Calculating -3db frequency for coupling cap in dual rectifier style schematic

[Porpoise Head]

Dear all,

I just introduced myself in the introduction section of the forum and thought it was about time for my second post 

I'd like to build a preamp with two relay switchable channels for which I attached a schematic created in LTspice. Channel A ('Thalassa') is basically a Matchless Lightning. Channel B ('Persephone') is basically a Mesa Dual Rectifier Rev. F orange channel.

I'd like to switch as little as possible because I don't want to much wires running back and forth with the high gain Persephone. Furthermore, I tried to keep the number of triodes down a bit. So the two channels share the input stage (matchless style) and share a mosfet output buffer. Two relays attached to one buss will switch inputs (ground unused input) and outputs (ground unused output).

The input stage of the actual dual rectifier has a 1uf cathode bypass cap parallel to a 1k8 cathode resistor. According to one dual rectifier expert (https://warpedmusician.wordpress.com) it tapers frequencies below around 95 Hz (cathode bypass cap) and further reduces frequencies below 32 Hz (anode coupling). I'd like to have the same response in my preamp. But I need a 10uf cathode cap for the matchless side of things (all guitar frequencies passed). I could switch cathode caps using a 3pdt relay but I'd rather keep the amount of wiring down. The most sensible thing (I think) is to reduce the value of the following coupling cap of the Persephone. In the dual rectifier it is 20nf which passes all guitar frequencies. I'd like to lower the value until I reach a cutoff frequency of ~100 Hz. To indicate the uncertainty I marked the cap as 999nf on the attached schematic.

The problem lies in determining the load 'downstream' of the coupling cap. This load is needed for both the cathode bypass capacitor calculator (Amp Books) and the coupling capacitor calculator (Amp Books). The load is called 'RV' in the coupling cap calculator (V for volume pot). In the Persephone, this resistance may consist of several parallel resistances, but I am unsure which ones to include. We have: a 510k and 470k in series with the signal path. We also have a 2M2 to ground for the filtering network prior to the gain pot and the gain pot itself which has a value of 250k and has audio taper. Is there anyone who can help me out?

Thanks in advance,

Hans

[pdf64]

The load is 2M2 // (510k + 250k)

The output impedance of the input stage may be around 20k.

[HotBluePlates]

... I attached a schematic created in LTspice. ... The problem lies in determining the load 'downstream' of the coupling cap. This load is needed for both the cathode bypass capacitor calculator (Amp Books) and the coupling capacitor calculator (Amp Books). The load is called 'RV' in the coupling cap calculator (V for volume pot). ...

If you're using LTspice, why would you not use it to figure out the frequency response and do the load calculation for you?

If you're not familiar with LTspice enough to do that, then skip the theorizing a bit and expect to adjust-on-test.  The capacitor can be changed after you build the amp and actually hear the effect (because using numbers only, you will likely guess wrong about what the response should be at any given frequency).

[Porpoise Head]

Thanks for the response pdf64 and HotBluePlates!

@ pdf64: does the 470k grid resistor not play into the load resistance?

@ HotBluePlates: good suggestion. I use LTspice all the time for opamp-based circuits. Tried using it for tubes before but got frustrated because it was not working. Your suggestion got me back on the horse. I downloaded RobRobinette's 5E3 LTspice circuit and started from there. Got it working this time. The power supply is nowhere near the actual one but this should not really matter for the AC analysis.

Curious what you think about the LTspice schematic and results (because with LTspice, garbage in = garbage out). I attached the two schematics and a comparison of the frequencies. The schematics have references to the colour of the lines.

Thanks in advance, Hans

[pdf64]

Thanks for the response pdf64 and HotBluePlates!

@ pdf64: does the 470k grid resistor not play into the load resistance?
…

In a simple linear analysis, no, the grid doesn’t draw any current, other than that required to charge its Miller capacitance.
See grid stopper http://aikenamps.com/index.php/grid-resistors-why-are-they-used
http://aikenamps.com/index.php/what-is-miller-capacitance

[Porpoise Head]

Thanks, that helps!