newb questions, advice needed, basicly help me with my first build

[danger-russ]

What I'm thinkiing about doing is building a moded 50 watt jcm 800, basicly I'm looking at the splitch reverb 4210 schematic and thinking about droping out the reverb and adding another distortion channel, maby solid state. I mainly play metal type music.
I'm also thinking about changing it from fixed bias to cathode bias, so I can use the Geezer bypass cap mod so I can lower the output power.

But I'm still trying to understand the basics.

Newb questions
I understand that The jcm 800's used either el34's or 6550's, But as far as I can tell the el34 is a 9 pin tube, and the 6550 is an 8 pin. So I'm wondering besides just a socket and wireing differance, what else is different?
tone/headroom/output/power requirements?

I'v asked around the local area and been told that 6550 would be better for what I want do you guys agree?, I need a very clean power amp section with most/all the distortion in the preamp.

more questions to come, thanks for looking and helping.

http://www.el34world.com/charts/Schematics/files/marshall/jcm800_splitch_rev_50w_4210.pdf

[TubeGeek]

Some things to consider:

For a first build it may be a better idea to follow something that is proven and exactly what you're looking for.  Based on what you describe, you may want to consider the jcm800 2204 schematic.  There are multiple schem's and layouts available over the web.  For reduced power output, look into VVR or power scaling.  This will be a great heavy metal amp.

EL84 are noval 9-pin tubes. ~12W

El34 tubes are 8 pin octal tubes.  ~25W

6550 tubes are 8 pin octal tubes. ~35W

[Tone Junkie]

danger-russ how much soldering experience do you have, there are some good tutorials on youtube they will be helpfull.
 First time it could be easier to do a kit my first was a 2204 jcm 800 great amp hard to start with, but you could do it there is a lot of help on this forum.
What I like most is they wont flame you for asking a dumb question, like some other forums. Ive asked alot of dumb questions and they still put up with me.
Bill

[Ed_Chambley]

I have 2 JCM-800 clones.  Both are running EL-34's.  One is 25 watts and the other is 50 watts.  For metal I would suspect El-34's (EL-34's are 8 pin octal).   The only difference between the 2 amps are the Transformers.  I have a friend who bought a Fargen Ole 800 and I just opened up his and copied it except I used Hybour PT's instead on the Mercury Magnetics Fargen uses.  I did spring for the Mercury OT's.  An additional gain stage would put you into Metal Territory.  There are lots of different tones players consider metal and it is just my opinion the JCM-800 is not a metal amp.  It is a great rock amp and I think the best.  They are very flexible.  I even use my 50 watter for country.  Fargen uses all the most expensive parts.  SoZo vintage mustards,  MM transformers.  Turret boards, pre-tinned teflon wire.  If you consider ACDC metal the 800 is it, but when I think of metal the Dual Rectifier MESA is what comes to mind.  Don't get me wrong, I love my Fargen clones and even got Jeff at JDesigns to make the head cabinets which is the same guy Ben Fargen gets is from.  I have done lots of thing experimenting with these heads but still have never heard that Mesa, Cornford, Diesel, Hayden or Engl sound coming from them.

[danger-russ]

ok looks I got a downloaded a spec sheet for the el84 thinking it was for the the el34.

I have the shematics for the 2204 too, I really like the channel switching circuit on the 4210.
So maby I will build the 2204 and then mod it a little at a time until it has the features i want.
less to trouble shoot if it doesn't work.

I have plenty of soldering experiance, and metal /wood working.

The 800 might be a little light on distortion, but I do like the 80's hard rock / metal sound, I'm hoping that a channel with a s/s distortion preamp might be able make up the difference.

I'm going too see what I can find on the net for info on some of the amps you mentioned.

 

[mpayne]

I just built an amp based on the JCM800 and I added a second gain control in the preamp section.  When I crank up both gains I think it has a good metal sound.  I am using an active pickup so I’m sure that helps.
I’ve attached the schematic in case you wanted to check it out.  The second gain is on page 2, R12.  In the original schematic it just had R13=470k and R14=470k.
 

[mpayne]

I just realized I had already posted that schematic in this thread "http://www.el34world.com/Forum/index.php?topic=11423.0".  Sorry for attaching it to another thread.

Mike

[danger-russ]

thanks for the info,

latley I'v been playing my esp cz-II, It's got emg active pick up's, bought it two years ago and haven't really touched my my other guitars since. I have an old esp m-100 thats going to get emg pickups when I get around to it.

ok couple more questions.

I want see if I understand whats going on in the pre amp section of a push-pull amp.
I know this is over simplified and ec83 are dual triode (two amp stages in one tube).
Not sure as to how they work as a PI?

so basicly on a clean channel the first tube ec83 is a pre amp to raise the signal voltage to feed the tone stack. Then the signal goes to second tube ec83 which would be the PI, the PI works as a double half wave rectifier seperating the positive side and the negitive side of the signal into two signals, each signal feeds one side of the power amp.

on a high gain channel the first tube ec83 would be used to over drive the first tube on the above example with a volume control inbetween acting as a gain control.

am I on the right path?

