Cathode Bias, can cap be switched in/out?

[GlideOn]

I have a push/pull bass pot in my amp and I currently have it utilized to parallel in the tonestack a .068 cap in with the existing .022, but it's not a very dramatic effect as I'd like.

While reading up on fixed vs cathode bias switching in the power amp, I read that having no cap can increase NFB and tighten up the output section, albeit at the expense of a bit of volume.

I do cathode switching via push/pull already on my V1 section and it works quite well, adds a dramatic voicing difference.

So I want to see if I can utilize my bass pot which is typography directly across my shared Cathode Bias RC and use it to add/subtract a cap in the Cathode Bias power section.

I understand there's at least 20-something volts here, but is that an issue that can be mitigated? Possibly use high W resistors in parallel here? Seems to not be an issue with Fixed/Cathode switching.

And If so, would the value of the unbypassed resistor need to be changed in order to maintain a steady linear volume in both push/pull modes?

[B Ingram]

While reading up on fixed vs cathode bias amp switching, I read that having no cap can increase NFB and tighten up the output section, albeit at the expense of a bit of volume.

I do cathode switching via push/pull already ... So I want to see if I can utilize my bass pot ... to add/subtract a cap in the Cathode bias power section.

I understand there's at least 20-something volts here, but is that an issue that can be mitigated? ...

Your output tube cathode bypass cap is in parallel with some cathode resistor of <500Ω, right? So try this:

• Leave the bypass cap connected at the tube cathode (which is also the not-grounded side of the cathode resistor)

• insert a 10-100kΩ resistor between the other side of the bypass cap & Ground.

• Use your push-pull switch to short-circuit across the added resistor to activate the bypass cap.

The series resistor allows the cap to charge, and helps eliminate popping when switch the bypass cap in/out.

20v across 10kΩ implies the resistor dissipates 20v x 20v / 10kΩ = 0.04 watts. Repeat the math to see a 100kΩ resistor dissipates even less. So we see that a 1/2w resistor is overkill in this role.

[Stevem]

You do not want to run output stage signal all the way back up into the preamp section to access the use of that pot switch because you will likely have a oscillating mess of a amp on your hands from that mod.

Get a rotary switch of the type that's makes before breaks as they are called to change out what you want, be it caps and or resistors.
Install it down near the output or power supply section

[GlideOn]

You do not want to run output stage signal all the way back up into the preamp section to access the use of that pot switch because you will likely have a oscillating mess of a amp on your hands from that mod.

Get a rotary switch of the type that's makes before breaks as they are called to change out what you want, be it caps and or resistors.
Install it down near the output or power supply section

Not a bad idea - I wouldn't be opposed to putting on simple switch on the back, but it would be intuitive and convenient to have it on the bass pot of all places. The pot is less than 5 inches away from the cathode (+) side and even closer to power bus ground on the board and I can use shielded wire of decent gauge to mitigate the issue, no?

While reading up on fixed vs cathode bias amp switching, I read that having no cap can increase NFB and tighten up the output section, albeit at the expense of a bit of volume.

I do cathode switching via push/pull already ... So I want to see if I can utilize my bass pot ... to add/subtract a cap in the Cathode bias power section.

I understand there's at least 20-something volts here, but is that an issue that can be mitigated? ...

Your output tube cathode bypass cap is in parallel with some cathode resistor of <500Ω, right? So try this:

• Leave the bypass cap connected at the tube cathode (which is also the not-grounded side of the cathode resistor)

• insert a 10-100kΩ resistor between the other side of the bypass cap & Ground.

• Use your push-pull switch to short-circuit across the added resistor to activate the bypass cap.

The series resistor allows the cap to charge, and helps eliminate popping when switch the bypass cap in/out.

20v across 10kΩ implies the resistor dissipates 20v x 20v / 10kΩ = 0.04 watts. Repeat the math to see a 100kΩ resistor dissipates even less. So we see that a 1/2w resistor is overkill in this role.

Yes, it's currently two EL34B with a shared cathode of 200r/100uf @ 100v.

I'm trying to visualize what you're saying - are you saying the 'anti-pop' resistor is at the positive (+) side of the cathode cap, the "not grounded side?"

This sounds similar to the parallel resistor across standby used to mitigate "in-rush" current, which I like the sound of very much.

All-in-all, this sounds like a very doable thing

Question is, what will be the side effects of going from a fully bypassed cathode (cap) to an unbypassed (resistor only) sound?

I suppose my concern is having equal perceived volume of the two settings

[B Ingram]

... are you saying the 'anti-pop' resistor is at the positive (+) side of the cathode cap, the "not grounded side?" ...

