Semi-Hoffman 2203 Build Hum

[3rdof3]

Hello All,


I gutted a Traynor YBA-1 and converted it to a Marshall JCM800 (2203) circuit, taking much of my layout inspiration from Doug's Plexi layout.  I've got some pretty nasty hum going on and would love some pointers toward eliminating it.  I moved the ground buss to the board, rather than floating it on the back of the pots as I usually design my boards that way.  Other than that, it should be pretty close.


I installed a DC heater supply for V1 - No change.


Thanks in advance!

[shaun]

Wow. What nice work! As a newbie who still makes spaghetti instead of amps, it's sort of weirdly comforting to know that even a great build comes with its own set of issues. But I aspire to your level, in any case. I might just stick that picture over my workbench 

[ncusack]

Just from a quick glance it looks like you may be missing a center tap groud reference on your heaters. If there is no wire for the 6.3v center tap add a 100r resistor from each heater wire to ground.

[3rdof3]

If you look between the two filter caps on the lower left, you'll see a yellow wire.  That's the HCT.


Shaun - Thanks!  Lead dress is key, but I'm almost certain it's a ground loop caused by not following the proper layout.

[3rdof3]

Wow. What nice work! As a newbie who still makes spaghetti instead of amps, it's sort of weirdly comforting to know that even a great build comes with its own set of issues. But I aspire to your level, in any case. I might just stick that picture over my workbench 

A great resource for turret board design are the layouts provided at AX84.com.  Those folks put a lot of thought and experimentation into their designs and the basics of it work for almost any type of amp.  That, plus Merlin Blencowe's book "Designing Tube Preamps for Guitar and Bass".

[3rdof3]

New info - The hum is present with no input plugged in, and does not change with adjustment of the gain control, only the Master Volume control.

[shooter]

try this
pull V1 and 2

MAKE SURE you ground the correct side!!!!!!!!

[3rdof3]

I'll try pulling the preamp tubes to try and narrow it down.


I don't understand that schematic ...  Why disconnect the tone stack from the CF and ground that line?

[shooter]

Your hum is there with nothing plugged in, by pulling the tubes and grounding the input to the TS you "eliminate" 1/2 the stuff, you've narrowed it with knobs, this sorta "verifies" that.

my guess is the TS/wiring/grounding so it's still part of the circuit "under test"

DON'T plug anything in, just power up and see if the hums still there

[3rdof3]

Found the dang culprit. Input tube.


New-ish Tung-Sol ECC83.  Odd thing is that’s the 2nd input tube to crap out. 1st one was a Ruby HG+ 12AX7 that fully failed.


Voltages are nominal for anodes and cathodes, so ...

[shooter]

what schematic did you use?  can you post it, NOT a layout. looks like 1 input jack?

Voltages are nominal

what's your nominal?

put a meter of V1a cathode, monitor VDC as you're playing hard, what kinda "swing" do you get?

[3rdof3]

Here's the schematic.  Our house is demolished right now from construction, but I'll post voltages soon.  Swapping the V1 tube cleared it up, but that's the 2nd tube that has failed in that position.

[DummyLoad]

see attached - NFB loop wired incorrect - not what's causing V1 to fail, but not working exactly like the original plan.


--pete


EDIT: the pot is wired reversed - transpose. better yet, refer to the marshall schema.

[shooter]

not what's causing V1 to fail

while you're fixing the NFB;

power down, discharge caps, verify your meter works, then ohm across each of the R's for V1 (R2,3,4,6,7,8)

ohm from tube sockets to "other side" of R's where they're soldered (Ex: Pin 3 to ground, pin 1 to PS tap B+5)

[3rdof3]

Ha!  That's what happens when I copy/paste circuit parts from various schematics I have.  That NFB is a variation of Mesa's Dual Recto NFB.  I'll swap that and check out the resistors per your instructions to see if anything stands out.