You may only need a low value pot for the leds ~ even 1k would likely be enough but use the lowest value you have on hand.
Yes odd ball tone design but if it works then leave it.
Phil.

I doubt you will blow the power amp chip by removing the leds.

I tend to think these little chips blow because quite often the modern metal shred players use very high out put pu's and thrash these chips to death. Often adding pedals to make it even louder.
Add to this it's a small amp and more often than not they have tiny heat sinks which are not up to task, so the combination of above will fry the chip.

At sane bedroom levels which these small amp are designed for you won't have issues.
So As Enzo mentioned lift the 2 Leds (or one at a time) and you will get more clean head room but as it's a small power chip it may not be much more because the power section will distort.

you can if you wish add a pot in series with the leds and that will allow you to dial in how early the leds clip.
(Leave C9 bridged from R13 to ground)

You could try better speakers but that will get expensive and still may not improve the SPL by much.

If you want more clean head room then you will need a bigger amp. Sorry
-------
Regards the C9 Q;
The cap is there to wipe off excess hifreq crud.
Valve amps wipe of hi freq by design while SS amps you have to add some hi freq roll off.
A lot of budget small amps don't bother to tweak this issue and hence sound very harsh Especially when in distortion mode.
Again lift it and hear the difference.
You will certainly have heaps more treble.
If the Amp is already bright then Leave it in place and just lift the Leds.
If you want, you can try different values on C9 to fine tune the top end. (lower values = more treble. higher = less bright)

One thing that may happen when you remove leds and engage the distortion.
The signal going to the reverb may start to distort the reverb drive. so you might have to raise the value of R201.

Just a note on the schematic;
Unless I'm missing something??? IC1B is labeled wrong.
Pin 5 is the Positive input and is normally grounded in this configuration,, It looks like it's + & - input labels are flipped. I can't read the label Numbers,, they both read 5 to me.
Hope it helps
Phil.

No worries chum,
 Just a couple of things that may help if you try this.

1/ The Master out has a very simple level shift foot switch. In use the trick is to adjust the master to balance the amount of volume lift you need.
Should be obvious that with the master full up there is little difference in volume, while with master at low level the volume jump is likely too much.
You just adjust the master to a point where the level shift suits your need.

2/
If I was to build this again I would change the switching sequence so that the trim section remained on all the time.
You just need to reconfigure the position of SW2.
3/
Depending on how bright or dark your Guitar/Amp system sounds you can play around with the cab sim values to fine tune it.

R3,R43 are there to drain the caps when switching.
Also add them on at C20 (switch side).
Sorry forgot to add them to Schematic. I think I added them on S3 as well. I forget  exactly where I put them but I assume most folks will bread board this before jumping in.
One day I'll try to get a video of my gear in action but my little camera has built in compression on the audio which does tend to slew what you hear.
Phil.

A larger value pot gives you more room to move over the lowest 10% of the rotation,, especially if using an Audio log taper pot.

The plots of log and linear responses are resistance vs, pot position.
The issue is that there are many different log Curves,, even reverse log. :loco

Ideally the closer you get to 10~15% of total resistance at half rotation point the better it will be for what you want in your application.

Google; *Log pot curves*,, That may help you get your head around it.
And also learn about how we all hear sound;
https://ozvalveamps.org/pots.htm

It may well be cheaper and less hassle to just mount a power amp Master level pot on the back panel. 50k pot should work fine.
Phil.

Quote from: Dino Boreanaz on March 20, 2022, 11:27:49 PM
I very much appreciate everyone's input on this as I'm quite new to the world of modifications.  I have a couple of questions for those with more experience that myself (which is just about everyone!):

As my end goal is to make the volume controls less sensitive at very low settings, is there something preferable about adding resistance (either fixed or a pot) before the power amp versus changing the volume pots from linear taper to log taper?

I have found volume settings that I am quite happy with, but I would really like to have finer control in this area and it seems like log taper pots would accomplish this without adding components or making permanent modifications.

