12SN7 MHz Oscillation

A 500pF just do a perfect job between ground and plate.

much better job than the gridresistor, actually
i don't understand why gridresistor is a more common technique...

thanks everybody here for the help. :thumb:

Go as low as you can, that 500 might rob some high end.

I was wondcering what the speed of your scope is, sometimes the clocks and A?D converters can put out a lot of crap.
We had to put 8 ea. 100 pf caps on the inputs of a National Semi ADC10158, the noise spikes were biger than the frickin rails!
They shoud stick to rectifier bridges and regulators.
They had a goat for a lawn mower one time, thats how cheap they are.

Are you sure you have oscillation, not random noise? If it's shifting around so much that you can't see a definite repetitive waveform on the scope, it might just be thermionic noise. 1mV of noise on the plate isn't that unreasonable...

It's done i find where it came from !

It was the shielding of the audio transformers

the shielding/cans of the audio transformers was connected
on the PIN1 of the XLR, which is connected directly to the chassis/box

Now connecting the audio transformers cans to the Audio 0v right
where the grid resistor, cathode resistor are = No more oscillation, or noise
without any shunting capacitors :green:

Thanks for all the helps !

[quote author="Jazzy_Pidjay"]It's done i find where it came from !

It was the shielding of the audio transformers

the shielding/cans of the audio transformers was connected
on the PIN1 of the XLR, which is connected directly to the chassis/box

Now connecting the audio transformers cans to the Audio 0v right
where the grid resistor, cathode resistor are = No more oscillation, or noise
without any shunting capacitors :green:

Thanks for all the helps ![/quote]

Hi,very good job :thumb: what kind of audio transformers did you used?

And can you give me more info about "how it's sound"?? :grin:

Thank's.

A+,Bruno.

I was waiting to finish the design completely to share it
on my website

i use LL1538 (1:5) and Sowter 6469f (1:7)

I like the Sowter sound, but the Sowter Side can clip
with hot mic and loud signal, the LL1538 side of the preamp
is almost impossible to overload... and sound clean but still a bit
"larger than life"

[quote author="Jazzy_Pidjay"]I was waiting to finish the design completely to share it
on my website

i use LL1538 (1:5) and Sowter 6469f (1:7)

I like the Sowter sound, but the Sowter Side can clip
with hot mic and loud signal, the LL1538 side of the preamp
is almost impossible to overload... and sound clean but still a bit
"larger than life"[/quote]

Hi,thank's for reply,I've used the 12sn7 in an Srpp output stage configuration,nice sound and so that's why I'm interested to used one in amplification just behind the Tx in (as you did) ...You said that's "it's very hard to overload it..." for me it's interesting to build a preamp LL1538+12sn7 for drums record .So,for this time I'm going to take my book,and data sheet of this nice tube. Merci,Bruno. :grin: