Candidate for weirdest problem ever.

[nandrewjackson]

https://drive.google.com/open?id=1w6Df5-R2afoI265F5JtFKBZEg6Zbp2By




push pull two 6L6.


The part of the circuit that's weird is the two selectable EQs.  When the "fender" EQ is engaged, all is fine.  When the "marshall" EQ is engaged there is slight muddy distortion and harmony overtones. 
What I'm calling harmony overtones is like hearing a 2nd fret A string B note while only playing a low E note.  And the *harmony note goes up as the actual note being played goes up too. 


The problem HAS to be in the *marshall EQ, right????  Each EQ has a LDR before and after it.  When the "Fender" EQ is engaged, everything is fine.  When the *marshall EQ is engaged, distorted harmony notes chime in.


It HAS to be in the *marshall EQ right???   


Has anybody every had an EQ make this kind of awful sound? 


It wasn't doing this since built.  I've got maybe 6 hours of playing on this amp, and this problem just started. 


The EQ controls seem to be doing what they're supposed to do, Bass Middle Treble. 


I do have enough parts to replace the entire *marshall EQ, and more LDR/LED roaches to replace them too, but if anybody knows a better angle at this, please let me know.


Thanks,
Noah.

[nandrewjackson]

https://drive.google.com/file/d/1iE-RWZC2lqEMIN200SkEP2c7j1ReW-d5/view?usp=drivesdk

[TIMBO]

Hey Noah, Your a brave man to use 23 LDRs.
I have a Mesa Boogie Dual rectifier type build that has been sitting for some time as I am unable to work out how the switching can be done.
What type of LDRs are you using?
The little I know about them is that they don't totally block the signal.
Could there be some signal bleeding through the LDRs in the marshall circuit??????

[nandrewjackson]

Hey Timbo,


The whole thing was working fine for about 6 hours, then some problems started up.


I'll post links to my switching circuits.  There's two types, one uses 2 BJT's and one uses 3 integrated chips.  The transistor circuit is easy to build, but it is very finnicky about the voltage, really likes about 4.6 to 4.8 VDC.


I was hoping someone would say " oh, that sound is because the bass pot is faulty, I've had that happen before "


https://drive.google.com/open?id=1bys2y-89hAMEbz2KMQ3BSgU1ptUuAp1r
https://drive.google.com/open?id=1OVqa4paubdsRK8R3Frx8jZBxHdQXVzcq


https://drive.google.com/open?id=1ptJQGjys2DIkmhTYwIoXI1x4XVcnpMRI

[nandrewjackson]

I'm using home-rolled LED/LDR combos. 


The LDRs are 5539 CDS. 5K on resistance4M off resistance.


3mm white LEDs

[nandrewjackson]

Apparently the random harmony note generator is present on the *fender EQ too. With the treble and mids turned down,  and bass up, the random harmony note generator is indeed present in both EQs
 
Some sort of bleed thru and whacky sh$% going on with all those LDRs.


It's kinda neat.  I almost want to figure out how to make the effect more prominent.  But I think I'll concentrate on making it go bye-bye.

[Ritchie200]

Almost sounds like one or more of your LDR’s is not working. I would first go through those and verify operation. Do the easy stuff first!  If that is the problem you can figure out what it’s doing wrong/right and add the Atomic Nandrew Boost Switch for the effect!


Jim

[nandrewjackson]

I've got it now. After quite a few hours of digging.


I removed the B+ from V2, V3, and V4,  and ran a jumper from ch1 volume swipe to phase inverter input. Bypassing all other channels and the 2 EQs and 4 master volumes . Sounded fine, no problems. 


I then took that jumper back a notch, to just before the mastrr volumes, there was now a bit of weirdness going on  .


I then took the jumper back another notch, to just before the 2 EQs. Now the *random harmony note generator was back fully.


I guess when there are 4 or 6 of these LDRs in parallel , even though they are at best,  5M , the overall  R was dropping to about 200K total (measured.)


I may roll some new ones, using 2 LDRs in series in each roach, or just wait on the new ones I ordered that supposedly have 10M dark resistance. 

[PRR]

Good logic.

But back up. How bright is your bench light? The LDRs only go to many-Meg in deep darkness. They may be in black shells, but maybe not perfectly opaque. When I used to mess with LDRs I had an inside room, turned off the room light, little teeny lamp on the meter with a board shadowing the LDR rig. Even then I frequently killed the small light just to be sure. If they used LDR shields good for normal light in a closed chassis, bright light on an open chassis may spoil things.

[nandrewjackson]

Yeah, I noticed how even room lighting can change the resistance of the LDRs.


The problem happens even with chassis in cabinet.


When I measured 200K across them at the junction of the 4 MVs, I had the room darkened.
I've got 10M dark resistance ones on order, fingers crossed.


I've seen  some with 20M dark resistance,  but can only find European sellers, which at least doubles cost due to shipping. (I'm in USA. )

[sluckey]

VTL5C1 goes up to about 50M dark. But at $8 a pop that's a lot of money for your amp.