20 Watt Marshall Dual Preamps / 6V6 & EL84

[plexi50]

I am building another Marshall 2204/Plexi AB switchable channel amp. This time i am using 6V6 & EL84 power tubes

I think i am going to use a footswitch to switch between each pair of tubes but i would also like to do what Egnater used on there Rebel 20 to blend both sets of tubes using a Mix pot

I have been thinking about how they are doing this but i dont know if i am in the ballpark or not

Could the Mix pot on the Egnater Rebel 20 be just a ground reference for the cathodes for the 6V6 & EL84 tubes

The pot wiper is the ground and the L&R terminals are cathode for each set of tubes

Some how that doesnt sound right

Preamp / http://triodeamplification.com/files/Marshall_Dual_800_2204_PDF.pdf

Power Section: http://triodeamplification.com/files/november.pdf

[jeff]

.

[Tone Junkie]

Man Im green even looking at it I cant figure it out.
Bill

[kagliostro]

About Rebel 20 schematic

To me it seems they are using four ganged pots in a PPIMV

I repeat it seems to me

if you use two double ganged pots you can decide the % for el84 and for 6v6 to be mixed


For Jeff

please, have you a file with the correspondent values of the caps - resistors and other components for the Rebel schematic you posted ? can you post it ? thanks

Kagliostro

[alerich]

If you used two double ganged pots it would allow you to set the percentage of each tube pair independently (two true post PI master volumes) but I don't understand the way the output plates are connected well enough to know if that is feasible in this circuit or not. The way it's configured it's an either/or situation with a midpoint 50/50 setting. I don't see how this amp would get any maxed out power tube distortion the way it is configured. Interesting how they paired one 6V6 and one EL84 on each side of the transformer.

I will say one thing about the Rebel 20 - it's the only overdriven EL84 amp I have ever heard that I really like and I love it. The amp tames down the harshness of the EL84's somehow. Maybe it's the OT. The drawback of this is that when you set it to the 6V6 side it's a little on the dull side compared to my other 6V6 amps (at least to my ears). I run mine most of the time either in the 1:00 sweet spot where both sets of tubes seem to be at their maximum or set fully to the EL84 side.

It was an impulse buy but every time I plug into I really dig it. As long as I don't have to dig "into" it. Compact little dude that doesn't look very maintenance friendly. My Peavey 6505 could have been one third smaller if they had wanted to. The Egnater should have been about a third bigger.

[plexi50]

I repaired a Rebel 20 last week that needed a new B+ fuse block. For 2 days i told myself to take it apart and get a bunch of pic's

I didnt. I only took pics of the work i did.

[jeff]

For Jeff

please, have you a file with the correspondent values of the caps - resistors and other components for the Rebel schematic you posted ? can you post it ? thanks

That'd be nice, but sadly no. It's TOP SECRET EYES ONLY and I don't have clearance. I got this schematic off of someone on this site. Don't know where it came from. It's just interesting to see how it's wired. Looks like the 6V6s and EL84s are on all the time and we're just adjusting the inupt signal to each pair. Wasn't expecting that. I was thinking it was something like what plexi50 was saying at first.

Generally we'll use a 8K transformer for a pair of 6V6s or EL84. Since we're using both at the same time do you think this is a 4K?

[jeff]

I think the mix knob could be rewired to use a simple stereo pot instead of four ganged. Think about it a variable resistor in series with a resistor is the same as a resistor in series with a varible resistor so switch them around. Then put the wiper to ground, one lug to the 6V6, one to the EL84 for each side of the PI. Not sure about this but I think it can be done. I'll draw it out but I don't have time right now. Maybe it has something to do with the taper or maybe they're different values though.

If so could you use independant stereo pot and adjust for a 6V6 push EL84 pull amp? No that's crazy. Getting carried away again.

[kagliostro]

Hi Jeff

That'd be nice, but sadly no. It's TOP SECRET EYES ONLY

I knew it was a forbidden question, but I could not resist

OK Thanks anyway

I think the mix knob could be rewired to use a simple stereo pot instead of four ganged.

