I'm working with a small 2 tube amplifier (12ax7, 7868). The supply voltage to the preamp is the correct value (280 V) from the schematic. The voltage on the first 12ax plate matches as well. However the second plate voltage is over double (260 V) what is written on the schematic.
I have tried two tubes (the original and a previously used tube) and swapped out plate resistors, but experimentation has yet to alter the plate 2/pin 6 voltage. Both coupling caps have been checked (C2, C4) and appear to be blocking dc.
I'm currently lost for why the same voltage can go across the same resistors and turn out 4 times higher on one side of my 12ax7s.
I have a few more questions once I jump this hurdle and the ultimate goal is to make the distortion of this amp usable. I am more than happy with the clean tone, but the amp breaks up early and is frighteningly awful sounding at that point.
Here's the schematic w/ notes
-phono and treble circuits are currently removed
-the R9 & R11 negative feedback circuit is removed
-I wired pot R5 as a master volume after C4 and can now generate terrible distortion at low volume
-diodes have been swapped with new
-the filter caps have not been replaced, but all voltages save the 110 V are good
http://imageshack.us/photo/my-images/819/bog1.png/
[img::]http://imageshack.us/photo/my-images/819/bog1.png[/img]
same HT, different plate voltages
Moderators: pompeiisneaks, Colossal
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markr14850
- Posts: 204
- Joined: Mon Jun 18, 2007 6:16 pm
Re: same HT, different plate voltages
What are the voltages at your cathodes?
Re: same HT, different plate voltages
There is a good chance you've either got a miswire or you have the wrong value resistor on the plate of the second triode. Lift one side of the 470K plate load resistor from the circuit and put a meter on it. There is a good chance it is 47K. I suggest this because you are getting about 10% of the expected voltage drop. I serves to reason that the dropping R is 10% of what it is supposed to be. There could be other causes. Give this a try.
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azatplayer
- Posts: 556
- Joined: Thu Feb 26, 2009 7:59 pm
- Location: Great Southland
Re: same HT, different plate voltages
Thats a real odd circuit. With 0 voltage at the cathode and the plate load and grid you have there, the first stage has a gain factor of over 86! And the second side has a gain factor over 76! Thats massive.
Very unusual feedback circuit too.
It would sound like a fiz factory.
Not familiar with the 7868, dunno its requirements, but that is pretty unusual 12AX7 operation.
I'd add a cathode R on V1a, that determines what you are seeing, plate current/voltage rises and falls based on how its biased.
And 470K plates are very high, typically see that on older octal triodes, B15 Ampegs and early tweed era stuff. Not oft in 12AX7s.
Very unusual feedback circuit too.
It would sound like a fiz factory.
Not familiar with the 7868, dunno its requirements, but that is pretty unusual 12AX7 operation.
I'd add a cathode R on V1a, that determines what you are seeing, plate current/voltage rises and falls based on how its biased.
And 470K plates are very high, typically see that on older octal triodes, B15 Ampegs and early tweed era stuff. Not oft in 12AX7s.
Re: same HT, different plate voltages
V1a is a "mu amp" which develops gain approximating the mu of the tube. What little bias there is is the result of electrons getting stuck on the grid (because of the larger than normal grid resistor). It's there strictly for maximum gain with a small input signal. It's noisy and easily overloaded so you can't drive it with a large signal; probably not even a humbucker.
You shouldn't expect the plate voltage to be the same just because they share plate resistor values; the other half of the equation is the value of the cathode resistor. On V1a there isn't one so a lot of current flows and you get a large voltage drop across Ra at idle. V1b will have less current flowing, therefore a smaller voltage drop, but given what you are seeing, I'd check the value of R8. This circuit will probably not ever clean up very much; it was not designed for versatility.
You shouldn't expect the plate voltage to be the same just because they share plate resistor values; the other half of the equation is the value of the cathode resistor. On V1a there isn't one so a lot of current flows and you get a large voltage drop across Ra at idle. V1b will have less current flowing, therefore a smaller voltage drop, but given what you are seeing, I'd check the value of R8. This circuit will probably not ever clean up very much; it was not designed for versatility.
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wallofsleep
- Posts: 2
- Joined: Mon Aug 01, 2011 8:01 pm
Re: same HT, different plate voltages
Great responses so far! I had completely neglected to take into account the different cathode resistances.
One of my next questions was going to be about the operating point of the 12ax. These values look to put the tube at a very low area of a plate characteristics graph.
At one point I had reworked the cathodes to mimic a champ and gotten much more clean volume to play with but have since reverted because I favor the tone from the grid-leak. It really does sound sweet as long as the amp is played bellow 10 o'clock. It sounds like I may have an adventure trying to make a usable distortion. The current distortion is more along the lines of 'flapping' than fizzing.
One of my next questions was going to be about the operating point of the 12ax. These values look to put the tube at a very low area of a plate characteristics graph.
At one point I had reworked the cathodes to mimic a champ and gotten much more clean volume to play with but have since reverted because I favor the tone from the grid-leak. It really does sound sweet as long as the amp is played bellow 10 o'clock. It sounds like I may have an adventure trying to make a usable distortion. The current distortion is more along the lines of 'flapping' than fizzing.