Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on January 26, 2011, 12:18:42 am

Title: Swiches and Switching cathode caps
Post by: J Rindt on January 26, 2011, 12:18:42 am

I looked in the library, and found info about switching gain stages, but not cathodes. If you want to make a by-pass cap switchable (perhaps with two different caps), can you simply switch the cat cap in and out of the circuit.?
If so, does it matter if you lift the ground side or the tube side of the by-pass cap.? Also is there likely to be a "popping" problem.?
Thank You

Title: Re: Swiches and Switching cathode caps
Post by: Geezer on January 26, 2011, 01:08:03 am

Quote

does it matter if you lift the ground side or the tube side of the by-pass cap.? Also is there likely to be a "popping" problem.?

You can do it either way, but for "anti-pop" you need to put a large(ish) resistor across the outside terminals of the switch. This "trickle bleeder" will pre-charge the cap to reduce the pop factor when switched into the circuit.

Here's a pic....HTH, G

Title: Re: Swiches and Switching cathode caps
Post by: kagliostro on January 26, 2011, 01:22:15 am

If you have enough space you can substitute the switch with a pot connected as rheostat and use a larger cap

so you have always the cap in charge and can insert it as a progressively boost to achieve different effects on sound

(all resistance ON = cap OFF / all resistance OFF = cap ON / Middle resistance ON = cap Middle Effect)

being in series with the cap there is no DC on the pot and it will not scratch

try a 4.7kohm pot and some different cap

Kagliostro

Title: Re: Swiches and Switching cathode caps
Post by: J Rindt on January 26, 2011, 09:45:52 am

Both of you guys ......Thanks.
That "anti-pop" resistor is what I was trying figure out.
Real estate is a big problem, or I would go for suggestion Number Two. Perhaps another time. I will print and save both schems for future use/education.
Thank You So Much

Title: Re: Swiches and Switching cathode caps
Post by: Fresh_Start on January 26, 2011, 12:21:24 pm

Just for fun, here are four different cathode switching schemes.  I use 82K resistors because I've got a bunch of 'em but 100K works just as well.  Note that all but one only need a single-pole/single-throw switch.  Also, an anti-pop resistor always connects each cap to ground regardless of switch position.  That way the cap stays charged and switching noise is minimized.

Note:  the bottom two approaches change the bias point of the tube and may create a "pop" or other noise despite my best efforts.  I've used all of these except for the lower, left one.

Cheers,

Chip

Title: Re: Swiches and Switching cathode caps
Post by: 38Super on January 26, 2011, 02:57:22 pm

Kagliostro,

How does your progressive bass cut circuit work?  The 250pF coupling cap will be swamped by a factor of nearly 200 by the 0.047u cap, whether the 1M pot is set to zero Ohms or 1M.

The triode/pentode morph control looks innovative.  Is there a noticable difference between the two modes?


thanks,

rob

Title: Re: Swiches and Switching cathode caps
Post by: kagliostro on January 26, 2011, 03:26:34 pm

Merlin docet  :wink:
 

that draw was revised in the tone control (not a great change)

what I want to show here was the progressive boost


Kagliostro

Title: Re: Swiches and Switching cathode caps
Post by: Backwoods Joe on January 26, 2011, 06:56:39 pm

Kagliostro, Please tell about the bass contol after V1 & the triode-pentode morph control. This stuff looks good on paper but how do you like it? Also I don't think Mr. Merlin would approve of the R values have fitted on V1. :wink:

Title: Re: Swiches and Switching cathode caps
Post by: Geezer on January 26, 2011, 10:47:30 pm

Quote from: Backwoods Joe on January 26, 2011, 06:56:39 pm

I don't think Mr. Merlin would approve of the R values have fitted on V1. :wink:

Those are typical "Voxy" values (AC15/30), although I use the ValveWizard values on my 5879 amps & love it! (see attachment)

Title: Re: Swiches and Switching cathode caps
Post by: kagliostro on January 27, 2011, 06:26:16 am

Quote

Those are typical "Voxy" values (AC15/30)

Sure

Quote

I use the ValveWizard values on my 5879 amps & love it!

which B+ do you use on plate resistor of the 5879, 240v ?

Kagliostro

Title: Re: Swiches and Switching cathode caps
Post by: Geezer on January 27, 2011, 06:35:49 am

Slightly lower (~~220v).....I may try lowering the final B+ dropping resistor value to get the voltage closer to 240v & see what happens (for better or worse).

G

Title: Re: Swiches and Switching cathode caps
Post by: J Rindt on January 27, 2011, 09:28:03 am

Fresh Start.....
Thanks for the great info/pics.
I Appreciate it.

Title: Re: Swiches and Switching cathode caps
Post by: Backwoods Joe on January 27, 2011, 11:29:19 am

Hey Geezer, Have you used diodes for biasing? If so, any audible differance?

Title: Re: Swiches and Switching cathode caps
Post by: Geezer on January 27, 2011, 12:00:29 pm

Quote from: Backwoods Joe on January 27, 2011, 11:29:19 am

Hey Geezer, Have you used diodes for biasing? If so, any audible differance?

No, I've not tried that.

G

Title: Re: Swiches and Switching cathode caps
Post by: kagliostro on January 27, 2011, 01:47:23 pm

thanks Geezer

Quote

The triode/pentode morph control looks innovative

as I told  wasn't flour of my bag

initially I've think to put a simply pentode/triode switch like in the Vox AC15HTVH and the complex bass shift - brillance control

then I find that Merlin's suggestions will give a better control and simplify construction about tone control

(may be it will be nice to use a stepped pot or a knob for multiturns pots - my son asked to add this knob in some of his pedal effects)

Kagliostro