DIY Tube Preamp

disneyjoe7

I been reading and looking around seems maybe your grid op amp maybe right, but you need a cathode resistor to bring it to ground. Note this resistor may act like a bias resistor to tube. The grid maybe act a low near ground and something just under ground (negative voltage)

Serendipity

disneyjoe7 wrote: »

I been reading and looking around seems maybe your grid op amp maybe right, but you need a cathode resistor to bring it to ground. Note this resistor may act like a bias resistor to tube. The grid maybe act a low near ground and something just under ground (negative voltage)

Yes, it appears that if I remove the second op amp to prevent it from draining the grid of the tube, there might be a chance of success.

Right now I am working with Electronics Workbench and PSPICE to see if I can get a simulation that functions properly; there's no use in wiring up the components if I know it's not going to work theoretically.

disneyjoe7

That a voltage reading between Cathode to Plate of tube, and report back. Also give me the plate voltage to ground.

disneyjoe7

I can't help you if you can't answer my questions.

Serendipity

disneyjoe7 wrote: »

I can't help you if you can't answer my questions.

Sorry, with other things going on I got a little sidetracked this week. I haven't done any further testing on the build and to a large extent won't be in front of a computer much the next few days...

disneyjoe7

Still need to figure out your tube design? Still need Plate, Cathode, and Grid voltage to ground.

Thanks for the update.

disneyjoe7

This is my thinking the plate voltage is 24vdc right? Then the cathode pin needs to be 12vdc, and you need to resistor that pin to ground what value don't know but what the tube draw to be class A amp then the resistor needs to be 5w or better and maybe 240ohm or so. The grid pin needs to be about 11.5v or -.5v under the cathode pin. You may need to rate up the + pin on the op amp so the grid voltage in correct or so.


Good luck lets know.

Serendipity

disneyjoe7 wrote: »

This is my thinking the plate voltage is 24vdc right? Then the cathode pin needs to be 12vdc, and you need to resistor that pin to ground what value don't know but what the tube draw to be class A amp then the resistor needs to be 5w or better and maybe 240ohm or so. The grid pin needs to be about 11.5v or -.5v under the cathode pin. You may need to rate up the + pin on the op amp so the grid voltage in correct or so.


Good luck lets know.

I added the resistor, not sure about the value yet but will determine it once I get back to the lab and run the simulation. I'm on vacation right now, so sorry for the late replies. Been a while since I touched this stuff as other things keep getting in the way. I will post back when I find out.

Serendipity

Update coming soon...

Looks like this is working out after all. Yay!

disneyjoe7

Like to see your new schematic.

Serendipity

disneyjoe7 wrote: »

Like to see your new schematic.

Ok, I added a plate resistor. And this time I am using only one opamp - the second one turned out to be detrimental to the circuit. You were right, along with what the others said in this thread. Thanks a lot for your help! I'll post the simulation when I get back to the lab. Right now I am on Winter Break so I have no access to classrooms/lab equipment.

Serendipity

disneyjoe7 wrote: »

Like to see your new schematic.

Basically I found out from the simulation software that my bias point was out of range. After that, everything should be smooth sailing.

Serendipity

Just an update - Bias point is correct, still not working.

disneyjoe7

Need to get a scope and find where the signal is dropping out.