Valvecaster - LM317T - 12au7 - no voltage when connected

[kissack101]

Hi,

I'm getting back into building pedals after a long time away. I've outlined below the problem I'm having. Attempting to drink the internet firehose (including attempting to read as much as possible of the ~180 page Valvecaster thread on this forum) suggests that the problem could be that I need to "soft start" the tube but I've never built anything with tubes before so I'm not sure what this means in practice.

The pedal is intended to be a birthday gift (for this weekend) so any help trouble-shooting this would be appreciated!

The pedal

Guitar ->
Cornish buffer (http://tagboardeffects.blogspot.com/2012/02/cornish-buffer.html) ->
Valvecaster (using this guide: https://cdn.instructables.com/ORIG/FXM/9B6H/IK1FLK5B/FXM9B6HIK1FLK5B.pdf) ->
Amp

The Valvecaster uses a NOS 12au7 tube (purchased from what appeared to be a reputable seller on eBay) which I'm trying to run at 6.3v DC using a LM317T with 6.3v going to pins 4 and 5 (connected together) and pin 9 going to ground. This was a variant on the schematic as I hadn't read good things anywhere about running a 12v tube at 9v.

Power supply

The (15 year old) power brick I'm using is rated 9v 450mA.

The problem

• When everything is plugged in and turned on, the tube isn't visibly heating and no sound is coming from the pedal.
• I tested the circuit with just the buffer (buffer output to the output jack, no Valvecaster circuit) and everything worked fine
• Using a trim-pot I've adjusted the output from the LM317 to be 6.3v (layout per p8 of the PDF linked above) but without the tube inserted
• When I insert the tube, the output voltage (leg 2) of the LM317 drops is zero, as does the 9v rail.

[mozz]

Sounds like your power supply doesn't have enough current. Or the voltage is dropping below the threshold the regulator needs to do it's job. You may need 12 volts in 500ma.

[antonis]

You have a power short to GND when tube inserted to socket..

[kissack101]

Thanks @antonis, @mozz.

Sounds like your power supply doesn't have enough current. Or the voltage is dropping below the threshold the regulator needs to do it's job. You may need 12 volts in 500ma.

This thought had occurred, though I don't have a 12v adapter to test the hypothesis (may need to jump on Amazon Prime and see if I can get one delivered in time).

My research suggested that the 12au7 requires 300mA at 6.3v which made me think that 450mA from the adapter would be sufficient (there is nothing else plugged into the brick when I'm testing).

You have a power short to GND when tube inserted to socket..

What could be the cause of this? A faulty tube? (Everything seems to work as expected without the tube in the socket). Is there any way I can test the tube using "standard bench tools" (i.e. my multimeter) to see if the tube is the issue?

[mozz]

A voltage regulator needs more volts in than out. A 9v may need 11-13v input to work correctly. Measure input and output with no tube. Insert tube and measure again. Or just bypass the regulator and see if it works.

[antonis]

A voltage regulator needs more volts in than out. A 9v may need 11-13v input to work correctly. Measure input and output with no tube. Insert tube and measure again. Or just bypass the regulator and see if it works.

A voltage regulator indeed needs more volts in than out but it's zeroing its out voltage only in case of input voltage equal or less than voltage difference needed.. 
e.g. a 9 volt regulator will lower its output to 5 Volts in case of 7 Volts input - give or take..
For zero volts output, input voltage should be marginally 2 Volts - give or take, again..

[amptramp]

When you are running the two heaters inside the tube in series, connecting to pins 4 and 5 and not using pin 9, the 12AU7 only needs 150 mA of heater current.  The problem with this is the tube heater draws more current cold than hot, so a power source that can maintain the current in an operating tube may be unable to start when it tries to supply the cold heater current.  The required starting current may be four times the running current.

Unless you have a very badly regulated supply, you don't need a regulator with it.  In fact, there is a current regulator connection with an LM317 where the output goes through a series resistor and the sense lead is attached to the output side of the resistor so the 1.25 volt difference is maintained by a constant current through the resistor.  This limits the inrush current to the tube heater but requires several volts more than the 12 volts you would need to operate the heater as a supply.

[kissack101]

Thanks for this.

When you are running the two heaters inside the tube in series, connecting to pins 4 and 5 and not using pin 9, the 12AU7 only needs 150 mA of heater current.  The problem with this is the tube heater draws more current cold than hot, so a power source that can maintain the current in an operating tube may be unable to start when it tries to supply the cold heater current.  The required starting current may be four times the running current.

Just to confirm, I'm using a 9v power source, not 12v (I'm using the LM317 to get a 6.3v supply, connecting to *both* pins 4 and 5 and then connecting 9 to ground).

Unless you have a very badly regulated supply, you don't need a regulator with it.  In fact, there is a current regulator connection with an LM317 where the output goes through a series resistor and the sense lead is attached to the output side of the resistor so the 1.25 volt difference is maintained by a constant current through the resistor.  This limits the inrush current to the tube heater but requires several volts more than the 12 volts you would need to operate the heater as a supply.

As above, if I'm operating the heaters at 6.3v, would a 9v supply give me the "several volts more" that you refer to? Do you have a link to a schematic that would show how to do this?

[antonis]

Ron is refered on using voltage regulator as current regulator (or limiter..)

https://diyaudioprojects.com/Technical/Current-Regulator/

[duck_arse]

have you measured the resitance of the heaters themselves? is there any resistance?