Resistor is on the negative side of the bypass cap. Between the cap & Ground.

... what will be the side effects of going from a fully bypassed cathode (cap) to an unbypassed (resistor only) sound? ...

Try it and find out. I believe you will find it doesn't do what you imagine it will, and that you'll only notice much when you push the power section hard.

I suppose my concern is having equal perceived volume of the two settings

Jump off that bridge when you get to it... You might be trying to mitigate something that won't even happen.

[GlideOn]

... are you saying the 'anti-pop' resistor is at the positive (+) side of the cathode cap, the "not grounded side?" ...

Resistor is on the negative side of the bypass cap. Between the cap & Ground.

... what will be the side effects of going from a fully bypassed cathode (cap) to an unbypassed (resistor only) sound? ...

Try it and find out. I believe you will find it doesn't do what you imagine it will, and that you'll only notice much when you push the power section hard.

I suppose my concern is having equal perceived volume of the two settings

Jump off that bridge when you get to it... You might be trying to mitigate something that won't even happen.

Thanks to this, I know it's not only feasible but a worthwhile thing to try out!

I have used bypassed/unbypassed switching on V1B stage of Marshall style amps, I'd have to say it's cleaner, more linear in the sense there's equal/flatter EQ of all frequencies and perhaps a slight 10% overall volume/output decrease, primarily in the lows and mid lows. Better IMHO if your aim in clean tones and I can see why some amps like Soldano use a straight, unbypassed 2.2k as their clean stage. I really like it.

However, I don't know if that translates to the power amp, hence the primary reason of the post.

Has anyone done or compared bypassed vs unbypassed power tube cathode?

[sluckey]

Has anyone done or compared bypassed vs unbypassed power tube cathode?

This is such a simple thing to do. In the time it takes to read this reply you can just do it. Then you'll have your own opinion rather than trying to decipher someone else's opinion.

[GlideOn]

Has anyone done or compared bypassed vs unbypassed power tube cathode?

This is such a simple thing to do. In the time it takes to read this reply you can just do it. Then you'll have your own opinion rather than trying to decipher someone else's opinion.

It may be a simple thing to try, but it's not quite as simple wiring it all in permanently.

So I need to know it is worth the effort before I commit.

[sluckey]

It may be a simple thing to try, but it's not quite as simple wiring it all in permanently.

So I need to know it is worth the effort before I commit.

Try it before you commit to anything. Unsolder the negative cap lead and play guitar for a while. Now use a gator clip test lead to reconnect the cap and play some more. Simply disconnect/reconnect the gator clip to get a good comparison between the sounds. Not much effort IMO. Until YOU do a simple comparison like this using YOUR own ears, you will never know if it's worth the effort for YOU to make it permanent.

[GlideOn]

Edit: I drew up a switching scheme as @B Ingram suggested, went with minimal 10k (all changes highlighted in Pink).

However, the added resistor is in parallel with the 200r and lowers the overall resistance to 196r-ish. Not a huge deal but it could make the bias a tad hot or over the 90% dissipation sweet spot.

Would there be any benefit to going 100k where we get 199r-ish? Would that have side effect of increased signal noise?

AX84 SEL Soldano Cathode Bais 3.PNG

[sluckey]

Edit: I drew up a switching scheme as @B Ingram suggested

That drawing does not represent what B Ingram said. All your circuit does is switch the 10K in parallel with the original cathode resistor. The cap never gets switched.

[GlideOn]

Edit: I drew up a switching scheme as @B Ingram suggested

That drawing does not represent what B Ingram said. All your circuit does is switch the 10K in parallel with the original cathode resistor. The cap never gets switched.

Fixed! Forgot the jumper wire.

[sluckey]

Still not right. The attached pic is what B Ingram suggested. You only need to use two terminals on the switch but I used three just to make it easy to mount the resistor directly on the switch. Shielded cable is not needed.

[GlideOn]

Okay. Thanks for the diagram btw.

I see now what he was eluding to, but doesn't a resistor in series with a cap limit the effectiveness of said cap? That's not necessarily something I'm looking for.

I've done the 10k + .1uf trick for Marshall Cold Clipper stage in a SIR36 mod to limit the gain and stop oscillation, but it does audibly cut some gain off, maybe in the realm of 10-15% worth.

[sluckey]

I see now what he was eluding to, but doesn't a resistor in series with a cap limit the effectiveness of said cap? That's not necessarily something I'm looking for...

Duh Huh!