18 Watt Questions

[jeff]

 Hello

 My bother's birthday's comming up and he has always been talking about getting a marshall 18 watt so I want to build him one. I've got a couple of questions.

He mentioned he's not interested in the trem channel so I'm going to build him a bare bone like Doug's schematic. I noticed that the orignal has the screens of both tubes sharing one 100 ohm screen resistor and Doug has seperate screen resistors.

The point, as I understand it, of the screen resistor is when the screens start drawing more current when pushed the current through that resistor will cause the voltage on the screens to drop. If they're tied together with one resistor then when one draws more current they both will drop. Is that bad?

I'd like to keep the amp as true to orignal as possible so I would rather use the 100 shared resistor but I will change to seperate resistors if this is bad for the tubes. Was this done to get a better sound/ more power/ etc. or for longetivity of the tubes?

    Thanks
      Jeff

[sluckey]

I'd like to keep the amp as true to orignal as possible so I would rather use the 100 shared resistor but I will change to seperate resistors if this is bad for the tubes. Was this done to get a better sound/ more power/ etc. or for longetivity of the tubes?

Do it with the shared resistor then. I did. If you want it to be more bullet proof, do it Doug's way. Marshall probably only used one resistor because of economics. Two screen resistors are more reliable but cost more.

[stratele52]

+1 sluckey,

Two resistors are better and more ohms too like 470 ohms, but did this change tone ? Not supose if you keep them under 1000 ohms each.

[stingray_65]

In general design mods are made for one of two reasons.

one to influence the tone of an amp

two to make an amp more reliable

it's a good question you asked, now I know why Doug designed it that way!

[phsyconoodler]

Here is a killer 18 watt lite layout.It's fabulous sounding.

[Rev D]

 +1, lite IIb's sound great, I gigged mine for like 4 years and recorded with it, sounded pretty decent to me. The circuit I did was off the old 18 watt board (before I quit going there, to much noise and gain in the forum for my taste ahem..), but it was very similar to Brown Notes version. Decent cleans and it would grind like crazy if you turned it up. They're a nice circuit, simple but effective and some like them just because there aren't a dozen knobs with sounds you rarely use.. Just a opinion of course... 

Regards,

Don

[jeff]

OK so it looks like they kept the shared screen resistor but upped it to 1K. But I still don't know if it's because of taste or safety.

Let me ask it this way:

 Is the purpose of the screen resistor for protection or for sound?

 If it's for protection, is a 100 ohm shared screen resistor big enough to protect?

 If not, what's small value(closest to original value of 100 ohm) I can safely use?

I want to stick with the original schematic as closely as possible, but not at the cost of the tubes.

[phsyconoodler]

safety.  I've built many with 100 ohm screens but had the voltage AT 300V  on the plates.EL84's don't like too high screen voltages.

Build it.Many have.

Quote:"I want to stick with the original schematic as closely as possible"
 
  Why? Have you ever played an original 18 watt?

[jeff]

No, but my brother has and it's for him.

safety.  I've built many with 100 ohm screens but had the voltage AT 300V  on the plates.EL84's don't like too high screen voltages.

I just have a few questions to get this right in my head.

Is the screen current more or less constant up to the point of overdriving?

Does the screen resistor has to do pre say with the screen voltage?
 Hear me out. Yes increasing the screen resistor will drop the screen voltage. But the B+ dropping resistor can be adjusted so that the screens are at the correct voltage. Imagine just the power tube, no preamp etc. If you used a 2K dropping resistor and a 100 ohm screen resistor, it idle the screen voltage would be the same if you used a 100 ohm dropping resistor and a 2K screen resistor. right?
(And I'm perfectally OK with using non stock dropping resistors, In fact I expect to because I'm eliminating the trem channel and using the stock values would alter the voltage since there is less current draw)

 Is the point of the screen resistor, not to drop the screen voltage, but to drop the screen voltage when the tube is being slammed? When the tube is pushed hard and it starts drawing more current? The more it draws, with a bigger resistor the more it drops. That's the saftey aspect.