I've a modified Vox AC30 project on my drawer

this is the solution I've think with bypass and 2 double ganged pots, one for el84 and one for 6v6

that give the possibility to decide the power independently for each type of tubes, so I can mix (as example) 30% el84 with 45% 6v6 (I hope so)

Since we're using both at the same time do you think this is a 4K?

I investigated very about that and the result is YES that is easily a 4Kohm OT

Kagliostro

EDIT:

p.s.: the Watt regulation is given from VR105 (connected to the cathode of PI) but I didn't think a resistor in parallel is enough to prevent scratch, happy to see that is a good solution

[jeff]

In the first picture the pots and resistors are reversed, which is the same thing. Also I've cut connections at X which is the same thing. So if one pot increases resistance to ground as the other decreases they can be put on the opposite sides of a pot with the wiper grounded. Sound right? (Assuming all four values of the pot are equal)

Kagliostro, I like yours better. That's a volume pot. This seems wierd because it looks like as you turn one down it's acting like a tone knob off the PI. Also in the 20's I don't think you can turn one set completely off, but maybe that's the point. Sure be great if we had values.

What's going on with the hot/warm switch. It looks to me like a DPDT switch but the botton is going to the same point in each position. Am I looking at this wrong or is this to break the connection while switching?

[jeff]

I will say one thing about the Rebel 20 - it's the only overdriven EL84 amp I have ever heard that I really like and I love it. The amp tames down the harshness of the EL84's somehow.

I don't think the 6V6's are ever completely off. I don't think this is a 6V6/blend/El84 amp. I think it's a 6V6+EL84 amp that you can blend the mix a little bit.

We really need to find the values of those resistors, They can't be too small or it'd kill the tone from the PI.

Those sure don't look like fun to work on!

[plexi50]

The only reason i did not remove the preamp board ,take lots of pic's and layout the Mix pot is because sometimes when you start fooling around with this stuff some thing is lible to come loose and then i would have created another problem

It wasnt my amp so i did the right thing by repairing it and sending it on it's way
I think the rebel 20 is one heck of an amplifier. There is nothing finer than a great guitar and amplifier to play with and go off on an hour or two journey

[jeff]

 What are you using for a 2.5 ohm load? Three 8s?

[plexi50]

Im not using a 2.5 ohm load.  Im using a Deluxe Reverb 6.6K primary OT with a 4&8 ohm load

[sluckey]

What are you using for a 2.5 ohm load? Three 8s?

That 2.5 ohm figure came from an old Fisher OT that I used to build my first November amp. I used a 4 ohm speaker load with it.

[kagliostro]

Hi friends today was a very long job day hope that now I don't say something wrong (be patient)

In the first picture the pots and resistors are reversed, which is the same thing. Also I've cut connections at X which is the same thing. So if one pot increases resistance to ground as the other decreases they can be put on the opposite sides of a pot with the wiper grounded. Sound right? (Assuming all four values of the pot are equal)

Sorry Jeff in this moment I'm not able to follow what you say (too tired), please can you add the tubes in your draw ?

That's a volume pot.

yes it is a double PPIMV (Larmar PPIMV if I'm not wrong)

What's going on with the hot/warm switch. It looks to me like a DPDT switch but the botton is going to the same point in each position. Am I looking at this wrong or is this to break the connection while switching?

there are two <center off DPDT> switch

they are independent and acting on one you select tree different status for the connected tubes

WARM - OFF - HOT

without the off position (and the 6v6 tubes) that is done in the Vox AC30CC2


Plexy50 hope this exchange of info is of your interest and you don't consider this an invasion of your space, if so, I apologize with you, wasn't my intention

Kagliostro

[plexi50]

All info is welcome. I wont implement all the warm settings and bias resistors. I like to keep it as simple as possable. But the way the tubes are blended has enlightned me. Refresh me seeing as i havent slept for 2 days. Did you say that the tubes are always on and that the mix pot is just sending (Attenuating) the signal from the inputs? It's a ZOMBIE day

[jeff]

Not 100% sure but that's what the schematic looks like to me. I give it another look.
If you follow from the PI to the grids it passes through Rebel Mix Mod V2 which looks like variable resistors to ground working in opposite directions turning one set up while the other down. R1-4 keeps the signal from being totally grounded and the caps(C1-4) just keep the bias off the pots.

