Plexi / Soldano project! I need some help !!!

[raza]

First of all... Hello everybody. It's My firts post. I am Rafa from Barcelona ( Catalonia). I am a guitar player and Electronic enginier. But I work in digital sector.

I like to build my own amps and this time I am building a Frankestein PLEXi/ SOLDANO amp.

I build this schematic



Its sound amazing but I want something that I dont know if is posible.

I want that in the lead Channel the gain pot moves from early clean sound, pass to crunch and after sounds like soldano lead that sounds now.
Now the gain at 9-10 oclock is with lot lot lot of gain. I want some more crunchy sound.

Whant can I do?

Also If itsnot posible there are some mod to put a switch to pass from crunch to lead?

[Dave H]

Now the gain at 9-10 oclock is with lot lot lot of gain. I want some more crunchy sound.

Whant can I do?

Welcome to the forum Rafa.

I'd be looking at the bright caps across the Lead Gain pot. It should be a log pot but even if it is it will be mostly bypassed at guitar frequencies by 1500p making the gain increase quickly. Try it without a bright cap to see if that helps.

[raza]

Welcome to the forum Rafa.

I'd be looking at the bright caps across the Lead Gain pot. It should be a log pot but even if it is it will be mostly bypassed at guitar frequencies by 1500p making the gain increase quickly. Try it without a bright cap to see if that helps.

i tried all about bright cap. Its true that i can reduce the gain a bit but far that the crunch i want.

[tedmich]

Are you using an audio taper pot? A log response would I thing be closer to what you want than a linear one. There are many stepped attenuator kits which allow you to build custom tapers that might work, if you don't mind non-continuous variability.

[raza]

Are you using an audio taper pot? A log response would I thing be closer to what you want than a linear one. There are many stepped attenuator kits which allow you to build custom tapers that might work, if you don't mind non-continuous variability.

Yes I use 1M log pot.
I want to do it my self from some idea of all to learn something more.

The amp la to high gain. I want it a litle more rocker when the gain is un the gira half.


Sorry for my english.

[tedmich]

here is a 24 position stepped series attenuator which you can use to approximate almost any potentiometer taper:
Prices
sadly its $67 and you'll need to solder a ton of resistors on it but you may be able to get the response you'd like.

Alternatively maybe you could make an AD5241 work as it will do 1M and you are digitally competent
http://www.analog.com/en/products/di...oduct-overview

[cbarrow7625]

With a single gain knob it is very difficult to get a 4 stage hi gain circuit to enter "crunch" territory, particularly in an SLO-type circuit that uses a cold cathode arrangement on the 3rd stage. The cold current stage pretty much does nothing but produce distortion / harmonics from the moment it is hit with a relatively small input signal. The single gain knob really can't lower the signal low enough to stop that 3rd stage from distorting the hell out of the signal.

Think about the first 3-stage Marshalls with the 10k cathode cold current stage. Where is the gain pot located? Right in front of the cold-cathode stage.

Cleaning up this circuit enough to get into crunch mode at some rotation position on the gain knob usually requires a pair of gain knobs in 2 different positions in the circuit (usually between stages 1&2 / 2&3, respectively. If you want to stick with a single gain knob, just use a dual-ganged 1M audio pot. Leave the original gain pot wired as-is. You have at least 2 choices for the 2nd pot: 1.) Just wire it as a variable resistor in parallel with the 470k grid leak resistor just prior to stage 3 (possibly with a 100k-200k limiting resistor in series; your choice)or, 2.) Wire it as a standard 1M volume pot following the 470k grid leak resistor. Experimenting will tell you which is a better choice for your circuit.

