Parallel EL34, SE 12 watts

[Ed_Chambley]

I played an amp this weekend.  Fargen Miniplex.  Got my interest.  For the most part it is just a small watt amp with power scaling.  It has a decade switch which is not very usable except set on 70's.  It is class A, single ended.  My friend would not let me open it up because he just got it, so I could not get any voltages.

I have never built a parallel SE amp, so I have a few questions.  An EL34 in class a at 265 volts produce 11 watts according the the philips data sheet.  I do not know what paralleling power tubes accomplish.  He had a el34 and a kt66 in it.  I put 2 el34's in it and could not tell any difference.

It has mercury PT and OT without numbers on them.  Oddly enough the OT is about twice the size of a 5e3 OT.  I have gotten no other info yet, but when he wears the new off it, I'll get that sucker out of the cabinet.  Until then I thought I would pose some questions.

I have been inside many Fargen amps as this all my friend plays.  They are all similar.  Sozo vintage, carbon film and wirewound.  Just expensive parts and clean builds.  Some values are different, but all are very similar to a Marshall 2204 except for the blackbird which is a AB763 lite with mustards.

Mainly what baffling is the Transformers and why do you parallel EL34 for 12 watts?
It seems to have a reverse tone cut for a middle, or at least from what a vox and matchless has.  The mid has an effect on the highs only and the amp never loses strong mids.  Any idea how?
No Choke.
Power Scale only on power tubes and it needs to be turned up plenty.

[HotBluePlates]

Mainly what baffling is the Transformers and why do you parallel EL34 for 12 watts?

Do you mean "why a pair of EL34's for for only 12w?"

I gather the point is to use very different output tubes for the tonal differences, but combined. Seems silly to use two of the same type in this case.

The mid has an effect on the highs only and the amp never loses strong mids.  Any idea how?

Voiced for high mids? High enough that when you adjust it, it seems to impact the top end of your tone. Just a guess.

[Willabe]

Mainly what baffling is the Transformers and why do you parallel EL34 for 12 watts?

Hi Ed,

You can parallel the output tubes for double the output power if the amp has the PSU and OT for it, which their not doing. Or you can run the 2 output tubes at less than max. Tubes last longer and with a margin of safety that running them full out does not have. Depending on how the output stage is set up and wired up you could use different output tubes for a different sound.

KOC has an amp called a 5100 (Marshall clone/type) in 1 of his TUT books that has only 50w output power whither you use 2 or 4 output tubes in it. He talks about how and why it gets the "sound" of a 100w at only 50w output power by using 4 power tubes.

IIRC it is because of the transconductance of 4 tubes verses 2. Gives the "feel" and muscle of 100w yet at only 50w.


                   Brad      

[Ed_Chambley]

Yes HBP, they are paired.  On his website he calls it parallel.  That is the reason for my question.  At that low voltage I don't know if you would notice changes in tone from tubes coming from the power section.  I didn't.  And in addition, the power scaling would not even bring the El34 to where it would have a heavily saturated tone.  It does sound is if it does.  This is where I am probably mistaken from my lack of experience of Class A amplification.

That said as willaby mentioned, the trem Muscle seems to describe the amp well.  It is a very unusual critter.  Every small watt amp I have ever played lacks a strong power chord focus whereas this one seems to pull it off.  Open back or closed, doesn't matter.

I just threw this out to see if anyone had ever done anything like this.  I will build one of these, but like I said, I may have to wait until the new wears off and I can have a look.

[Ed_Chambley]

Thanks for the lead on the Gibson amps.  I have gathered some more info on the amp.  It has parallel 12Ax7's, or course no phase inverter. leading to a Power Scaling for the cathode pair of power tubes.

For paralleling and looking at some Dr Z amps which have this and matchless as well you have to, cut value of grid stopper in half since input capacitance doubles. Halving the resistance keeps the same bandwidth.

Cut values of plate resistor and cathode resistor in half.

Double value of cathode bypass cap.

The statements above are questions.  Is this correct?

[thelonious]

Those statements look correct to me.

Grid stopper halved because of doubled input capacitance just as you said.

Plate resistor and cathode resistor values halved because double the current flowing through them means you only need half the resistance to drop the same voltage.

Double the cathode bypass cap value because the cathode resistor it works with is halved, and you again want to keep the same bandwidth.

[topbrent]

Here is a gutshot of a Fargen Mini Plex Mk-1.  Looks like a fun little amp.

V1 wired in parallel,
Mk1 style Decade switch is a 3 way cathode cap value switch on v1. .68-off-25uf
Tonestack looks pretty standard Marshall plexi to me.

Hammond 272FX PT
Hammond 125ESE OT

[Ed_Chambley]

Here is a gutshot of a Fargen mini plex.

Parallel setup on v1,
Decade switch is a 3 way cathode cap value switch on v1
Tonestack looks pretty standard Marshall to me, nothing exotic.

Thanks, that pretty much tells me what I was thinking.  Do you own this amp?

