Dual SE Plexi.

[DummyLoad]

good evening fellow amp junkies. i never have built a marshall; figured i could buy the genuine article for less scratch than building one, being that they're almost as common as dirt and all. :p you cannot however, get a high power class A single tube cathode bias marshall with the plexi preamp from marshall. that said, is where i'm stepping in now.

i was going to breadboard this but it seems simple enough, however, i may still since it is a significant investment in time and parts. the chassis and handful of parts would be the only real waste were i to scrap the project entirely, so it wouldn't be a total loss. fish, cut bait, cut bait, fish...err.

attached is a schema for a dual SE unit that should deliver about 30W of cathode biased SE sweetness...in theory. also attached is layout that's mostly complete, trivial things like filament, mains wiring, output jacking and switching are missing, however, the more unique and complex stuff is. many sets of eyes are better then just my old pair, so if any of you guys have the inclination to assist in checking, suggestions, mods, additions, etc.. please post. also missing is a cathode disconnect switch for the second power stage to halve the output power.

notice that view is looking in to the chassis and the transformers, caps, etc., are mounted topside; looking at the inside view the input jack is on the left; flip the drawing over and the input is on the right. lastly, there's room on the tagboards for a global NFB network and a (switchable) bypass cap for V3. my thoughts were V3 would sub for the lost gain of the LTPI - sort_of_kind_of_maybe_not since an LTPI does add tonal characteristics of it's own.   

link below to few shots of the chassis and parts in a mock-up. the PT is a hammond 372JX rated at 600VCT @ 250mA, the choke is a hammond 2.6H 300mA, and the pair of OT's are one electron UBT-2. tube compliment is 2 x 12AX7, 1 x 6AV6, 2 x KT88.   

http://s2.photobucket.com/user/pmitchel/library/Plexi-DualSE?page=1

respectfully,

--pete

[silverfox]

I've often wondered how it would sound if the two preamps were each voiced out of a separate output.

There is an amp builder that does that claiming it gives the amp a sound that guitarists obtain by playing through multiple amps.

Silverfox.

[DummyLoad]

there's only one pre in this design. the two output stages are for 30W of power. i could make 30W by paralleling KT88 into hammond 30W SE iron, but i don't have that particular OT any longer and i have the pair of UBT-2 OT, actually, 12 pieces of it... anybody want some?
 

respectfully,

--pete

[kagliostro]

Hi Pete

I see, you are cheating 

you are using a paralleled 12ax7 followed by two separated controls (VR1 & VR2)

I like that idea 

---

We are too far as to materialize my interest for those SE transformers

May be you can use one other pair building a "Two Amps in a Box" Stereo Guitar unit with same Power tube and different preamps

to be used with two speakers cabinets or bridged in a single speaker cabinet

Ciao

Franco

[Jack1962]

This is basically what I did with my Crate CR-1 retrofit , I built aa Marshall JMP {normal channel , low gain ) preamp into a modified fender SE output ( I used 6V6) .

[jazbo8]

I like the idea, although that is a lot of power! Anyway, isn't Rk of 470 kinda high for the KT88s? Are you running the OPT at 4.8K or 2.4K with them?

Jaz

[DummyLoad]

I like the idea, although that is a lot of power! Anyway, isn't Rk of 470 kinda high for the KT88s? Are you running the OPT at 4.8K or 2.4K with them?

Jaz

the real reason i'm using 470Rs? i don't have 430R power resistors in my inventory and that value would likely get closer to target of 75mA / bottle.

--pete

EDIT: sorry jaz.. 4.8K  missed that question.

[DummyLoad]

I see, you are cheating 

you are using a paralleled 12ax7 followed by two separated controls (VR1 & VR2)

I like that idea 


grazie K, but not my idea. - mark huss and his 6v6 plexi was my inspiration. the cold bias 3rd stage (CF driver) is my idea and could prove to be a flop. we'll see...

We are too far as to materialize my interest for those SE transformers

si! that, and italian postal service - you likely wouldn't get them for a couple of months... :-(

"Two Amps in a Box" Stereo Guitar unit with same Power tube and different preamps

on a stage stereo does no good. blend of two different preamps might be interesting as long as possibility of phase cancellation is dealt with in the design phase.

to be used with two speakers cabinets

two speakers in the same cabinets that are not connected together is the plan. alternative and during testing will be with two separate cabinets as you say.
the option to bridge the OT's together is always there and that may prove to be the simplest and workable plan.

