Fargen Mini-Plex style clone

[Big_Mike]

I am in the process of working on a Fender 5e3 style clone right now, but for my next project, I'd like to build a low-watt Marshall-type head similar to a Fargen Mini-Plex with power scaling.  This is a 12 watt amp utilizing two EL-34 tubes.  Does anyone have a schematic for this amp or possibly an idea of how to mod a standard Marshall head style kit?  I'd love to purchase a Fargen, but it is really expensive.  If I can make a knock-off or a similar amp and save a bunch of money, I'd love to do so.

[tubenit]

Paralleled V1 give about 30% more gain with no increase in noise.  I am NOT sure about the value for the shared cathode resistor.  Just an idea to consider based off an amp I built for my son.

Nevermind .......................... I got on the Fargen website and saw this is a single ended amp ............ so these suggestions probably would not work.

With respect, Tubenit

[SILVERGUN]

I'd like to build a low-watt Marshall-type head similar to a Fargen Mini-Plex with power scaling.  This is a 12 watt amp utilizing two EL-34 tubes.  Does anyone have a schematic for this amp or possibly an idea of how to mod a standard Marshall head style kit?  I'd love to purchase a Fargen, but it is really expensive.  If I can make a knock-off or a similar amp and save a bunch of money, I'd love to do so.

Welcome Big Mike,,,,here's my 2 cents...
The Mini-Plex runs 2 EL34's in parallel SE to achieve 12 watts.....if you want 'really low' watts, eliminate one output tube and just build a simple SE Plexi style (maybe with a switchable extra gain stage)....done...cheap.
You can use the standard cathode bias VVR control (generic power scaling), to get down to "bedroom" volume
http://www.hallamplification.com/main.html?src=%2F#2,2

I'm sure if you search around here a little you find some examples to help you...

Don't forget to check out the Hoffman Stout....you 'could' use that preamp into a standard SE output configuration...AND use Doug's board building tool to make your life easy: http://hoffmanamps.com/php/TurretBoard/TurretBoard.php

[SILVERGUN]

Here's some good stuff I found in the search bar....
http://el34world.com/Forum/index.php?topic=14216.msg134043#msg134043

Based off of the inside chassis pic alone, I'm pretty sure we've got the technology to get this pretty close

[Big_Mike]

Here's some good stuff I found in the search bar....
http://el34world.com/Forum/index.php?topic=14216.msg134043#msg134043

Based off of the inside chassis pic alone, I'm pretty sure we've got the technology to get this pretty close

Thanks for pointing me in the right direction.  Looking at the chassis, it does not look too complex.  Since I am a newbie at amp building, I will need some assistance on component values and turret board layout.  Also, is there a good supplier for these quality components?

[drew]

The Mini-Plex runs 2 EL34's in parallel SE to achieve 12 watts.....if you want 'really low' watts, eliminate one output tube and just build a simple SE Plexi style (maybe with a switchable extra gain stage)....done...cheap.

The old Allen Class Act might be worth a look - schems in this thread: http://www.thegearpage.net/board/showthread.php?t=691179

[Willabe]

Also, is there a good supplier for these quality components?

Sure our host Doug's on line store;

http://hoffmanamps.com/MyStore/catalog/Search.htm


             Brad     

[tubenit]

Layout here for a single ended Plexi:  http://www.scottgreiner.com/PlexiBeast/PlexiBeastV2.gif

NOTE:   As I am understanding this layout ................. it looks like only half of an LTPI is being used ???
                                               
Am I seeing that correctly??     The schematic attached is not the amp in the layout but I am using for illustration purposes.

With respect, Tubenit

[sluckey]

it looks like only half of an LTPI is being used ???

It might be better to say that only one output is being used.

The LTP is still being used as a differential amplifier just as in the original plexi. One triode receives the guitar signal. The other triode receives the NFB signal. The two signals are combined in the tubes and the difference of those signals appears on the plates. But only one output is needed since it's driving a single ended output stage.

This appears to be a modification of a standard push/pull plexi to a single ended output stage. The term LTP PI (phase inverter) that we normally use when talking about a push/pull amp doesn't really apply when talking about a single ended amp. So, in this case, just calling that circuit what it really is could be more appropriate and less confusing. That name is technically a differential amplifier.

[Big_Mike]

That Scott Greiner single ended plexi looks really interesting.  Is there a way to modify the layout to have two EL-34s in the output section?  If 2 EL-34s were used, would component values have to change on the turret board?

[sluckey]

That Scott Greiner single ended plexi looks really interesting.  Is there a way to modify the layout to have two EL-34s in the output section?  If 2 EL-34s were used, would component values have to change on the turret board?

