Beginner: marsha valve

[demym]

Hi to all,

after building the NPN boost (and it works :-)), i've tried to do the Marsha Valve... but, after checking connections, something strange happens: the circuit sounds the same (a very bad distortion, that i don't think is the sound that could be obtained) even if i take off the battery.... i've checked for shorts and so on, but cannot solve this problem.. 

if i connect off the battery no sound should be heard, correct ? (with the GUS NPN boost it worked this way..)..

I used J201 as jFET.

Sorry for the very basic and elementar question.

Thanks


UPDATE: i've discovered the following behaviour: when i clip on the battery i get sound, but not full volume and sound.... when i clip off the battery i get the sound that i think was intended to get, but it only lasts few seconds (i mean, obviously, the battery is off...)... to hear the sound the way it should be, i have to connect the battery and then take it off (and i can hear the effect in those few seconds, maybe thanks to the condenser ')..

I've checked schematics, and it seems to be correct.... does these symptoms help for debugging ?

Thanks

[jefe]

Which schematic are you using?

[demym]

This one (Marsha Valve revisited):

[jefe]

OK, thanks... what happens when you turn the 100k trim? Try it at different settings and see if that improves your sound.

[demym]

When i turn the 100k trim, basically i get various degrees of distortion (if trim is completely turned onw wa,y i get no signal)... but the sound i get is nasty, and it's level is very low, way lower that the non effected volume...

the only marshallish sound i get is when i remove the 100uF electrolitic cap and get out the battery (but, as i've said before, this 'right' sound lasts only a few seconds...and i can hear this sound using the same 'trick', that is, attaching the 9V and detaching it...)..

i used ceramic caps (didn't find the film ones) and an electrolitic, as shown in the schematics...

also, pins 2 and 3 (that is, center and one of the edges) of the trim must be connected together, right ?

feel so stupid to not being able to make it work, it's so a simple schematic, and i've already built the tillman, which ran immediately.... but, that's why i'me here, don't i ? :-)


already tried to invert the j201, but in one direction i get very small volume.... this already happened to me with the tillman, and rotating it fixed the output level.. so i think that the j201 is positioned correctly...

also, as i had only one mono jack (cause i'm still only breadboarding), i'm using a stereo jack for the input, and i connected ring to sleeve... shouldn't this turn it into a mono jack ?

another thing, i didn't find exact values for the capacitors, and i put in very close values caps..

thanks anyway for you help

[jefe]

Do you have a multimeter? I think you're gonna have to follow the "what to do when it doesn't work" sticky instructions. Post voltages, tell us what parts you substituted, etc.

"pins 2 and 3 (that is, center and one of the edges) of the trim must be connected together, right ?" - YES

"as i had only one mono jack (cause i'm still only breadboarding), i'm using a stereo jack for the input, and i connected ring to sleeve... shouldn't this turn it into a mono jack ?"  YES, or you don't even have to connect the ring at all, either way should work as a mono jack.

[bipedal]

Check again that you've got the correct pin-out orientation for Gate/Source/Drain on the j201.

I mention this only because I've made mistakes on this numerous times.   

(When I look at a schematic representation of a FET I automatically figure the Gate would be the center pin of the transistor, but center pin is usually the Source.)

Good luck.

[kurtlives]

You need to adjust the trimmer so you have 4.5V aprrox on the drain of your JFET.

If you dont know how to measure voltages it is in the FAQ at the top of the page.

Also make sure you got that transistor oriented correctly. Check the datasheet to confirm the pinout.

[demym]

An update, on the stereo jack it seemed i had wires connected wrong... i exchanged them, and now some sound is produces... anyway the problem is still not solved, as the sound coming out is the clean channel of my amp, exactly same volume and sound... but if i disconnect the battery i get no sound, and this to me seem to be correct...one step forward :-)

I'm pretty sure the J201 is oriented correctly, because i already didi this mistake with tillman's.... source is center anyway; and if i rotate the transistor, i get a very weak signal..

I now measured the drain voltage: but, with the trimmer all close, it measures about 9V, and when i open it up at the max, the minimum voltage i get is 7.63V (i measured drain source putting the multimeter between ground and drain pin of the 201... (the drain pin is that connected to the trimmer, and the source is connected to the 2k7 resistor/ 680n cap))... and the sound is always the clean channel of my amp, that i get even without the marsha....

I didn't hear samples of the marsha, but i imagine it is supposed to be a little groaring, right ?

