variable 3rd stage Rk?

[doctord02]

has anyone tried using a adjustable pot or switchable selection of resistors on the 3rd stage Rk? would it be possible to switch to lower value resistors (say 5k, 2.5k, etc) to produce a "clean" channel?

[loverocker]

I don't think reducing that R would help the clean tone at all. It would change the character of the distortion.

If I was loking for clean, I'd bypass the 3rd stage altogether. Or just dial the guitar's Volume pot down as KF intended

[tubedogsmith]

Try substituting a pot for the 150k grid drop resistor between the 2nd and 3rd stages. That can be pretty cool. You can use it almost like a master volume.

[ODwan]

Try substituting a pot for the 150k grid drop resistor between the 2nd and 3rd stages. That can be pretty cool. You can use it almost like a master volume.

I have a trimpot (500k) at that position. You could use it as a MV but it also messes with the cornerfreq of this RC.
For overall cleaner sounds I prefer the split plate load on the 2nd stage as per Express Var2/Komet schem. It doesn't change the response of the amp and makes for a nice bluesy tone.

[Robert]

ODwan,

What Express var2/Komet schematic are you referring to?

Thanks,
Robert

[toddyjoe]

Robert, I have not seen the schematic referenced but I do know the concept he is talking about: the split plate load trick.

Take a standard 100K plate load resistor value. Usually, the B+ goes through the 100K resistor to the plate and the signal comes directly off the plate and goes to the next tube stage through a blocking/coupling capacitor. But what if, instead of using a single 100K plate load resistor, you use an 80K resistor and a 20K resistor in series for the plate load. Go from the B+ first into the 80K resistor, then into the 20K resistor and then directly to the plate. Because the resistors are in series, you still have "100K" of effective plate load resistance between the B+ voltage and the plate so the gain and bias point of the tube does not change. However, instead of taking your signal for the next tube stage directly off the plate itself, take the signal off of the connection in between the 80K and 20K resistors. In other words, hook up your blocking/coupling capacitor to the junction of the 80K and 20K resistors rather than to the plate of the tube. Now your signal is getting reduced and attenuated by having to first go through 20K of resistance before going on to the next tube stage. This attenuation cleans up the sound a little by reducing preamp gain and distortion.

The Komet amp uses this trick to switch between a standard plate load setup and this split load setup. That is what the "Fast/Gradual" switch on Komets is all about. Brown Fender amps also used a variation of the same thing, although it was not switchable.

[doctord02]

Take a look at this schematic here for a visual...

http://maximoaudio.com/download/schemat ... iation.jpg

How does this sound for anyone that's tried it?

[ODwan]

Hi, all!

Sorry for my confused naming of the schem. It's that Express Preamp Variation that is closely resembling the Komet60. I tried it in my Express build and it sounded quite like them soundfiles @ Kometamps.com.
I liked the Express sound better though and changed it back except the split load switch.

Timo

[Robert]

Toddyjoe,

Thank you for providing such a nice explanation for the split plate load trick. I appreciate the time it took you to write the reply. I had looked at the Fast/Gradual switch but not understood it and dismissed it. However, your explanation makes me curious. I am half way through my Two Rock Ruby build and will consider incorporating this mod. With help like yours, I may actually learn this stuff.

Robert

[doctord02]

from Toddyjoe's description, it looks like Mark's schematic has the 22k and the 82k resister's reversed.

[toddyjoe]

I was just going from memory so if someone has a schematic that differs from my description, definitely follow the schematic. I have seen this type of split load resistance used with different values as well (two 47K resistors, for example), so play around and do not feel that you are stuck with 20K and 80K. Good luck!

[MSS]

Anyone tried using the 80k/20k type setup on other stages? How did it affect the sound?

[PeteRH]

tubedogsmith wrote:
Try substituting a pot for the 150k grid drop resistor between the 2nd and 3rd stages. That can be pretty cool. You can use it almost like a master volume.
odWAN WROTE:
I have a trimpot (500k) at that position. You could use it as a MV but it also messes with the cornerfreq of this RC.

The trick Tubedogsmith is talking about is using a 150k pot as a constant 150k to ground. The wiper is then fed to the third stage. What happens is that the 150k overall pot resistance does not change the cornerfrequency and the wiper affects gain to the next stage similar to a split load resistor. This acts as a master volume but IMO comes in more useful when you are dialing in different guitars.

Disclaimer: This is the only position I would use a pot as a split load as there is only small signal voltages. If you want to try split loads for plate resistors I suggest hard wiring a few resistors to some decent switches. Pots are not designed to take that wattage.

cheers
PeteRH

[dobbhill]

My 2cents:
I wired a 500K pot as a variable resistor in series after the coupling cap between stages 2 & 3. It acted somewhat like a master control, but introduced quite a bit of high frequency roll-off as I increased the resistance. I used a .01uf coupling cap there, so there was little effect on the bass response.
We seem to be coming up with quite a few ways to alter the gain structure of this preamp scheme:
1. Split loads
2. Master volumes
3. Trim pots/variable resistors in various configurations in various locations.

I'd like to see a collection of these mods and their effects.............
Dobb

[dms]

Hey... I'm admittedly more curious than knowledgeable, so why not use pots EVEN for the plate loads?? I have never seen 2 watt resistors recommended for the plates, and yet my bag of clarostat 10-turn wirewounds are rated at 2watts.... I would think as long as the pot is sufficiently rated, that a resistance is a resistance is a resistance, etc, at least for getting close. Wirewounds probably have better contact as well, so even having dc voltages around might not be a concern(?). If there's a question about nuanced differences in tone between fixed resistors and pots, one could first dial in close, then swap out to the corresponding fixed value. Assuming appropriately-rated pots, why would this not be the perfect experimenter's setup? Now if only variable capacitors were in the uF range too...... Davids