Surprising finding on BF Fender

[sbirkenstock]

Hi,

just put together my first own amp, transformers are BF Vibroverb style.
Tinkered around and put a switch after the .001 cap to the grid of the PI.
So I could switch back and forth and try out different values / brands / types.

On time I only had one cap installed and switched accidently to the other position. So the preamp was not connected to the PI any more.
But I still got signal!
Pretty low, but if I turn up the guitar and amp to 10, it is still loud as a kitchen radio.

To check for my project I have an open vintage BF Vibrolux sitting there and I soldered off the wire going to the PI grid.
Just to check out if I did something wrong.
Same result! The signal still get´s amplified, even if the preamp is not connected to the PI....

Any explanation on this?

Also I saw that the .001 in my vintage Vibrolux is a ceramic disc type.
Is this common for BF Fender?
Got a hole bunch of .001 (and .0012, .0018 etc) of blue molded as well.
I don´t like them much.
I actually prefer way bigger values, even a 0.1 orange drop works well for me.
Ether value does not seem to have a big effect on the bass response.
It is not getting boomy or anything even on higher volume.
I´m not sure if I did do anything wrong.
Any hints on this?

Thank you!

Stephan

[Stevem]

It's going thru the power supply,that's why when these caps go somewhat bad the amp can have a problem of oscillation when the gain stages can not decouple from each other enough!

[pdf64]

Any explanation on this?

Capacitence; it links everything.
Consider all the materials that are transmitting the pre-amp signal, at maybe 200V p-p, and the high impedance at the power amp input.
It only takes a few pF between them for signal to pass.
Good layout and lead dress help prevent it being a problem.

Also I saw that the .001 in my vintage Vibrolux is a ceramic disc type.
Is this common for BF Fender?

I believe so. There should be some photos of BF Fender circuit boards etc on the internet, should you wish to confirm this.

I actually prefer way bigger values, even a 0.1 orange drop works well for me.
Ether value does not seem to have a big effect on the bass response.

No reason it should. A high pass filter can only cut bass, and with an input impedance of a few M ohms, even a 1nF coupling cap is only knocking a dB or two off lowest guitar notes.
All that higher value caps do is avoid that loss.

My view is that the benefit of a lower value cap there comes into play when the amp is overdriven.
When that occurs, the power amp / LTP input impedance is reduced and the low end probably does get rolled off; this is probably beneficial mitigation for blocking distortion.
SF Fenders tended to have higher value caps at the power amp input, but this was balanced by having lower values for the coupling caps to the power tube control grids (and lower value grid leak resistors).
So to avoid bad results, consider being guided by that approach.