[Tyrannocaster]

What are those round black things by the tubes? I see those on all of the Fargen amp pix online but I don't know what they are.

[topbrent]

What are those round black things by the tubes? I see those on all of the Fargen amp pix online but I don't know what they are.

Those are simply plugs to fill unused holes.  It is far more cost effective to have 1 or 2 universal chassis designs drilled for several different amp arrangements rather than stock 20 different chassis.

[Ed_Chambley]

Those are just plastic plugs.  He uses the same chassis for all his amps.  Cheaper that way.  I am trying to find a suitable PT and OT for this thing.  Not having much luck.  I would assume a around 3.4k for the OT, but it runs other tubes as well.  I found a Hammond PT which is suitable for voltage and ma's.  My lack of knowledge on OT in single ended, cathode which will run 6l6, kt66, el34.......etc is confusing.  It has to have a low plate voltage for 12 watts.  I do however have a handful of marshall OT's, but the smallest is 50 watts.  Looking at it it seems like it is just a larger OT than would normally be in a 12 watt SE amp.  It is such a simple build there has to be something pricy in it as it is not cheap.
From the photo posted I have begun a schematic, but since I have no voltage readings I am shooting in the dark somewhat.  The photo is a older version of the amp.  The newer version has more switching to supposedly cover tweed territory.  It doesn't.  It changes the tone, but it does not sound like any small tweed I have ever played.  It is amazingly plexi sounding.

[Tyrannocaster]

It is such a simple build there has to be something pricy in it as it is not cheap.

Wrong! And I'm not saying it's not a good amp, either. But he should get some money for his design and construction work, and innovation counts for something, too. There are an awful lot of amps out there that cost a lot more and don't have anything magic in them (yeah, I'm talking to YOU, Komet) other than a good design and careful construction. Or check out the Area 51 amps, which are possibly the coolest looking overpriced tube amps I've ever seen.

Thanks for clarifying the black plug question; I was really scratching my head over that one! I'll be curious to see what you end up with your schematic.

[topbrent]

Hammond makes some powerful single ended output transformers.  Look at the 125fse or 125gse.  http://www.hammondmfg.com/pdf/5C_125SE.pdf

You could also try a Weber WSE25 Output Transformer: 25 Watt, Single Ended Output Transformer, 5K : 2,4,8, and 16

[HotBluePlates]

It is such a simple build there has to be something pricy in it as it is not cheap.

I don't build amps for one reason: I can't figure a way to build them at a price where I can consistently sell them at enough of a profit to pay myself enough.

How much does it cost you to build an amp? Unless you're buying on a big scale, it might cost you $500-800 to build a tweed Deluxe copy that's high enough quality to appear professional. But that's just the cost of parts, speaker, cabinet, tubes at retail or average wholesale prices. How much time did it take you to build it (honestly)? And how much are you paying yourself per hour?

If you get into business classes, they'll tell you your manufacturing cost should only be ~25% of the retail price. You gotta build in dealer profit, you gotta pay your employees (even if that's just you), you gotta have cash set aside for marketing...

I figured out a lot of production expedients when I used to build/sell Hoffman boards, but ultimately the boutique amp companies probably only exist because the market became used to paying bigger prices for "old toneful amps" because of the vintage amp market. You can't compete with the big manufacturers' economies of scale or their overseas outsourcing of labor and manufacturing, so the oldest boutique makers essentially built 50's amp knock-offs and had to sell the consumers on the idea they were intrinsically better and had more tone than modern stuff.

But when you get down to it, even though some makers have moved beyond tweed copy amps, there's not a lot of magic to be found. However, if they can convince the buyer that the tonal options available, even if simple to electronically achieve, are something special over other amps, then buyers will perceive added value and might be willing to pay the higher prices.

[HotBluePlates]

Philosophizing over, back to the question:

I am trying to find a suitable PT and OT for this thing.  Not having much luck.  I would assume a around 3.4k for the OT, but it runs other tubes as well.

Don't make it harder than it is. If you want to plug any common octal tube in, then you want to be sure the weakest link don't break.

So maybe you keep the supply voltage kinda low, 350v more or less. Maybe less. If a 6V6 is happiest with ~8k load for one tube single-ended, then 4k works for OT load. If the range of output tubes could require higher or lower cathode bias resistors, pick the highest value. That will keep your EL34 from achieving its best sensitivity, but will also keep your 6V6 from overheating.

You don't even try to build in enough B+ current to allow a 6550 or KT88 to make its max output power, but that's fine because you're replacing tubes for a tonal change, not an actual output power increase. So you just make sure the filament winding has enough reserve on tap to handle 2 of your hungriest output tube heaters.

Notice that Fargen appears to have put 1kΩ sandbox resistors in for screen resistors. This too is either catering to the expected big change in screen current under signal conditions for an EL34, or is blind copying. Either way, the move is to cover the bases for the tube choice needing the most consideration regarding this circuit position.

Straightforward enough?

[topbrent]

Newer MkII Fargen MiniPlex's are definitely more complex.  Some pretty clever things happening here. 
Power scaling, DC filaments.  The decade switch is more involved and switches the slope resistor and treble cap values.

