Preamp and Reverb Affect on Sine Waves...and Dreaded Parasitics
Posted: Mon 09/03/18 7:24 am
I recently completed my 36 Watt Superlite TMB with EL34s and Reverb. I'll post about it in the 36 Watt forum after I take some photos.
In the process I came across a very sneaky parasitic oscillation. The normal chopsticking around and other searching didn't help. I tried all sorts of things. It was frustrating. Using a differential probe (randomly testing different sections across different points, not always to ground), I was able to confirm my suspicion that the reverb section was responsible for the parasitic.
Parasitic oscillations can be caused by positive feedback, so I focused on the anode (plate) leads (pins 1 and 6) of the reverb tube, as well as the reverb input jack. Things started to happen! I had to use the back-end of the chopstick to get enough force to separate the anode leads from other leads, and press them down against the chassis. This was partly difficult because I made the leads short to avoid this sort of thing in the first place (maybe not quite so short next time). After much fiddling around, success! It's insane if you think about it. So much trouble caused by slight position changes of a couple wires. The amp was overheating, sounded like a helicopter was coming after you, and the preamp had no affect on the sine wave. Now it's all good.
Here's what the parasitic oscillation looked like on the scope:
Now that the parasitic is gone, we can see what affect the preamp has on the sine wave.
Gain: 50%, Treble: Full, Mid: 50%, Bass 50%, Master 40%, Reverb Off: Gain: 50%, Treble: 50%, Mid: 50%, Bass Full, Master 40%, Reverb Off: Gain: 50%, Treble: Full, Mid: 50%, Bass Full, Master 40%, Reverb Off: Gain: 50%, Treble: 50%, Mid: 50%, Bass 50%, Master 40%, Reverb Off: Gain: 40%, Treble: 25%, Mid: 25%, Bass 25%, Master 40%, Reverb Off: Gain: 50%, Treble: 50%, Mid: 50%, Bass 50%, Master 40%, Reverb Full:
Some interesting observations while playing with the controls connected to the scope, which you can see in those screenshots:
The Gain knob does affect how much the tone knobs shape the sound coming from the amp. With the Gain knob at 50% and higher, the tone knobs have a more profound affect on the shape of the sound. 35% and less, it isn't as profound.
The Treble and Bass knobs shape the bottom half of the sine differently, and if you turn them far, you can watch the bottom half of the sine swing in either direction depending on which tone pot you're turning.
The Mid knob didn't change the shape so much as to increase the amplitude, so I didn't bother with a screenshot.
Anyway, I thought it was interesting.
Thanks,
Josh
In the process I came across a very sneaky parasitic oscillation. The normal chopsticking around and other searching didn't help. I tried all sorts of things. It was frustrating. Using a differential probe (randomly testing different sections across different points, not always to ground), I was able to confirm my suspicion that the reverb section was responsible for the parasitic.
Parasitic oscillations can be caused by positive feedback, so I focused on the anode (plate) leads (pins 1 and 6) of the reverb tube, as well as the reverb input jack. Things started to happen! I had to use the back-end of the chopstick to get enough force to separate the anode leads from other leads, and press them down against the chassis. This was partly difficult because I made the leads short to avoid this sort of thing in the first place (maybe not quite so short next time). After much fiddling around, success! It's insane if you think about it. So much trouble caused by slight position changes of a couple wires. The amp was overheating, sounded like a helicopter was coming after you, and the preamp had no affect on the sine wave. Now it's all good.
Here's what the parasitic oscillation looked like on the scope:
Now that the parasitic is gone, we can see what affect the preamp has on the sine wave.
Gain: 50%, Treble: Full, Mid: 50%, Bass 50%, Master 40%, Reverb Off: Gain: 50%, Treble: 50%, Mid: 50%, Bass Full, Master 40%, Reverb Off: Gain: 50%, Treble: Full, Mid: 50%, Bass Full, Master 40%, Reverb Off: Gain: 50%, Treble: 50%, Mid: 50%, Bass 50%, Master 40%, Reverb Off: Gain: 40%, Treble: 25%, Mid: 25%, Bass 25%, Master 40%, Reverb Off: Gain: 50%, Treble: 50%, Mid: 50%, Bass 50%, Master 40%, Reverb Full:
Some interesting observations while playing with the controls connected to the scope, which you can see in those screenshots:
The Gain knob does affect how much the tone knobs shape the sound coming from the amp. With the Gain knob at 50% and higher, the tone knobs have a more profound affect on the shape of the sound. 35% and less, it isn't as profound.
The Treble and Bass knobs shape the bottom half of the sine differently, and if you turn them far, you can watch the bottom half of the sine swing in either direction depending on which tone pot you're turning.
The Mid knob didn't change the shape so much as to increase the amplitude, so I didn't bother with a screenshot.
Anyway, I thought it was interesting.
Thanks,
Josh