I just picked up a Knight 3235B PA amplifier at a local surplus shop for $25. Does anyone have any info on this amp? I'd like to get a schematic and service it... possibly turn it into a guitar amp.
I've repaired and built amps in the past from schematics, but I'm no-where near comfortable knowing what to expect without a schematic. I really like this amp, and don't want to smoke it. So I'd like to put it on a variac and test it at various points... but without the schematic I'll not know what to expect.
Also, I doubt this amp is unmodified. It came from my state universities electrical engineering department, which has the top lab in my state. I know that at the very least it's had a 3 prong plug added. Whomever did it was pro, their work was impeccable. They replaced at least a few wires, I'm not sure about components. The caps all look original, but I'm afraid to power it up before I have some test points to check.
Also, I doubt this amp is unmodified. It came from my state universities electrical engineering department, which has the top lab in my state. I know that at the very least it's had a 3 prong plug added. Whomever did it was pro, their work was impeccable. They replaced at least a few wires, I'm not sure about components. The caps all look original, but I'm afraid to power it up before I have some test points to check.
Under the skin tube amps are pretty much all the same idea. They are single ended or push-pull output stage. If push-pull, it will have a phase inverter stage, of which there are only a small number of types. The gain stages are usually pretty similar and there are a few styles of tone control circuits.
I say this because if you are familiar with many amps, you can pretty much draw a push pull pentode stage already and fill in the values from your amp. Either there is a cathode resistor to the power tubes or they are grounded and a fixed bias voltage supply exists. You can tell by looking at the parts if your phase inverter is a split load type (concertina) or a long tailed pair type, or something else. Pick a drawing of something else with the same type phase inverter, and you will see the form your drawing should take, but fill in the values. It really isn't hard.
Certain tubes I have in my head, but if I see a 6SK7, say, I dont remember the innards, so I look it up in the book, and draw just the tube on the page, then I can draw small parts as I go around the socket pin by pin.
In the absence of drawings, pull all the tubes. I prefer a variac and current meter, but the classic "light bulb limiter" will work also. I would power on the unit tubeless, and ramp up the variac watching for current draw. There should be very little. Or your bulb tester should remain dim. If so we move on. If it draws current, we might isolate the power transformer to see if it draws current by itself.
But assuming we can put the full 120v main on the amp, we then check to see that heater voltages make it to the sockets, we look for any bias supply if it uses one, we verify the high voltage secondary. If it has a solid state rectifier for B+, then check to see if B+ voltages are present, and relatively free of ripple.
If it uses a rectifier tube, install only it. Bring the amp up slowly again, does the addition of the rectifier cause issues? If you can bring it all the way up, check B+ voltages in the amp.
ANything is possible, but for the most part, small tubes like the preamp uses don't have a good way to burn something up. So at this point I'd probably install the preamp tubes, and bring the amp up again.
There will usually be an initial current surge as the caps charge and the tubes heat. But the current should quickly settle down, or your bulb should flash some but dim back down.
You can do all that without a schematic.
I say this because if you are familiar with many amps, you can pretty much draw a push pull pentode stage already and fill in the values from your amp. Either there is a cathode resistor to the power tubes or they are grounded and a fixed bias voltage supply exists. You can tell by looking at the parts if your phase inverter is a split load type (concertina) or a long tailed pair type, or something else. Pick a drawing of something else with the same type phase inverter, and you will see the form your drawing should take, but fill in the values. It really isn't hard.
Certain tubes I have in my head, but if I see a 6SK7, say, I dont remember the innards, so I look it up in the book, and draw just the tube on the page, then I can draw small parts as I go around the socket pin by pin.
In the absence of drawings, pull all the tubes. I prefer a variac and current meter, but the classic "light bulb limiter" will work also. I would power on the unit tubeless, and ramp up the variac watching for current draw. There should be very little. Or your bulb tester should remain dim. If so we move on. If it draws current, we might isolate the power transformer to see if it draws current by itself.
But assuming we can put the full 120v main on the amp, we then check to see that heater voltages make it to the sockets, we look for any bias supply if it uses one, we verify the high voltage secondary. If it has a solid state rectifier for B+, then check to see if B+ voltages are present, and relatively free of ripple.
If it uses a rectifier tube, install only it. Bring the amp up slowly again, does the addition of the rectifier cause issues? If you can bring it all the way up, check B+ voltages in the amp.
ANything is possible, but for the most part, small tubes like the preamp uses don't have a good way to burn something up. So at this point I'd probably install the preamp tubes, and bring the amp up again.
There will usually be an initial current surge as the caps charge and the tubes heat. But the current should quickly settle down, or your bulb should flash some but dim back down.
You can do all that without a schematic.
It's a bit late, so I'll not be doing all that tonight. 🙂
I did find this: Knight Public Address System KN 3235C Ampl/Mixer Allied Radi
which is almost identical to mine other than the color. Mine's a 3235B and that link is to a 3235C and the only thing difference is mine's silver and black and has old school screw-on connector instead of 1/4" jack inputs.
I can't dig it out now without waking the wife but those tube designations look right:
2x 12AX7A
1x 6AV6
2x 6L6GC
1x 5AS4
and yes, it's tube rectified.
That big can capacitor is my first concern. I think I'm just going to order replacements before I even touch it.
I did find this: Knight Public Address System KN 3235C Ampl/Mixer Allied Radi
which is almost identical to mine other than the color. Mine's a 3235B and that link is to a 3235C and the only thing difference is mine's silver and black and has old school screw-on connector instead of 1/4" jack inputs.
I can't dig it out now without waking the wife but those tube designations look right:
2x 12AX7A
1x 6AV6
2x 6L6GC
1x 5AS4
and yes, it's tube rectified.
That big can capacitor is my first concern. I think I'm just going to order replacements before I even touch it.
Looking at the schematic Jazbo8 posted, if you turn it into a guitar amp, you will definitely want to add screen grid resistors for the 6L6s. Otherwise this amp is likely to destroy output valves when overdriven, because there is nothing to limit screen grid current.
This schematic also uses grid leak bias (10M resistors from grid to ground for the 12AX7 stages). This is an old and unreliable biasing method, I suggest lowering those 10M resistors to 1M, and adding an appropriate cathode resistor to set the cathode (and therefore anode) current.
-Gnobuddy
I found the Schematic!!! It's not the best quality but it's definitely readable. It still needs the grid resistors, if anyone has any other tips, I'd appreciate it!
edit: that quality was awful, I fixed it.
edit: that quality was awful, I fixed it.
Last edited:
- Status
- Not open for further replies.