Just add another octal socket. You would need to change the value and wattage of the 330Ω cathode resistor.

[SILVERGUN]

That Scott Greiner single ended plexi looks really interesting.

Based on the gut shots I have seen of the Fargen I don't believe that they are using a third 9-pin tube socket,,,so I don't believe that they are using the differential amplifier design....
We might be making this more difficult than it has to be (for Big Mike's request)

If 2 EL-34s were used, would component values have to change on the turret board?

Look at the pic below...I circled the separate cathode connections for the 2- EL34s
If you use one output tube, you'll only need one set,,,,two and you need two

My main reason for trying to steer you towards a one EL34 version is to be able to use a simple, cheaper OT.....if it will have to be used in a band setting than the 2 output tube version would probably make more sense

Here's my GUESS at a preamp schematic for you.....the cathode connections on the paralleled V1 'could' be switchable so you could choose one or the other (it looks like he does that in one of the pics I saw)
**The print was borrowed from the Mark Huss 6V6 Plexi, and adjusted for a parallel V1 (done quickly and open to debate)

On another note....Doug's been working on a new chassis design that 'might' be a good candidate for this build 
http://el34world.com/Forum/index.php?topic=17502.0

[Willabe]

That's a good example of how NOT to mount the K bypass caps.

It will cook them and shorten their life sitting right on the K R's.


                      Brad     

[SILVERGUN]

That's a good example of how NOT to mount the K bypass caps.

It will cook them and shorten their life sitting right on the K R's.


                      Brad     

Excellent point Sir,
Last one I did, I did it like this Big Mike:

[Willabe]

Now that's nice. (Fargen should know better.)


                 Brad   

[SILVERGUN]

I decided to wire this up on the board tonight and here's some thoughts....
- I wired it up with a single EL34 @ 310 V plates with a 250R cath. (bypassed w/ 100uf) for about 72mA
- I think it sounds great as a 'classic' pushed Marshall with some nice harmonic overtones and great range of clean to mean
- With the cathode bypass cap engaged on V2 it had PLENTY of thick gain, and when lifted it cleaned up nicely, but still retained enough punch to be considered 'high gain'
- I tried a 5881 in there and liked certain qualities about that better than the EL34
- The power tube being pushed into overdrive is a big contributing factor to the best sound possible from this amp,,,at lower Master settings it starts to sound a little anemic....cranked, it's REALLY good

- I thought that if I'm putting out approx. 6ish watts with one output tube, and it is this loud (where I cant sit in front of it for too long), then I would stop here if I was only going to be using it in the basement/bedroom
BUT
- I went back out and put a 5881 in parallel with the EL34, and really liked that combination,,,and it wasn't 'all that much louder'
- With 305v plates and the same cath. values I was estimating approx. 44mA per tube, so I dropped the R value to 135ohm and that really woke it up
- I was reading 265 VDC from plate to cathode, while dropping 19 VDC across that R, so that tells me each tube is at around 69mA
- And now I see WHY you would want to do parallel SE, and mix tube types.....it sounds better!....it's a way to push the output tubes into overdrive without pushing BIG wattage, AND get the tonal qualities of 2 different tube types
- It just made me want to play VH riffs for about 20 minutes straight 
- I'm sure the VVR will help a lot with the overall volume, and I'm waiting for some Mosfets to come in to try it myself...

Just thought I should report some findings....
Thanks for bringing this up Big Mike,,,,I've been putting this one off and this gave me a reason to get excited again! 

[Big_Mike]

Great work Silvergun!  This is a really interesting project, so any insights you have will be much appreciated.

As a comparison to this project, I currently have a Valve 105 Bimbo amp (www.the-valve.com).  It is a 5 watt SE EL-34 amp with power scaling.  What I don't like about it is that it only has one tone knob, and the range of tone is very limited.  Second, I'd like a little more power for clean headroom.  So I am intrigued with the 2 power amp setup of a Fargen Miniplex clone and whether I can coax any more clean headroom out of it.

I am mostly a home player, so a home amp is what I'm looking to build. 

[Ed_Chambley]

What kind of OT are you testing with SG?  I mean what ratio.  Is it more suited to a single 5881?

BTW, I have played the Fargen and it is really slick and seems to have a powerful feel.  Muscle.  It has dc heaters and it is studio quiet.  Nice amp.  I did not like the tonestack.  Little too dark, muddy easily.  Also, if you added a touch of verb it would go a long way to making it feel larger.

That is Ed's thought for the moment.

[SILVERGUN]

Great work Silvergun!  This is a really interesting project, so any insights you have will be much appreciated.