I just noticed something. I thought the Rebel Mix Mod V2 was connected between the PI and the power tubes because that's where it is on the schematic, but the conncetors are labeled differently. The PI and power section have 6 pin connectors and the mix pot has only 5, I don't know if we're missing something, like page 2.

I got take a harder look at this.

Center off, now I get it! Didn't think of that.

[plexi50]

I am here and awake but my brain is on the counter next to me

[kagliostro]

The PI and power section have 6 pin connectors and the mix pot has only 5

Yes, but did you noticed that two of the pin in the 6 pin connector are shorted ?

EDIT:

I've just find this, may be it can help

The Watts control is indeed a pot in series with the PI cathode and 470R PI bias resistor. It is 25kB with 33k in parallel (to give a log response). This simply cold-biases the PI as you turn it down, effectively reducing the magnitude of the signal going to the power section. Quite useful. Things do start to get warmer, more compressed, and a bit muffled as you go down, but a worthy sacrifice to reduce the amp to lower practice levels. I implemented this on a Marshally build and found it a good tool. A variation of this control can be found on the Carlsbro 60T amp where the PI grid leaks are made variable.

The Egnater tube blend circuit is a 4-ganged custom 500k pot that is tapped; custom piece. There are two output coupling caps per "leg" on the PI, one for each set of tubes (EL84s and 6V6s). Essentially the blend control acts as a balance control creating an upside down parabola if you were to graph it. Pretty cool, but not easily cloned without a tapped 4-gang pot.

Kagliostro

[jeff]

Yes, but did you noticed that two of the pin in the 6 pin connector are shorted ?

Yes, but it's how they're labled that confused me. The mix out is labled CN2 and the effects loop is labeled CN2B but those don't have the same amount of pins either. I wasn't sure if I jumped the gun in assuming the mix did what I thought it did. I didn't have time to trace it out and for a second I thought, because the CN2 and CN2B, maybe that knob was to attunuate the input ond output of the effects loop. Now that I look at it the connectors next to each being connected together is only way it makes sense to me. I'm just confused because if it is connected as layed out I can't see how you could ever shut one set completely off.

 Has anyone out there with a rebel 20 ever pulled out the 6v6's and see if you can turn off the amp with the blend knob or would that royaly screw things up? If so then I'll take back what I said about not being able to shut one set completely off. To me, looking at the schematic, each set is never completely off.

"The Egnater tube blend circuit is a 4-ganged custom 500k pot that is tapped"
I can't see from the schematic that it's a tapped pot. I thought a tapped pot has a terminal somewhere along it's resistive strip between the two lugs. Can anyone enlighten me?

[kagliostro]

I can't see from the schematic that it's a tapped pot. I thought a tapped pot has a terminal somewhere along it's resistive strip between the two lugs. Can anyone enlighten me?

I Hope someone can give further explanations

however consider that the schematic we have seen is the Mix Mod V2

may be there are many different versions of that mixer or at last two different versions

also I think we can consider they use a resistor in one side of the pot to obtain something they previously obtained with a tapped pot, this, of course, is only a supposition

I've also read that firstly the circuit was represented only by a switch to swap between the el84 and the 6v6 tubes - later they (Egnater) were constricted by Mesa to don't use the switch swap mode because it was patented by Mesa, so they reached this Mix solution with the pots - I'm not sure about this, but I've read that this is the story of this Mix circuit.

Kagliostro

[jeff]

however consider that the schematic we have seen is the Mix Mod V2

Good point

I've also read that firstly the circuit was represented only by a switch to swap between the el84 and the 6v6 tubes - later they (Egnater) were constricted by Mesa to don't use the switch swap mode because it was patented by Mesa, so they reached this Mix solution with the pots - I'm not sure about this, but I've read that this is the story of this Mix circuit.

That's complete BS, shame on you Mesa, and shame on you pattent office. I understand certian designs are unique and patentable, but a switch between output tubes, BS, I mean come on what's next?

