Problem with 832A PP amp

[db!]

I recently finished building my first tube amp (832a PP) and I've already encountered a number of problems. I have managed to solve most of them, but one problem still persists. It may also be worth mentioning that the amplifier design is the very common audiohobbyist 832a PP.

The problem is that the output of the amp is just slightly less than the input. In simple terms, the signal passes through but it's not being amplified. I have changed sources using everything from an ipod to a an already amplified signal. At this point, I think it's a biasing issue with the power tube. I will try probing between the preamp and final stages tomorrow. I also did use a slightly different power transformer though(220VAC instead of the 200VAC specified); I don't think this should affect the circuit too much.

In the mean time, has anyone who has built this amp ever encountered this problem before? If not, how would I go about biasing the tube correctly?

Please bear in mind that my knowledge of tubes is pretty much identifying the components(grid, screen, plate, etc). I have almost no knowledge about their actual workings.

Any information would be greatly appreciated!

[mavric]

I wish I could help, you soundd sincere, however, tube amps take time and and alot of patience, i am sure someone on this site will direct you to fix your problem, I wish I could help.

[aardvarkash10]

Hi DB - can you please post the EXACT schematic as built ie with the values of components that you built with. We can go from there by guiding you through the various test points and the values that you should expect at each of them.

Ya gotta learn somehow !

[db!]

Thanks for the positive responses guys. The schematic can be found at:

http://www.audiohobbyist.com/images/832/832sch.jpg

The amp was built exactly as seen in the schematic, I haven't changed any of the component values. The power transformer is the only part changed; I used a 220vac instead of the 200vac specified...

[aardvarkash10]

ok, can you confirm the voltages at the cathode and the anode of the both sections of the 12at7 please. Take care - the anode should be at close to your B+ voltage and will NOT tickle if you come into contact...

[torrence]

I have been playing around with the 832 myself, and I noticed they have a tendency to oscillate like crazy (at RF frequencies) if you don't have stoppers on both the grid and the screen inputs. Do you have resistors connected right at pins 2, 3, and 6 of the 832 socket (with leads no more than 1 cm)?

[db!]

Originally Posted by aardvarkash10

ok, can you confirm the voltages at the cathode and the anode of the both sections of the 12at7 please. Take care - the anode should be at close to your B+ voltage and will NOT tickle if you come into contact...

The plate voltage on the 12AT7 is 24v while the cathode is at 300v; This strikes me as a bit odd since I would expect the cathode to be at a much lower voltage than the plate. I will try to retake these measurements as soon as I replace the filter cap(Seriously underated). These measurements were taken directly from the socket with the preamp tube disconnected, I'm not sure if leaving them connected would have made a difference.

I have also tried putting a test tone through and scoping at various stages of the circuit. The test tone was a 3000Hz signal at 600mv P-P. I then proceeded to probe the grid of the power tube(pin 2 and 6) and I found it to be 200mv P-P. The final output with a speaker connected is 300mv P-P. The other channel also behaved similarly.

Does this indicate a biasing problem with the preamp? Also, the power tube appears to be amplifying very little(200mv to 300mv). Is this normal/expected? I would expect the final to amplify a bit more than it does...

Thanks in advance for any advice/help!

Please keep in mind my knowledge of tube operation/theory/biasing is very limited. I apologize in advance for any stupid/trivial questions I may be asking.

[db!]

Originally Posted by torrence

I have been playing around with the 832 myself, and I noticed they have a tendency to oscillate like crazy (at RF frequencies) if you don't have stoppers on both the grid and the screen inputs. Do you have resistors connected right at pins 2, 3, and 6 of the 832 socket (with leads no more than 1 cm)?

I did not airwire the components; the amp was essentially built on PCBs with wires connecting the sockets, etc. As for the oscillations, I have not experienced such(probably since my amp isn't amplifiying).

Since you're probably dabbling into RF stuff, you may want to put a parasitic choke on the plate to "choke off" any parasitic oscillations; This is quite common in HF power tubes as parasitics can quickly kill the tube.

[aardvarkash10]

No, this is NOT a biasing issue - your voltages as you have given them are the exact opposite of what you should have - odd, as you suggest.

ok - pull ALL the tubes out. We need to confirm that the circuit is actually wired up correctly. We'll worry about the voltage amp and phase splitter first.

With the tubes out, you should have around B+ voltage at pin 1 and 6 of the 12AT7. You should have ground or near as dammit at pin 3 and 8. You should have ground or NAD at pin 2 and around B+ at Pin 7. All these are referenced to supply ground.

Confirm please. Also, a quick heads up - the pin numbering is from the BOTTOM of the tube ie looking upward...

[Loren42]

Originally Posted by db!

The plate voltage on the 12AT7 is 24v while the cathode is at 300v; This strikes me as a bit odd since I would expect the cathode to be at a much lower voltage than the plate. I will try to retake these measurements as soon as I replace the filter cap(Seriously underated). These measurements were taken directly from the socket with the preamp tube disconnected, I'm not sure if leaving them connected would have made a difference.

I have also tried putting a test tone through and scoping at various stages of the circuit. The test tone was a 3000Hz signal at 600mv P-P. I then proceeded to probe the grid of the power tube(pin 2 and 6) and I found it to be 200mv P-P. The final output with a speaker connected is 300mv P-P. The other channel also behaved similarly.

Does this indicate a biasing problem with the preamp? Also, the power tube appears to be amplifying very little(200mv to 300mv). Is this normal/expected? I would expect the final to amplify a bit more than it does...

Thanks in advance for any advice/help!

Please keep in mind my knowledge of tube operation/theory/biasing is very limited. I apologize in advance for any stupid/trivial questions I may be asking.

Okay, anything that strange tells me that my test instruments may not be connected correctly. You may be chasing your tail.

First, determine you are measuring the voltage correctly. Take a battery or a known voltage source and determine if the meter is working correctly.

Verify that the ground connection for the meter is properly connected.

Verify that the meter is set to the correct function. How many times have I done that?

Once you do that, start with testing or measuring the power supply. Start at the rectifier and move down the chain. Record all the voltages by writing them on the schematic.

If that seems okay, move onto the amp portion of the schematic and record those voltages.

An oscilloscope is a great way to trace signals. You will see exactly which stage is defective very quickly.

Always keep an open mind when debugging and expect the unexpected.

Share your results when you get them.

That circuit is a cathode biased output, so as long as the components are correct and correctly wired in you should be golden. My guess is the amp is miswired, so recheck everything again.