Preamp cathodes options..

[chocopower]

Hi there!!

Just a question..
In the tipical 12ax7 preamp tube there are 2 cathodes.
Pin3 and Pin 8.
Diferent Rk/Ck values give diferent sounds...ok..

In a tipical Marshall combination:

820ohm/350uf - Pin3
2700ohm/.68uf - Pin8

Any change if i reverse the values?

820ohm/350uf - Pin8
2700ohm/.68uf - Pin3

Just trying to learn something....

Thanks!!

[Enzo]

Which section of the tube is not what matters, what matters is where IN THE CIRCUIT each section of the tube lies. If you were building an amp from scratch, you could select whichever side of the tube you wanted.

If one triode is the input stage, and the other the tone stack recovery, then each triode is doing a very different job, so it is natural to expect to find different values. Even if both triodes were used as channel input stages, chances are they designed the channels to sound different, so again, there is the difference.

Looking at the 1959 for example, the 820 ohm stage is the normal channel, while the 2.7k stage is the bright channel. If you swapped them, you would be partially moving the channels.

But all in all, if you are curious, do it.

[MWJB]

I suspect option 2 would be brighter, more aggressive than option 1 - higher voltage, more highs, better fidelity at the input triode, some grind from the 820 at the 2nd stage. I don't know if there is really much benefit in having the .68 cap in front of the 350uf, the 350uf will pass more lows, but probably not if limited by the .68 in the earlier stage? A smaller 2nd stage cap maybe just as good as 350uf?

It would seem to make sense to have introduce the grind & fullnes at the first stage (820/350uf) & then temper it with the 2nd (2.7K/.68).

Perhaps try it, & let us know.

[chocopower]

All this is for my Pro-reverb ex "normal chanell"...

Now, is like that:

Input stage-single volumen control-2nd gain stage....

and the cathodes are like that:

Pin3: 1500ohnm/25uf.

Pin8: Rotary switch with: 1500ohm-----2700ohm/.68uf-----1500ohm/25uf

First position (like a tweed champ) and second on (marshall like) are relly usable. The 3rd one...... to much for my Godin LGX.....

We talk about this here.
http://music-electronics-forum.com/s...ead.php?t=5048

I,m using a 3 position 4 circuits rotary switch, so i,m looking for the 820ohm/ 250 or 350uf to use with the central position (the marshalliced one....).

The layout for the switch is like this:



A-B-C-D in green, are the center poles of the switch, no the positions.....

tomorow i´ll chek and come back next week with some more conments....

[stokes]

To try to simplify things so you can understand what is going on,tweaking the cathode resistor value is going to alter the overall gain characteristic of the triode.Tweaking the bypass cap is going to alter the frequencies that get bypassed,which will also affect the gain of the tube to some degree.

[chocopower]

Hi!!

Drooping resistor value, increase gain and diminishes the cap effect.

If i raise the resistor size, it´ll diminishes the gain and increase the cap effect.

isn´t?


Test it yesterday... and I could not make a decision... I´m gonna use my tone section empty holes to install some switches and test different options. When i decide what i like, i put all in the rotary switch....

thanks everybody...... and happy christmas....

[Satamax]

All this is for my Pro-reverb ex "normal chanell"...

Now, is like that:

Input stage-single volumen control-2nd gain stage....

and the cathodes are like that:

Pin3: 1500ohnm/25uf.

Pin8: Rotary switch with: 1500ohm-----2700ohm/.68uf-----1500ohm/25uf

First position (like a tweed champ) and second on (marshall like) are relly usable. The 3rd one...... to much for my Godin LGX.....

We talk about this here.
http://music-electronics-forum.com/s...ead.php?t=5048

I,m using a 3 position 4 circuits rotary switch, so i,m looking for the 820ohm/ 250 or 350uf to use with the central position (the marshalliced one....).

The layout for the switch is like this:



A-B-C-D in green, are the center poles of the switch, no the positions.....

tomorow i´ll chek and come back next week with some more conments....

Whuh? Are you sure that you're talking about 250 or 350µf???

[chocopower]

Yes. I´m sure. Electrolitics. Of course....

[CarlZ]

Ok, here's what you're probably going to get by doing the swap, which you've probably already found out to one degree or another with that switch...

Given that the entire preamp is representative of a plexi type design, the bright channel has a bypass cap across the volume pot, AND a 470pF/470k RC pair after the pot, resulting in a very bright channel. The normal channel is more or less a straight shot from the plate through a pot(no cap) and into the V2 grid. This gives you a dark tone.

So, by swapping the RC pairs on the cathodes you're going to brighten the normal channel and darken the bright channel. While it's not a complete wash out, the two channels are going to be more closely matched tone-wise than in the original configuration. Bright channel not quite so biting, and normal channel not quite as woofy.

-Carl

P.S. Yes Satamax, that 330uF is the right value for a plexi Marshall

[chocopower]

So, by swapping the RC pairs on the cathodes you're going to brighten the normal channel and darken the bright channel. While it's not a complete wash out, the two channels are going to be more closely matched tone-wise than in the original configuration. Bright channel not quite so biting, and normal channel not quite as woofy.

-Carl

I understand it...but i,m gonna use this in a single input channel with 3 gain stages and a lonely volumen control betwen first and second gain stg.

the 3rd gain stg. is shared with the other channel..(Stock black-face channel).. so this mods are only for the 1 and 2 g.stgs...

Thanks every body....!!