Best cap to cathode tube bypass is …..

[Katapum]

Hi,

Try it with an unbypassed resistor.

I´m electronic amateur but I think it will decrease a little gain stage, increase output resistance and add negative feedback. Am I right? And is it good?

According Morgan Jones book the stage will needs a 680uF.😱

Katapum

 

[wildswan]

I've been considering using a negative supply to generate the bias voltage for a bass preamp I'm working on at the moment.

I'm trying to eliminate feedback and electrolytics entirely in my design for sonic reasons, and this is the solution I decided on.

The power supply could be very simple and clean (1 diode + RC filter) , because of the low currents involved.

Has anyone tried this method with preamp tubes?

 

[lndm]

It could be done with a battery. Then, instead of putting it in the grid circuit, you could put it in the cathode circuit. You could use a NiCad cell or two as required, and the cathode current will keep them charged.

As the cells are a short circuit (virtually) to AC, they behave as a well bypassed resistor.

Further, there is the LED, which I am choosing to use. It drops a voltage and is a short for AC. SY is leading a thread about it which may be of interest.

Hope this helps.


P.S. you may want to look again at the anode circuit in your schematic, is that what you wanted?

 

[Tubelab_com]

For many simple input circuits a cathode capacitor is a neccessary evil. If a negative voltage is available, I found a simple (but sandy) way to eliminate it and kill Miller effects at the same time. It doesn't work with all tubes though. I can't post a schematic from work, I will post it this weekend from home.

I have tried many flavors of cathode bypass caps and for low voltage input stages I prefer a solid tantalum cap. Low ESR up into the MHz region. Small size, medium cost. A 330uF 6.3V cap is about $8 from DigiKey (478-1898-ND). 150 uF 6.3 volt caps are available (478-1862-ND).

 

[Rodeodave]

I think Morgan Jones (Valve Amps 2nd Edition) points out that for cathode bypassing OSCONs are the preferred part because of the lower leakage current due to the different type of electronlytic-thingy used within these caps (see RIAA-amp).

 

[fdegrove]

Hi,

I have tried many flavors of cathode bypass caps and for low voltage input stages I prefer a solid tantalum cap. Low ESR up into the MHz region.

If you must use a bypass then these tants aren't all that bad but be ware, they're very sensitive to the slightest overvoltage.

Cheers, 😉

 

[coffeedj]

Cathode ByPass Cap

I use surplus Techtronix selected non-polar 25 or 50uf 50V caps for bypass. Just gang as needed. Accuracy is 4 decimal places.

These were built for scopes and are ultra-ultra high quality. Absolutely neutral.

Used to get them from Wacky Willies, but the biz changed hands and now they are not available.

I only have about 50 left. Anybody know where to get these?

 

[maxpou]

Hi Katapum,
i carried out several test with this type of circuit and the best result for cathode bypass cap was elna cerafine and wima 0.1uf. For the value cap does it with listening. But the option of negative bias is very good like describe it Wildswan. Now my circuit have negative bias and it's my best result. Bye!

excuse me for my english.

 

[David Davenport]

With a CCS on the plate do you need a cathode bypass cap?

Try it with an unbypassed resistor. You'll be surprised at how poorly it works.

Hi SY,
I have noticed this. It would seem that a bypass capacitor is not needed but you are right, it works poorly without it. What is going on here?

Dave

 

[SY]

Interesting question. My presumption has been that it's a matter of the effective plate resistance increase. But the distortion seems disproportionate.

 

[David Davenport]

Interesting question. My presumption has been that it's a matter of the effective plate resistance increase. But the distortion seems disproportionate.

In addition to the distortion, in one circuit I noticed a large amount of high frequency roll off: like -3dB at 10kHZ. A small, 0.47uF, capacitor across the cathode bias resistor cured this.

The interesting thing is that this was an Ultrapath, or WE, connection with a CCS in the plate and the parallel load connected between the plate and cathode. Theoretically there should have been no signal current through the cathode bias resistor so the capacitor should have had nothing to bypass.

Dave