I'm analyzing the Mesa Dual Rectifier

[warpedmusician]

Hey everybody, I have begun some analyses for the Mesa DR 3 Channel. Some of the content is for lay people; some of the content is about general concepts behind the design and what is happening. The goal is too help people understand it for use or to understand it for future amp design (good or bad). Right now, I've become very ill and injured and I have to pause my work on this. Since I have to wait, I figured I could make sure facts are actually facts and have a chance to recheck my work when I am able to proceed.

If anyone would like to take a look at what I've done so far and supply feedback, I'd appreciate it. I am mainly looking for corrections regarding the filtering or my interpretations if I've made any errors about objective, factual matters. If there is a difference regarding my opinions, please save fighting with me. I'm not interested in that. If there is potential to address something that makes changes in approach or statements to current (and future) content, I'd like to consider it.

Since my work is mainly about filtering and concepts , it's light on math, because a person would be able to calculate it or download an EE calculator, but it's sometimes not clear to newer enthusiasts what the filtering is meant to accomplish.

Thanks in advance and I appreciate your time.

https://warpedmusician.wordpress.com

[Dave H]

Welcome warpedmusician. If you want to calculate more accurate values for the -3dB points you have to take into account the impedance of the tube's cathode in parallel with the cathode resistor which could make the -3dB point about an octave higher (150Hz ?) for the 1k8, 1u example.

[warpedmusician]

Thank you, Dave. I appreciate that. I have something that's come up, but I will apply that suggestion and credit you with a thanks when I am able to do so.

[pdf64]

Just to note that the cathode bypass isn't what may normally be considered a high pass filter; rather it's a shelving type.
A high pass filter rolls off the low end at -6dB (or -12, -18 etc) per octave and so would have zero output at 0Hz, whereas the lower shelf of the cathode bypass response extends down the 0Hz.
In regular tube stages, there can only be a few dB between the shelves of a cathode bypass response, so it's not as effective a response shaping tool as, eg, a coupling cap RC high pass filter.

The overdriven tonality of a bypassed cathode stage and (otherwise identical) unbypassed cathode stage can be rather different.
In the case of the partial bypassing used by the Dual Rectifier etc, the frequencies affected by the different shelves may have differing overdriven tonalities. The 100 ohm resistors in series with the cathode bypass caps may act to even that out.

[Dave H]

Just to note that the cathode bypass isn't what may normally be considered a high pass filter; rather it's a shelving type.

Here's the 1k8 1u frequency response.

[warpedmusician]

Thanks for reminding me about the filter type. The 100 ohm resistors do even out the response, but I had a difficult time finding info on it.

Dave, what program is that?

[Dave H]

Dave, what program is that?

LTSpice, the top download here

[warpedmusician]

Thanks. I have that. When I first got it, I was working 60 hours a week. I didn't have time to learn it and forgot I had it. LOL. I'll have to putz around and figure it out when I can. The help from both of you is much appreciated.