If I may ask another question to further my understanding of this circuit:  It seems like the two volume pots (VR10 and VR11) are not located between their op-amp's input pins and output pins, and so should not affect the gain of these op-amp stages.  Is it correct to say that these two volume pots control the signal passed to the power amp as voltage dividers rather than by altering the gain (and therefore clipping behaviour) of the op-amps?

Thank you again for all your input and insight.

Yes A log pot may well alter how early the volume jumps but I think a larger value pot (Log or Lin) would give a more progressive rise in volume. 5k would make it jump up in volume with only little rotation, as you have noted.

I have a hunch 50k pots might resolve the issue far better,   then log or lin would not matter much.
That may resolve this better than working on adding an extra master pot.

Regards to VR10 & VR11.
Yes they are just voltage dividers and do not effect the gain.
Phil.

My advice would be to cut the input cable to power amp as per TassieV's idea and mount a 50k~100k pot in the back panel and that becomes the power amp Master volume. Looks like enough room for a pot mount next to the Socket PCB in the back panel.
Phil.

At the request of member *TassieViking* here is the schematic for my little black box. The PhAttBox is a compressor and a cab sim with a few extras.
Now if you want to build this I strongly recommend that you bread board it before you heat up the iron. you may want to tweak the cab sim to suit your sound.
If I was to build this again I would likely change a few things but it's been working fine for the past 7 years and played many gigs with it, so I'm not about to reinvent it again.

I run mine on 18V for that little extra head room but 9 will work just watch for big signals.

You can make the LDR by using 2 green low power leds, an LDR and a bit of shrink tube. Don't use hi brite leds.
In Australia Altronics and Jaycar sell cheap LDR's you may have to experiment with other types in your part of the world.
Check FSB for other LDR options here; https://www.freestompboxes.org/viewtopic.php?t=8581&sid=18c2603e67d802c43c8f08b2b3b38269

If you have insane hi output PU's you may need to up the value of R4
My PU's are low output and this works out fine.
Some builders have complained about a bit of hi end loss and loop a small cap around R4 but I've had no issues with my build.
Have fun,, Phil.

Quote from: Tassieviking on March 17, 2022, 11:19:52 PM
My curious little eyes saw a Phatt Box with compression and OD in it.
What would a man have to do to get that circuit ?  Beg, borrow,  ??
Thanks for the FabbTone circuit Phatt, I might have to make it next.
Michael

Yep that took a long time to come together.
That is a simple light triggered Comp with and extra drive circuit wedged between the comp and the Cab sim.
The compressor is from FSB site (Free Stomp boxes) A retired teck chap lives on that site called *Mictester* came up with a *Really Cheap Compressor*. A very clever chap. :dbtu:

It is unique in that Unlike a lot of comps this one actually adds grit the harder you drive it. much like the triode rattle you get from hard driven triodes.
I've owned a few Comps and the Boss ones are just way to clean and except for the CS3 the earlier CS2/1  units are noise prone, and they are prone to pumping. :trouble
I even tried to build a few but I gave up because the noise issues are a pain.  :grr
I went right off Comps for some years but then Mic came up with this one and It worked a treat.
Super clean comps are for studio but don't work for Rock guitar sound. IMHO. xP

Link is here;
https://www.freestompboxes.org/viewtopic.php?t=8581&sid=18c2603e67d802c43c8f08b2b3b38269

The Extra drive is just a simple diode clipper stage for an extra bit of grit.

The Cab sim idea was a highly tweaked Nobels SST1 (think rockman rip off) I only used the Cab sim part.
The output has a basic top cut filter (Edge) as well as a simple level shift on the output, for when you just need a slight volume lift which has no effect on distortion.
-----
Re the PhAbbtone.
The Schematic assumes you know how to wire up input switching and such as well as add a Led.
As mine is on all the time then No need for a stomp switch. a toggle is cheap.

As should be obvious I'm old and don't care much for fancy painted boxes and flashy knobs. Heck all that extra work and cost and it won't sound any better, so flat black is kool for me, wink.

My tone circuit has been tweaked to improve a few little issues because it's only running from low voltage.
If you wish to use the original HiWatt values there will be some problems so I'd suggest you stay close the schem values.

