Scope pattern

[mr2racer]

Hey Anyone,

I built Pete Millet's uniamp using 2A3 outputs. The voltages are all good but it is really distorted? My scope is a goner so I borrowed an old Simpson . The Simpson uses banana plugs for the inputs and I don't have any probes or leads for it so I just used speaker wire from the output. And when taking a digital picture of the scope you can't see the pattern at all. Maybe it will work better in daylight? Anyway, I drew a crude sketch of the pattern at the output and have never seen this pattern before. Does anyone have any ideas? I plan to make up a set of leads tomorrow to find out where the distortion is occuring but in the mean time any ideas?

 

[JonSnell Electronic]

Is the feedback, if any, the correct phase? What signal are you feeding it, looks like square wave and what frequency?
Looks like a partially integrated square wave.

 

[Matt BH]

What cathode resistor value are you using for your 2A3's?

Cheers
Matt

 

[DF96]

Looks like bass boost, combined with a little overshoot.

 

[mr2racer]

Thanks guys, there is no feedback. The signal is a square wave at 1k. cathode resistor is a ten ohm, the amp uses fixed bias. The outputs are set at 40ma. And yes the low frequencies seem to be more distrotred.

 

[Matt BH]

Ah ok, fixed bias and square wave. It just looked like way under biased and cutting off one valve either side with the notches which I guess are around zero crossing.

Anyway a sine wave would tell more for now.

Cheers
Matt.

 

[hooman]

can check sine wave ?

 

[mr2racer]

I checked the amp with a sine wave and found it to be clipping the bottom of the wave form. Then I traced it to a problem with the way I wired the input transformer. Thanks for the help!

Kevin

 

[disco]

Yep,, thought so
It's a classic 'too low bandwith' sign, according to Valley & Wallman.