someone in Las Vegas picks it up once I get it finished. Should be a good market there, loads of working musicians, many of which are 80's style rockers.

[Willabe]

Very sorry to hear of your mothers passing.

(95 is a good long life but it still hurts.)



                Brad       

[AZJimC]

Very sorry to hear of your mothers passing.

(95 is a good long life but it still hurts.)



                Brad   

Thanks Brad, very thoughtful of you to express.

After a long trip home, stopping for a couple weeks to assist a friend replacing his Evap coil on his a/c system, I finally got rested up a bit and started this build. As with anything, no plan survives contact with the enemy. Everything will be a tight fit, and a circuit board and switch cannot occupy the same space, so now I need to move my standoffs about 1/4" away from the rear of chassis. Hoping a flat washer will cover the holes. Other option would be to slot the holes in the board, but then one side would be barely on the standoffs. Also, I really am wishing for some 3/8" length standoffs. The 1/2" are just a bit tall for my taste, so I may just grind them down a bit.

OH, and also, would someone be kind enough to get me an editable copy of the files that make up the 6V6 Plexi PDF? I hope to carefully document my deviation from the original plan. IMO an undocumented amp is not worth as much as one with full docs.

....and thirdly,
I'm looking at transformers, and am looking at three options.
(1) Mojo758 (Tremolux) for power, and a good 40W 6K6 OT, or maybe the 35W trainwreck type, with 6600ohm primary. I think 300ma is a little overkill on HT winding, but it would work, and fit with little modification.
(2) Call Edcor and see what they can do for a set. Wait time is a pain, but I do like that blue paint. (Does Edcor make laydown mounting trannys?)
(3) Grab a set of Classic Tone iron. Their Tremolux tranny has 180ma on HT winding, and that seems a little more sensible than 300ma.

Realistically, cost will likely dictate my choice, but I'd like to hear your thoughts. Anyway, I've gotten enough done that I now have a list of the things I don't have, so I'll try to get an order together for the missing bits.

Can't wait to do the next one in the new Stout Chassis. Loads more options, without modification, and a better fit as well. Great job Doug!

[sluckey]

OH, and also, would someone be kind enough to get me an editable copy of the files that make up the 6V6 Plexi PDF?

You'll need to contact Doug for that. And you'll need Visio to edit the file.

[AZJimC]

OH, and also, would someone be kind enough to get me an editable copy of the files that make up the 6V6 Plexi PDF?

You'll need to contact Doug for that. And you'll need Visio to edit the file.

Cool, I'll msg Doug. I have Visio, and your parts stencils for it. I was hoping not to have to redraw the whole thing, when I only need a modification here and there..

Also, I am trying to figure, while I work, a good way to split the two channels to two over/under input jacks, and maybe a switch to recombine them when desired. My concern is that if I have a 1M resistor on each jack, then combine, it would change the input impedance by 50%?? The idea is to be able to use an ab switch to swap channels, which should make the most of the flexibility of two channels.

[sluckey]

My concern is that if I have a 1M resistor on each jack, then combine, it would change the input impedance by 50%??

That shouldn't be a concern. Your guitar output impedance is much lower than the input impedance of the amp. That 1M is more about grid leak for the tube than impedance matching. 500K works just as well.

If you want to split the channels, just look at the original 1987 schematic. The hardest part will be trying to squeeze the 2 over 2 jacks onto a 12" wide control panel.

[AZJimC]

If you want to split the channels, just look at the original 1987 schematic. The hardest part will be trying to squeeze the 2 over 2 jacks onto a 12" wide control panel.

 I'm thinking the Hi jack only, CH1 on top, CH2 on bottom. Won't be a problem to implement if the 500k input is okay. I thought about a ground reference for each triode, and the thought of switching that took me down the bunny trail,  .

[sluckey]

I thought about a ground reference for each triode, and the thought of switching that took me down the bunny trail,

Just connect the ground lugs of the two input jacks together. No need to switch ground.

Maybe I don't understand what you mean?

[AZJimC]

I thought about a ground reference for each triode, and the thought of switching that took me down the bunny trail,

Just connect the ground lugs of the two input jacks together. No need to switch ground.

Maybe I don't understand what you mean?

Switching the 1M ground reference resistors to maintain 1Mohm, rather than 500K, when combining the two inputs. Glad I don't have to do that.

[AZJimC]

Here's a question. The Fisher HiFi amps used a 100ohm screen resistor, while others used none. I will use them, as a safety fuse if nothing else. Should I go with the 100, or 470R like Fender, or 1KR? These tubes seem to push easier than 6L6, or even EL34's, like something between an EL34 and EL84. Would the 1K, like some use on EL84's, be more appropriate? (If it doesn't matter, I have 560R, 680R, 820R, 1KR, 1K2R, etc, but no 470R).


