Help with the 3rd stage cathode bypass

[Turret]

I might be on a bit of an amp modding spree, and I need your help again.

I have a Bogner 3534, the 35W version. I don’t want to change anything about channels 1 and 2, but the third channel is just too hot for my taste. I tried to tame it by using a 5751 in V2, which affects both the second gain stage (channel 2 and 3) and the third gain stage of channel 3. It sounds great on channel 3, but I'm not entirely happy with the impact on channel 2. The cathode resistor for the third stage is 3.3k ohms in parallel with a 680nF bypass capacitor. I’m considering adding a 3-way switch that center-off to modify this stage.

My current idea is to use a 10k ohm resistor with no bypass capacitor in the center position, to achieve as neutral a stage operation as possible. For the first position of the toggle, I’d like to use the R/C values as close to the stock configuration of the 3534 as I have now, which is a 680nF capacitor with a 5.1k ohm resistor. In parallel with the 10k resistor, this will yield around 3.3k ohms. I believe that the 3.3k ohm and 680nF combo boosts the midrange. For the third position, I’d like an R/C combination that adds medium gain and boosts the full frequency range.

So here are my questions:
1. Will a 10k ohm cathode resistor with no bypass capacitor give me a fairly neutral gain stage in terms of both tone and gain?
2. What R/C combination would you suggest for medium gain and full frequency boost?

[Turret]

Initial idea, thoughts?

[martin manning]

What is the plate resistor value? if it's a typical 100k to 220k, the 10k Rk will bias it cold, and you will get early clipping.
For reference a neutral stage with full frequency response would be 100k Ra, 1k5 Rk, with a 22u bypass cap. If you want to do some reading, get a copy of Blencowe's "The Triode Gain Stage" from the Valve Wizard site: https://valvewizard.co.uk/gainstage.html

[Turret]

Thanks, Martin.

The plate resistor in my 3534 is 100k, just like in the larger models. Looking at the schematic, the third stage always had a 10k cathode resistor paired with the 100k plate resistor. They later added a 2.7k resistor in series with a 220nF capacitor in parallel with the 10k cathode resistor. After 2004, this combination became the standard resistor/capacitor configuration for the Ecstasy.

Schematic attached

Ecstasy correct.jpg

In my amp, I have a 3.3k resistor and a 680nF capacitor, which is mentally on the hot side. Apparently, using a 2.7k resistor and a 220nF capacitor in parallel with a 10k resistor is still fairly hot. CEA was modifying the third stage in the XTC to use a 1µF capacitor and a 10k resistor in parallel with the 10k cathode bypass for less gain. I'm considering arranging my switch in that way, as it kindoff aligns historically.

Schematic with new "idea" also attached

I was reading Merlin's materials, but it's a bit much for my basic understanding.

[Turret]

Martin, thoughts on arrangment in schematic 2? Thank you

[martin manning]

Here are a couple of plots from a simulation of that stage by itself. Input is 500 Hz, 1, 2, 3, and 4V.
- A 10k Rk by itself is cold biased, but the NFB is so large that it stays clean, and has a wide bandwidth.
- The 10k//10k-1u has 4.5 dB more gain, and the 3 and 4V input traces are clipped.
- The 10k//2k7-220n has another 4 dB gain and reduced LF response, with the 2 through 4V input traces clipped. This would be a distorted lead tone.
Note all the clipping is on the negative swing of the input signal, reflecting the off-center bias of the 10k Rk (the DC path to ground).

[Turret]

Thank you, Martin, for taking the time to explain and for the graphs.

I might still go ahead with the three-way cathode switch in the future. Yet, the urge was too strong, so I already made some modifications to the third stage this afternoon. It’s a simple mod: I clipped off the 680nF cathode bypass capacitor while leaving the 3k3 resistor unchanged. The change was positive—there is significantly less drive, but the definition is great. In a way, it has a similar flavour to the blue channel, just with more drive, right in the middle — where I like it — the gain control is now usable across the entire rotation range.

I have kept the 100k 2W carbon film plate resistor for now, but I have ordered the 220k and 330k resistors to see if I like them even better