I have a weird one I could use some brain trust ideas with.
Many moons ago I built an SSE to see if I liked it. It worked beautifully and I built another one with a nice enclosure and that has been my primary amp since around 2011.
The first one went on the shelf until a friend's (and tube lover) house burned down in CO last year and I sent it to him along with two bookshelf speakers, so he could have some tunes while he and his family got back on their feet.
He just noticed that the amp doesn't really play stereo. Unplugging one of the channels still produces sound on both speakers; it just gets a bit louder when both are plugged in.
Suspecting a faulty 12AT7, he replaced it. No change. Measuring resistance across signal inputs on the board (10 and 50 on the board) shows no continuity at all, so inputs are not shorted.
Next, he took volume pot out of the equation and hard-wired input RCA jacks directly to the board. No change. Single channel input drives both output tubes.
Looking at the schematic, I don't really see any other way for the two channels to interfere like that, except for the 12AT7 plate supply via the CCS IC. Is it possible that those failed in a way that would cause them to pass audio from the other channel?
What am I missing in terms of troubleshooting steps? I am trying to help remotely, but I am running out of ideas.
Any thoughts are welcome!
Happy 4th to all!
Stefan
Many moons ago I built an SSE to see if I liked it. It worked beautifully and I built another one with a nice enclosure and that has been my primary amp since around 2011.
The first one went on the shelf until a friend's (and tube lover) house burned down in CO last year and I sent it to him along with two bookshelf speakers, so he could have some tunes while he and his family got back on their feet.
He just noticed that the amp doesn't really play stereo. Unplugging one of the channels still produces sound on both speakers; it just gets a bit louder when both are plugged in.
Suspecting a faulty 12AT7, he replaced it. No change. Measuring resistance across signal inputs on the board (10 and 50 on the board) shows no continuity at all, so inputs are not shorted.
Next, he took volume pot out of the equation and hard-wired input RCA jacks directly to the board. No change. Single channel input drives both output tubes.
Looking at the schematic, I don't really see any other way for the two channels to interfere like that, except for the 12AT7 plate supply via the CCS IC. Is it possible that those failed in a way that would cause them to pass audio from the other channel?
What am I missing in terms of troubleshooting steps? I am trying to help remotely, but I am running out of ideas.
Any thoughts are welcome!
Happy 4th to all!
Stefan
Is it possible that the amp was always like this?
I have seen three examples where an amp got sound out of both speakers with only one input connected. All involved wiring issues. If the screen UL tap wires on the OPT's are swapped side to side, there will be coupling from one channel to the other in UL mode. If the secondary feedback to the output cathodes are swapped, there will be coupling from one channel to the other. It's not hard to get this wrong if UL or CFB switches were installed during the build.
I have also seen an open ground on the interconnects cause channel to channel coupling, but there was also some hum. This turned out to be the input jacks themselves as they were mounted in a wood panel and the ground lug only touched the plastic part of the jack.
Try pulling one output tube and testing for sound. If both speakers still play, there is likely a wiring error around the OPT's. You could also lift one end of a coupling cap. This will tell you if the problem is in the driver or output stage.
I have seen three examples where an amp got sound out of both speakers with only one input connected. All involved wiring issues. If the screen UL tap wires on the OPT's are swapped side to side, there will be coupling from one channel to the other in UL mode. If the secondary feedback to the output cathodes are swapped, there will be coupling from one channel to the other. It's not hard to get this wrong if UL or CFB switches were installed during the build.
I have also seen an open ground on the interconnects cause channel to channel coupling, but there was also some hum. This turned out to be the input jacks themselves as they were mounted in a wood panel and the ground lug only touched the plastic part of the jack.
Try pulling one output tube and testing for sound. If both speakers still play, there is likely a wiring error around the OPT's. You could also lift one end of a coupling cap. This will tell you if the problem is in the driver or output stage.
Thanks for your response George, I appreciate it!
I built the amp, so everything is possible... 😛 I may have been so excited that it "worked" at the time that I never noticed myself.
The CFB terminals are shorted, so that's not an issue, but at this point I think it's very likely that I accidentally swapped the UL tap wires when I initially hooked up the UL switch, and maintained that screwery when I disconnected the UL switch and made it permanent. Now that you say it, it makes complete sense as to how channel signals can present in this way.
Interconnects are next, but I feel pretty comfortable that grounding and shielding on signal wires is OK as I spend a lot of time on that topic, the amp is in a metal enclosure and it was dead silent.
I'll asked my friend to trace UL tap wires and check that they are not swapped, and report back.
Stefan
I built the amp, so everything is possible... 😛 I may have been so excited that it "worked" at the time that I never noticed myself.
The CFB terminals are shorted, so that's not an issue, but at this point I think it's very likely that I accidentally swapped the UL tap wires when I initially hooked up the UL switch, and maintained that screwery when I disconnected the UL switch and made it permanent. Now that you say it, it makes complete sense as to how channel signals can present in this way.
Interconnects are next, but I feel pretty comfortable that grounding and shielding on signal wires is OK as I spend a lot of time on that topic, the amp is in a metal enclosure and it was dead silent.
I'll asked my friend to trace UL tap wires and check that they are not swapped, and report back.
Stefan
BOOM! George, you were spot on with "wiring error". I just got word that I apparently had one of the OPT output wires crossed between channels. Not sure how I never noticed this during testing. Only explanation is that I must have never unplugged inputs and the amp appeared to otherwise work perfectly.
Rookie mistake when going from breadboard to enclosure.
THANK YOU for giving the essential hint!!
Happy 4th to everyone!
Rookie mistake when going from breadboard to enclosure.
THANK YOU for giving the essential hint!!
Happy 4th to everyone!