JE lab 2a3 , fizzy, squeal in one channel, help

[spgjmf]

hi folks,
Last year I built a JE labs 2a3 amp. I did not change anything from the
schematic except added a hum pot for the 2a3s. It has started to whine,squeal, fizzle in one channel - very quite, but on my fostex FR I can hear it 4 feet away. I have swapped the driver tube, no affect, swiapping the 2a3s, no affect and when the driver tube is not installed the whine ii still there. Any idea where to look for the problem? the other channel is fine.

could it be bad component, bad solder joint, socket? it gota be down stream of the driver tube, so there is not much there to change. oh the 2a3 are cheap shino brand - no affect when touching or tapping the tubes either.

any suggestions where to look for the problem are appreciated.

thanks!
Jeff

[M Gregg]

Hi,

Can you post a schematic including any mods. You may get a better response when people have something to work with. You say you can hear it with the output tube removed! Through the speaker?

Is the power supply for both channels? Or Mono block different supply.

If you can hear it with no O/P tube then its probably a short either on the Board or socket. Worst case swap the O/P Tx over on the channels. If it moves its a short in the O/P Tx!

Do you have any suppressor networks across the HT? Mov etc!

Regards
M. Gregg

[spgjmf]

Originally Posted by M Gregg

Hi,

>Can you post a schematic including any mods. You may get a better response when people have something to work with. >You say you can hear it with the output tube removed! Through the speaker?

ok...thanks for your suggestions.

here is a link to the scheamtic
simple45


>Is the power supply for both channels? Or Mono block different supply.


amp uses one PS
>If you can hear it with no O/P tube then its probably a short either on the Board or socket. Worst case swap the O/P Tx >over on the channels. If it moves its a short in the O/P Tx!

I'll give it a try

>Do you have any suppressor networks across the HT? Mov etc!

yes, a cap and a MOV.

after messes with it more, I thinks it the 2a3 sockets. I was able to get the noise to go away, if I pushed, rotated the tube. or maybe the tube base is weak..
I'm going to upgrade the sockets to something better - teflon and machines round pins(???)


-Best,
JEff


Regards
M. Gregg

[M Gregg]

spgjmf,

Check that when you push the socket you are not moving some cables or components away from the chassis, if you have a failure of insulation it will "track" through the insulation to Gnd. It will only be a pin hole. If you are careful you may see it under the chassis in the dark "tracking".
Best way to do it is get into position so you are looking at the base / cables (Power on) and get someone to turn the lights off "don't move untill they turn them back on again!"

Hopefully you have found it! (On the tube base).

If you look at the schematic there is nothing between the O/P Tx and the tube base except cable!

let us know how you get on.

Regards
M. Gregg

[M Gregg]

Plese note,

When I refer to Tube base It should say Base/socket.
Also take care of tube position when powered and checking.

If it was a tube base, If you swap the tubes over the fault should move to the other channel. It does sound like a socket fault as you say. try changing the "faulty" socket first and see if the fault stops!

Regards
M. Gregg

[spgjmf]

Originally Posted by M Gregg

Plese note,

When I refer to Tube base It should say Base/socket.
Also take care of tube position when powered and checking.

If it was a tube base, If you swap the tubes over the fault should move to the other channel. It does sound like a socket fault as you say. try changing the "faulty" socket first and see if the fault stops!

Regards
M. Gregg

yes, good point about tube base vs. socket( female part on chassis). I think it is the socket. with power off and caps drained, I carefuly pinched the pins in the socket and that channel is now dead quite. The other side has a bit of noise, so it will get the same pinch.
I ordered up some new cmc sockets and will install them as soon as they arrive. saving up for some new 2a3 tubes as well. I hope the fix is this simple.
ha...

thanks for your interest and help

JEff

[Jaap]

Bad solderjoint (on the socket) ?

[spgjmf]

following up.
I redid all the socket solder connections and the squeal is less.
I ordered new cmc sockets for the 2a3s. The new sockets are smaller than the current punch outs, so I have to build an adapter plate and such.

I'll see if this helps the problem - if not, new tubes! sovket seem to be the best bang for the buck.

-Jeff

[spgjmf]

update -

I installed the new cmc sockets - boy are they nice. I used electrical conduit reducing washers to mount in the larger existing holes. worked rather well.

The squeal is still there, but now at least it follows the tube if I switch the tubes between channels. After 10 minutes or so, the noise stops - So I think it is a bad tube. arg! but the new sockets are nice and I cleaned up some wiring and parts placement. amp seems smoother over all, so the work was worth while. next new tubes.

thanks for all the input folks,
Jeff

[M Gregg]

Just to let you know, we are following your progress!

If it follows the tube then I agree, time to try a new tube.
Can you post a pic of the socket mod. Thank's



Regards
M. Gregg