Adding a 2 second cold clipping stage in JCM800 style amp

[Rod]

Hello!

What would be the expected result of adding a second cold clipping stage after the frist 10K cold clipping preamp stage in a JCM800 style amp ?

What would be the advantages and disadvantages?

If in the second cold clipping instead of 10K i used a 4.7K resistor it will improve assymetric clipping?

[Chuck H]

Hello!

What would be the expected result of adding a second cold clipping stage after the frist 10K cold clipping preamp stage in a JCM800 style amp ?

What would be the advantages and disadvantages?

If in the second cold clipping instead of 10K i used a 4.7K resistor it will improve assymetric clipping?

Are you talking about actually adding a triode, or changing the 820R cathode resistor for the triode behind the CF?

[Rod]

Hello Chuck!

Im talking about adding a triode, after the first 10K cold clipping and before the 820r cathode resistor.

[Chuck H]

Well you probably already know that the gain is going to increase A LOT without considerable padding. As far as symmetry of clipping there are a lot of caveats. How each successive gain stage is affected by the one before it is all relative. In the case of the classic 800 with the cascade preamp and the 10k second stage cathode, yes the clipping is asymmetrical at lower levels. But when the PI begins to clip (LTPs clip asymmetrical) the duty cycle starts to even out. When cranked up those amps have a fairly symmetrical clipped output. It's part of their mojo. In fact in this case adding a gain stage with ANY level of bias will actually make the clipping more asymmetrical than the stock amp when operated like this. That said...

If you are keeping the master volume low and what you're after is more symmetrical clipping from the preamp alone, without PI and power tube clipping then there's nothing to be lost by adding a triode. Not only can the preamp clipping be made more symmetrical, but you will increase gain that's lost by not clipping later amplification stages. But in that case you could just remodel the preamp after something like a Soldano, which is designed for good preamp clipping. You'll have the same number of gain stages if you add one to the JCM800 preamp.

[Rod]

Can we say that two cold clipping 10K stages in series will be the same and equal to have one cold clipping stage with 20K in the cathode in terms of assymmetric clipping?

[Chuck H]

Not at all. They do offset each other somewhat. With one clipping one swing and then the phase is inverted and the next would clip the other swing. Probably what you expected. It just happens that a bit of asymmetry in the preamp compliments the power amp for this model. IMHO that model hasn't been famous for it's "master down preamp clipping" Any tone junky will tell you that the master on those amps should be epoxied in place.

[Enzo]

Um...

2205? 4100? Older ones like 1959 or 1987?

[Rod]

2203

[Chuck H]

2203

That was my assumption.

So... Are you you hoping for more symmetrical clipping from the preamp when master levels are set low? In other words, are you wanting to make this design into a modern high gain amp like a Soldano or Engle, etc? The opening inquiry is a blunt crayon for painting the goal.