Yeah, the supply voltages have never been checked. I it worked upon building it so assembling it wad the last thing I did before it started exhibiting this issue. How would one check supply voltages, and if the tube's heater voltage remain constant over time, and if the other voltage supplies remain constant over time? This would all be new to me?
To check for consistent voltage supply values over time, simple test them a moment or two after firing it up, (so everything as time to stabilize), then again after a few more minutes. Then continue to check the voltages every 5 minutes or so. If the heater voltage on the tube dips too low, that could cause the gating.
Consider how the Si diodes have a lower Fvd than the LEDs and how that difference will affect the signal going into the tube. If any of the tube's supply voltages are drifting, it could alter the performance of the tube and cause it to exhibit some sort of signal threshold difference (insufficient signal level?) between the diodes and LEDs.
Yes thank you! When I asked about "checking supply voltages" and the tube's "heater voltage" I meant literally, lol. Where do I measure? I assume with the DMM. I started as building pedals with no soldering or electrical experience 6 years ago and even though I have learned a lot from this forum, I'm still just a step above "paint by numbers."
Thanks for this also. It's great to get more of an understanding of what's going on in a circuit.
Sounds good! Now that you say this. Is it possible that the tube is the problem? The one I have in there is possibly one that was causing a problem in an amp and got mixed up in my (small) stash. I think I have another one could try.
The tube heater in Lab Rat comes straight off the supply voltage, so it will be your main rail on the cathode of your 1N5817. Should be just a little lower than your supply.
From the schematic you've posted, look at the "VIN" point on the 12AX7, should be pins #4 & #5. I'd also verify that the 250v supply is there. (Pin #1 or #6.)
Yeah. Easiest test would be to swap the tube.
Changing the tube didn’t fix the issue.
Yeah. The Fvd (0.35V) of the 1N5817 accounts for the difference between the DC jack and after that diode.
Those seem to be right in the ballpark. So now you've removed suspicion from power supply and the tube.
I have a tone generator app on my iPad that I can choose 1000Hz but I don’t see any adjustment options for voltage. Am I going get one that has that feature?
Tone generators on Apple devices typically output based on the device's volume control. Knowing the sig gen's voltage output will help verify what you read throughout the circuit's signal path when measuring. So you may want to measure the unloaded output of the iPad's signal generation so you know ahead of testing time what you're starting with.
1.3
Maxing the volume on the iPad I only get less than a volt on the tip of the input jack so that’s not good. But here’s the DC and AC measurements on pin #2:
This would be removing the LEDs and toggle from the circuit, so just using the SI diodes? If it operates normally, as it does when selected to the SI diodes, it would localize the problem to the LEDs or toggle, or their current soldering, yes?
