JCM800 Preamp Clone - Wierd Distortion (oscilloscope pic)

[yosemitesam144]

Hi, I'm experiencing a strange distortion on this preamp.
It looks like asymmetrical hard clipping..very harsh sounding; especially when boosted, so I decided to investigate.

All scoping was done using a 440Hz Sine wave.

V1a (gain stage 1) is perfectly fine, no distortion. A little bit of compression when the signal is over 800mV, but thats to be expected. I can provide scope pictures if required.

V1b (gain stage 2) this stage has been slightly modified, the cathode resistor is 4.7k on this stage. other than that is stock. this is where it gets wierd (pic attached to post)


theres a sort of hard clipping on one side of the wave form, the other is slightly overdriven. Is this normal? I'm fairly new to scoping etc.

I can provide any more scopes or info required. Hope someone can help!

[g1]

Are you saying your input signal is 800mV? Is that waveform shown at the plate? What is it's amplitude? What kind of output level can you get at the spkr. jack cleanly?

[J M Fahey]

when the signal is over 800mV

*Where* do you have those 800 mV?
On the input, on the first triode plate, on the second one?
Please post the schematic, the no-signal DC voltages and the AC signal voltage on different parts when you get what you've shown.
By the way, that's a very normal and lightly clipped (by guitar amp standards) signal.
It will sound like slight crunching.
Welcome to the real world of scoping, you'll find that *most* of what's posted on "other" Forums, (not here) about tube clipping is worth its weight in .... horse manure.
¿Remember what those "rounded clipping" drawn tube waveforms look like?
Well, now you know the truth.

[Bruce / Mission Amps]

Well... beat to it but that is not really hard clipping... that is squanky tone guitar clipped.
With 800mvac input at the first preamp grid, and the volume up to anything over quiet playing, I would expect to see that after the second gain stage and onto the cathode follower.
Roll your drive signal down to more reasonable level, maybe 100-200mvac and try again.
List some AC voltages at the coupling cap too.... normal could be around 6-7vac after the first stage with a much smaller drive level.

[Chuck H]

All that+

With the 4.7k cathode resistor on V1b it is likely biased cold. This means you SHOULD expect to see asymetrical clipping as the tube begins to overdrive (as naturally it will at the extreme drive level). This does not in itself make for harsh tone, unless by harsh you mean distorted or clipped. It's actually quite common in high gain preamps to intentionally cold bias a stage to induce exactly the behavior your getting on your scope.

Chuck