Preamp and Reverb Affect on Sine Waves...and Dreaded Parasitics

General Tech Discussion - Anything amp-ish goes!

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 2040
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Preamp and Reverb Affect on Sine Waves...and Dreaded Parasitics

Post by JMPGuitars »

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:
SDS00011.png

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:
SDS00029.png
Gain: 50%, Treble: 50%, Mid: 50%, Bass Full, Master 40%, Reverb Off:
SDS00028.png
Gain: 50%, Treble: Full, Mid: 50%, Bass Full, Master 40%, Reverb Off:
SDS00030.png
Gain: 50%, Treble: 50%, Mid: 50%, Bass 50%, Master 40%, Reverb Off:
SDS00031.png
Gain: 40%, Treble: 25%, Mid: 25%, Bass 25%, Master 40%, Reverb Off:
SDS00033.png
Gain: 50%, Treble: 50%, Mid: 50%, Bass 50%, Master 40%, Reverb Full:
SDS00034.png

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
You do not have the required permissions to view the files attached to this post.
2 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

geoff 1965
Frequent poster
Frequent poster
Posts: 732
Joined: Mon 01/04/16 3:52 pm

Re: Preamp and Reverb Affect on Sine Waves...and Dreaded Parasitics

Post by geoff 1965 »

very interesting Josh!
that's it! i won't rest now until i have an oscilloscope to play with!
you can clearly see the little parasitic spikes and i like the way the different settings alter the signwave,thank's for posting this.
looking forward to you posting pics of your el34 superlite,did you draw up a schematic & layout for it?
bye the way,was that oscilation clearly audible or did you find it with the scope?
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 2040
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: Preamp and Reverb Affect on Sine Waves...and Dreaded Parasitics

Post by JMPGuitars »

geoff 1965 wrote:
Mon 09/03/18 8:35 am
very interesting Josh!
that's it! i won't rest now until i have an oscilloscope to play with!
you can clearly see the little parasitic spikes and i like the way the different settings alter the signwave,thank's for posting this.
looking forward to you posting pics of your el34 superlite,did you draw up a schematic & layout for it?
bye the way,was that oscilation clearly audible or did you find it with the scope?
Thanks. I did draw a really nice layout and schematic, but I haven't decided if/when I will share them. There's a PTP FX loop in there too. ;)

The parasitic was clearly audible, and it was causing the amp to overheat, as parasitics tend to do. It sounded like a helicopter, but only when you crank the reverb knob past a certain point; that was my first clue it was the reverb part of the circuit causing the issue.

It was interesting because a shielding plate over the amp was a temporary fix, and the amp sounded great anyway. Of course, now that it's fixed, it sounds even better.

I'm working on a second amp with basically the same problem now. Both of these are based on a modified single valve reverb method I did with ZP's help, but it's susceptible to parasitics. This other amp I'm working on now had much tighter space and a much different layout, so I'm flipping the direction of the socket now to get everything at better angles. I'm curious to see the results.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

Post Reply