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Old 11th January 2021, 07:03 PM   #1
jwags818 is offline jwags818  United States
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Location: Wilsonville Oregon, Corvallis Oregon native (You can't lick our beavers!).
Default Need some help with a problem

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.
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Jwags818 - Wilsonville, Oregon
D50s - 300B/D3A SET - 12in Audio Nirvana Speakers
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Old 11th January 2021, 07:05 PM   #2
rayma is online now rayma  United States
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Sounds like a high frequency oscillation or instability. Can you post the schematic and photos of the board?

Last edited by rayma; 11th January 2021 at 07:08 PM.
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Old 11th January 2021, 07:30 PM   #3
jwags818 is offline jwags818  United States
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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.
Attached Images
File Type: gif scha.gif (92.0 KB, 148 views)
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D50s - 300B/D3A SET - 12in Audio Nirvana Speakers
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Old 11th January 2021, 07:44 PM   #4
rayma is online now rayma  United States
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You are adding stray capacitance, which can cause/cure/change instabilities in a circuit.
Could be layout related. Does it still happen without the 300B?

Last edited by rayma; 11th January 2021 at 07:51 PM.
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Old 11th January 2021, 10:23 PM   #5
jwags818 is offline jwags818  United States
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Location: Wilsonville Oregon, Corvallis Oregon native (You can't lick our beavers!).
Rewiring ground made no difference. I changed tubes to 2 new ones and its still screwed up but putting my hand by the tube does nothing now. It must be the boards. It has to be.
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Jwags818 - Wilsonville, Oregon
D50s - 300B/D3A SET - 12in Audio Nirvana Speakers
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Old 11th January 2021, 10:42 PM   #6
rayma is online now rayma  United States
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Can you post photos of the board?
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Old 12th January 2021, 03:26 PM   #7
jwags818 is offline jwags818  United States
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Join Date: Aug 2005
Location: Wilsonville Oregon, Corvallis Oregon native (You can't lick our beavers!).
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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Jwags818 - Wilsonville, Oregon
D50s - 300B/D3A SET - 12in Audio Nirvana Speakers
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Old 12th January 2021, 08:20 PM   #8
fossa is offline fossa  United States
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In-line ferrite bead is recommended for almost all the pins of D3a
to impede ringing.
It is easy for point-to-point wiring, not sure
how to do this for planar PCB technology ?
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Old 12th January 2021, 08:48 PM   #9
rayma is online now rayma  United States
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Quote:
Originally Posted by jwags818 View Post
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.

Last edited by rayma; 12th January 2021 at 08:50 PM.
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Old 12th January 2021, 11:41 PM   #10
jwags818 is offline jwags818  United States
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Join Date: Aug 2005
Location: Wilsonville Oregon, Corvallis Oregon native (You can't lick our beavers!).
No this board is new and has never worked
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