As a comparison to this project, I currently have a Valve 105 Bimbo amp (www.the-valve.com).  It is a 5 watt SE EL-34 amp with power scaling.  What I don't like about it is that it only has one tone knob, and the range of tone is very limited.  Second, I'd like a little more power for clean headroom.  So I am intrigued with the 2 power amp setup of a Fargen Miniplex clone and whether I can coax any more clean headroom out of it.

I am mostly a home player, so a home amp is what I'm looking to build.

Thanks Big Mike,,,,,it's really kind of self serving so I can't take too much credit....

I spent moist of the night trying to get a great distorted tone out of it, so I didn't concern myself much with clean headroom.
If I had I probably would have removed the cathode cap from V1 because I really liked the tone I was getting with the master all the way up,,,and the preamp would get pretty gritty in a hurry...so removing that cap might clean it up nice
Based on some of the info I have gathered I think his 'decade' switch may have been switching out the caps on that first stage.
Here's my guess as to how to get close:

[SILVERGUN]

What kind of OT are you testing with SG?  I mean what ratio.  Is it more suited to a single 5881?

BTW, I have played the Fargen and it is really slick and seems to have a powerful feel.  Muscle.  It has dc heaters and it is studio quiet.  Nice amp.  I did not like the tonestack.  Little too dark, muddy easily.  Also, if you added a touch of verb it would go a long way to making it feel larger.

That is Ed's thought for the moment.

I had one of those 5K---4/8/16----15 watters laying around....and it was getting HOT!
Since I'm still having a hard time grasping the output curves, I kept it low voltage to stay "safe"
I tried splitting the pri K  by plugging my 8 ohm cab into the 4 ohm tap....and I thought it was good

Funny you mention the tonestack, because I still had the tonestack from the TBM5879 on this board and I used it for the mid boost,,,and then I bypassed it altogether,,, and then I hooked up the std. TMB......I preferred the TMB with a 56K slope, 500p, .022, .022 (pretty standard stuff)

I think the model with the DC heaters is the MarkII....and I've based most of my assumptions off of this gut shot of the MarkI,,, so maybe he changed the tonestack.....he definitely added tonestack switching as seen in the second pic

Check out the 'bridged' grid Rs with the shielded cable zip tied to em  feeding the power tube sockets

[SILVERGUN]

I went back out there for a few minutes tonight and lifted the bypass caps on V1 and V2 and it did clean up nicely...and I really like that specific clean tone a lot,,,right on the edge of grit
I also switched the pri load back to 5K by returning the 8 ohm speaker to the 8 ohm tap, and I thought that sounded better, and did provide more clean headroom.

Then I went the other way and reconnected the preamp bypass caps and tried to 'brown' it down by dropping the plate voltage to 220VDC (to simulate VVR) and I really liked that a lot...cranked the gain and master, and had great response with a more manageable volume...so I'm all for including VVR.
Tried the speaker in both spots, and again, preferred it in the 8 ohm jack.

That's all I've got for now....here comes the weekend!

[Big_Mike]

Which master volume circuit is used on the miniplex circuit?  The reviews seem to indicate that it is outstanding.  Very curious which one was used.

[SILVERGUN]

Which master volume circuit is used on the miniplex circuit?  The reviews seem to indicate that it is outstanding.  Very curious which one was used.

Based on what I can see in the MkI gut shot, I used this very common Master (as seen in many marshall MVs)

[Big_Mike]

I found another gut shot of the MKII online, I'll try and upload it later today once I figure out how to do so on this forum. 

[SILVERGUN]

I found another gut shot of the MKII online, I'll try and upload it later today once I figure out how to do so on this forum. 

I've seen those...and the good news is, it's very similar to the Mk1 , just with the changes that they advertise

For example...
-The VVR is introduced as a Power control on the front panel and 'might' be the master that you were referring to.
-There appears to be a DC heater circuit added, which is a simple,,and omittable feature
-The 'decade' control is on a rotary switch and now appears to include tone stack switching options as I mentioned in an above post

Other changes that I think I see:
- V1 is no longer paralleled (all the time)
- The addition of a rear panel 'VOS or BRITISH' voicing switch, which is somehow tied into the decade switch wiring,,,and would appear to control the second triode of V1
- Smaller value dropping resistors in the power supply rail for the preamp
- The addition of a  LOW/HIGH power switch

The VOS switch setting is a carry over from his Velvet Overdrive Special and I would have to guess that it would be cascading V1a into V1b

I'm gonna continue to experiment with it, when I get some more free time....I'm in the middle of one other build right now.