What may not be obvious is that these are Hiz tone circuits and you can plug a passive PU guitar straight in. You could use a buffer in front but as it works so well why bother.

Watch your layout as like ALL HiZ stuff they are prone to pickup noise.
Anything above R1 & R3 is sensitive.
so use of expensive ultra low noise opamps won't help much.
If you have a lot of pedals which introduce a lot of gain then I suggest you rethink your layout.

My pedal board is deathly quite because of good supply grounding and insightful use of gain settings on dirt pedals. 8|
The phAbbtone used into other hot rodded Amps can be a bit hissy but again then so are most hi gain amps.

re schematics for the PhAtt Box,, give me time I'll have to dig for the files. I have so many schematic files it's just getting on top of me now.
I think the original is here somewhere on these pages but that was long time back.
Most of my successful circuits are here on SSGuit

Phil.

Quote from: Claulogic on March 17, 2022, 08:50:41 PM
ok thank you very much for the information, and all the review, but it is not really the answer you expect but if I am interested in doing the phabb tone, it is good to have a tone shapping in a small format, really I have not bought the boss katana yet, simply I tried it and said for 25w it sounds decent, and well I had the idea of making a preamp to use it there although the preamp that I like the most is the hiwatt one. The reason why I exposed the Alembic or Fender Bassman one is because it is simple as far as it goes and I more or less wanted them to explain to me how to find the cutoff voltage and all the other parameters to be able to carry out other projects later. but the pahbb tone to start is excellent I will do it.

PD:but now that he's telling me about the katana, I think I'll back off and look for another amp that meets my expectations, thanks, it just saved me from a tantrum hahaha

Thanks phill

That's ok, sorry if I'm not understanding your request.
i.e. What do you mean when you ask for *Cutoff voltage*?
It's a basic fender tone stack which will work from about 250Volts quite well. Which itself means more real estate for the Hi voltage supply on your board. You also need a heater supply to run the Valves. not an easy build. 8|

So you posted the Alembic with FET mod and I see zero benefit because you need one triode so you save no space by adding a mosfet. and you have 1/2 an AX7 sitting there doing nothing.
If you want all SS then look for KMG's postings here as he has made complete Amps from FET's,, Very impressive stuff but not simple for someone with only basic Leky skills.

BTW If you like HiWatt then the PhAbbtone IS the HiWatt Tone stack I just refined/reworked it into a pedal format. Runs from 9 to 18Volts.

I have simulated as well as real world tested many of the tone stacks and I came to the conclusion that Dave Reeves HiWatt tone circuit gives the best results. Even Mark Huss will agree that the Tone section of HiWatt Amps is a big part of the sound.
Even the famed Dumble tone stack with it fancy Rock/Jazz switching still suffers from the interaction issues of the fender tone stacks.

The best I can make out is, For what you wish to do you don't need valves, You need knowledge which is far more useful than just assuming you need a messa stack to be a great guitar player.
You can blow big money chasing brand names If you catch my drift. winky.
Phil.

That's Ok that helps a bit,, but you have a heck of a lot going on there and I could fill pages trying to explain the issues you are about to face and you still may not fully grap the issues.
But I'll try.

1/ what is the alembic magic box?
Well it's a fair bet it just a Tone control unit and because famous people used them it's now a legendary device and now likely overhyped.

Truth is you can use many tone shaping devices that will deliver the same result. (may even be better)
The Alembic is simply an extra tone control in the signal path and ANY extra tone control device will work because most guit amps cannot alter the tone shape Enough to deliver the sounds for the modern player.

I assume you are basically by passing the preamps of the Katana and just using the power amp and speaker?
If so then Know this;
The power stage is likely class D and as sure as the pope is catholic the speaker is a super efficient hi SPL driver and they most often sound very harsh. With a lot of distortion the sound becomes very brittle.

Clue to Amp Speaker combinations.
If the Amp is dark you use a bright speaker.
If the amp is very bright you use a dark speaker.

