Twin Reverb RI question

[gui_tarzan]

Does anyone have the faintest idea why someone would remove a cathode capacitor (C14) in a reverb circuit? This one is missing the cap, the leads were cut off at the top of the PCB. The rest are all there and I don't see anything else that's non-stock, but it puzzles me why someone would remove that part.

[matt h]

(deleted)

[gui_tarzan]

Interesting. Thanks!

But I should also ask (quite humbly) why would removing it cut the power of that circuit in half?

[tubeswell]

Interesting. Thanks!

But I should also ask (quite humbly) why would removing it cut the power of that circuit in half?

Because an unbypassed cathode resistor is subject to cathode current feedback, which (all other things being equal) reduces the gain of the stage. (And a gain stage with an unbypassed Rk is about 1/2 the voltage gain of a stage with fully bypassed Rk)

[pops]

While it gives 1/2 the gain, it gives a flatter frequency response.

[Reeltarded]

And you can kill frequencies by shunting instead of bypassing. You might drop the resistor as you do this and find a sweet spot.

Throwing out options.

[pdf64]

The reverb driver cathode bypass cap causes horrid distortion in that channel when overdriven.
It's not a problem as the power amp overdrives way before that, masking the nasty.
Perhaps a previous owner was tinkering with a master volume, use of which tends to make the yucktone apparent.

[rp]

Would this then be a good place for a diode/led to bias?

[martin manning]

Diode bias also eliminates the local NFB and stabilizes the cathode voltage. It's not the cap itself, it's the increased gain.

Pops, frequency response is unaffected if the bypass cap is large enough to bypass all audio frequencies.

[gui_tarzan]

That's all even more interesting. So I'm thinking the previous owner may have not liked having the distortion going through his reverb and that was one way to cut it down.

[Reeltarded]

I prefer less input to the reverb than those amps provide at all those spiky frequencies. Smoother and give it hell. Much nicer.

[gui_tarzan]

After running this for some time it developed a whoooosh sound that got real loud. Since it's almost twenty years old and they used IC caps I figured it wouldn't hurt to replace the cathode caps in case one was acting up and might as well do the filter caps and bias cap along with them. Cheap insurance, right?

When I was removing the bias cap I found that one leg wasn't soldered. Hm! I did some digging on another site (MEF) and found that someone else had the same problem back in 2012 - one leg not soldered on the bias cap. I wonder how many left the factory like this?