[plexi50]

I read that where Mesa has a patent on the footswitch mode between tubes. That was a surprise 

[kagliostro]

Sorry Plexy50

I forgot to answer your question

Did you say that the tubes are always on and that the mix pot is just sending (Attenuating) the signal from the inputs?

Yes, the pot is just attenuating the signal given to the power tubes

Kagliostro

[plexi50]

Thanks kagliostro. Thats a pretty cool idea in itself  

I threw a PT 230-0-230 @50ma i knew wouldnt do the job but i did it anyway for fun

Really low voltages are struggling to put out but man it's got some frekin tone

That is if you can hear a 1/2 watt. Thats what it sounds like anyway

I have been putting off ordering a PT for this as the bulk of the build was hard labor and a PT wasnt on hand to use so i sort of reverse built this

I will say that Steve Luckey sure has some great PDF layouts and so i am destined to turn to plastic forever more  

They just sound so dam good. I didnt choose my user name years ago because i was marshallfied. It just turned out that way

 

[kagliostro]

Thats a pretty cool idea in itself

Oh yes, I don't know who had that idea, but also I consider PPIMV a good thing

I will say that Steve Luckey sure has some great PDF layouts and so i am destined to turn to plastic forever more

English isn't my language but I think I've understand what you mean  

Steve jobs ( works, pardon) are always very accurate and his documentation is fantastic
much better than a lot of trademarks, no ......... that isn't exact, it is much better than commercial !

this night I was making some consideration about this:

I can't see from the schematic that it's a tapped pot. I thought a tapped pot has a terminal somewhere along it's resistive strip between the two lugs.

Well, I think that if someone that has seen the amp told:

tube blend circuit is a 4-ganged custom 500k pot that is tapped

may be that Mix Mod V1 was something like in this my draw

what do you think ?

here the .jsch file if someone want share his idea


Kagliostro

[plexi50]

Im confused. (My favorite slang saying)

The diagram is using master controls for each set of tubes?

This is how the tubes signals are mixed?

Are we using a single dual gang master pot to swing between the pair of tubes?

How would you go about making a single dual gang master pot do this (Cant)

Wire each gang pot opposite of the other so the master would swing in opposite directions for each set of tubes?

At center position on the Dual ganged pot you would have both set of tubes operating at half there potential signal strength

Thus the Mix?  Scratch all that. Do they make a 1MA single 4 gang pot? Im gona go look  

Heres  long shot! Cut the shaft and or threading as needed to butt the shaft to the inside rear of the front pot

Solder 4-5 heavy gauge buss strips across the pots like a cage to keep them in line

JB Weld the shaft to the rear wheel of the front pot

I dont see much online about 4 gang pots other than they are very exspensive hi-fi stuff and are rarely available when they have them

And the common values are in the 50-100K  range

[kagliostro]

May be there was a bit of confusion, I've read "four ganged tapped pot" as a pot with intermediate intakes (phisiological intakes - one for each pot) but now I seriously doubt about that.

The diagram is using master controls for each set of tubes?

This is how the tubes signals are mixed?

Yes

Are we using a single dual gang master pot to swing between the pair of tubes?

No we must use two dual ganged pots

Heres  long shot! Cut the shaft and or threading as needed to butt the shaft to the inside rear of the front pot

Solder 4-5 heavy gauge buss strips across the pots like a cage to keep them in line

JB Weld the shaft to the rear wheel of the front pot

Yes,  using pots like those in the photo you posted (no all pots are good for that mod) think that you can obtain a 4 ganged pot

there is also another solution, you can use composable pots (they are produced by Tyco)
http://it.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0393885&cm_vc=prev_it
http://it.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0393807

But I prefer the solution I posted on my first schematic (that solution was suggested to me by HotBluePlates sometime ago for my modified Vox AC30 schematic).

that way you can decide the percentage of power to be mixed independently for each kind of tubes (the % you want of el84 power + the % you want of 6v6 power) that (for me) is a nice option, also don't have to use (and find) special 4 ganged pots - yes, I know this way you must pay the solution with one more hole in the chassis, one more knob and the necessary space, but to me seems that advantages are more than disadvantages.