This whole game is about a thing I call system tone. You have to build each section so as to enhance the right
frequencies in the right place.
As example;
If you plugged the Alembic into the front end of the Katana you will never sound like Pink Floyd simply because the system tone of the Kat will have way to much bandwidth. They are as harsh as hell. 
Why do I know this? I've been asked by a sound teck recently to help him get a decent tone from his 100W Katana. As he has envied my setup which is my own design perfected over the last 30 Years of building land fill, but once in a while I have had success. <3)
BTW, this hobby takes a long to time to perfect but once you learn to understand Whole System Tone it becomes obvious that you are far better to subtract that to keep on adding.
As example;
Distortion is easy to get with modern equipment but the tone shaping is the key,, try plugging a dirt pedal into a hi fi amp with full bandwidth and the bass will fart and the extreme treble will rip your ears off.
Good guitar circuit design requires a deep understanding of the freq spectrum.

learn to understand the most basic observations;
Train your ears to recognize what freq you are hearing.

note that 95% of any note you can play on a guitar is below 1,000 CPS (1kHz) add a couple of octaves for the  harmonics = 4kHz. past that it becomes crap and just destroys any chance of note definition.

The speaker in your Kat is likely well over 100Db at 5khz. That is why they sound so in your face harsh.
But they are extremely loud.. Wow!! which of course impresses the novice who has tin ears but it sells amps to the masses and roland gets rich,, meanwhile the kids still don't know why they can't reproduce the magic that they hear from the big players.
---
Ok your signal chain;

FWIW, I have dirt pedals at the front end then> mod FX into a compressor> then Cab sim> into a tone control.
The Amp I use is an 80's SS basic Keyboard Amp (No freaky tone alterations designed by fools who thinks more dist is better)
The reason My compressor is AFTER the Distortion pedals is to keep the levels within a set SPL range.
The issue with nearly all dirt pedals is if you stomp on one the volume often jumps way too high. (or not high enough)
So if you have a Compressor After Dirt it keeps everything within a set SPL range. The other way does not work for me as I play live So I can't stop and re-adjust on the fly. :duh

Now back to your tone control;
My tone control is my own Design (as is most of my gear) This will deliver a big round Bass and a Treble that is crisp but not harsh. I've built many now for happy players.
You are welcome to the design.
Assuming the Alembic is just a stock Fender tone stack then it will suffer the same drawbacks as the Fender tone circuits.
My tone circuit was designed to overcome the limitations of the Fender tone controls. the big difference is ALL 3 knobs actually Work.
Mid cut actually cuts while bass stays big and round.
Fender tone circuits suffer from too much interaction between the 3 controls. (Marshall stack is even worse)
Cavet, there will be exceptions to this general observation but this is what I've found with most of the amps I've been around.

If you are new to building circuits then there are DIY ways to save time and money. I only build stuff that is single sided PCB and use the Laser print method to make circuit boards. My eyes are to old to start trying to make tiny PCBs with surface mounted bits you can't even read  xP
I hope some of the above helps you on your tone chasing endeavors,
Phil.

Some comments about the PhAbbtone here;
https://www.ssguitar.com/index.php?topic=1136.0
But build the V4 version posted here.
Adding piks of my pedal setup

I'm a bit lost ,,What For

Fig 1A in the link shows 2 triodes, 3 R's and 1 Cap.
that's a 4 part count circuit. = simple

Fig 1B shows 1 tridoe 1 LND150,, 5 R's 1 Cap and 7 Diodes. = Not so simple

total 13 part count verses 4.

All to emulate a triode,,, Likely easier to use a triode.
That way you know what you will get.
Phil.

If it has an FX loop just insert a volume pedal in that loop.
That then becomes the master volume for the power amp.

As Mr Jazz Bass said, Don't touch the Amp circuit as it could lead to tears.
Phil.

