Help with switchable cathode resistor

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framos
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Help with switchable cathode resistor

Post by framos »

Hi,

I am starting a new 2204 build (it's only my second one, sorry for the newbie question) and I'd like to add a switch to select between two different values for the cathode resistor of v1b.

The attached picture shows the two arrangements I am considering.
20171013_115411.jpg
Although the two versions should be the same in practice, in (B) there the cathode is always connected to ground through a resistor, even during the switching, while in (A) the connection to ground would be momentarily broken during the switching. Is this an issue ?

I ask this because if possible I'd like to use A as it is cleaner and easier to play with the values.

Thanks!!
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xtian
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Re: Help with switchable cathode resistor

Post by xtian »

B is better. Very clever.
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Stevem
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Re: Help with switchable cathode resistor

Post by Stevem »

First and foremost Cathode resistor changes of more then 25% should be accompanied by a change in the plate resistor value !
If your base line Cathode resistor is 10k and you want to drop down to 4.7k then switching in another 10k resistor in parallel would be the way to go.
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framos
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Re: Help with switchable cathode resistor

Post by framos »

Stevem,

What do you mean by the need to also adjust the plate resistor ? Could you please elaborate a bit?

Thanks!
tictac
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Re: Help with switchable cathode resistor

Post by tictac »

Looks like you're going to have noise issues with dc on those pots....
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M Fowler
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Re: Help with switchable cathode resistor

Post by M Fowler »

I used Stephen's #39 10k/10k switching witch is very effective on the 2nd gain stage.
Darkbluemurder high gain amp.gif
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tictac
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Re: Help with switchable cathode resistor

Post by tictac »

:oops: oh duh..... they do look like pots the way the picture is drawn though...

I've actually tried that approach before. I don't think you'll hear much difference between 4k7 and 10k. A 33k to 39k would yield a bit more contrast with the 4k7 I would think....
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Re: Help with switchable cathode resistor

Post by Stevem »

You want to keep the tube biased in the center of its amplification range so that when driven hard enough with signal both the top and bottom of the wave form saturate and cut off ( clip) at near the same time and that's what calls for a change in the plate resistor .
Go find the RCA 1975 tube Manuel on line and go to the resistance coupled amplifier chapter and read up, I think it starts on page 671 , but you want the section that has the charts!
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martin manning
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Re: Help with switchable cathode resistor

Post by martin manning »

I believe an un-centered operating point is the goal here, to get asymmetric clipping. See for example the second gain stage in Marshall JCM800, with 100k Ra and 10k Rk. Here the switch will turn down that effect by reducing Rk by half.
sluckey
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Re: Help with switchable cathode resistor

Post by sluckey »

Go find the RCA 1975 tube Manuel on line and go to the resistance coupled amplifier chapter and read up, I think it starts on page 671 , but you want the section that has the charts!
I guess Marshall missed that chapter. :wink:
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framos
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Re: Help with switchable cathode resistor

Post by framos »

As far as I understand, on the 2204 the 10k on the second stage is on purpose for the asymmetric clipping ('cold clipper' as they call it).
RockinRocket
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Re: Help with switchable cathode resistor

Post by RockinRocket »

Yes the 10k is unusual unless its a Wreck or 2204 circuit. Certainly a bit part of the tone of those amps.
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M Fowler
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Re: Help with switchable cathode resistor

Post by M Fowler »

Here is how that 10k/10k are switched to choose 5k or 10k quite a nice boost.
10k10k switched.jpg
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jjman
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Re: Help with switchable cathode resistor

Post by jjman »

"B" looks better since there is no chance for a period of time with the cathode-path "open." But I'm thinking both would cause at least some "pop."
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