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Need some help with a problem

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Ever since I had some new boards made for my amp I haven't been able to get it to work right. The signal is static on it but if I take my fingers and touch the driver tubes it gets loud and the static quits. I am touching the glass only. I was pushing on the tubes and noticed it change and I thought there was some internal issues with the circuit board but it affects it without putting any pressure on the tubes. Just touching the glass is enough.
I thought maybe a bad ground and its maybe working because its grounding thru me? The audio ground I mean. I have the rca jacks on the back connected to star ground but I am going to try running them straight to the pot and see if that matters. Any ideas are welcome.

D3a drivers with 300B SE output tubes.
 
I am going to try moving the ground directly to the pot like i always have in the past.
Why would an oscillation stop when you touch the tubes? That is something I know next to nothing about about.
schematic attached but its pretty simple.
 

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I will when i get a chance ill post pics. I design pcbs for a living. I am thinking its an internal defect, not a layout issue. I routinly design boards with 16 layers, impedance matching, mixed analog and digital grounding etc. This is a simple 4 layer board. But its possible. I have tried several of the 5 boards I had made and same exact issue with every one. I have already changed it to a 2 layer board, increased clearances, reduced ground pours to a single plane, increased size of holes in a few places and increased keep out areas around all mounting holes. So the new boards just plain have to work. Want me to post gerbers? picture of each layer? Or ???
Also this amp has been working for 2 years. I just got tired of the old chassis because working on anything was a pain in the *** and after shocking myself with 500v 3 times I grew tired of it. 500v hurts! Hell it blew a hole in my skin and made me bleed once. Didn't think it possble to bleed form being shocked but alas it is.
 
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Joined 2011
Also this amp has been working for 2 years. I just got tired of the old chassis

If the board worked before being installed in the new chassis, it is probably ok.

Check the input and output wiring, and the common connections. Keep the input socket and input wiring
away from other parts of the circuit, especially away from the output transformer leads and the output
terminals. Bring the input common to the supply common separately from the output stage common
and the speaker common.
 
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My new boards will be here in a day or two.
I reduced the 4 layers to 2
Increased clearances
Increased keep out zone around all mounting points
Connected only one sides pour to ground ( I usually connect them all and stitch the hell out of it with vias unless it a dig/analog board then I lay it out carefully and only connect the two grounds in one spot using 0 ohm resistors so we can put caps across them if needed.)
Among the things I tried with the current board:
populated 3 of the 5 I have. All were the same.
At one point I had it lying on top of the chassis with no screws in it and it worked for 4 hours. Sounded great. But then it digressed,
swapped driver tubes back and forth
swapper output tubes back and forth.
bought new coupling caps ( mostly because I wanted to_
Pulled the amp 100% apart and rebuilt in two chassis seperating the power supply.
I am quite sick of working on this thing.
 
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Joined 2011
If both channels of the board are unstable (with different layouts), the D3A could as mentioned
need ferrite bead(s). This is a very wide bandwidth tube. You could cut the traces close to the
tube and add a jumper with the bead, for each tube terminal. I would start with the screen 2,
then the grid, cathode, plate. Do one bead at a time.

Also I would try local bypassing across each filament, 0.1uF.

Last thing to try is adding local decoupling on the board for the power supplies,
a larger electrolytic and a smaller ceramic, for each channel.
 
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WOO HOO New boards fixed it bigtime...
I have my stereo back ....

breaking in some new Deulend caps. got the cast copper since they were on sale. Also swapped out the resistors on the input to Vishay Precision Z foils.
I have been slumming it with a chinese el84 amp and man what a difference...
Also have a brand new pair of telefunken D3a tubes in it. But not a lick of noise. None...

Thanks to those that attempted to help.

Now I need to finish designing the final chassis for it. going with a couple mono block chassis. Need to buy another power transformers...


Jeff
 
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