I looked at the Ampeg G-15, and it uses the 470R with the 7591A tubes, same tube in octal basing.

[DummyLoad]

1K or 2K common.


--pete

[AZJimC]

I'm looking at OT choices, and considering space limitations etc. With the right iron, these tubes will run 44 watts at 2.5% THD I think it was, so it seems a 45 watt 6K6R tranny would be in order, however those are getting too big to be a comfortable fit on the stout chassis. I have looked at a lot of specs, and if I go with edcor, their 30W iron is about as big as I can get mounted.

Has someone here used enough edcor to say how much I could push the OT? Their CXPP series say they're good from 20hz-20Khz, and I'm thinking that for guitar I don't really need that much bandwidth. What If I got them to wind that iron for 200ma? Would the result of running at greater wattage than the iron mass suits be simply more distortion, less frequency range, or both?

Also, I was looking at their UL tapped trannys. what if I got a 10K primary, with 45% UL taps, and used the resulting 6K5R as normal loading, with the option of going to 10K loading, for instance, in triode mode?

Also looking at the Hammond 1650G, I think it was, and it's a bit big, but might be wedged in, and has 250R and 500R taps. Has anyone ever used those higher impedance taps like for a line out? Could I feed the 500R into say a 1k pot, and send the wiper to a line out jack, all while disconnecting the speaker load, or not?

Anyone know of any little bitty 50W 6K6R output trannys?

[DummyLoad]

doug's mojotone 791 - 35W 6K6:4/8/16 model will fit the stout chassis. minor machining to fit and has room for large fender choke. actually, a hammond 193B will fit as well should more inductance be desired.

https://www.dropbox.com/sh/ryu4hqhwazllp0d/AACw1Fjqsko_EoQVJrvzLq46a?dl=0

one can be had for 77.00 + ship.

more info in link below.

http://el34world.com/Forum/index.php?topic=17631.msg180985#msg180985

--pete

[AZJimC]

Thanks Pete, that looks like a workable option.

Making some progress, and I'm getting anxious to fire this beast up.

[AZJimC]

Trannys have been ordered, the last components needed to get this thing running.

I decided to go with the Mojo758 for power and the Mojo791 for output. Now I'm going to forget this project until parts show up. That way I won't be sitting here pecking out rhythms on the table with my homegrown guitar picks (fingernails)

[AZJimC]

Thanks to everyone for their input. Thanks Pete for the tranny suggestions, It seems I ended up with those used on the boilermaker, I think it was.

Okay, I got the tranny's mounted, wired, and the board wired in, and I'm about ready to test fire. The last holdup is the inputs. I have two jacks, intended to be wired to the two channels, but I've come across issues in trying to figure out the switch logic.

The switch is meant to have the normal and bright jacks, a single jack each, jumpered one way, and individual when switched the other direction. Here is the problem I see. Either jack may be switched to ground on the tip, depending on its condition. No jack, and it's shorted, effectively making the remaining jack a Low input if paralleled, due to the 68k being grounded. So... a SPDT simply switching the two jacks together won't work right in every condition.

Normally I can figure out pretty complex switching arrangements, but I've got brain block this time.

The intent is to be able to have two jacks that will work for each channel, and be able to switch them parallel, with a switch, while being okay even if one jack is empty, or even shorted by an AB footswitch.

Help me out here, I wanna boil these 7868's.

Thanks,
Jim

[sluckey]

Look at the 'jump' mod in this pdf...

     http://home.comcast.net/~seluckey/amps/VAC15/Vox_AC15_mods.pdf

[DummyLoad]

Look at the 'jump' mod in this pdf...

     http://home.comcast.net/~seluckey/amps/VAC15/Vox_AC15_mods.pdf

is that what he's asking? i think he's stating that he wants channels in parallel, not series but bonded to one input jack or split when the second input is used. 


--pete

[sluckey]

is that what he's asking? i think he's stating that he wants channels in parallel

The jump mod shown on page 1 puts the two channels in parallel. Maybe you were looking at the cascade mod on page 2?

I ran into the same problem with the jump mod, ie, a SPDT switch will not break the grounded tip on the unused input jack. My solution was to use a DPDT that would lift the ground connection to the switch terminal of the jacks when in 'jump' (parallel) mode.

Maybe he is looking for something more sophisticated that uses only the switches on the jacks and doesn't require an extra toggle switch???

