My 6 week old amp has started doing something different...
I now get random static at a fairly low level. It comes and it goes. It literally sounds like an old radio tuner in between stations. It also seems to be getting progressively worse over time.
I've tightened up every connection possible, disconnected the input (still there) and tried different output tubes.
I'm thinking that maybe my input tube is going bad?
thoughts?
I now get random static at a fairly low level. It comes and it goes. It literally sounds like an old radio tuner in between stations. It also seems to be getting progressively worse over time.
I've tightened up every connection possible, disconnected the input (still there) and tried different output tubes.
I'm thinking that maybe my input tube is going bad?
thoughts?
If this is in both channels, check power supply filter caps. best way to check them is to replace all if you have a spare or one by one till it goes away. If you can show me schematic, it can help to figure this out. The problem seems to be common to both channels, thus any circuit that is common to both is suspect.
Cheers,
Robertaudio
Cheers,
Robertaudio
I agree it is most likely the power supply. Does wiggling the power switch do anything? Tickling the fuse? Those green phoenix connectors have a reputation for working a bit loose after a bunch of heat-cycling. Go over them and torque them all down.
I can't remember if you have a switch on the SS rectifier, but that is another possibility as is a switch for UL/triode modes. If no dice, start easy by disconnecting the external cap.
I can't remember if you have a switch on the SS rectifier, but that is another possibility as is a switch for UL/triode modes. If no dice, start easy by disconnecting the external cap.
Ok - I don't have spares of any of the caps - but I can remove the the big oil can to see if it might be a problem. Seems unlikely since it's way over rated to begin with and brand new.
I did tap on the 12at7 (lightly of course) and I was able to affect the static... still leaning towards the tube - of course I don't have a spare one...
I did tap on the 12at7 (lightly of course) and I was able to affect the static... still leaning towards the tube - of course I don't have a spare one...
replaced the volume pot today... it was on the list to do anyway, the RS pot was not exactly balanced between channels.
That wasn't it either.
Taking a closer look at the 12AT7 - I can barely see the heaters glowing. Is that normal for these tubes?
Next I'll try removing the big cap, and then I'll see if I can measure the B+ rail with my scope. Any recommended values for the divider? I was thinking a divide by 10 would make easy math, but I don't want to use values so high that the scope's capacitance becomes a factor.
That wasn't it either.
Taking a closer look at the 12AT7 - I can barely see the heaters glowing. Is that normal for these tubes?
Next I'll try removing the big cap, and then I'll see if I can measure the B+ rail with my scope. Any recommended values for the divider? I was thinking a divide by 10 would make easy math, but I don't want to use values so high that the scope's capacitance becomes a factor.
I guess it depends on the impedance of your scope. 
Mine is simply 90k + 9k + 1k 1/2W to give x10 and x100. Not the best for sniffing around high impedance stuff, but it works fine for PSU work and other poking around. Was going to build a 900k+90k+10k for that.
The heaters should be fairly visible. Who's AT7 is that? Check your solder joints on that noval socket.
Mine is simply 90k + 9k + 1k 1/2W to give x10 and x100. Not the best for sniffing around high impedance stuff, but it works fine for PSU work and other poking around. Was going to build a 900k+90k+10k for that.The heaters should be fairly visible. Who's AT7 is that? Check your solder joints on that noval socket.
Good, I'll see if I can find those values at the local parts store...
The tube is an Electro Harmonix... bought at AES. It could very well be the socket. The underside joints looked perfect, and to get at the top side of the PCB would be pretty difficult. But I can ohm out the socket to the PCB...
Sigh, I'd rather be working on the SE chassis.
The tube is an Electro Harmonix... bought at AES. It could very well be the socket. The underside joints looked perfect, and to get at the top side of the PCB would be pretty difficult. But I can ohm out the socket to the PCB...
Sigh, I'd rather be working on the SE chassis.
socket resoldered. The static is better, but still there at a much lower level. Almost bearable.
Replacing the socket will be a pain, but mostly because I will have to disassemble the amp to do so.
I may try a new 12AT7 first, or perhaps bend the leads on the one I have to make better contact.
Replacing the socket will be a pain, but mostly because I will have to disassemble the amp to do so.
I may try a new 12AT7 first, or perhaps bend the leads on the one I have to make better contact.
oldmanStrat said:
I'd try a new 12AT7A or equivalent before replacing that socket - it could very well be that tube. Hamfests can be a pretty good source of not too unreasonably priced NOS tubes including 12AT7A/WA or 6201.
You might also try cleaning the tube pins (not socket) very carefully with a very fine emery board. I have run across a few tubes with so much glaze or oxidization on the pins that they were noisy.
Looks like you have resolved this to upstream of the input socket. So, it might be the socket, the tube, the volume pot, the input rca shorting to chassis or the wiring between any of these.
I have not built an amp with a volume control, all mine are controlled from a preamp or from the source. I have read people having similar problems if the body of the volume pot is not grounded.
Something also worth checking while waiting for a new tube would be your source and the cables from it to your amp. I have spent hours trying to sort out an amp problem only to find that an external component was to blame...
Best of luck!
Chris
I have not built an amp with a volume control, all mine are controlled from a preamp or from the source. I have read people having similar problems if the body of the volume pot is not grounded.
Something also worth checking while waiting for a new tube would be your source and the cables from it to your amp. I have spent hours trying to sort out an amp problem only to find that an external component was to blame...
Best of luck!
Chris
Wouldn't hurt to have an extra 12AT7 on hand in any case. You can probably tighten the socket contacts by carefully slipping a fine screw driver between the ceramic and the outside of the pins. I have my doubts that it's the socket contacts themselves...you can try to leave the tube in the socket crooked to see if that makes a difference. What happens to the static if you gently lean the tube while the amp is on?
What happens? You will have an old man hopping up and down saying "Thats HOT" lol jk.
Ty - I have a couple hundred tubes here, but they are mostly 6Al5, 6CB6, 6AQ5, 6CX8, 6J6... almost nothing in the 12 series....
wait take that back, just found a 12AT6. Yikes, thats a totally different tube - rectifier + gain stage...
I'll play with the tube pins and socket tonight.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.