The trimmer is a 100k, as indicated into the schematics...for the moment i leaved out the 1M volume potentiometer, but that shouldn't affect drain voltage measurement, isn't it ?

Very helpful guys, thanks a lot !

[Baktown]

The Marsha Valve was one of my first builds that actually worked, and i gave it to a friend of mine who uses it as a boost for leads, and he loves it. 

A great sounding simple pedal, in my opinion.

Rick J

[demym]

good to know, thanks Rick... that question was because i dind't hear it still..

I'm new to electronics, but i've already one of the midibox kits (actually, a simple midi controlled 1fx looper, as the advance midiloop), and the GUS NPN... this thing is addictive, anyway !!

I'm in the phase of trying different little circuits, but my goal is to build a 3 channel floor preamp (a la H&K Tubeman...), if midi controlled (using the midibox core that i've already built), better !... speaking about that, is there someone that built something similar ? The three channels i would like to be clean, crunch (marshall) and lead/heavy (dr.boogey ?).... I know it will need some time before i get to that goal, but things have to be started, right ? ....:-)


In the meanwhile, i still get 7.63V on the drain at the maximum trimmer opening..... no way to get the suggested 4.5V..

Thx, bye

Demy

http://myspace.com/demym
http://www.mondomargot.it

[petemoore]

  Something is pulling the drain toward + or something isn't connected which brings the drain toward 1/2v.
  Measure the drain resistor between the drain and +.
  Measure the gate resistor.
  Measure the source resistor.
  Measure from 'distant points'...follow through the schematic to the most distant connected point and test all the in-between connections at the same time.
  Pinout of the Jfet...is often enough the problem with Jfet circuits though.

[demym]

Great tips ! I've found the following:

Measure the source resistor.

2.2M (was not able to measure it as my multimeter has a 2M maximum scale, but by colors it's a 2.2M (red,red,green)... here the problem, presumibilly...

replaced with a 3k as by schematics, and guess ? magic biasing, got 4.5V, but then adjusted with enjoyment by ear.

and here is the marshall channel, you know it's it :-)  !!

Thanks guys !

How can i increase distortion (or add distortion) to the channel ? I've tried some of the simple mods with diodes, but they didn't seem to work..
Also, it is possible to simply duplicate a gain stage and have a stronger dist ? It would be nice to be switchable and pottable..


Thanks once, have a nice time

[kurtlives]

Great tips ! I've found the following:

Measure the source resistor.

2.2M (was not able to measure it as my multimeter has a 2M maximum scale, but by colors it's a 2.2M (red,red,green)... here the problem, presumibilly...

replaced with a 3k as by schematics, and guess ? magic biasing, got 4.5V, but then adjusted with enjoyment by ear.

and here is the marshall channel, you know it's it :-)  !!

Thanks guys !

How can i increase distortion (or add distortion) to the channel ? I've tried some of the simple mods with diodes, but they didn't seem to work..
Also, it is possible to simply duplicate a gain stage and have a stronger dist ? It would be nice to be switchable and pottable..


Thanks once, have a nice time

That would be the best option. Add a gain control (wired like the volume control) between the two stages!

[demym]

Great, thanks.,.


So, i've added a second stage (identical to the first, starting from the 68k resistor), and i obtained a more strong crunch !

Then, taken by euphory, i've added an identical third stage... more and more crunch, but the sound vanishes after a short while.... is it because more than two stages, in this configuration, is not possible to do ?

also, do i have to include the 1M resistor to ground at every stage ?

Thanks a lot, have a nice time

[kurtlives]

Great, thanks.,.


So, i've added a second stage (identical to the first, starting from the 68k resistor), and i obtained a more strong crunch !

Then, taken by euphory, i've added an identical third stage... more and more crunch, but the sound vanishes after a short while.... is it because more than two stages, in this configuration, is not possible to do ?

also, do i have to include the 1M resistor to ground at every stage ?

Thanks a lot, have a nice time

Yes, thats probably why your getting the odd sustain/fade.

I know in tubes it prevents the grid from becoming charged positively, probably similar with FETs.

[demym]

You're absoluteli right ! Adding the 1M restistor 'normalizes' the sound.... I'm at the 4th gain stage....I imagine there's no other way to increase crunch without adding all these stages, isn't it ...?

Then i would need a tone/eq section.... i've read about the tonemender.... is there something similar and maybe simpler to do for this purpose ?
And in case i had to add the tonemender, in which point should i add it ?

Thanks very much ! You're making my first experience a very cool thing !

Cheers

[Renegadrian]

Hope that helps!!!