Mercury Mag PT Axiom FM45 PLB  270v-0-270v
Mecury Mag OT: Axiom SE 3K

Cool little amp, for sure.

Pictures courtesy of google searching, btw... thanks to whoever took these pics.  We salute you.

[Ed_Chambley]

Philosophizing over, back to the question:

I am trying to find a suitable PT and OT for this thing.  Not having much luck.  I would assume a around 3.4k for the OT, but it runs other tubes as well.

Don't make it harder than it is. If you want to plug any common octal tube in, then you want to be sure the weakest link don't break.

So maybe you keep the supply voltage kinda low, 350v more or less. Maybe less. If a 6V6 is happiest with ~8k load for one tube single-ended, then 4k works for OT load. If the range of output tubes could require higher or lower cathode bias resistors, pick the highest value. That will keep your EL34 from achieving its best sensitivity, but will also keep your 6V6 from overheating.

You don't even try to build in enough B+ current to allow a 6550 or KT88 to make its max output power, but that's fine because you're replacing tubes for a tonal change, not an actual output power increase. So you just make sure the filament winding has enough reserve on tap to handle 2 of your hungriest output tube heaters.

Notice that Fargen appears to have put 1kΩ sandbox resistors in for screen resistors. This too is either catering to the expected big change in screen current under signal conditions for an EL34, or is blind copying. Either way, the move is to cover the bases for the tube choice needing the most consideration regarding this circuit position.

Straightforward enough?

Straightforward.  and on you philosophy.  In real life I own a manufacturing company and do understand the figures for profitability.  My comments on price was that it is not a overseas manufactured pcb type of amp.  I really have no problem with amps in this category.  Egnater amps are surprisingly nice.  The problem is I am somewhat of a hack in amp building.  Most of my experience in amp work, or any electrical work comes from need with no desire for selling.  In no way do I even want to calculate the hours I spent with my last build.  In reality it would be best for me to purchase the amp if I consider my hourly value in business as opposed to working in my shop at home.

I began building amps as it is something which is serene to me and interesting.  I guess it is like golf to others.  It is truly a hobby which began years ago with a simple blackface conversion.  My background is in Graphic Arts and Commercial Printing.  You would be surprised at what you can learn about electronics when you have an electrical problem on a 6 color press at 9 pm and you have a job that must be delivered the next day.

Problem I have now is I have some great information gathered and no time.  My business is running overtime which is a good thing, but it is getting in the way of my hobby, so I will store the info and get to it asap.

You guys really go out of your way to help this hack and I feel a certain responsibility to learn and share as well.  My comment about price was not meant to be taken as the amp is overpriced, just that it is a handwired amp and really cool.  The reason I am attracted to wire over PCB is not tone, it is simply easier for me.  I hate working on PCB boards.  The first delay pedal I ever built assured me that the price for Brian Wamplers pedals are really not that bad.  Neither is Fargen's amps, but the price will tell you it is not an Egnater Tweaker either.

[HotBluePlates]

I began building amps as it is something which is serene to me and interesting.  I guess it is like golf to others. 

Me too.

Part of my comments are that people occasionally tell me I ought to do this stuff for a living. If someone was hiring tube guitar amp designers, and I could convince someone that not having an EE degree doesn't preclude me from knowing how to design good amps, maybe I'd do it for a living. Then again, I once had a smart person ask why I'd ever make a job out of a hobby, when it the becomes work not fun. 

The reason I am attracted to wire over PCB is not tone, it is simply easier for me.  I hate working on PCB boards. 

Funny enough, I have a good amount of tube-based test gear, not because there's not better solid-state lab gear available, but the tube-based stuff is easier for me to fix if needed.

[tubeswell]

Since we're off the track already - its strictly an (obsessive) hobby for me too (altho' I have fixed the occasional tube amp for friends at parts-cost, and off-loaded some of my home brews at parts-cost to willing buyers). Its not something that you can easily make a business out of when you have to buy the bits and pieces from everyone else.

[Ed_Chambley]

Newer MkII Fargen MiniPlex's are definitely more complex.  Some pretty clever things happening here. 
Power scaling, DC filaments.  The decade switch is more involved and switches the slope resistor and treble cap values.

Mercury Mag PT Axiom FM45 PLB  270v-0-270v
Mecury Mag OT: Axiom SE 3K

Cool little amp, for sure.

Pictures courtesy of google searching, btw... thanks to whoever took these pics.  We salute you.

Here are the specs from Mercury for the Transformers.  Seems the PT may be a waste for US use only.  It is centertapped as well as a bias supply and European voltages.

[Ed_Chambley]

Thanks, I do have that book in pdf.  I will review to get a better understanding.  There is something in this amp I have never done.  DC heaters.  I looked at Doug's 30 early version.  The components are not clearly marked, but I have seen many methods for this, from rectifying existing PT leads to using separate transformers. The only "benefit" I have read is in certain applications it will reduce hum.  Are there any particular benefits?