Dual La2a - [silent:arts] version

[[silent:arts]]

when I ever will do a rev2 board I will include it.

from my understanding the grid stoppers are there to prevent oscillation at high gain settings (like we have with the Sowter Output transformers)
try the potentiometer mod I did first, for the grid stoppers you would have to modify the PCB. worked for me.
and I can't remember any hum before, only the oscillation ...

Kubi would be the man to explain grid stoppers

[waldorfcave]

ok i found a 22k pot and assembled it. gain range is better now, but hum is still there.

now i measure the voltage of the tube pins

[waldorfcave]

..also did the grid stopper mod...nothing happens

highvoltages for the tubes:

v101: pin 1: 119
pin 6: 123

v102: pin1: 104
pin6: 230

v103: pin1: 94
pin6: 95

v104: pin5: 196
pin6: 76

i think they are in the normal range.

DAMM the comp works sound good but the fucking hum peak 105hz @ -74db is to loud.


i rewired my ground lines, pushed all transformers checked everything.

slowly i´m out of my nerves....

if nothing worx i will put a pad behind the output. so i have to use more gain but the humfloor will be less i think.

[[silent:arts]]

putting out my unit out of the rack to compare at the moment ...

DAMM the comp works sound good but the fucking hum peak 105hz @ -74db is to loud.

this is -74dBfs in your DAW, right ? what's your headroom ?
if it is -74dBu all is fine (mine is -67dBu at max gain)

will post my voltages soon ...

[[silent:arts]]

mhm, most of my voltages are lower (and I only had a 5 minutes warm up):

v101: pin 1: 119 (mine: 93V)
pin 6: 123 (mine: 100V)

v102: pin1: 104 (mine: 91V
pin6: 230 (mine: 200V)

v103: pin1: 94 (mine: 73V)
pin6: 95 (mine: 73V)

v104: pin5: 196 (mine: 113V)
pin6: 76 (mine: 108V)

what Tubes are you using ? will try to find some normal ECC83 in my box (since I used NOS Telefunken ECC803S)

[waldorfcave]

maybe this is an idea?

i use jj ECC803 and 12bh7 from electroharmonix all brand new

[[silent:arts]]

Lars, I think we are on the right way

I checked different Tubes in the place of V101, and there were significant differences in overall noise, noise and hum spectrum.
sadly I can't do any FFT at home, but I used a Neutrik Minilyzer and my ears.

the Tubes I found and have tested are:

- Telefunken ECC803S (my "Stock" Tube in there)
- Valvo ECC83
- Siemens ECC83
- Sovtek 12AX7LPS
- JJ ECC83S
- JJ ECC83
- Japan 12AX7A

with the JJ ECC83S I had significant Hum and a Noise Floor of only -50 dBu. will be fun to do some real measuring at work with different tubes.
(but this will take some time ...)

do you have any other ECC83 to check ?

[waldorfcave]

no ihave no different tubes

is - 50 dbu good or worse ?

ok. then i need new tubes, which should i prefer?

great volker

[[silent:arts]]

is - 50 dbu good or worse ?

it is 17dB worse than -67dBu

ok. then i need new tubes, which should i prefer?

interesting thing is the JJ ECC83 was better than the JJ ECC83S in my little test. should be the other way in my understanding.
I have only used old tubes yet, don't know what I should recommend.

it is always handy to have some spare tubes ...

[waldorfcave]

ok.

but what should i do now

[[silent:arts]]

I will send you the V101 and V201 from my unit (but I want them back )
this way we can make sure if the hum goes away with other tubes.

[waldorfcave]

you make my day

[[silent:arts]]

I have 4 older Mallory Caps (silver cans) that someone gave to me for free. Send me you address and I will mail you 2 of them! I will use the other two on mine. I measured them with my Fluke MM and 2 came out as ~13uF and the other 2 came out as ~12uF so I will pair them up that way.

Not sure if they are good. They are new old stock but they were free ... so.

I will throw them in the post for you.

-ChuckD

received them today, thanks very much.

[waldorfcave]

Hi Volker,

received your tubes....and i´m totally shocked

now i have some hum behind -98dbfs, this far enough for me.

changed my tubes yesterday also to electro harmonix, with result of -80dbfs hum. nearly ok.

But i need this telefunken tubes

maybe somebody know where i can buy them



- oh man this thing is really crazy for me. i´ve been looking for hours for some faults.

now i know what differences are in tube

[[silent:arts]]

now i have some hum behind -98dbfs, this far enough for me.

good to hear - so it is not the PCB or your build

if you are looking for those Telefunken tubes:
ebay

ich sollte meine Altersvorsorge mal aus dem Sidechain entfernen, da tut es auch was anderes ...