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Dead 12ax7 gain stage  -  Help...
Dead 12ax7 gain stage  -  Help...
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Old 20th March 2009, 02:00 PM   #11
mouldiwarp is offline mouldiwarp  United Kingdom
diyAudio Member
Join Date: Mar 2009
Originally posted by jmbee
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
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:

Click the image to open in full size.
If it works, but I dont know why it works... is that a bad thing??
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Old 20th March 2009, 02:10 PM   #12
Allen Wright is offline Allen Wright  Switzerland
Join Date: Dec 2001
Location: Schaffhausen Switzerland
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)
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Old 20th March 2009, 02:35 PM   #13
mouldiwarp is offline mouldiwarp  United Kingdom
diyAudio Member
Join Date: Mar 2009
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
If it works, but I dont know why it works... is that a bad thing??
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Old 22nd March 2009, 10:25 PM   #14
mouldiwarp is offline mouldiwarp  United Kingdom
diyAudio Member
Join Date: Mar 2009
Brilliant, A big thanks to everyone on the forum that contributed to this thread. 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!!
If it works, but I dont know why it works... is that a bad thing??
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