Teo's test

[greiswig]

In the thread on OD brightness, Teo had made the following statements:

At least you can always try this: dime every control to max but turn the OD ratio/volume to zero and then play -> can you hear some subtle thin sound out of the speaker?

cheers
teo

-and-

if you do this test, and you hear something, that means that you've got undesired coupling somewhere?

yes

it could be a sign of coupling, a bad grounding, or something else that isn't working properly

you can test that way because with controls maxed the preamp signal reaches 250vpk-pk very easily and with lots of high freq harmonics

I put signal into my input from a CD player, playing comparably loud on the clean channel to what a guitar would be. I didn't dime any of the controls. Switching to the OD channel with the "Level" pot turned all the way down and the "Drive" pot turned up part way, there was distorted signal coming out of the speaker. Reversing it ("Level" part way up, "Drive" down) there was still some signal, although it was clearly not as distorted.

So...does this represent some kind of undesirable coupling? How does one begin to track down where it comes from if so? Are there builds of this same amp (D'Lite) where this does not occur? And other than additional HF content in the OD channel, what other effects might this have?

Thank you!

[Fischerman]

I don't really know for sure but when I first read that in the other thread my first thought was; "You're always gonna get some bleed." If you have all that signal going through all four preamp triodes it seems likely that at least some will bleed into the PI (I editted this sentence cuz I worded it opposite of what I meant).

I also think that if you do this you have to make sure that your OD Level pot actually goes all the way to zero...often they'll only go down to say 60-100 ohms.

But that's just based on limited experience and a little intuition...I'd like to hear from others if it's realistic (or even possible) for there to be no bleed at all. I've never tried this on my ODS but when I completely disconnected the preamp from the PI in a Marshall amp of mine I had quite a bit of bleed and all the knowledgable folks on Ampage said "some bleed is totally normal".

[Fischerman]

Also, your experiment with turning down the Drive pot sounds perfectly reasonable. Now the 4th triode isn't getting anything and that's the triode that does the bulk of the distorting and usually has a ginormous signal on it's plate...now it's plate is just idling.

[greiswig]

Thanks, Fischerman. You're getting at the heart of what I'm wondering: how normal is this? What level of bleed is actually detrimental?

[llemtt]

Apart from level considerations I can think also that there should be some "good" bleed and some "bad" bleed, It could explain why some amps sound good and some don't, but to me this is just a little more than a new born idea...

Phase relationship between coupling signal could be an important element because it will determine behaviors such as enhancement or cancellation of some frequency and even oscillations.

I remember some people, Ayan maybe one among them, suggesting to use some chopsticks to move wires and thing around while playing the amp just to discover what changes the sound.

Teo