Adding a gain stage

[fossilshark]

I put an extra gain stage (one half 12ax7)  right before the gain pot, and it made the amp really quiet (turned all the way up only getting low volume). I have a 100k on the plate and an 820 ohm on the cathode with 310v on the plate. Are these values wrong or did i mess up building the addon circuit? The only thing connected to the grid is the 22nf cap off the 12ax7 before it and the plate also has a 22nf cap going to the gain pot.

[MakerDP]

You added a gain stage into that schematic or that schematic is what you built?

If that's what you built, it looks ok... probably a wiring issue somewhere.

If you added another stage to that schematic how you described, you don't have a grid leak path before your new stage. Add a voltage divider, try 470k/470k and go from there so they add up to close to 1M, or another 1M-A gain pot for maximum flexibility.

A 33k grid stopper is always advisable between your grid leak (or gain pot) and the input grid as well.

[fossilshark]

So the output of previous stage is going to 470k, then another 470k going to ground, in between the 2 resistors goes to the grid of the addon stage? I did this and it did not help.

[sluckey]

Are the filaments lit in the tube?

[fossilshark]

Yes, the tube is ok as the other half is being used for the phase inverter

[fossilshark]

I found this online, would this work better?

[sluckey]

I found this online, would this work better?

No. What you describe should work. I suspect a wiring error. What are the voltages on the plate, grid, and cathode of the extra tube?

[fossilshark]

206v on plate
0v cathode
0v grid

I tested continuity betwee the cathode, cathode resistor and ground and everything is wire right, is there supposed to be 0v on the cathode?

[sluckey]

0 volts on the cathode mean the tube is not conducting any current at all. Check resistance from the cathode to chassis. With power off, put one meter lead directly on the tube socket pin (3 or 8, whichever is correct) and the other probe directly on the chassis. What have you?

Also, you have a 100K connected to the plate. Where does the other end of that 100K connect? Seeing a complete schematic would be helpful.

[fossilshark]

Cathode to chassis resistance is 821 ohms. B+ is 386v (would 309v be better?) Sorry about the hand drawn schematic

[sluckey]

Your voltage readings do not agree. Your B+ is 386 and the plate voltage is 206. That means 180 is dropped across that 100K plate resistor. That means there must be 1.8mA flowing through that 100K and that must also flow through the tube and must also flow through that 820Ω cathode resistor. So that would mean there should be 1.47V dropped across that 820Ω resistor. But you measure zero.

Please recheck your cathode voltage.

[fossilshark]

1.504v on the cathode, had the meter set wrong  (not one of my finest moments). Does this mean bad tube? Since im getting some (extremely low) volume i would assume the phase inverter is working and i havnt had a problem with the tubes before.

[kagliostro]

Bad solder ?

Franco

[sluckey]

That tube stage is probably working just fine. You can try another tube. I would have done that even before I posted this thread. But I think you just have something wrong with the way you spliced it into the amp. What you described sounds right, but that don't make it right. I still want to see a schematic of the WHOLE AMP + what you have done. Hand drawn is fine. Heck, you don't even have to use your stencil. Maybe a hi-rez pic would help too.

[MakerDP]

Well he didn't have a grid leak path at first. Can running a triode without one ruin the triode?

I'd try a known working tube before anything else.

[sluckey]

Well he didn't have a grid leak path at first. Can running a triode without one ruin the triode?

I really don't know. But the fact that he had that error just means there's a possibility of another error.

I'm still curious about the schematic. Especially since he had a spare triode in the PI tube.

[MakerDP]

Agreed.. hard to diagnose any further at this point without a full schematic and a good pic of the wiring.

[PRR]

Love the stencil.

But even free-hand crayon scribbles are often 1,000 times better than words.

[fossilshark]

I just the voltage divider upside down  thanks guys for the help

[sluckey]

I just the voltage divider upside down  thanks guys for the help

Really! Both resistors are equal value, ie, 470K. How can it be upside down? To me, upside down would mean you got the two resistors switched around. But since the resistors are the same value it doesn't matter which is which.

Or do you mean something else? Please explain.

[tubenit]

Really! Both resistors are equal value, ie, 470K. How can it be upside down? do you mean something else? Please explain

Yeah, I am curious about that statement also?   

Are you saying it was wired incorrectly and that you extra gain stage is now working properly?

With respect, Tubenti

[Crate50watt]

Love the stencil.

But even free-hand crayon scribbles are often 1,000 times better than words.

Haha, I just spent fifteen minutes on ebay looking for one of those stencils! I had no idea that even existed!