Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: zircontweezer on July 10, 2021, 02:24:47 am

Title: Coupling capacitors in series?
Post by: zircontweezer on July 10, 2021, 02:24:47 am

So I've got a six way rotary switch cap selector off the preamp pentode with 130Vdc on the rotary switch. Is it possible to have a coupling cap before the rotary switch selector that passes the full frequency AC but blocks the HT? I just don't like the idea of the HT next to the input jacks and front panel. The only examples I can think of are bright caps over volume pots and dividers.

Title: Re: Coupling capacitors in series?
Post by: kagliostro on July 10, 2021, 03:23:55 am

Yes, you can do that, only remember that the added cap will be in series with the cap on the rotary switch so the value of the caps on the rotary switch must be changed according to the in series cap value to obtain the standard cap frequency cut response (an in series very large value cap will not affect very much the circuit)

Something similar was on the Matchless, look here the
Rotary Selectortone (other use a button instead of rotary switch but principle Is the same)

Franco

Title: Re: Coupling capacitors in series?
Post by: zircontweezer on July 10, 2021, 03:39:34 am

Thanks Franco the smallest value on the rotary is 360pf and the largest 0.01uf. Something "quite similar" to the Matchless

Title: Re: Coupling capacitors in series?
Post by: tubeswell on July 10, 2021, 03:41:43 am

Quote from: zircontweezer on July 10, 2021, 02:24:47 am

... Is it possible to have a coupling cap before the rotary switch selector that passes the full frequency AC but blocks the HT? ...

Yes but C in series; 1/Cs = 1/C1 + 1/C2 so your 'full frequency' cap wants to be a least 5 x the capacitance of your following cap in order to keep the frequency response about the same as what it would be without the 'full frequency' cap (if you follow me)

Title: Re: Coupling capacitors in series?
Post by: sluckey on July 10, 2021, 05:13:24 am

I used a .1µF blocking cap on my amps. Here's one that is probably exactly what you describe (switched caps are different values)...

     http://sluckeyamps.com/dual_lite/dual_lite.pdf

     http://sluckeyamps.com/dual_lite/big_guts.jpg

Title: Re: Coupling capacitors in series?
Post by: pdf64 on July 10, 2021, 06:06:15 am

Quote from: zircontweezer on July 10, 2021, 02:24:47 am

… I just don't like the idea of the HT next to the input jacks and front panel…

Consider how tiny the spacing between the input valve’s grid and anode is - a fraction of a mm. And the grid circuit goes out to a cable jack plug and the guitar, (almost invariably) without any DC blocking cap to mitigate the risk of a grid-anode short, so a much higher likelihood of user contact.
But an input stage anode has almost zero risk of electrocution or even injury. May cause an unpleasant mild tingle via hand contact. You’d probably need to lick the jack plug to actually get a burn  :huh:

Title: Re: Coupling capacitors in series?
Post by: kagliostro on July 10, 2021, 09:39:23 am

I agree with pdf64

however I think that an in series added cap isn't bad practice

Franco

Title: Re: Coupling capacitors in series?
Post by: thetragichero on July 10, 2021, 12:00:32 pm

makes me think of the FAC switch: https://el34world.com/charts/Schematics/files/Orange/Orange_graphic_mkii_120.pdf