1969/70 Vibrochamp high note modulation

[Voxbox]

Hello,


Hope this finds you all well and in one piece.


I've been replacing the board in a Vibrochamp from 1969/70. The original had gone conductive so I've replaced it with a fibreglass board as in the pics.


I really like the sound of the amp and how it is quite touch responsive. I've never played one of these amps so really don't know what to expect from it.


The thing is, when a higher note, especially up round the 12th fret on the higher strings is played, its fine on its own, but if , say the open 6th string is played, the high note modulates in sympathy was the lower note.
I've swapped out the main PSU multi-cap for a new one from JJ, but no real difference.
The only difference I can see is that the sag recovers quicker with the newer multi-cap, but audibly there is no real difference in that respect, i.e. the sag.


So as I am inexperienced with these amps, I'd like to know it  is it a "feature"of the amp to have the higher notes modulated by the lower notes?
BTW, are the orange drops original?


Thanks
Cheers, VB

[PRR]

> the high note modulates in sympathy was the lower note.

I'm not sure what you are hearing. But yes, if you play a high note at moderate level, and a low note at high level, simultaneously, as the low note distorts it cramps the high note. If the low note is not steady, this crampage will also vary.

This used to be standard audio teaching (1938) but "intermodulation" is so pervasive that today it is hard to find a "music audio" example.

https://en.wikipedia.org/wiki/Intermodulation
https://www.rfcafe.com/references/electronics-world/intermodulation-distortion-electronics-world-february-1960.htm
https://sound-au.com/articles/intermodulation2.htm

Here's a hasty sim. Two tones and a diode distorter. I used one diode b/c I think your VChamp is SE? As the combined waveform approaches the ~~1.2V threshold of the diodes, the amplitude stops rising. This makes little difference on the big 100Hz wave but really smashes the small 1kHz wave.

[acheld]

The Orange Drops are not original.  Nor are the silver electrolytic caps -- are they Sprague? hard for me to read the brand.   

Most of the wires do not appear original to me, but I have worked on only one Vibrochamp of that vintage in the flesh. 

[Voxbox]

Thanks PRR, that quite informative from the engineering theory POV.


I'd expect some IMD, its just the quantity I'm interested in from a practical POV. e.g. my 5E3 exhibits IMD, but at a much lesser level than the Champ. So I'm wonder what peoples hands-on experience is of the Champ in this respect?


Yep, Acheld, I didnt think the ODs were original, as there is also one blue anode cap in there as well. I think perhaps someone has mistakenly changed the anode caps whilst trying to eradicate the scratchy sounding controls, which is a bit of a waste of effort when the circuit board has volts all over it..... 


Anyone with hands-on experience of the Champ in this respect?


Cheers, VB.

[66Strat]

Is that the original can capacitor in place? Ghost notes are a symptom of failing filter caps.

[pdf64]

The IM issue may be a characteristic of the little speaker in the VC - does it happen if the amp is connected to a speaker that you're familiar with?

[Voxbox]

66 Strat,
It happens with a new JJ multi-filter cap as well the original cap unfortunately, with no difference in the effect. I think I know what you mean about the ghost notes being related to a failing filter cap. From recollection, if a high single note (above the 12th fret?) is played, then there can appear to be a following lower note if the cap is failing? The single notes are fine though, its just the noticeable modulation by the open E string, or A string for that matter.
BTW, There is little difference in sag between the two filter caps, the new just recovers quicker. Practically, i.e. from a playing perspective there is no discernible difference to these ears!


pdf64,
interesting point, I tried the Jensen P12 in the 5E3 Deluxe but it exhibits the same symptom.


I've put a recording in the  zip file. The first three notes are without the bass note, the next few notes have bass note added so you can hear the modulation. Shoulda tuned the thing first, but it will do......


Cheers, VB

[66Strat]

66 Strat,
It happens with a new JJ multi-filter cap as well the original cap unfortunately, with no difference in the effect. I think I know what you mean about the ghost notes being related to a failing filter cap. From recollection, if a high single note (above the 12th fret?) is played, then there can appear to be a following lower note if the cap is failing? The single notes are fine though, its just the noticeable modulation by the open E string, or A string for that matter.
BTW, There is little difference in sag between the two filter caps, the new just recovers quicker. Practically, i.e. from a playing perspective there is no discernible difference to these ears!


pdf64,
interesting point, I tried the Jensen P12 in the 5E3 Deluxe but it exhibits the same symptom.


I've put a recording in the  zip file. The first three notes are without the bass note, the next few notes have bass note added so you can hear the modulation. Shoulda tuned the thing first, but it will do......


Cheers, VB

I don't hear a problem. It sounds good to me.