Kagliostro

[sluckey]

Plexi, don't overthink this. One dual gang pot is a MV for the EL84s. The other dual gang pot is a MV for the 6V6s. Turning the EL84 MV to zero allows you to have only 6V6 output and vice versa. Setting the two MVs to any other setting allows you to control the level of signal that you send to each PP pair. The pots need to be controlled separately to be effective.

[kagliostro]

One dual gang pot is a MV for the EL84s. The other dual gang pot is a MV for the 6V6s. Turning the EL84 MV to zero allows you to have only 6V6 output and vice versa. Setting the two MVs to any other setting allows you to control the level of signal that you send to each PP pair.

Sorry if I don't explained it clearly and give the way to misunderstand

Going on reading about the Watt control I've find this that was told by Alerich here into the forum:

When you move the watts control there is a slight cracking - designed that way since Egnater says there is a small amount of DC on the pot.

so the (only ?) way is an LDR in that position for wattage control purpose, the paralleled resistor isn't enough to prevent all scratch

Kagliostro

[plexi50]

Your explaining it just fine.

[plexi50]

I got the dual gang 1M Mix pots wired up and it WORKS!!!  

I still need to order the right PT for it but it sound like it is going to be a sweet machine
 
The mix pots work well and does really define the amps tone between the 6V6 & EL84 tubes very nicely

I built this using the chassis from an old Crate G40C SS amp

Not pretty but good enough for me / Now to cap off all those holes with some christmas lights

[kagliostro]

Hi Plexy

Have you considered to use a thin aluminium strip to cover the front and back of the chassis ?

which schematic did you adopted at the end ?

the Rebel version with the series resistors or "my" version without ?

did you put the condenser EDIT: cap before and after the PPIMV ?

after the PPIMV they are not strictly necessary if the amp is cathode biased, they are needed only for fixed bias amps

when you finish your job I hope you post some clip

Kagliostro

[kagliostro]

Ah Plexy

I forgot, I've a further complication for your 6v6 - el84 tube selector

Firstly you told you want use a foot switch to swap tubes, than you have think to Rebel solution

as you know also Rebel at the begin had a foot switch to swap tubes and was forced by Mesa to change that solution because of Mesa patent

well I've think to a different solution

you can use a footswitch to swap between two different mixing levels

that can be done simply, you must modify the schematic that I attach here (that was a Geezer's "patent idea" to have pseudo channel switch)

and instead to swap the Volume and gain pots you swap between two different couples PPIMV dual ganged pots

EDIT: I added the schematic of this idea

Kagliostro

p.s.: here the original tread about pseudo channel switching:

http://www.el34world.com/Forum/index.php?topic=10933.0

[plexi50]

I used your mix circuit minus the tone cut. The cathode resistor and cap values are as shown below. (Last Pic)

Do you mean capacitor (Condenser)? Yes i probably will get a piece of bronze door kick panel and make a faceplate

Dam kick panels were $30.00 last year at ACE. Quess i could just go to a sheet metal shop and get two pieces cut for a lot less

I keep forgeting about the economy and gas prices causing everything else to go up

As far as the footswitch goes the last time i built one of these i used a push switch to lift the cathodes of each pair of tubes to switch between them

At this time i am forgetting about the footswitch until i get the new PT that i have ordered for this new build

The first build 2+ years ago does not show the finished product as i had to make many changes because of front panel space

I have the finished pics' some where either on PB or youtube.

My first combo 6V6/EL84 build below

 

[kagliostro]

Hi Plexy

Do you mean capacitor (Condenser)?

yes I mean capacitor, sorry but english isn't my language

Nice job your previous amp what did you used for the faceplate print ?

about the schematic of this last amp is it something like this for the preamp ?

http://home.comcast.net/~seluckey/amps/misc/Marshall_Dual_50.pdf

and what for the PI and power section ?

I think you used something like my schematic with the cathode and bypass resistor values you draw, but what about other components (grid resistors and so on .....)

