Dirty Shirley Wonky sine wave

[Microamp]

Hello Amp Garage,

this is my first post here.

I recently converted a JTM45 I built about 10 years ago into a Dirty Shirley after watching Jason Tong's video on youtube about the conversion process.

I also recently acquired an oscilloscope, and found a post on the Marshall amp forum about biasing an amplifier using and O-scope.

That post directed the reading to be taken across a resistor that matched the output of the amp, in my case, across a 16 ohm, 100W resistor, with the impedance selector on 16 ohm.

these were the resultant sine waves:

master vol 50%:

sinewave1.jpg

master vol full:

sinewave2.jpg

This doesn't look right to me. I was expecting a healthy sine wave.

are any of you well-versed in oscilloscopes and signal generators that could chime in on this?

I am using an HP 200CD as a signal generator.

[LOUDthud]

Does the amp have a bad Mains frequency hum ? Or there could be a bad ground connection between amp and generator ? Try setting the Trigger Source to Line and slow the sweep speed down to 20ms or so to see if your sine wave is riding on a Mains frequency wave.

[Stevem]

That’s ringing taking place and is due to your build layout more then likely.

One quick thing you might try is to double the resistance of your 1/2 watt resistors on pin 5 of each output tube and see if that makes a noticeable improvement.

[Microamp]

Thank you both for the replies!

I think my oscillator is the problem, but It gave me a clean sine wave a few days ago. It's old, I borrowed it from work and it probably hasn't been used in at least 20 years.

as a sanity check, does my connection seem legit?:

the oscillator:

hposcillator.jpg

I have a cut guitar cable with center lead in the middle, cable shield on the right and the shield clipped to the ground terminal:

connection.jpg

reading the other end of the guitar cable:

sinewave3.jpg

I used this oscillator the other day to trace through an EQ pedal and it gave me a clean wave. I just wonder if I'm not using this correctly.

Any thoughts?

[sluckey]

Center conductor should connect to the right lug, shield connects to the center lug, and jumper between the left lug and the center lug. Those HP oscillators original had a factory jumper/strap/link that came installed on the unit. The 600Ω outputs can be left floating (no ground strap) for balanced line circuits or you can connect the ground link for unbalanced circuits such as most guitar amp inputs.

[Microamp]

thank you, everyone for the assistance.

here's got finally got me straight:

1. as I was pondering what to do about all this, I had my phone connected to a little bluetooth amp I have in my shop. I noticed as the music played that strange signals would appear on my scope in time with the music. somehow the power supply of the bluetooth amp was causing some sort of interference.

2. I had to use a ground isolating plug for the oscillator.

Problem solved, clean sine wave.

Thanks again for the thoughts and assistance!