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Re: LDR Switching Questions

1/22/2004 5:07 AM
Re: LDR Switching Questions
after the coupling cap on the first stage is where you want to put the LDR's, so no DC gets in there.  
to get the voltage needed fo rthe LDR, i rectify the 6.3, then use a 5v regulator, then current limit resistors on the LDR's. 165 to 270 ohm is great for them.  
take a look at the soldano X88r preamp schematic for one way of using them. it is an incorrect schem. you have to add LDR b4 the grids of the indivdual channels.  
you cold even use 2 LDR's for each channel. there is some leakage going on, even with 50M or more resistance in the off state.  
the footswitch, you want to send the ground of the channels LDR's out to the footswitch. if you want ot be able to do switching from the front panel or off the footswitch, use a switching 1/4" inch jack.  
1/22/2004 11:05 AM

When using ldr's for switching, should you use two ldr's per "switch". I.E. when the channel is switched off, the now unused channel gets shunted to ground at each switching ldr.
1/22/2004 8:46 PM
Just out of curiosity, why do you need the common gain stage after the input ?  
What would happen if you had the channel switching right after the input and left the two preamp channels completely independent ?
1/23/2004 4:38 AM
Omar N.

Using a common first gain stage saves adding another 12AX7. I'm up to 5 12AX7s for this amp already.  
1/23/2004 1:00 AM

if i used 2 ldrs for each switch section, i would put them in series with the signal. how i do mine ( i only use one), and i have no problems at all. putting them to ground, you might have problems. run em in series.  
1/23/2004 5:06 AM
Omar N.

Thanks for the info. Putting the LDRs in series seems to make more sense so the unused circuit is open at the beginning of the channel and after the channel's master volume.  
Would you happen to have a schematic for the DC circuit with the footswitch. I looked at the X88 schematic I have and I didn't see it. I looked over the SLO schematic and I still don't quite understand it fully.  
1/23/2004 7:20 PM

well. here's my drawings of the Twin Reverb II after I had modified it to be somthing like what is planned here. I separated the original schem into three sections and edited it to reflect my mods, as usual the pictures look big on the monitor, or faint when reduced to screen size, but print ideally when fit to regular 8.5x11 paper:  
The same LDR's are shown on both the Audio & FX sheets, and the LDR voltages are not exactly at points T & U as indicated, instead they are measured on the LDR side of the resistors leading to those points.  
If you compare these to the original schem, you will see where I rewired the existing LDR's a little bit to improve performance.  
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