Hello
I am currently working on an 6v6 SE amp from leftover parts (thats why some of the component values are different from the originals). The amp is up and runnning and it sounds amazing except for some excessive static noise. The amp is based around the 5f2a but with a parallell matchlsess-ish first gainstage. I think theres some details from the ax84 stuff as well (I cant quite remember - its been about a year since I drew up the main ideas). Anyway, the layout is attached - do you see any obvious "errors" that can cause this static noise? I suspect the power transformer which is ripped out of an old tube radio, what do you think?
Edit: I uploaded a pic of the inside, the only change I've made since the picture was taken is that I tried to connect pin 1 of the 6v6 to groud which left me with no sound at all... Is the tube bad (JJ 6v6 s), or is this as expected?
Edit 2: Dont mind the second octal tube socket - its just used to hold the power diodes at the moment. I'll probably get a tube rectifier in there when and if I make this amp usable.
Thank you
Jostein
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Last edited by goldenGeek on Mon Feb 09, 2015 4:59 pm, edited 1 time in total.
Is there a reason you are connecting pin 1 to ground? JJ's data sheet doesn't show a connection for pin 1. You might pull it and check whether it's got continuity with any other pins. If so, you may want to remove that connection to ground.
johnnyreece wrote:Is there a reason you are connecting pin 1 to ground? JJ's data sheet doesn't show a connection for pin 1. You might pull it and check whether it's got continuity with any other pins. If so, you may want to remove that connection to ground.
I removed it as soon as I noticed theres no sound with it connected. I also tried a new TAD tube and the static noise is still present.
i know it might not fix anything and im not sure because of the angle of the photo but id lift the red wire coming off what i think is pin 8 on the power tube closest to the pre-amp tube up so that its not laying on those filament wires. i know they are 90 degrees but hey thats just me.
Also might want to use some shielded cable for the signal paths coming of the 33k since they are crossing some heater wires and 2nd pre-amp tube.
johnnyreece wrote:Do any of the controls change the noise? Have you tried chopsticking to see if maybe there's a cold solder joint somewhere?
I havent poked around yet, I'll give that a try. Maybe I'll reflow all the joints as well just to be sure. The controls dont change the noise, but the "master" turns down the noise as well as the sound so I guess its in the pre-amp (i'll have to re-check just to be sure, I just got 2 minutes with it earlier today before I had to go so I haven't really tested that much). The tone pot also filters out some of the noise when turned down.
Ill also bend that red wire away from the heaters and solder the 33k directly to the tube socket and add a shielded wire (it cant hurt anyway, I ususally do this on my builds).
try a different preamp tube.. then maybe look at changing any of the carbon comp resistors. Even if they are new, they can sometimes cause the static noises. I'd start with the 100k
Changed preamp tubes, replaced all carbon comp resistors and moved the 33k to the tube socket and added shielded wire from the input (in which I also replaced the 1m and the actual jack input).
It seems like its picking up noise all along the signalpath up to the power amp tube. The shielded wire on the input got rid of some minor noise but the static is still present just like it was before. If I turn down the master the noise goes away, with the master up it gets worse as the gain pot is turned up. It doesnt go away with the gain pot set to 0. Can a bad power transformer cause this noise or do you still think its in the components/wiring?
I think this one will end up in the pile of junk. I rewired and changed components so that Its almost identical to the 5f2a (except for 22 and 16uf filter caps instead of 16 and 8uf). Now it sounds a lot duller but that static noise is still there. The voltages are a bit high since that's an old transformer for 220V, nowadays theres about 235V in my house. It's rated 265-0-265 but it puts out 290-0-290 which gets me about 390V after ss rectification. Is that too high? The final thing I'll try before I ditch the project is to replace the pre-amp tube socket. It seems like the tube is difficult to insert and pull out so there might be something going on there.
Edit: I see that I forgot a couple of things... I left out the negative feedback (22k) and I didn't remove the 220k in parallel with the first filter cap. Worth a try, or waste of time?
Now I've replaced all components (including filter caps) but the power transformer, output transformer and the power tube socket. Should I bother with the pt socket or do I just get a proper power transformer?
Have you checked the board to see if it is conductive? maybe check from the MV board connection over to the preamp. Check around those eyelets to see if the board is conductive. If you do find a bad spot, you could lift the connection from the board spot, and float that connection off the board.
Richie wrote:Have you checked the board to see if it is conductive? maybe check from the MV board connection over to the preamp. Check around those eyelets to see if the board is conductive. If you do find a bad spot, you could lift the connection from the board spot, and float that connection off the board.
I haven't checked that. Good suggestion, I'll try that later today.
One other thing that might give a clue... I wired in the negative feedback resistor yesterday (I used 33k instead of 22k). With that resistor in place I had an insane squeal/feedback so it had to be removed. Does that make any sense, is this a common problem?
I checked the board for voltages but that seems okay.
Okaaaay... so I replaced the power transformer with another one I had around. This one is only 225V - 0 on the secondary (no CT) so I put in two more diodes and a stump of wire to ground for full wave (?) rectification. The 6.3V on the other hand has a CT which I forgot to wire up at first and the noise was the same as before. But when I realized that I forgot the CT to ground... the noise went away. I still have a pretty strong 50hZ hum with this one, do you think more filtering (choke?) will make the hum more subtle?
This brings me back to think about the first transformer... that one had a CT on the secondary, but not on the 5V or 6.3V. Could that be the reason this noise was there? Should I have used rectification and DC-heating?
