Tweet
Well,
I am overhauling this old Univox-japanese amp from the 60's. It has reverb, too. No model number. It has 3 inputs (2 low gain & one higher gain). It also has tremolo.
The problem is the input sensitivity is verrry low. With a guitar you have to turn the volume all the way up & you still aren't even 1/2 way to 15watts.
I've replaced all the electrolytics as well as all the oil base caps. I'm 99.99% certain everything was replaced correctly. I also double checked.
Unfortunately, it has been too long since I originally checked it out to recall how it originally worked when it came in.
The amp has 3x 12AX7's. 1 & 1/2 of them seem to exclusively work the reverb section.
For power, it has 2ea 6973 power tubes. It will put out about 15watts at clipping. I have no schemo & have drawn a portion of it.
The first stage 12AX7 has a gain of about 15 if I'm calculating that correctly (Av= Vout/Vin?) been too long to really remember that stuff. Basically with about 400MV on the grid, I get 6volts p-p on the plate. I'm thinking that is wayyyy low.
Another unusual item is the plate voltage. It's about 55Vdc. Seems wayyy low to me, but the 220K load resistor is good & goes to a source of about 200V (when loaded). The cathode is bypassed & has about 1.8Vdc on it. that seems normal to me.
The 2nd stage...the tone circuit recovery...has equally low plate voltage (about 75Vdc) through a 100K plate load resistor that is also good. It terminates to a source of about 150V loaded (that sounds low). That stage has a gain of about 15 (.1vp-p in & 1.5v p-p out).
This stage then feeds the PI, which is basically a gain of 1 ckt using just the one triode section. The other half seems to be the tremolo ckt.
What could I be missing here. Dang, I wish I had paid better attention back in tech school in 1973. They were still teaching tube design along with trasistors, too.
Bruce, any ideas? Thanx, glen
I am overhauling this old Univox-japanese amp from the 60's. It has reverb, too. No model number. It has 3 inputs (2 low gain & one higher gain). It also has tremolo.
The problem is the input sensitivity is verrry low. With a guitar you have to turn the volume all the way up & you still aren't even 1/2 way to 15watts.
I've replaced all the electrolytics as well as all the oil base caps. I'm 99.99% certain everything was replaced correctly. I also double checked.
Unfortunately, it has been too long since I originally checked it out to recall how it originally worked when it came in.
The amp has 3x 12AX7's. 1 & 1/2 of them seem to exclusively work the reverb section.
For power, it has 2ea 6973 power tubes. It will put out about 15watts at clipping. I have no schemo & have drawn a portion of it.
The first stage 12AX7 has a gain of about 15 if I'm calculating that correctly (Av= Vout/Vin?) been too long to really remember that stuff. Basically with about 400MV on the grid, I get 6volts p-p on the plate. I'm thinking that is wayyyy low.
Another unusual item is the plate voltage. It's about 55Vdc. Seems wayyy low to me, but the 220K load resistor is good & goes to a source of about 200V (when loaded). The cathode is bypassed & has about 1.8Vdc on it. that seems normal to me.
The 2nd stage...the tone circuit recovery...has equally low plate voltage (about 75Vdc) through a 100K plate load resistor that is also good. It terminates to a source of about 150V loaded (that sounds low). That stage has a gain of about 15 (.1vp-p in & 1.5v p-p out).
This stage then feeds the PI, which is basically a gain of 1 ckt using just the one triode section. The other half seems to be the tremolo ckt.
What could I be missing here. Dang, I wish I had paid better attention back in tech school in 1973. They were still teaching tube design along with trasistors, too.
Bruce, any ideas? Thanx, glen
Comment