CascoSieg1
TDPRI Member
I'm an enthusiastic novice with enough tools and knowhow to be dangerous. Have built three imperfect guitar amps I am none-the-less pretty happy with, and prototyping the next one, but getting test results I just don't understand. Scroll down to "crux" of the issue if you want to skip the context...
<background>
Design requirements are basically: I'm aiming for simple to grab and go, with a few onboard fx so I don't have to keep track of pedals, built-in OD option, stereo SE EL84 power section. I also want to try some things I've haven't done yet, so, the current design is a single channel 12ax7 preamp, driving a 2-knob tone stack, with a switchable one-tube OD, into a split 12ax7 driver, into the power tubes. I'd like to implement a Sluckey trem-o-nator, and a Belton Brick for stereo reverb, but really want to build and test things out ahead of time so I can get through the build easier. So, I've breadboarded the fundamental parts of the amp and a single output channel, and all that works Ok.
I realized/discovered that the single input pre, while it's fine for cleans, wouldn't drive the OD very hard, so added a JFET (2N4416) boost to the front of that sub-circuit (a new thing for me to try), with it's power supply based on the Ceriaton 183 (http://www.ceriatone.com/ceriatone/wp-content/uploads/2019/05/OTS-183-50W-010519.jpg)... it needs tweeking but seems to work fine. That B+ filter node - the 5th node in a standard-enough power supply design - makes 25vdc (see schem attached).
Rather than using tubes for the input buffer and make up stages (one for each side) of the stereo reverb, I figured I'd use the TL072 opamps from a donor amp for those various uses, and power them with the same 25v B+ I'm making for the OD boost - 25v is overkill, but below max for those units.
<crux>
I put together one side of an opamp circuit for testing (see schem attached), and used a 25v battery (three 9-volts) since that was more convenient for my workspace at this stage, and passed a clean signal, doubling it as expected in the makeup stage. Then I fired up the amp and connected the power supply IN and GND of my circuit to the amp's 25v node and GND, instead of the battery and nothing works. So I probed for the voltage during operation at pin 8 and got 4.5v from there to GND. What the... ? So I go back and check the battery and for sure it continues to read 25v during operation. So, I check the working JFET boost circuit, and it also continues to read 25v during operation. I Also checked between pins 8 and 4 of a different TL072 - unconnected to any circuit - and got the same behavior: 25v when battery connected and 4.5 from the amp's B+.
Can anyone help me understand what's going on? Why does my voltage go down with the filtered DC supply compared to a battery?
Thanks for any help !!
<background>
Design requirements are basically: I'm aiming for simple to grab and go, with a few onboard fx so I don't have to keep track of pedals, built-in OD option, stereo SE EL84 power section. I also want to try some things I've haven't done yet, so, the current design is a single channel 12ax7 preamp, driving a 2-knob tone stack, with a switchable one-tube OD, into a split 12ax7 driver, into the power tubes. I'd like to implement a Sluckey trem-o-nator, and a Belton Brick for stereo reverb, but really want to build and test things out ahead of time so I can get through the build easier. So, I've breadboarded the fundamental parts of the amp and a single output channel, and all that works Ok.
I realized/discovered that the single input pre, while it's fine for cleans, wouldn't drive the OD very hard, so added a JFET (2N4416) boost to the front of that sub-circuit (a new thing for me to try), with it's power supply based on the Ceriaton 183 (http://www.ceriatone.com/ceriatone/wp-content/uploads/2019/05/OTS-183-50W-010519.jpg)... it needs tweeking but seems to work fine. That B+ filter node - the 5th node in a standard-enough power supply design - makes 25vdc (see schem attached).
Rather than using tubes for the input buffer and make up stages (one for each side) of the stereo reverb, I figured I'd use the TL072 opamps from a donor amp for those various uses, and power them with the same 25v B+ I'm making for the OD boost - 25v is overkill, but below max for those units.
<crux>
I put together one side of an opamp circuit for testing (see schem attached), and used a 25v battery (three 9-volts) since that was more convenient for my workspace at this stage, and passed a clean signal, doubling it as expected in the makeup stage. Then I fired up the amp and connected the power supply IN and GND of my circuit to the amp's 25v node and GND, instead of the battery and nothing works. So I probed for the voltage during operation at pin 8 and got 4.5v from there to GND. What the... ? So I go back and check the battery and for sure it continues to read 25v during operation. So, I check the working JFET boost circuit, and it also continues to read 25v during operation. I Also checked between pins 8 and 4 of a different TL072 - unconnected to any circuit - and got the same behavior: 25v when battery connected and 4.5 from the amp's B+.
Can anyone help me understand what's going on? Why does my voltage go down with the filtered DC supply compared to a battery?
Thanks for any help !!