 There are two reasons I want to use one resistor that is as close to the 100 as possible so the amp will have as close as the responce of an original while still being safe.

 I assume there is a difference between using one shared, or one each. When using one, when the push side of the PP amp is overdriven, then the screens of both the push and pull side drop, and I assume that affects the amps responce. So I want to stick with one if possible.

 I assume the bigger you make the screen resistor the more the screens drop when pushed hard, also changing the responce.

 I want it as close to the original as possible, both screens drop at the same time, both screens drop as close to using a 100 ohm as safely possible.

I don't know how to calculate what that is and I don't know if 1K was used because it is the closest you can safely go or so the screens will drop more giving a better responce.

Build it.Many have.

Maybe I worry to much but that was my reasoning.

[sluckey]

Maybe I worry to much ...

I agree. Just build it with the single 100Ω. Heck, many famous amps, including Deluxe 5E3 and Twin 5E8 had no screen limiting resistor!

[jeff]

"Heck, many famous amps, including Deluxe 5E3 and Twin 5E8 had no screen limiting resistor!
"
...but didn't those amps also have two prong AC cords and death cap switchs? 

Sluckey thanks for the head check. Your right I worry too much. I'll use the 100 ohm resistor.

(I guess it would just make me feel better if Doug would explain the reasoning behind his change: was 100 ohm inadaquit or did seperate bigger ones sound better.)
 
   Thanks
     Jeff

[phsyconoodler]

It just affects the screen current and the voltage slightly.The larger the screen resistance the lower the output level.  The 18 watt lite blows the normal 1974x out of the water for tone because of the paralleled input.

[jeff]

 Yeah, I bet it does. If it was for me that's what I'd do. But it's not. I called my brother trying to gather intel without giving it away that I'm building this and one of the questions I asked was "Is the lo channel different that the high, would you ever use the low input?" He seems to think there's a big tone difference between the two. I wired this one with a push pull volume knob which wires it as if you were plugged into  hi or low and to me the only difference I hear is a slight volume difference. Maybe when cranked it really stands out, I don't know, because I was afraid to crank it until I figured out the screen resistor thing, but it sounds the same only one number lower on the dial. Like low on 3 sounds the same as high on 2. It's kinda usless, instead of pulling the knob, turn it down a little- sounds the same to me. But maybe it's psychological. Pullin' the cord out and sticking it into another input must sound different and you hear what you expect to.

[EL34]

I guess it would just make me feel better if Doug would explain the reasoning behind his change

I probably spent 2 seconds thinking about it....then I gave each tube it's own screen resistor.
Same with shared cathode resistors and caps...... 2 seconds up.... give each preamp tube it's own cathode resistor and cap.
Same with Hi-Lo input jacks.... don't ever use the low...out it goes.

There's no huge master plan behind any of it
Parts are cheap, why share them

[stingray_65]

(I guess it would just make me feel better if Doug would explain the reasoning behind his change: was 100 ohm inadaquit or did seperate bigger ones sound better.)

I really like to think of this forum as a place of higher education in amp construction and design.

wouldn't be much of a forum without questions and replies

Besides, Doug has so much on his proverbial plate, a career, online amp shop (best customer service ever),few webpages (great ones at that), this forum (free by the way), his hobbies like biking and gardening, OH and playing and recording his guitars... LOL I'm exhausted imaging how much he gets done in a day!

Personally I'm grateful that he shares his ideas as fully and freely as he does, his layouts and ground schemes are legendary, most people would consider those as proprietary and keep those secrets for themselves.

I'm sure you never meant your comment as derogatory, and I really don't want this reply to sound condescending or scolding, written words can sometimes loose inflection and be misinterpreted.

