Does removing a cathode cap add hum?

[frank57]

o if I remove a cathode cap that's in a gain stage on a 12ax7,
does it increase hum?
I know the gain goes down but there seems to be a split on whether the hum is increased.

[phsyconoodler]

It shouldn't.Unless something is amiss in the layout or something along those lines.Solder joints etc.

[overtone]

My understanding, and I have only recently picked this up, is that the bypass (cathode) cap in the first triode stage does contribute to reducing noise. (Merlin Blencowe and others recommend the first triode stage to be fully bypassed to reduce noise.) So far I have not understood why this is the case and whether "noise" in this case is the same as hum, or perhaps another issue altogether.
Not a great help I know but maybe someone else can expand on this one way or the other.

[DummyLoad]

from experience:  YES.

--DL

[HotBluePlates]

Food for thought: Why would a tweed Bassman need a 250uF cap on the first stage, when a 25uF cap will effectively bypass the stage well below the range of hearing?

It is possible to have hum in a tube due to leakage from the heater to the cathode. There are 2 ways to think about the mechanism by which is can happen, either in terms of current or voltage, but regardless, the amount of leakage (and therefore hum due to this cause) varies from tube to tube. You eliminate the hum by either specially selecting a tube with low leakage (as determined by use in the amp) or you ensure that the cathode is very effectively shorted to ground for any a.c. frequency.

A tweed Bassman was intended to be used by bass players, and any hum or noise in the low end is a problem. The 12AY7 was marketed in the 50's as a special low-hum tube for audio applications, and it is my belief Fender hedged their bet (and avoided specially selected tubes) by adding an extra-big bypass cap to the first stage.

[octal]

The output impedance of a triode gain stage goes up when the cathode bypass cap is removed. So, it's more prone to any hum coupled in from the heaters, lead dress, the supply etc. The high impedance of tube circuits is why we need to (sometimes) be so careful with lead dress; when you're driving a 20K volume pot with a SS circuit, you might hear some low volume hum if you touch a signal lead. When you're driving a 1M vol pot with a tube circuit, all you have to do is get NEAR a signal lead with your finger.

[jjasilli]

Why would a tweed Bassman need a 250uF cap on the first stage, when a 25uF cap will effectively bypass the stage well below the range of hearing?. . .A tweed Bassman was intended to be used by bass players, and any hum or noise in the low end is a problem. The 12AY7 was marketed in the 50's as a special low-hum tube for audio applications, and it is my belief Fender hedged their bet (and avoided specially selected tubes) by adding an extra-big bypass cap to the first stage.

Thanks for this explanation.  I've often wondered about this.

[The_Gaz]

I think the idea is to get the noise floor as low as possible by increasing the gain of the first stage (where signal is lowest, and most prone to noise). If you bypass the first stage, the signal can get that  much higher than the noise. In practice, it really depends on the rest of the amp. In a low/mid gain amp, I can't imagine much noise would materialize if everything else was okay.

[plexi50]

Interesting thought. I never thought about hum being present without a cathode bypass cap. I always thought of the bypass cap as beefing up the tone. Now that you mention this i can see that without a bypass cap where you would have a more sterile tone without a buffer (cap) to smooth out any hum or buzz. Cool thinking

[HotBluePlates]

The output impedance of a triode gain stage goes up when the cathode bypass cap is removed.

Good point. Just remember that in this case, we're talking an apparent increase of the internal plate resistance of the triode, whereas your other example was a higher input impedance.

But let's think about B+ ripple, the plate load resistor and the internal plate resistance. To get a measured gain from a triode stage as close to mu as possible, you want a very large plate load resistor and/or a very small internal plate resistance. The two essentially form a voltage divider which causes actual tube gain to be lower than the triode's mu.

If you have an unbypassed cathode resistor, the internal plate resistance goes up, and measured triode gain is lowered. You could say the internal plate resistance is a larger portion of the sum of the plate load resistor and internal resistance. That also means you could say the plate load resistor is a smaller portion of the sum of the two.

Ripple voltage at the filter cap feeding the stage is attenuated before reaching the coupling cap by a relatively large plate load resistor and relatively small internal plate resistance (same conditions as for high gain in a triode). The increased internal plate resistance of the unbypassed triode means the voltage divider effect to B+ ripple is not as effective in reducing the ripple voltage, so more reaches the coupling cap (and is then passed to the next stage's input).

I didn't bother mentioning it before, because in most Fender-style stages, B+ ripple is sufficiently small that I wouldn't consider this a big source of hum.

[frank57]

In the case of this little hiwatt I tried this experiment and was able to get rid of a lot
of the hum and bzz issues. When flipping the cathode cap between the two v2 stages,
hum goes down a bit when you put the cathode where it was originally and also get a bit of buzz with the gain high.
Flip over to the next v2a and the hum goes up a little with the master past noon, and less bzz or noise from gain.
The gain goes down a little which in this case is a good thing.
Is it a case of gain plus more gain in the original cap  position causing some extra noise because of the bad layout etc?
Can any problems arise from the bigger 47uf cap in the preamp ?

[Geezer]

Can any problems arise from the bigger 47uf cap in the preamp ?

No "problems", but the preamp may be a bit "stiffer", which may (or may not) be what you want......let your ears decide, but it won't hurt anything.

G

[phsyconoodler]

hmmmm....just built an added gain stage on a PR circuit and it has two un-bypassed cathode resistors and it doesn't hum.I'm thinking WHERE that cathode bypass cap is is rather important.The input side of the triode is obviously where it does the most good.
  I have also built a lot of 2204's that have an un-bypassed cathode on one triode on V1 and they don't hum either.

[frank57]

I think the cathode cap hum(more or less) can be subtle.
I think you're right about the where part.
You might not notice it  as much on a better built better designed amp.
It might show up more when you turn up the master way up lets say.
The 47uf cap seems to be ok I think.
I think the distortion is a little harsher, but maybe because I didn't put back in the roll off cap on the plate.
 I do have more bass overall but that's in case I reconnect the efx loop which will cause problems and I might have to lower r8.

I did have an interesting very Hiwatt feel two stage version.But very tough to play on.You really had to whack it.I couldn't figure out how to get more gain out of it.
Maybe raise the resistors for the power tubes from 100 to 1k.
http://www.el34world.com/Forum/index.php?topic=11524.0
It sounded like this:
http://www.mediafire.com/?0f4ela4im33a6eb
Overdrive now is similar sounding.

[HotBluePlates]

hmmmm....just built an added gain stage on a PR circuit and it has two un-bypassed cathode resistors and it doesn't hum.I'm thinking WHERE that cathode bypass cap is is rather important.The input side of the triode is obviously where it does the most good.

Well, let's not get crazy with the idea.

A lack of a bypass cap might increase hum. Or it might not. Or it might increase it, but not enough to really matter.

Any noise injected at the first stage of your preamp is going to be more trouble than the same noise voltage applied at the output tubes. Your small guitar signal is competing with that noise voltage, and the noise is being amplifed by the entire amp as much as your intended signal is. A 50mV noise at the first stage is very big when your intended signal is maybe 100mV; it's nothing at the output tube when the intended signal is 32v RMS hitting the output tube grid.