ROG amp sim trimpot use for higher voltage

[Threefish]

Hi everyone,

I'm going to build a Professor Tweed to try it as a practice amp preamp, and run it at around 20v for a bit more headroom. Can anyone suggest about what size trimpot I should use? I'm guessing that the 100k used for the 9v version might not get me to the er...10v-ish half voltage I'll need.

Thanks

[d95err]

Usually, 10-20k is enough for a 9V circuit, so 100k should be fine for 20V.

See this thread: http://www.diystompboxes.com/smfforum/index.php?topic=49578.0

[davebungo]

Bear in mind that (for a given device) if you use a higher value of drain resistance to reduce the drain voltage, you will get more gain and hence less headroom!  Could be counter productive.  Gain is proportional to Rd.

[davebungo]

Bear in mind that (for a given device) if you use a higher value of drain resistance to reduce the drain voltage, you will get more gain and hence less headroom!  Could be counter productive.  Gain is proportional to Rd.

I wasn't really thinking when I wrote this.  Of course, you will have more headroom in the circuit, just that it may be eaten up by extra gain.  Sorry, if I misled anyone.

[d95err]

Bear in mind that (for a given device) if you use a higher value of drain resistance to reduce the drain voltage, you will get more gain and hence less headroom!  Could be counter productive.  Gain is proportional to Rd.

I wasn't really thinking when I wrote this.  Of course, you will have more headroom in the circuit, just that it may be eaten up by extra gain.  Sorry, if I misled anyone.

I never really thought about the relationship between the trimmer, gain and headroom in that perspective before. Thanks for pointing that out. Come to think of it, this is the key to how ROG style ampsims work. The gain of each FET stage is much lower than in the corresponding tube circuit. This is compensated by the much lower headroom. Gain and headroom take each other out, so the amount of distortion in the circuit is similar regardless of voltage.

However, some circuit parts have attenuation that is not dependent on voltage, like a voltage divider between two gain stages. The lower gain of the ampsim does not fully compensate for this. So, in theory it should mean that a ROG style ampsim should have a bit less gain than the corresponding tube circuit. Most practical reports seems to indicate that ROG ampsims have a bit more gain (distortion) than the tube circuit. I assume this is due to the different clipping characteristics between tubes and FETs.

Perhaps one needs to compensate cathode resistors and fixed attenuation resistors to get the right response, rather than using the exact same values as the tube circuit.

[Threefish]

Thanks for your input everyone, much appreciated, but , er....if I did want a bit more headroom with a bit less distortion, where do I go from here?

[davebungo]

The Professor Tweed has volume and output level controls.  Use the front end volume control and you will get less overall gain, and still have lots of headroom.  Other than that, you are into adjusting the bias of the FET stages to allow you to use smaller drain resistor values and/or adding some resistance in series with the source bypass caps which will also reduce gain to an extent (limited by the existing source resistors).