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Old 19th March 2009, 10:18 PM   #1
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Default Dead 12ax7 gain stage - Help...

I have built a 5W valve amp using a single 12ax7 pre-amp and el34 power valve. All works well but I really want to put in another gain stage both for the sound and the experience of doing it.

I am learning more and more every day on this project but I have hit a problem that has me stumped.

Here is my diagram (with some voltages marked in red taken with valves inserted in the socket - there is also a picture of my circuit / peg board design)

Click the image to open in full size.

In addition to the circuit board I have jumpered valve pin numbers 6 and 2 using bare wire, and, 1 and 6 using a 100k resistor. I am applying 250v DC load directly to pin 1.

I have included the additional stage by tapping into the output from mu existing pre-amp stage taking the feed frrm the anode side of the valve and am returning the output form my new stage directly into a failry standard tone stack. The thing that confuses me (as you can see in the diagram) is the voltages seem completely wrong on the first triode. When I run the circuit with this stage connected up I loose all output form the amp. If I run the amp with the input disconnected and the original signal path in place (and the output of this stage tied to the tone stack) I hear a slight buzzy nature in the sound but overall the output is weak and lifeless. (does that make sense). I am running 6.5V to the heaters but can not for the life of me see what is going wrong here. I guess an anode voltage of 19V is probably my problem here but I cant work out how i loose 230V over the 100K resistor in this circuit..

Any help... Or idea's where I am going wrong... or where to look next perhaps??
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Old 20th March 2009, 10:48 AM   #2
Funker is offline Funker  Germany
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Hi Mouldi,


you wrote:
..I guess an anode voltage of 19V is probably my problem ....


Are you sure that the Amp is properly wired? More positive volts on grid against kathode means normally the system is turned on "full current" and that means the voltage on R6 should reach its maximum of about 200 volts.
in your case the grid is positive but there is only a very low current in R6, that make me wonder. The input amp seems similar to me. 100A in the Anode resistor should give a voltage drop of 10 V on R2 , in other words 240V on the plate .

Look for a faulty wiring a defective socket and so on. I built serveral gain blocks like this . At 250V + B with your values should exspect about 150V at the plate and 1V at the kathode . The kathode Voltage at the 2.nd system must be about 1 volt less as the voltage at the grid .

Did you checked the valve ? is they ok.
How about C4 , probably short ?
How the voltage on the socket with the valve being pulled ?


regards from Hamburg
Wolfgang
DF6ZC
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Old 20th March 2009, 11:06 AM   #3
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OK, I have found some great newbie resources online (i dont know if I am allowed to post them here for anyone interested, but if I get the Ok from the moderators then I will add them to this thread) and I am thinking now that the problem with my first triode is that I am failing to get the grid -'ve relative to the cathode. This will be causing significant electron flow through the grid and into the Anode plate causing a reduction in the voltage at plate (and relative to the supply)?? am i right??.

I am also thinking that the grid and cathode resistor values contribute to the overall gain of the stage.

What I have not worked out yet is how / why my cathode voltage is so small. I guess I should probably increase the value of Rk relative to Rc (or reduce Rc relative to Rk) to get the grid pushing the electrons back and slowing the flow through the valve.

The more I think about this the more I think I have something wrong....
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Old 20th March 2009, 11:15 AM   #4
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Thanks for the reply,

The voltages on the socket with the valve out look ok (seeing 250+ on the anode)

But I will re-check tonight when I get chance. I am confident that my power is coming from the right place as I am seeing the right voltage on the second triode plate.

I just dont understand why I am seeing such a low V difference between grid and cathode... Am i right in that this could be causeing electron flow and reduce the voltage relative at the anode??

As a test I will pull the cap from the Ck resistor to make sure there is not a problem there. Reduced gain and lower distortion but if i get some sound, it is an improvement
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Old 20th March 2009, 11:25 AM   #5
SY is offline SY  United States
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There's a bunch of problems that you could have. First, disconnect the cathode follower to see if the voltages come back to normal on the first stage. If it does, you've cut the problem in half. If it doesn't, recheck your wiring. Disconnect the cathode bypass cap and make sure that the resistance from cathode to ground is actually 1k.

Second, once you're done troubleshooting this, put some protection on the cathode follower grid so that you don't kill the tube at turn-on (grids don't like 250V suddenly whacked on them); a reverse-biased diode from grid to cathode is the common method. And make sure that the heater is biased up high enough not to abuse the heater-to-cathode voltage rating.
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Old 20th March 2009, 11:37 AM   #6
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Thanks, will look at that. As for the reversed biased diode between cathode and grid, Is this necessary if I have a standby switch to allow the heaters to come to temperature before I load them...

I have seen schematics in which the cathode bias is provided by a LED instead of a resistor but not got the section on grid protection yet. On that front, it doesnt seem common in the circuits I am finding online. Is it less common to bias using diodes, if so, why?? (I presume a standard LED would give a 1.7V -ve bias to the grid putting it a lot closer to the value I would be expecting in this circuit??

I will add into the diagram what I think you mean before I go ahead and do anything (having to learn fast here). I will pull the cathode cap tonight and run some tests valve in and valve out, also disconnecting the cathode follower as suggested. I will re-post my circuit with the new measured voltages.

Really appreciate the help.!!
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Old 20th March 2009, 11:39 AM   #7
SY is offline SY  United States
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Quote:
Is it less common to bias using diodes, if so, why??
Yes and tradition.

The protection diode is a 5 cent part. And can save a relatively expensive tube.
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Old 20th March 2009, 12:12 PM   #8
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OK, found a description of the grid protection at the Valve Wizard site. He describes a 1n400* diode in series with a resistor between grid and Cathode. I am slowly understanding what he has written and it makes sense. I will look it up in 'the Valve Bible', that is commonly referenced here, when I get home to see if I can find it referenced in there.

What I will do is I will completely re-wire the socket and move the connections away from the back of the valve socket and put them onto a board. That way it will make things easier to swap out and test without running the risk of a short across the socket terminals.

Apart from the valve not working (which it doesnt already ) would there be any obvious symptoms to watch out for with a damaged grid?
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Old 20th March 2009, 12:31 PM   #9
jmbee is offline jmbee  France
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hello Mouldy,
I think your diagram is good, just a mistake in wirering because the current
running in R2 has to go to the cathode , nowhere else.
But the value of C4 leads to Fc near 160 Hz must be ten times bigger at least
10 to 47 F , probably also increase C3.
good luck
jmb
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Old 20th March 2009, 12:35 PM   #10
SY is offline SY  United States
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Mouldi, the grid can fail by shorting to the cathode (easy to detect) or by going partially open (hard to detect). Separate the two stages and see which half your problem is in. If it's in the CF stage, then it might be worth subbing in another tube after donning the recommended protective gear.
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