How does REMOVING a 25uf cathode cap make things nutty?

[dehughes]

I'm totally frustrated and totally baffled....

I have an AC30ish amp that now has the 1st 12AX7 (V1) run with both triodes in parallel (I just had one triode wired up...but recently decided to try running them both in parallel as I had no other use for the unused triode). Okay, fair enough....the triodes in parallel thicken things up a bit, sounds kinda nice. Yes. Okay, well, let's try reducing the gain a bit as I seem to have more than I really need....hmmm....okay, let's try REMOVING the 25uf cathode cap on V1 and just leave the 1k5 cathode resistor. That should do it, yes?

WRONG. Well, it did something, but that something was to introduce a very grating, BLATTY, super annyoing gimpy distortion when the preamp gain is turned up. Gain down, no problem. Heck, even running a 12AY7 in there (way less gain...) didn't seem to cause trouble, but boy, put that 12AX7 back in there and crank the preamp gain and you have a world of suck.

Seriously, what is going on? I've re-flowed the solder joints by the cathode resistor and replaced it with a brand new one, and I'm still having issues. How is it possible to REMOVE a 25uf cathode cap and get what seems to be MORE distortion? This makes no sense....it sounded just fine with the triodes in parallel AND the 25uf cathode cap in there. I guess that's what you get for working on your amp when you're at home sick...

Your help is, as always, greatly appreciated... I'm totally stumped...

[dehughes]

OH, and yes, I've tried other 12AX7s in this position, so I know it isn't a tube issue. Odd...

[tictac]

sounds like you may have a grounding problem, check the grounding of the cathode resistor with an ohm meter. So why would you remove the 25uf anyway? If you need to lower the gain you can raise the cathode resistor value and keep the cathode bypass cap in the circuit.

TT

[tictac]

do you have grid, cathode, plate of both tubes in parallel ?

[jelle]

what happens when you bypass it with 1uf or .1uf or even 100pf?

The bypass cap also stabilises the triode....
Jelle

[dehughes]

sounds like you may have a grounding problem, check the grounding of the cathode resistor with an ohm meter. So why would you remove the 25uf anyway? If you need to lower the gain you can raise the cathode resistor value and keep the cathode bypass cap in the circuit.

TT

Possibly....I've checked and re-checked and reflowed the grounds, so I'm pretty sure they are correct. Thanks.

A couple gents on the 18w Forum suggested that I'm experiencing blocking distortion due to removing the cap, paralleling the triodes, and not adjusting the cathode or anode resistors to set things straight. I'm inclined to think this is more of a possibility...

[dehughes]

do you have grid, cathode, plate of both tubes in parallel ?

Yup!

[dehughes]

what happens when you bypass it with 1uf or .1uf or even 100pf?

The bypass cap also stabilises the triode....
Jelle

Dunno...I'll go find out.

I've heard that the cap stabilizes the triode...so I may be experiencing a rowdy gain stage and not realizing it. Thanks!

[Allynmey]

The bypass cap also gives errant AC voltages a path to ground. Otherwise, Bias can be altered by a superimposed AC voltage on the cathode.

[unklmickey]

...A couple gents on the 18w Forum suggested that I'm experiencing blocking distortion due to removing the cap, paralleling the triodes, and not adjusting the cathode or anode resistors to set things straight. I'm inclined to think this is more of a possibility...

i don't know the proper definition of blocking distortion, so i won't even use that term.

when you paralleled the 2 triodes, you now should have twice as much current.
actually, the increase in cathode current will increase increase the drop across the cathode resistor, and the difference between the grid and the cathode, so you really don't.
more like half the original current through each triode.
so you may be in a highly non-linear region, near cutoff?

you might try half the values for Rp and Rk, and see if that clears things up.


cheers,

unk

[dehughes]

It was the removal of the cap that did it. Apparently not having the cap in there made the stage too unstable.... So, now I know...paralleling triodes is a whole 'nuther ball game compared to conventional triode wiring in an AC30...

THANKS GUYS!

[jelle]

Thanks Allynmey for the background on this. Most times I forget the reason behind these things...

dehughes, do the smaller caps solve things as well or did you put the 25uF cap back?

Jelle

[dehughes]

Thanks Allynmey for the background on this. Most times I forget the reason behind these things...

dehughes, do the smaller caps solve things as well or did you put the 25uF cap back?

Jelle

I dunno....should have tried that, but I just put the 25uf cap back in. SUPPOSEDLY you can just run any value of cap in there to stabilize the stage, but I've not experimented with that (and am too chicken now to go de-soldering what now works so well...).

[mhuss]

Bias can be altered by a superimposed AC voltage on the cathode.

Bias (in my mind) represents the static, steady-state, no-input condition. Once you add input (AC), then all points that are not AC grounds will change with the input.

As KOC pointed out in one or more of his TUTs, the gain and frequency response of each stage interact to produce the sound heard at the output. Change any one stage, and the sound will change, sometimes (as you discovered) for the worse! I suspect by killing the gain of the first stage, you made another stage work harder, and the overall effect was not positive.

--mark

[FUCHSAUDIO]

Let's not forget that the cathode is another signal input into the tube. It's possible that you could be getting some kind of feedback from the output of the power amp, or another circuit as well. Just a thought....