--pete

[kagliostro]

About italian post

last week my father asked me to send a letter for him, at the stamps resale I must buy stamps for 20c more as due

because the italian post didn't give the stamps to the reseller shop to be sold and they hadn't the correct value

This morning my father wanted to send a letter to a friend, he go to ask for stamps but the reseller told him they have no stamps ....

this happen because italian post are no longer state company, now are private an they didn't pay to the state the rights for the stamp

so

the state company who print stamps didn't give it to the post offices and the italian post didn't give it to the resellers

---

As you can see here are times of great confusion and bewilderment 

We are hoping for a new govern as quick as possible or things will go very badly here (not that now they go very good)

Franco

[Willabe]

We have the same idiots running the country here too.              

We don't have bunga-bunga parties, but probable won't be long till we have those too.     


                Brad      

[silverfox]

You haven't heard about the, "Down Low Club"?.

Let's see.. how can I make this about tubes...

Fox.

[jazbo8]

I like the idea, although that is a lot of power! Anyway, isn't Rk of 470 kinda high for the KT88s? Are you running the OPT at 4.8K or 2.4K with them?

Jaz

the real reason i'm using 470Rs? i don't have 430R power resistors in my inventory and that value would likely get closer to target of 75mA / bottle.

--pete

EDIT: sorry jaz.. 4.8K  missed that question.

I asked because I was looking at the schematic for the AX84 SEL where the Rk is 220R, and Vg=-30V, so it's biased much hotter than your 75mA. So I guess this is as good a place to ask as in another thread - what is the "ideal" KT88 operating condition? What sounds better? Is there an "optimal" load line, not in terms of output power or distortion, but in terms of tone - I know everyone's different, but is there a general consensus?

TIA,
Jaz

[PRR]

V/I ~~= Zload

[DummyLoad]

AX84 SEL where the Rk is 220R

which amp?

http://www.ax84.com/static/sel/AX84_SEL_101004.pdf

the print i have (link above) shows 430R for Rk. the OT is a hammond 125ESE with white wire sec. tap loaded at 8 ohm is 2.5K Zpri. BUT, they're running 350V to plates less 30V Vk to gnd. so Va-k is ~315V. Ik is 30.5/430R  ~71mA. so then Va-k= 315V/71mA = Zpri ~4400ohms. that design is not optimally loaded, but it does sound good. i have built that output stage on my breadboard.

to me so far, the sweet spot with KT88 in SE is around 400V with a ~5k Zpri load with plate diss. between 25-30W.

respectfully,

--pete

[jazbo8]

AX84 SEL where the Rk is 220R

which amp?

http://www.ax84.com/static/sel/AX84_SEL_101004.pdf

the print i have (link above) shows 430R for Rk. the OT is a hammond 125ESE with white wire sec. tap loaded at 8 ohm is 2.5K Zpri. BUT, they're running 350V to plates less 30V Vk to gnd. so Va-k is ~315V. Ik is 30.5/430R  ~71mA. so then Va-k= 315V/71mA = Zpri ~4400ohms. that design is not optimally loaded, but it does sound good. i have built that output stage on my breadboard.

to me so far, the sweet spot with KT88 in SE is around 400V with a ~5k Zpri load with plate diss. between 25-30W.

respectfully,

--pete

Doh! I was looking at the SEL variable bias option page, where the 220R appeared, sorry about that... And thanks for the tip on the "sweet spot".

Jaz

[DummyLoad]

on the breadboard now. thought i'd test drive before commit. also, a revised wiring diagram is attached.

