This May Be "Old News"

[PCollen]

Awhile back if saw the YT video of Andy Baylor demoing his Ceriatone Epression, and noticed that there was a mod to the PI in that amp: a 22K trim pot in series with the PI tail resistor. This, of cours, would serve to reduce the PI headroom and cause earlier PI breakup.

See YouTube demo http://www.youtube.com/watch?v=472K5yGVKeQ
You can see the trim-pot on the tail of the PI in the video ( @ 1:06, 2:12 and 2:26) if you watch close.

[M Fowler]

Yeah that limiter mod has been around for a long time and currently seeing action the Egnator Rebel 20.

25kL pot with 33k/2w across the pot. It was also used in the Carlsboro 60TC

[JamesHealey]

ahh the limiter control on the old 60TC.. I used to have one until I butchered it into a wreck.

it's Carlsbro btw

[PCollen]

Yeah that limiter mod has been around for a long time and currently seeing action the Egnator Rebel 20.

25kL pot with 33k/2w across the pot. It was also used in the Carlsboro 60TC

[M Fowler]

This limiter control has been successfully used in 18watt and 50watt amps.
The resistor across the 25kL pot removes the scratchiness.

Mark

[PCollen]

This limiter control has been successfully used in 18watt and 50watt amps.
The resistor across the 25kL pot removes the scratchiness.

Mark

It also lowers the effective value of the pot, doesn't it (33K || 25K) ?

[Colossal]

It also lowers the effective value of the pot, doesn't it (33K || 25K) ?

Yes it does. See attached PDF. I have installed this control in a 22W 6V6 JCM800 as well as a client's 50W Marshall. It is a very effective and simple means of taking the edge off the amp. I am quite amazed at how it retains the distortion characteristic of the amp while reducing the sound pressure to manageable levels. In the 50W Marshall 25kB||33k reduced the the dimed volume to "still-loud" levels but took the ridiculous volume (and painful upper treble) out while keeping the awesome feel. Man, that was awesome. In the 22W amp, 25kB||33k will take it down to "loud" but completely manageable for a large bedroom and a 2x12. The tone will get slightly compressed at lower levels and lose just a bit of dynamic feel to it but then you are pushing the speakers less. It's just like with a VVR: part of the amp's tonal equation is moving air. There's always a price to pay in some form. However, for the price of a pot, the result is quite useful in my opinion.

As Mark mentioned, this method is called the "Watts" control in the Egnater Rebel 20. Please also note that this method is slightly different than the way it was done in the Carlsbro amp. I much prefer the 25kB||33k attached directly to the pins 3 and 8 of the PI tube, then the tail resistor. You can use other pots, parallel or not-parallel the resistor but I find that the non-linear sweep that the paralleled resistor gives is excellent and sensitive.

[PCollen]

It also lowers the effective value of the pot, doesn't it (33K || 25K) ?

Yes it does. (snip)...

As Mark mentioned, this method is called the "Watts" control in the Egnater Rebel 20. Please also note that this method is slightly different than the way it was done in the Carlsbro amp. I much prefer the 25kB||33k attached directly to the pins 3 and 8 of the PI tube, then the tail resistor. You can use other pots, parallel or not-parallel the resistor but I find that the non-linear sweep that the paralleled resistor gives is excellent and sensitive.

So, what you are doing is effectively changing the value of the bias resistor ..putting the pot/resistor in place of the 470 ohm, or in parallel to it ? I guess I'd have to see it on a schematic to understand. Back to the Ceriatone mod in question, I think the 22K trim-pot is in series with the 10K tail resistor to up it's effective value to between 10K and 32K to reduce PI headroom

[Colossal]

So, what you are doing is effectively changing the value of the bias resistor

Yes, adding to it.

..putting the pot/resistor in place of the 470 ohm, or in parallel to it ?

In series with it. Turning it up cold biases the PI. Great for reducing volume while retaining the distortion characteristic. In a client's '74 Marshall it was very useful to take the over-the-top cranked volume and just knock back it very slightly. The amp retained all of the feel, touch sensitivity and dynamics while reducing the incredible sound pressure of a full tilt 50W.

Back to the Ceriatone mod in question, I think the 22K trim-pot is in series with the 10K tail resistor to up it's effective value to between 10K and 32K to reduce PI headroom

I haven't seen that mod but it is slightly different than what I've described.

[RevD]

Personally I don't care for the tone on the clip very much, and this is just my taste, sounds to buzzy and not very open sounding, but thats just an opinion and you know about those.

Regards,

D.

[ChrisM]

I am a bit confused as to how this mod would be implemented in a non NFB amp. I looked at that Carlsbro schem but didn't quite get what it was doing.