Switching plate resistors on the fly

[daz]

The second stage in my cascaded pre has a split load plate. But there are times i'd like more gain and i'd like to put the coupling cap back at the plate instead of between the resistors. But i want to know if there is a way to accomplish this without having to turn the amp off or to standby first. Or is it not gonna happen? (i'm thinking the latter but I had to ask)

Oh, and as to the thread title, i couldn't word it right in the small space so i just said switching resistors. But they would actually not be aswitched, just the coupling cap. But i imagine thats still gonna pop like crazy and cause who knows what else.

[Chuck H]

It's actually not that hard. Don't switch the cap placement. Run two coupling caps. One from either point on the plate load resistor string. Include a largish load on the output of each cap so that there will be a ground reference but it won't detriment the tone as it is. Then just switch between the output and loaded ends of the caps. Does that make sense? Did I describe that well enough?

Chuck

[cbarrow7625]

I second that Chuck. That is the way to go. For added versatility you could wire a pot between the two coupling caps and make it a variable gain control. That would be trickier to implement and get sounding right. Switching between two caps is the easiest.

[daz]

Ok, but you said a load at the cap output, but i already have that in my preamp design so i think it's ok as is in the pic below, no?

http://i24.photobucket.com/albums/c39/dazco/plate.jpg

[Chuck H]

Not like that. The way you have it the switch is still making/breaking the high voltage and the caps hang on end so they will still pop.

Do it like this. Notice that the caps are part of a closed circuit on both ends and they are located in the circuit in a way that isolates the switch from the high voltage. The value of the original load resistor (now marked ?) would be changed so that your end result has the same gain as before you added the two 1M load resistors. Probably around 120K.

Chuck

[daz]

Thanks for that Chuck. The only thing i'm unsure about is that with a 1m resistor in parallel with the 100k, according to my electronics calculator this would be over a meg. That would leave the amp with way too much gain and blocking distortion. I chose the 100k because it was about as low as i could go w/o killing the tone because there was much too much gain otherwise. I don't want to alter the tone because the amp is perfect as is. Is there another way i can do this w/o changing the ground ref to the grid so much?

[cbarrow7625]

Daz,

1M in parallel with 100K is about 91K. Resistance lowers when put in parallel. You will likely have to increase the grid resistor to around 120k (with 2 x 1M in parallel) to get right back to around 100K total resistance. If you leave the 100K and just add the 2 x 1M, you will reduce the grid resistor to somewhere around 85k or so (don't have time to calculate exactly at the moment). This will be beneficial to help stop blocking distortion.

[daz]

i know !!!! I just realized when i checked "parallel in the program i guess the cursor wasn't centered and it stayed at "series". so my mistake. I thought that was odd but i didn't even think to double check where it had not changed when i checked it. Doh ! Thanks.

[cbarrow7625]

It happens. Let us know how it turns out.

[daz]

Ok. Won't get to any of the things i've been asking about till next week, but i'll let ya know. Thanks.

[Chuck H]

The 100k grid load (marked in my schem with a ? because I knew it would need to be changed) is actually only in "parallel" with one 1M load resistor at a time. But, with the .022 cap and it's own 1M load on the either end of the plate resistor string, there will still be some loss because of both 1M loads. The 120k value I mentioned for the grid load was just a near guess. I think listening tests and a temporary pot in it's place would be the best way to choose the new value.

FWIW I've done the dual output/split plate load thing before. It was a really cool circuit that used a dual ganged pot for the volume control. The "B" pot on the dual gang was used to graduate from one circuit on the plate resistor string to the other. I acually used different value pots and simple voicing circuits for each. So the amp could "morph" as the volume control was increased from pretty shiny clean tones down lower to more agressive grind in the middle and big fat distortion at full tilt. The guy who I built it for says it's his favorite amp

Chuck