Stick around! your chance will come to share your experience here with someone else who doesn't quite get what he's seeing. :)

[plexi50]

I think im going to build the  Brown Note Lite. I have everything i need to build it. I love looking at circuits and the simplicity of some of them. It looks like it would be a nice sounding lower gain amp

[phsyconoodler]

quote:"It looks like it would be a nice sounding lower gain amp"

It's so nice it will rip your face off when you crank it up.EL84's have their own gain thrown into the mix and the paralleled triode is killer.

Nice.Yeah,right.

[jeff]

Doug, Thank you for taking the time to respond.

 Sorry If I came off as derogatory. Trust me that was never my intent. I worry a lot. I saw there was a change and just wanted to know why. It wasn't meant to be said in a bad way. I'm still learning.

 I too like to think of this forum as a place of higher education in amp construction and design. I was just trying to better understand the role of the screen resistor and why different values are used on different amps/ how they affect sound and responce of the amp and if things can be inproved for saftey.

 I love this site and recpect everyone on it for sharing there piece of the puzzle with me. Sometimes words on the page don't convay the intent. Sorry if I came of as an ass.


          Thank you,
              Jeff

[dude]

quote:"It looks like it would be a nice sounding lower gain amp"

It's so nice it will rip your face off when you crank it up.EL84's have their own gain thrown into the mix and the paralleled triode is killer.

Nice.Yeah,right.

I'm agree, I'm big believer in the 18 watt style amps. I've made them with 6V6s and EL84. Of the two, IMO those EL84 win hands down especially if you can get your hands on some Phillips, Holland made el84's,

The lite and the TMB 18 watt give you so many different tones, clean - super blues tone and Led Zepplin crunch. The VVR even gives you more choices by lowering the voltage. MY 18 watt TMB is my favor amp and my wife says I have way too many, ha.

al

[EL34]

Sorry If I came off as derogatory

No, I did not take it that way at all.

I just thought you wanted some high and mighty cork sniffer type explanation as to why I did that way

I don not the cork sniffer attire (wtf was that?)    (<-- friday night at the computer with beer in hand)

I am just very logical and practical when it comes to design.
I don't care how Leo or Jim Marshall did it, if it makes sense to give each tube it's own cathode or screen resistor, I do it.

[jeff]

 Cool
 Just wondering if 100 ohm corks were known to come loose and have the pressure blow it out and ruin the wine.... ...and I don't want to lose an eye.

I'm sure both corks would smell just as good. Trying to aviod imminent catastrpohic failure if possible.

[panhead]

I'm building a variation on the 18w as a 1x12" combo right now. I ditched the tremolo channel, and other than that it is pretty much the same. I'm keeping the Hi and Lo jacks because of the way plugging into the Hi jack effectively parallels the fist 12AX7 or ECC83. I know that's easy enough to do without the second jack, but I have a few around anyway. I'm using Doug's 18w output transformer and I bought an Edcor PT. I used a single 250 ohm/5 watt resistor for the screens, because that's what I had around. I'm looking forward to hearing what it sounds like. Good luck on your brother's.

[sluckey]

Jeff, I'm just a hobbyist but I've built two 18 watters. Both amps used a single/shared screen resistor. I still have both amps and the tubes are holding up well. I probably play these amps more than any of my other amps. The simplicity of these amps make them very easy to use.

The first is a full blown model 1974 Marshall 18W that is true to the original. This was a total scratch build. The second is a 18 Watt lite, 98% like the Brownnote posted by phsyconoodler. This was converted from a Hammond organ AO-39 reverb amp. BTW, the AO-39 is an excellent donor for the amp you want to build.

These links may give you some ideas...
http://home.comcast.net/~seluckey/amps/18w/18w.htm
http://home.comcast.net/~seluckey/amps/hammond/hammond.htm

[Willabe]

Jeff, I'm just a hobbyist but I've built two 18 watters.

Yeah, thats right, he really runs a home for wayward squirrels.

Look here;

http://www.el34world.com/Forum/index.php?topic=12076.0

                       Brad