--pete

[DummyLoad]

thanks to chip, he sent me some forum links to threads discussing various marshall SE concepts. what rattled my cage was the recent thread on the m. huss 6V6 plexi, and the AX84 hi-octane i put a spin on into a old crate hull. with reference to a "purists" plexi, i made the following changes: no mid control; colder bias on the 2nd stage, bonded the cathodes on the input stage gain pair; i have no 470pF bypass across the "brite" channel summing resistor; no bypass caps on the 2nd gain stage or the third gain stage (although i have plans to add a switchable bypass cap to the second gain stage); the coupling caps to the output pentodes are 47nF rather than 22nF; and NO_stinking_NFB! :p

basically what i did was make some minor plexi-like changes to the gibson GA-70 i had on the breadboard, scrapped the 1/2 the LTPI and it's tail got the boot... lol! 

following the philosophy of "usually the simplest solutions are best". with high-gain circuits, to me, that phrase has almost become a mantra.

this thing sounds damned nice. now that's probably just "new amp fever" talking, but it's definitely getting put in a chassis. added bonus: i'm playing through two cabinets; 1  cannabis rex and 1 jbl k-120. btw, this thing is a nice way to make A-B comparison of cabinet and combinations of cabs/speakers.

attached schematic as-is on the breadboard with telemetry.

respectfully,

--pete

[SILVERGUN]

on the breadboard now. thought i'd test drive before commit. also, a revised wiring diagram is attached.

Wholey Crap!!!!             .....
THAT BREADBOARD MADE MY HEART RACE!

Gotta get me one of them!!!.....that is seriously the most bad-ass tool I could imagine

If you were me and had to build one of those again (which I just might try)......what would you change or do differently??

WOW DL......WOW

[DummyLoad]

thanks SG. there are actually 3 of those. richard and i collaborated on this project. i have the first two built. after some time, working with them revealed some flaws;
the number of connection points and spacing,
the 9 pin sockets we cobbled together in an 11pin relay socket are cantankerous to build and work with,
we didn't need all the filament power options [in reality, 6.3V and 5V would have sufficed],
we wanted DC for working with preamps,
we needed a dedicated and flexible power supply proto area,
we needed more board space.

so my off_the_cuff shortsighted initial spec of 20" x 30" was enlarged to 24" x 36" and about the maximum size most modern machine shops can handle on a CNC V-mill. we built the third that richard has, and that was a MAJOR PITA to mill on my CNC machine - we got halfway through the fourth, botched about 75 bux worth (at the time) of aluminum plate and that was that. zero interest since. if i were to do it all over again, i'd probably go with .375 garolite and just punch it out with a drill press. the angled mixing console style cabinet was richards idea. i designed some PCB sockets to be used in the second gen, most of my half of PCBs we had made are still sitting unfinished in a pile somewhere. someday i'll revisit the project, i sent copies of the plans to HBP, willabe and 6-el-6. i think 6el6 is attempting to cobble one together. see the thread in Amp Tools about the barrier strips. all the info i have on what to buy is in that thread. PM an email address if you want a copy of the plans. 

respectfully,

--DL

[DummyLoad]

if anyone was following...this project is dead. not going to build it.

has a nasty anomaly that i can't seem to track down. disconnecting one of the power amps does not resolve the issue. only happens when playing near or at full up. this amp isn't any fun unless it's cranked, so i wouldn't recommend it.

--pete

[Willabe]

 

[kagliostro]

Many thanks for sharing your experience Pete

we are sorry to hear that things did not go as wanted

and we hope that luck come back with your next project 

K

[silverfox]

What happens if you sub a different preamp into the circuit. A different power amp. At least that would tell the half that is dysfunctional. If it sounds great under certain circumstances I don't know that it should be abandoned?

Seems as though there's a lot of Iron for the power supply but I can't say for sure.


Further- Please be aware that- Individuals like myself actively follow these posts as part of our learning process and your experiments and sometimes failures along the way are quite beneficial to the serious newbies. For example: I saved pictures of your breadboard so that I can  build something like that, perhaps, in the future and most likely it would take a different form but your inspiration was the impetus. A little at a time the discussion enlighten us.

How many filaments did Edison try? 1000?

Head up and onward. And thanks for your work.

Silverfox.

[DummyLoad]

it is in the pre-amp. specifically the cathode follower and the DC coupled driver. hacked on it for for a few days. sometimes you just have to start over. i had a gut feeling to breadboard the concept first, and i'm glad i did. btw, it won't be the last failed concept and it certainly isn't the first.   

 

respectfully,

--pete