Cathode Bypass Caps

[lions den]

I am finishing up my Rocket build and want to implement a multiple cathode bypass cap for different gain settings.

Planning to have single quarter inch jack to a Normally Open footswitch that can real time switch the bypass caps in our out. In addition I want to put a small switch on the amp to choose between a single 20uF or 2X20uF caps in parallel. this would not be switchable via pedals, but a make it and go switch setting.

As I draw it out, I was planning to come off the cathode of V1 pin 3, go to the positive side of both 20uF capacitors, then have the small switch control whether the negative side of both capacitors or just one is sent to the footswitch jack which will ultimately complete the path to ground for these caps.

So is it better to put the switch on the negative side of the caps or should I put it on the positive side?

Would one create more of a chance of an audible pop when engaging, etc.
thanks,
sean

[Jana]

Put the switch on the negative side. To reduce pops, have a 47K resistor from the negative of the cap to ground and have the switch in parallel with the resistor--you are either shorting out the resistor or not.

Having said that, I doubt you are going to notice any appreciable difference between 20uf or 40uf in this position. A better choice, if you are set on switching a cathode cap, would be a .68uf and a 25uf. Switch the 25uf, leave the .68 in all the time.

[lions den]

Jana,
thanks for the quick feedback. Pic is always worth a thousand words so here it goes. Please excuse my poor skills in powerpoint/pdf, i'm new at this

here is what I was planning: This ends up floating the bypass caps via the open footswitch jack, then grounding them when needed. The toggle switch would be between 2 different cap values, so in essence you have unbypassed, cap 1 value, then cap 2 value. the plan for cap 2 value was to have 2 caps in parallel for a different setting.

where would the 47k resistor go?

thanks again, I can play around with cap values to taste after i get the configuration dialed in on how to implement.
thanks again,
sean

[greekie]

Floating leads to loud pops, like Jana wrote.

The 47K is from the negative of the cap to ground. Positive is connected at all times. Your footswitch is connected to both sides of the 47K and shorts out the resistor when engaged.

Jake

[lions den]

thanks guys, I get it now.

my original thought was that I didn't want to leave the 47K in place but with it in place the 1.5K R5 provides the easiest path so in effect nothing goes down the side with the caps. Just had a lightbulb moment.

thanks
sean

[tubeswell]

You can even try a bigger snubber than 47k (like 1M or 2M2, which is effectively infinite resistance in this situation)

[Super_Reverb]

The resistor in series with bypass cap allows the capacitor to precharge, so that when you switch it in, there is no sudden change in the DC operating point. When you switch in an uncharged cap into the cathode circuit, the cap initially looks like a short and causes the cathode voltage to momentarily sag, while the cap charges up to Vk. The value needs to be large enough so that the series combination of the R and bypass cap does not affect the Rk (800-2700 Ohms).

The configurable low frequency corner you describe is a good idea. The -3dB low frequency (corner) occurs where the capacitive reactance equals Rk. Neglecting minor effects, you can calculate this with f=1/(2*pi*R*C). So, with an 820 Ohm Rk and 0.68u, the LF corner from that gain stage is ~300Hz. With a 22u and 820, f~9Hz. If you double Rk, f ~ 5 Hz. In your experimentation, you also need to consider the low frequency poles created by other gain stages (bypass caps and coupling caps)

When you consider the amount of bypass capacitance required to audibly affect low frequency response of a triode gain stage and the fact that the open E string is ~82Hz, a 22u borders on overkill.

One thing you get with lower values of bypass cap is reduced emphasis on low bass of E, A, and D strings. Timbre of low bass notes is changed by demphasizing fundamental frequencies, bring up harmonics of the note you're playing.

cheers,

rob

[edit] p.s., having the bypass cap grounded through a cord to your remote footswitch may introduce noise, depending on how you do it. A higher tech apprach would be footswitch controlling relays or some other approach to avoid sending the signal out of the amp and back.

[lions den]

Rob
Thanks for the feedback and concerns around the footswitch.

It will be a short run from the back of the amp maybe 12 inches at most to a rack where I have a GCX rack piece with loops. The loop will be set to make/break the short around the resistor. So no long cable runs etc.

I'll give it a try and see how things go. And experiment with cap values for sure.

Thanks again
Sean

[Structo]

Something I did on my 6V6 Rocket was instead of messing with different power tube bypass caps, which will probably more subtle than you want, is to switch in the the optional bypass cap on the second preamp stage.

It's been a while but I think I used a 22uF cap.
I used a footswitch to add it to the circuit and it added a nice boost with more grind.

C4 on the schematic.

[rp]

I assume you're talking about V1/2 and the optional cap? I really don't think you need more than one boost setting so try .68 - 25, find what you like best and use a simple switch. I can't really hear a big difference after 4.7mf so you can prolly try .68, 2.2, 4.7 and use films. Try it with both SC and HB pickups too before deciding and at the volume you use most.

I've been reading up on Rockets, old posts seem to weigh in on no bypass on V1 pin 3, so you may want to live with it a bit before drilling holes.

[Structo]

Well it's definitely a gain boost, as usual season to taste.

[roberto]

I'd go for a 680n-4.7µF on-on switch on first cathode,
and a 680n-unbypassed-4.7µF on-off-on switch on second cathode.

[lions den]

Tom, you are exactly right that I'm switching in the optional C4 on the Rocket schematic. I've been using a 25uF cap and it adds just the right amount of gain.

After playing around with several cap values I'm going to stick with one and ditch the other switch idea. This gives a quick boost with a simple footswitch.

Love the versatility it adds.

Thanks
Sean