You could do some more advanced things like using the second 1M audio pot to simultaneously reduce the grid leak resistor while also increasing the 3rd stage cathode resistor to 100k by wiring the wiper of the pot to ground with the top lug wired to the grid of stage 3 (as a variable resistor), as in option #1 above. The bottom lug of the pot would be wired to the "ground" end of the 10k resistor currently on the stage 3 cathode with a 100k resistor wired in as the "fixed" cathode resistor. With the pot on "10", the circuit would pretty much act as it does now - the wiper would ground the 10K cathode resistor while simultaneously presenting only an additional 1M resistor in parallel with the g 470k grid leak resistor (increasing that 470k to 1M would give you a total combined grid leak of about 470k-500k; exactly as it is currently). As you rotate the pot down not only does the grid leak resistor become smaller, creating a greater voltage divider at the stage 3 grid (thus reducing drive), but it would also increase the cathode resistor until it essentially becomes about 100k (equal to the plate resistor), reducing the gain of the stage to about 1 and reducing the distortion / harmonic creation of the cold-cathode stage that results from the typical 10k-39k cold-cathode arrangement. 2 gain reductions for the price of one. This option may be more precarious to pull off and may also sound crappier (or better - who knows - depends on your amp and what you are after).

To summarize: Use a dual-gang volume pot. Figure out where / how in the circuit the second half of the pot contributes appropriately to get you into "crunch" territory as the pot is rotated. Nothing new. There are plenty of examples of it (not all of the executed tremendously well).

Good luck.

[defaced]

If I understand what you're looking for correctly, the Fryette Deliverance can do what you want. It uses a 10k cold stage, but there's alot of other voodoo going on in that amp that I'm sure helps its ability to clean up. You can find the schematic easily.

[nsubulysses]

You can find the schematic easily.

???

Can you post it please I have never been able to find it

[defaced]

Google fryette deliverance schematic. Look in the images.

You will find at least three different versions:
Images of hand written schematic: original rev of amp. Hard to read some values but it is correct.
Cad PDF of schematic: transcription of hand written schem above. Easier to read but contains errors. I believe a couple cap, resistor values and a missing ground on the presence pot
CAD image of schematic: New rev of amp. This is the current production version without the choke and lower screen filtering. Some small changes were made in the preamp. I think all of them are proximal to the gain pots.

Both versions of the amp clean up very well with the volume knob.

[cbarrow7625]

Well, volume knobs (plural), technically. It has dual gain knobs, although both between the 1st & 2nd stages, but dual gain knobs none the less. To get a full range of tonal pallates out of a 4 stage design, dual gain knobs are more or less essential.

There are two main features that set it apart from other SLO-type knock-offs (that don't clean up into "crunch" very well). First, the Deliverance gives the operator the option of limiting the drive early on in the circuit (between stages 1 & 2) using two different gain reducing "techniques" (just two different types of voltage dividers & HF bypass caps used on Gains 1 & 2). It is really a terrific implementation and allows the operator to create a wide range of "voices" and drive levels. In the end, it accomplishes the same task I described in my first post, but implemented in a different way; the pair of gain knobs produce settings that prevent the downstream stages 3 & 4 from being massively overdriven, thus allowing the signal to clean up enough to achieve "crunch" tones when both are set fairly low.

The second big difference between the Deliverance design and "typical" hi gain amps is the use of modest local negative feedback around stage 4. It looks like it should limit the overall gain of the 4th stage to around 10x in the low & mid frequencies (cleaning & tightening that frequency range up quite a bit) while allowing a portion of HF signal (and harmonics) to essentially "see" the normal open loop gain of the stage; working in very much the same way as the typical presence control does to allow open loop HF around the power amp stages. Brilliant, once again. Huge respect from me for Mr. Freyette. Every amp of his I've ever played through has blown me away.

So, to go from "screaming distortion" down to "crunch" the basic principal remain: Use an additional gain pot (somewhere in the circuit) to provide a secondary means of signal reduction to prevent stages 3 & 4 from getting driven too hard as usually happens in a 4 stage amp with a single gain knob. The advanced principal the Deliverance schematic shows is to limit the gain in a third way by using frequency-selective local negative feedback late in the circuit to limit the amount of "excessive" distortion that is contributed from the later stage(s). I'm honestly surprised we don't see that technique used more often.