[Matty_V]

+1 for the 2204. My first leap into tube amps was rewiring my 2204. Not too difficult, just at the time I had no idea what anything was. The basic flow of it is this: Gain stage, volume control, cold biased gain stage, warm biased gain stage, cathode follower to provide a low source impedance, tone stack, master volume, phase inverter (which is part of the power section), power tubes. The low input jack bypasses the first gain stage.

This particular inverter is called a long tail pair. The purpose is to provide 2 equal signals that are 180 degrees out of phase with each other for the power tubes (one to push, one to pull). The other common inverter you will see is a cathodyne. It only uses one triode section but it loses gain. Hope this helps you out some.

[danger-russ]

Ok that was a big help, I couldn't figure out how the pi did what I thought it did.
But now I got it common cathode on one side common grid on the other.

Yeah I'm going to start by building 2204, then mod the preamp alittle at a time to get a more modern metal sound out of it.

I want to try to understand as much as I can before I start the project. So when I make the chassis I can make provisions for the mods.
 
I have to do some more reading so I can ask some more questions.

[danger-russ]

Ok I think I'm ready to start gathering parts. Probably going to take a couple months to get everything together.

The plan is build the 2204 per the schematic. But add two extra sockets for preamp tubes, one for two more gain stages and one for an effects loop. To be added after I get the amp up and running.

Would 18 Gage galvanized steel be good to build the chassis out of.
The caps on the b+ power are cans would it be better to use singles? From looking around it's a wash cost wise.

I'm assuming that I will have to add another power node when I add the other tubes.

[danger-russ]

I'v been working on darwing up the schematics, I have the original preamp section drawn up, I'm working on how I want to mod it, I drew up a switch for the boost and a A/B switch for two channels right now both channels are the same just want to see if I'm going about the switching right.

[stingray_65]

Ok I think I'm ready to start gathering parts. Probably going to take a couple months to get everything together.

The plan is build the 2204 per the schematic. But add two extra sockets for preamp tubes, one for two more gain stages and one for an effects loop. To be added after I get the amp up and running.

Would 18 Gage galvanized steel be good to build the chassis out of.
The caps on the b+ power are cans would it be better to use singles? From looking around it's a wash cost wise.

I'm assuming that I will have to add another power node when I add the other tubes.

Russ,
You're asking for opinions, so here's mine.
adding extra sockets is ok,
18 ga is a bit lite, the trannies are heavy. 18ga is adequate for head type amp, questionable for combo.
can caps save room on the board, you plan on modding, board space is at a premium. can caps may have some drawback for some people, but Marshalls regularly use them.

Ray

[danger-russ]

thanks stingray, found a place that sells a chassis I like, so my left over 18ga sheet will go to something else. I'm thinking I'm going to use cans for the power supply.

Figured out that my above schem. wouldn't work so try number two this one only has one more tube than stock, another clipping stage and a shared tone stack. I'm hoping I got it right this time.

Can I use the output from master volume to go the send of the fx loop and then use a cathode driver into th pi?
 I'm not sure what effect the extra cathode driver after the tone stack would have? maby just drive the pi a little harder with nothing in the loop?

[stingray_65]

I just checked out your new schematic, Interesting switching scheme.

It looks like you have 1 switch throwing in 2 gain stages at once? or are these two separate switches (or relays)

If it's just 1 switch (4PDT) thats a lot of gain! You may encounter "popping" issues, but they can be remedied fairly easy.

FX loop, Just an opinion here, why go active? a simple passive in/out like Tubenit uses on most of his builds is pretty effective. If needed, you can build a stand alone active 1 tube box that plugs in there. Tubenit again I think has built and used them with success. Just trying to keep it simple.

And yes, right there off the MAS VOL, before the PI is where it would go.

I cannot comment with any credibility on the use of a cathode driver.

I'm not well versed at all in hi gain amp designs, but I do have questions about where your tone stack is, I'm not at all sure it is in an optimal point in your design. But again I'll defer to those better qualified.

I hope some others will review your schematic and chime in on my questions.

Ray

[danger-russ]

sorry forgot to rename the second switch. fixed it

basicly it's just like the original 2204, it has two input jacks a high gain and a low gain. I got rid of the second jack and put in a switch for the gain stage, the second switch will just add in a clipping stage after the original warm biased stage, it wont have much gain but will clip pretty hard. At first it will just be toggles but later I want to go to relays and a foot switch. I'm also thinking about putting a switch between two tone stacks, at first to a/b different ideas, but after the relays are added to follow the second switch so I have one setting with the original clipping and another for the harder clipped channel.

The reason I'm thinking active on the fx loop is because I have digitech gnx3000 workstation, that replaced my whole pedal board several years ago, I'm thinking I may want bypass the preamp and run that strait into the power amp. I want to make sure the levels going to the pi stay consistant. I'm not really sure if it is needed or not. I saw Tubenit's loops but haven't had time to look in too much detail.