Tweaks on a Grommes LJ7 modded for guitar

[goldstache]

Fooled around with this Grommes Little Jewel (LJ7)


Here is where its at currently.  Sounds pretty good.  The EQ is a bit whimpy.  Wondering if anyone has any tips on getting a more pronounced EQ effect with similar insertion loss. I also included the original Schematic.


The schematic shows voltages for a 12at7, but i am using a 12ay7 now.  It does that cool trainwreck express thing, where the notes blossom into feedback as long as the gain control is set high and the Master Volume is at about half.  Maxed out it's still controllable.


As I tinker with existing design add-ons I realize I know very little about stage to stage impedance.  Can anyone help me with the basics. I have been reading several books on the subject.  But I keep getting lost.


Also, digging this self balancing paraphase inverter, wondering if that is some of the charm of this amps beautiful Overdrive??? 


Thanks Everyone.

[Platefire]

Hello goldstache

You show a modification sketch schematic and the original schematic. At what stage is it in right now? Is it still all stock or partially modified or fully modified like the schematic? Need to find out where you are before advising you where to go. Some pictures of the existing circuit would be helpful. Platefire

[goldstache]

Hello Platefire, thanks for the response. It's in the exact state as the hand drawn schematic.  The original grommes schematic is included for reference of what it started as.  The impedance relationships of the plate-EQ-grid section is my quandary.  I tried several variations of tonestacks, to no avail!

[Platefire]

goldstash

I can't really help you with impedance. I just try stuff and hope it will works . There are plenty here that can help you address that if they will just chime in. Just be patient! My first thought was since you have a place for a treble and Bass control, find a vintage Fender Treble/Bass tone stack and try that---like the attached Pro Sch. Then there is the Baxandall tone stack. You may have already done that? Also the old tweed 5E3 one pot tone control works pretty good if you need more gain. Platefire

[jjasilli]

Some thoughts: 


*  If the Fender tonestack itself is wired correctly, then it should not be the cause of poor EQ performance.
*  The Fender tonestack does not need a coupling cap.  Try jumpering it.  If you still really want one, use a larger value than .047uF, like 0.1uF up to 0.22uF or more.
*  Try a higher Mu tube in the first position.
*  At the Input, the 68K & 1M resistors are forming a voltage divider which reduces performance of the low Mu tube.  Try temporally jumpering the 68K resistor for more input signal voltage.
*  Not sure about the 47K resistor under the 1st stage bypass cap.  Try jumpering that.
*  Not sure about the NFB circuit.  Try disconnecting it.  If that helps EQ performance, but you still want NFB, then the NFB circuit can re-designed.

[goldstache]

Thanks for the excellent ideas. The input is drawn wrong, it's reversed 1M grid leak into 68k grid stopper a'la normal fender.  It's the stock blackface tonestack.  I have tried most standard tone stacks.  I have tried other tube variations as well.  More Mu gave me oscillations.  May need to put a voltage divider where the 560k is into the PI.

I will fool around and see what tumbles out!  Thanks!

[jjasilli]

OK, that narrows down some possible issues. 


BTW:  " It's the stock blackface tonestack." I'm not sure there is such a creature:
*  Tone cap values range from .022 to .033 to .047.  That's also why your .047 coupling cap may be an issue.  It's in series with those caps & hence substantially reduces their stated value;
*  Also, the standard slope resistor is 100k, except for cathode-driven tonestacks where its 33K. 


You may wish to post your tonestack schematic.

[PRR]

Sticking a master volume, inside a paraphase driver, inside a NFB loop, makes my head hurt. That's not right!

Turning-down will screw-up the push-pull balance; also reduce the NFB.

However since the goal is LESS power output but still distorted, I suppose it may work OK, even well. I'll leave it.

Input cathode: the 47K is in the wrong place. Won't change gain (2%, not audible). Short the 47K. Now gain changes, but the switch pops. Put the 47K back but across the switch, so that the DC will equalize when switch is open.

The 560K into V2a seems large and begs for instability. More like 50K seems wise.

The 220K out of the MV into the 220K/270K grid resistors is a heavy loss at a high level point. I'd short those, but put a smaller resistor right AT the power tube grids.

215V at the guitar preamp is lower than we usually use. Take the 47K dropper to "215V" down to 10K to get 280V or so.

You "should" be able to run 12AX7 in the first stage. If not, study the layout, see where high-level signals in later stages is sneaking back to low-level stages.

The Blackface Fender tonestack can go right there. The overall layout is JUST like many fenders, gain will be ample, overload profile will be classic.

[2deaf]

Turning-down will screw-up the push-pull balance; also reduce the NFB.

If he gets rid of the bottom half of the dual-ganged pot. so that he only has a single pot. after the top splitter stage, it looks like it should have a balanced signal.  If there is trouble with the common cathode connection, the bottom stage could go straight to ground.  NFB is jeopardized in any event.

Wouldn't it be better to move the 1M connection at the input to the junction of the jack and the 68K?   

[goldstache]

Thanks guys it all seems so clear now!
I modded this before I really understood how amps work and how to draw schematics (I.e.- v1A cathode cap is wired right but written wrong! . I will tamper with the suggested mods and report the findings. 

[Platefire]

goldstache

Don't know how much experience you had in electronics or drawing schematics but I thought you did rather good on
your schematic sketch even though it may have had a mistake or two--I'd say very good! I remember when I was learning schematics, was a little tough. Not sure if anyone has told you but on this forum we use a free program off the internet called Express PCB/Sch that we draw our schematic electronically. It's a good simple program and most folks here have it on their computer so we can access each others schematics and we all share our schematics. Good Stuff, Platefire

Follow this link for a download
http://el34world.com/Forum/index.php?topic=590.0

[goldstache]

Thanks Platefire!


I only have mac, so Im not sure it will work for me. I think they make emulator programs that allow you to run windows based software.  But Im not that techy when it comes to computers!     I think I have a program called JSchem that has tube symbols in the library.  Probably should give it a whirl.  Though I see the advantage in working with a common program for editing across the forum.   I've also never used any emulation software.  If anyone has any recommendations for me, that would be swell!


Gonna try and dig back into the Grommes within the next day.  Thanks for all the priceless input!

[Willabe]

IIRC, not long back someone here (Doug?) posted that Sch now works with windows?


                          Brad