Tube pedal build issues?

[tackleberry]

I built a pedal along the line of the valvecaster. Only I used a 6GM8 tube instead of the 12au7. The 6GM8 is for use in car radios its a 6v tube with max at the plate of 30v Im running off a 15v regulator. Im using cathode bias with, if my math is correct, 820ohm cathode resister and 100k plate at 15v. But theres no signal getting thru. I tested some voltages, 6v to the heater and 12.5v at the plate resisters. The voltages for the second stage at the plate is 12.5v and stays steady. But the first stage when the tube warms up the plate drops steadily from 12.5v until its next to nothing. I tried all 3 tubes I have they do the same thing.

[frequencycentral]

Can you post a schematic of exactly what you have? How are you deriving the heater voltage? The tube wants ~6.3v @ 330ma. The pinout looks the same as the 12AU7. The plate voltages you quote seem unusually high when compared to Valvecaster plate voltages, which are about 25% of the power supply at the first triode and 66% of the power supply at the second triode. Plate voltages that high on a 12au7 would suggest too low plate resistor values. Why not test it a stage at a time? Replace the first. stage plate resistor with a 1M pot and tweak it until you get a decent signal after the coupling cap, then do the same with the second stage. 

http://www.classiccmp.org/rtellason/tubedata/6GM8.pdf

[Renegadrian]

Very strange issue indeed...

[frequencycentral]

Very strange issue indeed...

Yeah, this is the wierd bit:

But the first stage when the tube warms up the plate drops steadily from 12.5v until its next to nothing.

[Ripthorn]

Seems like there might be a short of some sort (through a resistor or something).  Not sure though, never seen it myself.

[tackleberry]

I using a 24vAC transformer rectified to DC then a 15v regulator for the plates and a 1A 6v regulator for the heaters. I have other regulators so could run the plates at a lower voltage if needed. Just strange the 1st stage loses voltage this way but the second doesnt. Id like to post a pic of the schematic but cant seem to get pics to post on this site. I see some tube pedals use cathode bias and others use fixed. Is it better to use 1 or other with low voltage. 

[Ripthorn]

Cathode bias is nice because it will automatically adjust when the plate voltage is changed.  Fixed bias requires a fixed plate voltage, from what I understand.

[Lurco]

you`re sure it is a 100k on the first plate? (colourcode?)

[tackleberry]

Yep checked same color code even checked em with a meter 100k for both plates. Checked the bias set for both stages same codes same resistance. Swapped the input wire to pin 7 and got signal to the amp. So only the second stage works. Its not a complicated thing, input - .022 cap - 1M to ground then pin 2. And there is a .022 cap between stages then out to a tonemender circuit for vol. 

[Ripthorn]

Maybe test the resistance between the supply side of the plate resistor and the ground side of the cathode resistor.  I would probe around and see if your juice is somehow going somewhere without telling you.

[petemoore]

  Tube Pinout differences ?
 

[frequencycentral]

  Tube Pinout differences ?
 

Nope

The pinout looks the same as the 12AU7.

http://www.classiccmp.org/rtellason/tubedata/6GM8.pdf

[tackleberry]

Doesnt have some strange pin out. I will mess with it more when I have the time. Leaking thru the from the plate to the grid has crossed my mind. The second stage just has a cap from 1st to 2nd no resister to ground like the first stage does. When I had it working with the first stage bypassed it had a ton of distortion not what I was looking for. I was looking at more of a clean to warm my signal. Might be time to look at other options.

[MohiZ]

I think there's something wrong with your math. I'd say try 6k8 as the plate resistor and 1k2 resistor at the cathode for this tube. That oughta put you close to center biasing.

The way you have it now, the first stage actually starts to "work" when the tube warms up, the plate current tries to rise to the quiescent point set by the cathode resistor and due to the massive (for this tube) plate resistor the voltage drops to almost zero. The second stage won't work properly if you don't have it dc coupled to ground by a cathode resistor.

[Jered]

 I think mohiz is right. Try the resistors he recommends.