Bieworm wrote: ↑Sat 02/19/22 3:38 am
tfrost33 wrote: ↑Fri 02/18/22 2:23 pm
Been lurking here a while and just became a member. Thanks! This thread in particular interests me because chalk up another Mojo TMB kit that squeals like a banshee! I did everything this thread has instructed to do except for one thing. That is moving the 68k resistor from the input jack to pin 2 of V1. At least that how I read the mojo schematic. I see other schematics are different from the mojo one. But I didn't realize that until I had already purchased the kit and built the amp.
After performing all the other things ( new ground scheme, shielded wire, touch up a few poor solder joints, etc) I still get some squeal, although not as bad as it was. I have to crank the treble, gain past 3 O'clock-ish, MV past noon-ish and that lets the banshee out!
Anyway I am having trouble wrapping my head around rewiring the input jack for the tmb channel and moving the 68k resistor to the tube socket. Mostly just how to wire the jack after moving the resistor. I think the resistor is pretty straight forward.
looking at other schematics it seems pretty simple, but can the schematics be intertwined? For instance the Valvestorm Scheme (if I am reading it correctly?) has it going to pin 7 of V1.... Could I just wire it the valvestorm way except land it on pin 2 (where it currently resides)
Other than that, I do say the amp sounds great up to the point it starts to squeal, then it still sounds good, but it just squeals! And the normal channel is spectacular!
Thanks everyone!
Tony
Hey Tony! Welcome to the forum.
For the correct wiring of the input jacks you can go to the Downloads area and take any layout from Josh. Look for 18W modern classic .. that one will correspond largely with what you need.
Maybe the oscillation is from something else in the circuit. If you can post detailed Hi res pics of the build we can have a look under the hood
Grtz Bjorn
THANK YOU!!!
I will see what I can do as far as posting some pics, I think you may all enjoy the cabinets that I build? However, your guidance was the final nail in the coffin. No more squeal! I wired the MV (High Gain Channel) input as per Josh's diagram. I can't thank this forum enough!
Just for the record I performed everything this thread provided and a little extra. I am not sure if it made a difference or not, but I went through the tedious process using shielded cable on the Treble/Gain/and Master pots. Then moved the said resistors to the tubes. Previous to that I had touched up a couple questionable solder joints, but that didn't really seems to make a difference. I also re-did the ground scheme per Josh's grounding...
In order of operations:
1: re-touch some solder joints (little to no change)
2: redo the ground scheme (made a difference but still had squeal after turning the T/G/MV pots up past 2 oclickish)
3: Moved normal channel resistors to tubes (not relevant but I did it anyway) No help here with squeal on the other channel
4: Rewired the T/G/MV pots with shielded cable and moved the 470 resistor to the tube ( major help!) Could turn everything up to about 9 oclock before the squeal started
5: moved the input resistor (68k for Pin 2 at V1) rewired the input jack per Josh's scheme and used shielded cable here as well. Now I can crank all knobs with no squeal, but MAN can I get me some on demand feedback
Put the amp through its paces today and it sounds absolutely incredible. If I were to do this again, and I just might, I would definitely not get another Mojo kit... This is the 12th amp I have built now and this is the first time I have had an issue (except for a minor standby switch issue with a Metro JTM45 kit I did geez back in 2005-ish already)
Anyway, hope these details can help someone else in the future... Oh and one last thing, once I get some pics posted I know about the overheated resistor at the cap can and its been changed!
Thank you again everyone! Now I have a lot reading to catch up on here at this amazing 18 watt site!!!
Thanks
Tony