Oliver Archut custom made v78 pre for mastering - HPF switch not working?

[Darcy]

Hi all,

Would really appreciate some help here. I have a TAB Funkenwerk v78 preamp that Oliver made custom for a mastering engineer. Uses a different tube in the second stage, looks like it has some special xofrmers etc - whatever it is it sounds great. But the 120Hz HPF isn't working.

I traced the circuit of the LPF section. Also attached is original v72 circuit for reference.

Any reason this isn't working? I thought was faulty cap so replaced but same issue. Switch seems to be working fine too.

As a heads up I am a producer with basic electronic knowledge and super basic tube amplifier understanding. I might be missing something obvious? I don't know how to calculate the RC values so maybe it's doing some kind of 20Hz rolloff since it's for mastering? If I wanted to make it 120 - should I change cap or resistor?

Again any help greatly appreciated - thank you!





=

 

[radardoug]

If it is wired as drawn, you have a short across the switch. It will never work.

 

[capacitorless]

Just my .02 and I'm dead tired, but I think the connection between 1 & 2 is to avoid a pop when switching between HPF on and off. When the switch bridges 2 & 3, the .22uf cap is bypassed. Edit: there may well still be an interaction with the input transformer secondary and the cap in this position.

The 210k resistor could be a reference for the grid (eg grid leak?), but unsure of this and how it might interact with the .22uf cap.

My vote would be to increase the value of the .22uf cap. This is what I'd do in a guitar amp to increase the low frequency bandwidth (and vice versa for removing bass from a signal).

 

[dmp]

The 3dB high pass frequency for 0.22uF & 210k is about 3 Hz. Formula is 1/(2*pi*c*r)
To get to 120 Hz you would use a smaller capacitor. About 6 nF.