On conjunctive filters

[Bruce / Mission Amps]

Fried Bacon

The .0015uF and 10K RC filter should have a F3 of around 10.5KHz to 11KHz.
That seems like a decent choice to me for most guitar amp speakers and my experience with visible noise on my scope at around 12KHz to 15KHZ.
Although I didn't hear a gigantic difference in overall tone, I recently used a 15 watt 12K resistance with two .0047uF/630v caps in series,
all across the 8K OT primary on a 15 watt 6V6 "harp tuned" amp and my customer/player really liked it with the filter in all the time... so I left it.
The F3 on that filter should have been around or <6KHz.
Yes, the 15 watt resistor gets slightly warm to the touch.

[Chuck H]

Thanks Bruce. That's nice reinforcement and support. I'll repost if the 10WR/1kv cap combo fails.

And no, there isn't much audible change. But with bright speakers there's enough to merit the filter. Also, it's an aesthetic thing. It's just very satisfying seeing a nice symetrical waveform

I've seen it posted here that CF's are a crutch or a band-aid for an otherwise inferior build. I disagree. When you consider the sometimes extreme circumstances we put guitar amps through anything you can do to make them behave is Ok by me. Plus it comes down to the question' How does it sound?'. I'm not going deny a design option just because it was a crutch for cheap transformers in old table radios. Some of the coolest circuits for tube gear came from the golden age when whacky dudes in white coats with bushy eyebrows considered this technology important. When I see some of the innovations from that era I can't help but think that these same designers could just as easily been "gurus" and heroes today.

Chuck

[Old Tele man]

..the conjunctive filter was shown in the 1933 RCA Tube Manual...so, it's been around for a LONG time.

[octal]

I've seen it posted here that CF's are a crutch or a band-aid for an otherwise inferior build. I disagree. When you consider the sometimes extreme circumstances we put guitar amps through anything you can do to make them behave is Ok by me. Plus it comes down to the question' How does it sound?'.
Chuck

I'm with you on this, Chuck. Let's face it- if you're going for output tube overdrive, your opportunities for "post distortion EQ" consist of 1.) a conjunctive filter 2.) designed in HF roll off in the output transformer and 3.)speaker selection. Unfortunately, global NFB may be a bit incompatible with a more aggressive conjunctive filter or an OPT with strong HF roll off; i.e. if you have too much phase shift in the passband, the amp might ring or be unstable.