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Dead 12ax7 gain stage - Help...

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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)

An externally hosted image should be here but it was not working when we last tested it.


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??
 
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. 100µA 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
 
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....
 
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 :)
 
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.
 
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.!!
 
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 :bawling: ) would there be any obvious symptoms to watch out for with a damaged grid?
 
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 ùF , probably also increase C3.
good luck
jmb
 
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.
 
jmbee said:
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 ùF , probably also increase C3.
good luck
jmb

jmbee, I am using the 1uf cap assist in boosting the upper frequencies (as is common in guitar amplifier pre-amp stages) otherwise the sound can be a little flat.

As for the addition of a diode between the anode follower and cathode of the second triode, here is my updated schematic. There are references to the addition of a 47k resistor in the connection, but, there are also references to leaving it out of the circuit. I will go with the original recommendation from the forum unless someone can help me understand why I might need it :)

New Circuit:

An externally hosted image should be here but it was not working when we last tested it.
 
If you have 0.1V on the first cathode, then that tube has 100uA running through it.

Therefore, if the anode load R is really 100k, then you should only lose 10V in it, giving a anode volatge of 250 - 10 = 240.

But you only have 19V, so I suspect your 100k anode R isn't 100k at all, more like 2-3megohms. Check it out, resistors can be wrongly marked, or go open from handling.

Regards, Allen (Vacuum State)
 
I had thought somewhere along thos lines... I will take a look (I am going to remove the erisitor from the back of the valve socket) and re-wire with a new resistor on the turret board. I have new bag new 100k resistors somewhere. I did check the resistor value before soldering it in, but, I am not always that tidy so I could have picked up the wrong one... I am fairly sure it is OK though from the marknigs (I dont currently own anything bigger than 1M that I am aware of) unless it has been damaged in some way.

I am building a great list of to-do's here. Thanks all, keep em coming please.

When I get to work on this I will run voltage checks at all points with and without valves in place and tie them to each test.

This really is a great place for people like me to learn. Thanks
 
Brilliant, A big thanks to everyone on the forum that contributed to this thread. :grouphug: With the exception of leaving out the cap on the cathode of the first half of the stage it is in as per the above schematic and working a treat. I am almost embarassed to say what the problem appears to have been, but It was kind a wiring problem. Since re-wiring the valve socket the voltages all seemed to sort themselves out but still no sound... A little tweaking and studying the schematics and I realised the problem as not with the stage itself but how I was taking the output from the stage back into the EQ circuit.

Now I have to work out how to try and eliminate the slight background hum I am picking up in the extra gain stage. Time to dig out some shielded cable I think, and unfortunately because of a lack of space in the chassis I have had to position the extra circuit close to and parallell to the AC supply for the heaters giving me a bit of cyclic hum... Perhaps a simple diode recifier to convert to DC would be a great call here.


Thanks again!!
 
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