I’ve got a 2 watt DIY SET amp built on the Decware zKit Schematic (including the anniversary modifications/bypass capacitors)
https://www.decware.com/newsite/ZKIT1.pdf
I’ve been using this amp for about three years and when I shut it off if the source is still playing the music fades out over the course of 10 seconds or so and starts to break up before going silent. The last couple of weeks I have noticed that it started cutting out instantly when switched off. Yesterday I switched it off and it cut out instantly; I changed interconnects and turned it back on. The tubes lit up and everything looked normal but it wouldn’t produce any sound. I let it sit about 10 minutes and tried again. Still no sound. I powered it on today, about 16 hours later, and it is working normally again. Anyone have any ideas on a diagnosis for these symptoms?
https://www.decware.com/newsite/ZKIT1.pdf
I’ve been using this amp for about three years and when I shut it off if the source is still playing the music fades out over the course of 10 seconds or so and starts to break up before going silent. The last couple of weeks I have noticed that it started cutting out instantly when switched off. Yesterday I switched it off and it cut out instantly; I changed interconnects and turned it back on. The tubes lit up and everything looked normal but it wouldn’t produce any sound. I let it sit about 10 minutes and tried again. Still no sound. I powered it on today, about 16 hours later, and it is working normally again. Anyone have any ideas on a diagnosis for these symptoms?
Thanks, I was thinking the same thing as far as the caps draining but wasn’t sure it would work if they had failed. I’ll start there.
No, it is not fused.
No, it is not fused.
You really should fuse it. If a filter cap goes short, the fuse will blow and save the expensive components. The schematic suggests a 3A fuse, but I'd opt for a 1.5A slow blow fuse.
Yeah, easy enough to do. I used to have one wired it but removed since it wasn’t tube rectified. Why wouldn’t it turn back in after shutting off last night though?
If the filter caps have gone bad I'd expect a pretty big impact on the sound quality, and plenty of humm in a SE.
Maybe just the second capacitor is bad or disconnected.
Checking with a DVM during turnoff should find the problem easily.
Checking with a DVM during turnoff should find the problem easily.
I had an odd problem when I added a DC blocking capacitor to the input of a Quad amplifier. I forgot to add another grid resistor to ground, and over time the capacitor charged then blocked the input signal. If I switched the amp back on it would not play, but then the following day it was OK again. I think the input tube had lived with a positive grid voltage for some time and had to be replaced.
mannytheseacow,
If the problem is still unsolved, you can help us help you.
Draw a complete and accurate schematic, with numeric values, such as capacitance and voltage rating, power supply DC volts, resistances, etc.
If the problem is still unsolved, you can help us help you.
Draw a complete and accurate schematic, with numeric values, such as capacitance and voltage rating, power supply DC volts, resistances, etc.
The OP provided a link to the schematics.
I'd second the theory of an open grid leak R.
Power up / power down could then freely pull the grid voltage way up (tube fully on) / down (fully cut off) thru the coupling cap.
But that would affect only one channel ... unless both grid leak Rs shared a common solder joint / wire to gnd which is broken.
By the way, since when is it custom to connect G3 of a pentode with a cap to cathode ? Never seen that before ...
I'd second the theory of an open grid leak R.
Power up / power down could then freely pull the grid voltage way up (tube fully on) / down (fully cut off) thru the coupling cap.
But that would affect only one channel ... unless both grid leak Rs shared a common solder joint / wire to gnd which is broken.
By the way, since when is it custom to connect G3 of a pentode with a cap to cathode ? Never seen that before ...
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Yesterday I tried to diagnose and the amp was working fine again, no cutout when shut off, all voltages in spec, no bad solder joints I could find.
As seen (first post now contains the link to the schematic), no grid leak resistors in first stage, only potentiometer.
The common mistake is to believe that input potentiometer/volume controller always solves the grid leak problem.
The common mistake is to believe that input potentiometer/volume controller always solves the grid leak problem.
Thanks for pointing that out. I actually don’t have a volume pot in the circuit and use 10k grid leak resistors on the inputs.
10k is unnecessary load for previous stage (source).
I always use for small tubes 100-470k.