[DummyLoad]

is that what he's asking? i think he's stating that he wants channels in parallel

The jump mod shown on page 1 puts the two channels in parallel. Maybe you were looking at the cascade mod on page 2?

i was... sorry!  :-)


--pete

[DummyLoad]

Maybe he is looking for something more sophisticated that uses only the switches on the jacks and doesn't require an extra toggle switch???

that's what i interpret as well... :-\ 

--pete

[AZJimC]

Look at the 'jump' mod in this pdf...

     http://home.comcast.net/~seluckey/amps/VAC15/Vox_AC15_mods.pdf

Yes, that is what I was looking for. I've considered a cascade mod, but not there yet. I have a switch hole, between the two jacks, with a mini toggle in it,

My concern was that, if I do use a foot switch as intended, to go between channels on the fly, that the grounding switches in the AB FS work as intended when the two jacks are separate. When used paralleled by the switch, I'd just be returning to the circuit as Doug laid it out for the single jack.

[AZJimC]

I ran into the same problem with the jump mod, ie, a SPDT switch will not break the grounded tip on the unused input jack. My solution was to use a DPDT that would lift the ground connection to the switch terminal of the jacks when in 'jump' (parallel) mode.

Exactly. Nice solution, that I couldn't manage to figure out. Thanks Steve.

I'm assuming the 68k resistors would be on the socket grids?

The only other thing, If I have a Foot switch that grounds the unused jack, I'd not be able to use parallel setting without unplugging the FS. I can live with that, I guess.

[AZJimC]

Thank You, Thank You, Thank You, Thank You THANK YOU!!! (From Dr. Hook, Bankrupt Album, Millionaire)

This thing is a screaming crunch monster! I can only try it through a pair of Celestion Vintage 30's in an open back cabinet right now, but that's enough to see how it screams. Can't wait to hook it up to a 4x12 closed cabinet. It's 80's all the way. Angus Young, to Gary Rossington, and "all points in between".

Oh, and that's only pushing it with a fender, my Les Paul isn't at home, I've been leaving it at the church.

I ended up with 475v on plates loaded, showing about 37.5ma per tube on the cathodes at idle. I had to tweak the bias supply, by swapping to a 33k in place of the 15k feeding the pot, and 15k on the tail of the pot, in place of the 47k. That left me with the grids at -25.6v at very near the center of the pot.

I added a PPIMV and below 5 position, it drops volume, but above that, it seems to set how hard I'm pushing the 7868's. It will hit them pretty hard, for single coils.

I have a pretty microphonic v1 in it right now, so I get a bit of squeal at max vol on Brite channel. I'll steal one later to swap out and I'm sure it will stop. Also a bit of hiss on Brite chan, but that's likely tube related also. Everything else is running great.

The switching is cool, the Normal jack is used to go parallel, and I suppose the brite would kick in alone with an AB footswitch while jumped, kind of redundant though with a FS to control it. I haven't had time to hook that up yet.

Ugh, now the hard part, stop playing it long enough to get the docs modified as built. Also Need to get on the faceplates. A buddy is making a cabinet for me, a shiny hardwood model.

I'm gonna plan to put a small box, velcro'd inside the cabinet, with a new set of outputs, and an extra preamp tube or two, maybe a 12ay7 to get a bit more headroom. Since these 7868's won't be at a guitar center, extras might be handy.

I nibbled the hole out, and put the IEC with fuseholder in, so the fuse hole will get a switch, maybe triode or who knows, I'm not sure yet. I was considering a cascade switch on the front, but I'm not sure that is needed. I can get to some pretty metal'ish overdrive as is, just jumped.

Anyway,
Again, Thanks Guys!

Jim

[AZJimC]

Gut Shot
Just about done inside. I still have the fender type fuse holder in the back, unused, but I'll use that hole for something as soon as I figure out what.

I also did the MV differently than my previous. I replaced the bias resistors on board with wire, and rather than tying those two lugs together, I wired from the end where the bias would have been tied on the original layout directly to the Output grids. I added a turret and connected the bias voltage there. From that point I ran a single wire to the MV dual pot, and soldered it to the left side of both MV pots. Across each of the gangs, I put a 2.2m resistor from left to wiper, and used the lead to jump to the right lugs of the pot. From these two right side lugs, (From rear view) I ran a short wire to each of the Output Tube grid terminals, pin 9 in this case. The three wire hookup looks much better, and I can't see how it would differ from the conventional LarMar MV 4 wire hookup electrically. It works as expected, and provides a solid path to bias in pot failure.

I also moved the cap can, and placed the neon pilot between the presence and the MV. It looks intuitive from the front even without labeling.

Most wiring from the board is ran underneath, with the wire ran to top of turret and hooked over the top to prevent falling out if components are changed. I kept signal and tone wires on top of the board, since they mostly come from the edge.

[Willabe]

Very nice and neat looking build!   


                   

[DummyLoad]

I decided to go with the Mojo758 for power and the Mojo791 for output.

killer combination. B+ is just right for plexi-ish builds and the PT runs cool.

excellent workmanship and looking forward to seeing the completed project.

sound clips? 

--pete