Kagliostro

[plexi50]

It uses the PI and power section of the schematic you posted but it uses the Marshall JMP Series 1987 and JCM-800 2204 Switchable preamp. It is very similar to the one you posted. I used a program called FPS (front panel designer) back then to print out the faceplate on a large photo paper sheet and then used a 1/16" polycarbonate clear plastic to go over and mount to the panel.

[kagliostro]

polycarbonate clear plastic  Plexy........glass ........... Plexiglas® 

I used a program called FPS (front panel designer) back then to print out the faceplate on a large photo paper sheet and then used a 1/16" polycarbonate clear plastic to go over and mount to the panel.

that way isn't strictly necessary to use a metal cover for the front and back panel, did you think to change in this amp ?

Kagliostro

Thanks for the info about preamp

[plexi50]

Clear poly laminate is much cheaper but i have a thing for gold or bronze face plates. They look great. Maybe i need to learn more about making nice quality faceplates for myself. Get a method worked out that works well everytime. There was a thread on here a year ago all about faceplates,how to make them and the different methods used. There were a few cool variations

I think Geezer had a good way as did anothers

[kagliostro]

A friend of mine has experimented the laser toner print transfer to metal for faceplate

the same method used at home for transfer the circuit to the copper strip for PCB

you can also use color laser print and the transferred toner seems to be quite resistant (also can be covered with a thin layer of clear coat)

the big problem that has met is about flatness of the support, a little imperfection and the toner won't transfer to the metal

a way to obtain a flat metal panel may be to use a copper base (those for PCB) that are also easily workable, but copper will oxidize easily, so must be protected in some way

may be an electroplating cover will be the solution, another my friend told he want to have some experimentation about it, but till now nothing concrete has been done


Kagliostro

[plexi50]

It is really hard for me to change modes and go from amp repair and circuits to faceplates. When you do some thing all day 18-7 it's hard to shutdown and go do some thing else. Especially when your work area is not that big and your work station is setup for electrical work only. The amps always creep back into thought and screw up my other plans. Just like CL. I dont get much time to get on CL (Craigslist) but i have to find time because im a CL whore and am always looking to buy some unique piece of gear.   

[kagliostro]

It is really hard for me to change modes and go from amp repair and circuits to faceplates.

Yes Plexy

you are right

is better to do one thing very well than to do more but badly

Kagliostro

[plexi50]

It sounds great Willabe. The EL84 's have that nice shine to them but the 6V6 's are killer sounding. I lowered the 10K resistor on V1B to 6.8K and added a 100uf bypass cap. This is the 2204 input and has some real nice smooth to ripping grind

The Plexi channel sounds pretty nice. It's not as bright as i think it should be so i have to work on that / At half volume this channel sounds clean but after half way it gets a little muddy and the bright pot isnt comming on with the bright like i think it should

Could just be the tubes are starting to breakup at that volume setting but i have always had no problem with brightness using a bright pot to get the maximum shine. This is acting different. I will go back over this when i get time

Otherwise i love it! If i had taken more time in thinking of the possabilities before i wound up adding the EL84 tubes i would have made a better layout for the tube sockets

I wound up with (2) PPIM master controls for the EL84 & 6V6 tubes. The blend was ok but was muddy at times and really not all that benificial in the tone dialing

I added a DPDT switch on the front panel to lift the cathodes of each pair of tubes to turn the bias on or off to switch between them

It worked out better this way to me as far as the difference between the tube pairs tone being noticably different

A blend is what i wanted but i didnt like what i was hearing / It rocks

Small chassis to start with. 20 watts will make you hard of hearing. My ears are still ringing

Panels someday /

[TIMBO]

Hi Plexi, Any chance of posting a final schem of your circuit as i am thinking of adding the 18W power amp(EL84s) to another peramp circuit. It sounds like you have nailed it with adding the 6V6s. Thanks

[plexi50]

TIMBO i used the Steve Luckey Marshall dual preamp schematic and layout at the top of this thread. At some poing in time i will try and make one myself to sho a few diffences

I changed the 330uf bypass cap out for a 250uf and the bright channel sounds much better.