got a YBA-1 - worked perfectly some time ago but sitting for a few years. ---
brought it up slow on my variac. At first no sound at all. Replaced the phase inverter tube and got a tiny bit of guitar sound - but when flipping the ground switch, plenty of hum, so the output stage seems good. Voltage at the output transformer's CT looked ballpark. I re-did the basic power supply caps some time ago. Coupling caps are original (from late 1960's - early 70s)
I could get some loud microphonic sounds when tapping on the first two gain stages but still not much out of the guitar - will check with a different guitar cord. (My multimeter has gone crazy on resistance range)
What would be most likely to fail ? what would you use for new coupling caps? (orange drop?)
brought it up slow on my variac. At first no sound at all. Replaced the phase inverter tube and got a tiny bit of guitar sound - but when flipping the ground switch, plenty of hum, so the output stage seems good. Voltage at the output transformer's CT looked ballpark. I re-did the basic power supply caps some time ago. Coupling caps are original (from late 1960's - early 70s)
I could get some loud microphonic sounds when tapping on the first two gain stages but still not much out of the guitar - will check with a different guitar cord. (My multimeter has gone crazy on resistance range)
What would be most likely to fail ? what would you use for new coupling caps? (orange drop?)
Look at your input jack and wiring, and make sure the guitar and cord work when plugged into something else. If tinking on the first tube makes good sound then it sounds like the amps is mostly working.
thanks - will do - what sound is getting through is very small and distorted - maybe a coupling cap failure (?) - I need to draw out the schematic on paper just to see the path. There's maybe a chance of shorting input jack not opening (?)
If it is a coupling cap(s) failure - what's the best way to troubleshoot ?
I think those I've circled in "red" would be the most likely positions
to cause trouble
I need a new meter as mine has gone screwy on resistance - sole income got shut off after last Nov. so have to wait and hope it resumes in March.
If it is a coupling cap(s) failure - what's the best way to troubleshoot ?
I think those I've circled in "red" would be the most likely positions
to cause trouble
I need a new meter as mine has gone screwy on resistance - sole income got shut off after last Nov. so have to wait and hope it resumes in March.
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Don't think about it as a bunch of parts, think about it as a system. Does this low output happen on both channels? If so, it seems real unlikely that both C2 and C4 would fail, you know?
First, look into every tube, are BOTH little orange heaters glowing in each small tube? Look at pins 1 and 6 of each 12AX7, is there high voltage? Anything from 100-300v even.
Let's use your meter as an antenna. Turn the volume controls up midway. DO not ground your black probe, only use the red one. With the amp running, touch pins 2 and 7 of each small tube. When you touch that grid pin, you normally would hear hum out the speaker. SO on V1 pins 2 and 7, do you hear hum? If so, the input wiring is suspect. If no hum, then move to V2, touching pins 2 and 7 cause hum? And finally V3, does pin 7 make hum.
And you say you get a real low volume signal out? Do the tone controls sound like they are working even though it sounds crappy? For example, if the treble control fails, you get no output.
First, look into every tube, are BOTH little orange heaters glowing in each small tube? Look at pins 1 and 6 of each 12AX7, is there high voltage? Anything from 100-300v even.
Let's use your meter as an antenna. Turn the volume controls up midway. DO not ground your black probe, only use the red one. With the amp running, touch pins 2 and 7 of each small tube. When you touch that grid pin, you normally would hear hum out the speaker. SO on V1 pins 2 and 7, do you hear hum? If so, the input wiring is suspect. If no hum, then move to V2, touching pins 2 and 7 cause hum? And finally V3, does pin 7 make hum.
And you say you get a real low volume signal out? Do the tone controls sound like they are working even though it sounds crappy? For example, if the treble control fails, you get no output.
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