Regards cost of a valve amp you can purchase a lamington kit for $350 at Valve heaven.
https://valveheaven.com/  (by Grant Wills in barossa valley SAust)

If you wish to do it even cheaper go here; https://ozvalveamps.org/

DIY ideas here;  https://ozvalveamps.org/ava100/ava100project.html

The site is the work of an amazing aussie chap (Roly Roper) who collected almost every Aussie amp circuit that was made in Aust and NZ.
Plus a ton of high voltage info. :tu:

Regards the "Valve mojo thing"
Valve amps by design (at least the older ones) distorted simply because they were very simple basic design. The HiFi brigade would never dream of building such a pathetic design.
But as guitar has a relatively limited range it actually beneficial to have a limited bandwidth especially if you are wanting the classic OD breakup effect.
Distortion through a full bandwidth circuit is the last thing you want for electric guitar.
So rule of thumb is, if you want a great Amp don't build a hifi spec circuit. That's half the reason why a lot of later digital stuff sounds Krass as they often have way to much hi Freq content. so they sound harsh and fake.
Yes there are good ones that deal with bandwidth but they will cost bigger bucks.
So your little Gorilla amp is an ideal setup as it's older and by the looks of the schematic you posted will sound sweeter than the latest hi teck digital stuff.

Yes Valves are great but I've found you can pull some stunning sounds just using a few well designed pedals and some basic amplification.

My main Amp is actually a Keyboard amp and with some pedals and some reworking of the Amp I'm happy.
I have basically designed most of my system by trial and error (lots of Errors but that is how one learns). you don't need hi teck,, you need to learn to understand what your Ears Are Actually hearing.

Take distortion,,, dead easy to do that but it's the TONE shaping that makes it sing.
Example, a common Guit player complaint; I want to cut through I need more MID.
      WRONG!!!  what he really needs is less bass.
The trick is Subtraction THEN turn up what is left and it cuts through.

It's what you DON'T hear that makes great guitar sound/tone
This goes for both SS and Glass
Phil.

Just google nixie smps there are many showing the adjustment pot.
Just make sure you use an Ultra fast diode.
Silly chip valve kit was the preamp one, can't remember the name? But the 555 is far easier and less parts.
I'll have a dig through all my elecro junk box. Major flooding up here right now in south east Qld but we are up high, just roads cut.
Phil.

Some years back I built and messed around with this idea and yes the 555 circuit works.
I also built the Silicon Chip Valve kit and integrated it into a circuit.
Just keep in mind that there are a few things to consider.

A valve pedal or stand alone preamp is not going to have the same interaction for the player as a full valve Amp.

The magic of a full rig is partly due to the Power supply and the way in which it sags under load. (more notable with lower power rigs)
As the power section supplies the WHOLE amp then there is a lot of interaction with the preamps as well as the power section. It all works together to create the magic.

Just altering the voltage drops from Power stage to preamps can alter the way in which it responds to signal.
You get the triodes as well as the power stage all interacting.
This interaction does not happen if the preamps are run from a totally separate power supply.

To make matters worse SMode supplies don't sag as th feed back keeps it Stiff. yes you still get a triode rattle but it's Stiffer which is harsher to my ears. (for want of a better description)

I have done the above test and it was obvious that running the preamp section from my Smode supply while the power stage run from it own separate supply,,changed the way the Amp responded.
With all else same, I then reconnected the preamp back to main Amp supply and the magic returned.
the Smode voltage was close to same reading as main Amp, about 240volts.

If you care to compare all the early Fender Deluxe schematics you will note the power supply voltage drops changed a lot over time.
Those first ones were lower wattage due to voltage drop/sag, add to that they were K biased.
Over time voltage drops were changed to give more watts,, and eventually they used Fixed bias giving cleaner power more headroom.
There is a point where you can have a nice balance where the Amp runs clean but still breaks up at just the right place and it's magic.

You can make a Valve circuit as stiff as a big SS rig and it won't sag but by the time it does start to deliver some valve mojo it's ear bleeding loud.

As a Great teck chap once told me when I first started messing around with circuits, "Small Cathode biased amps are great because they break up at just the right place and are not too loud."
I now see why that is so.

But try it out if you wish,, oh yeah I'll have a dig around for the 555 circuit i built as it has a pot which allows adjustment of the output voltage. That might take a while as I've got tons of schematics I've built over the years.
Phil.