can anyone explain ....

[iknowjohnny]

where exactly the excess gain is causing blocking in the schematic below? Note that as it is in the schematic theres no blocking. But by adding a bit of gain by locating the .0047uf at the plate instead of in the middle of a split load, the amp goes silent with blocking distortion when the 1M gain pot is up past about 3:00. I could just add grid blockers but i don't want to do that if i can do it by changing some value somewhere. but because i don't understand theory well enough i'm not sure whats going one due to the fact my last amp (exactly the same circuit) never did that till the gain was much higher than this. so can anyone tell me in detail what is likely happening?


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

[David Root]

You need a grid resistor on V1B and possibly also the PI entrance, when taking the full plate gain. May not need to be real big, I would start at 10K and take it from there.

Component variations could explain the difference in the two builds.

[RB]

The purpose of the split plate load is to decrease the signal level go to the next stage. I am sure you know that but just thinking outloud. So when you go to the top 47k with the coupling cap you are seeing the full voltage swing which makes the drive to the next stage greater. I dont think this is the actual issue with the circuit. I think the 220k resistor you have connected to the top of 1M gain pot should be connected to the grid of that tube you are driving not the pot. I think the way its wired you do not have a fixed grid load for that tube and its making it impossible for tube to bias properly when you change the gain setting. It looks like the pot is acting like its a 200k or so pot because you have it wired in parallel with the 220k effectivly leaving the tube without a fixed grid load or as some call it around here a grid leak resistor. If you ever left out the grid load on a stage like I have on ocasion you would experiece somethin similar to what you are descbing. Just a wild ass guess but thats what i woud change frst.

[iknowjohnny]

Sorry about that, it's a mistake....the 220k isn't there in the amp, it was from a older schematic i used to make this one. But as far as a load, the pot does that, right? I mean, most circuits like this just have the pot and thats it, which is what i have now.

[iknowjohnny]

Well, i figured out that for whatever reason, if i went to the plate with the cap on V1A instead of dropping the gain with a split load, the blocking was horrible and i couldn't even have as much gain as i wanted. yet in the other amp with the same circuit i had to reduce the gain to get it where i wanted ! I could even use no split load on both sides and it wouldn't get blocking ! I have no idea why, but in any case I then put the cap on V1B to the plate instead of using the split load there, and wham.....gorgeous tone with all the gain i want ! The gain is actually the least of the goodness that resulted. I have had, and then then lost that super fluid resonant and harmonically rich tone that you get when you know everything is in balance, and i captured that again with this simple change. A change that if i did on V1a instead of B caused horrible blocking. this time i'm leaving it, assuming after a few days i still feel it's the shite. Right now i'm in tone heaven. this is how this amp is supposed to be.