Kustom wanna-be-tube

[Lex]

Hello all you lovers of tube* amp's,

already been discussed about this little quirky amp https://www.ssguitar.com/index.php?topic=1356.0
so not to go back to that topic what is too difficult (read: technical) for me to apply all-or-some of mod's,
I went for the very first and easiest mod: replaced ax7 with au7 lamp.

Now is Gain more controllable and that typical rattling is gone.
It sounds really good. Removed the head and running it with Roland's external cab.
Happy days ..

Schematic was a problem as I can read it so I found one. 

I would like to apply Gain to the clean channel so if someone know-to ..  my eyes  are wide open.

tube* - well it's a hybrid.

Cheers

[phatt]

Hi Lex,,
You can only do that with a 2 channel circuit.
your unit is single channel with switching for OD as such whatever changes you make effects both clean and drive.

If it's feasible you could separate SW1B from SW1A into 2 separate switches then on clean just open SW1B will remove diodes from the circuit.
As the diodes are the main source of distortion then removing them with SW1A still engaged in drive mode the signal will be a lot cleaner.
How well that would work in practice is anyone's guess.

One thing I have learned in many years of trying to refine multi channel circuits is the balance between Clean and Drive is ever so hard to get right.
Very few Amps I've had the pleasure and often Displeasure of using are NOT well balanced. It took me years of building dud circuits and amp mods ,, to finally realize that as much as I hate pedal boards I now use them as it's so much easier to work into a clean amp and do all the tricks on the pedal boards.

I'm now quite happy with my setup and hardly ever bother to tweak the amp.
At least half of my setup is all my own designs as I've learned from years of research what works,,, a long road but all done on a budget.
Phil.

[Lex]

Hi Phil, thanx,

Im getting what you wrote, this about single / double circuit is logical to me.

I was thinking that the easy solution would be to make a switch to clipping diodes.
However, from the factory those 4 diodes are covered in some white plastic/hard glue.
Like they want to prevent people modifying it.
I suppose it would be possible to melt that stuff with soldering iron.

But also, just to try OD channel without clipping diodes Im thinking to 'jump' them instead of removing them from the circuit (much less work).
Is this something as DO it or NO No nooo ...  cant imagine that I could burn something as is the same thing as removing diodes all together and placing a jumper.

OD channel with gain on 0 behaves as the clean channel.

Im keeping everything on 'the budget' side while learning. Im not expecting huge changes, but even little ones will boost my enthusiasm.

Of course pedals are the way to go, thats clear to me.

Cheers

[phatt]

No don't bridge the diodes that will just leave the 1k5 resistor (R14) across the audio path.
Likely low volume will be the result.

Removing means open circuit which is the complete opposite of jumpering them with a wire, closed circuit.

Likely easier to replace R14 with a pot and dial in how much diode effect (distortion) you want.
I don't know how well that would work but I'd guess somewhere between 10k up to 100k pot would work.
Phil.

[Lex]

ok Phil, learned something again, 

That something is actually - im getting more confused what is a clear sign of my elec knowledge around amp circuits.

Does it work like this;
- SW1A (Gain) is ON > so it gives the power trough 2 > then trough the tube V1A > and then sends it to 1 > and further to the tone stack?
- signal goes back to the tube > V2A > amp > speaker ?
- if this is correct, I cant see from the circuit how OD works only when Gain is on

about the pot - well sins I started being interested how an amp works, been thinking of: "why is there only off-on switch for OD instead of a pot.."
So you gave me a good idea that I should try to implement.

Thanx for the time and willing to explain stuff,

Cheers

[phatt]

Hi Lex,
        I can only give some explanation here as it's beyond the scope of a forum to teach 101 electronics.
Start to search the net using keywords such as Diode clippers. or Audio design with opamps.
Anything you don't understand just search the net as there are mountains of info to be found.
some are better than others.
As your amp uses valves then some very good info can be found at the Valve Wizard site.
If you like the site you can purchase his book, it's very well written.
As for SS amp designs Teemu's book has tons of good reading that will help,, it's free and can be found right here on SSGuitar,,https://www.ssguitar.com/index.php?topic=711.0

regards to your questions;
Yes Audio path runs left to right.
In Clean mode SW1A and SW1B are open therefore VR1 is a 10k fixed resistor which sets clean gain of IC1A.

With SW1A closed (OD mode) VR1 becomes a Variable resistor (a Pot).
maximum gain is when VR1 grounds R5. (the CW (clockwise) sign tells us which way the pot turns.

Ok now the signal is much bigger so after the signal passes through the triode V1A it runs into the diodes D3,4,5,6.
SW1B has also been switched on as it's part of SW1A so now the diodes will clamp/clip/distort the signal at the threshold of the diodes set by the amount of diodes in the string. When the Gain pot is high the signal is clamped by the diodes. The amount of clipping is set by the number of diodes and the value of R14.
If you want less distortion make R14 larger. or As I mentioned replace R14 with a pot wired as a variable resistor then you can crank the Gain pot and then dial in the amount of clipping you want.
Hope it helps, Cheers,, Phil.

[Lex]

HI,
Im aware about my question(s) and 101.
However I reed somewhere on this forum: there are no stupid questions ... so Im taking my chance.

Found The Valve Wizard site, thanx. Havent see it before and I did an extensive research.
Found also a pdf, downloaded .. but the more I read (learn) - the more questions I have.
It could be a typical syndrome when you know some stuff but not having a proper foundation. Could be like learning to write with the opposite hand, or learning to play guitar with the opposite hand.

Thanx for the circuit explanation Phil !

I'll give a pot a place instead of R14.
And a question about 10K vs 100K vs ***K;
 -  pot's are going from 0 to their max value. So if I take 250K (for an example) and I turn it only till 10k or 100K, its still OK?
Or im missing here (again) some basic electro knowledge ?

Cheers

Lex

[phatt]

Yes Lex,  It all depends on how much you already know about electronics as to how fast you learn.

Some stuff here might help
opampbasics
https://www.neatcircuits.com/op_amp.htm
Main page
https://www.neatcircuits.com/

Also a ton of stuff here;
https://www.sound-au.com/

Re the pot value.
You can use any size pot but larger values will be less useful.
Of course a 1k pot will be way too small so yes start with 250k if you have that and report how well it works.
With pot at minimum resistance the distortion will be at maximum but the signal level will start to suffer.

Regarding understanding circuits remember this;
When you look at a schematic you are basically looking at 2 interconnected circuits.
The Signal (what you hear) is AC but to make it work you have a DC supply.

Most circuits read with the AC running left to right and DC is at the top and runs through active devices to Common or ground. Common is often for both DC and the AC but not always. The Valve part of you circuit reads that way.
When designing a simple valve or transistor circuit one has to set the DC voltages up so as to pass the best AC signal.

Along with a lot of reading if you want to speed up your learning the best way is to grab some Circuit sim software (many are free) Most come with examples to help you learn.
Add a breadboard and make some simple guitar pedals and you will learn so much faster.

The best part of Simulation is that you can't blow stuff up the worst that can happen is you crash the program. lol.

And yes of course you can still ask stupid Q's if you are not sure. 
Phil.

[Lex]

With pot at minimum resistance the distortion will be at maximum but the signal level will start to suffer.

yes got it ! this is the right information that explains to me the roll of this resistor.

Hi Phil thanx again for your writing.

I have some Sim already installed on my comp. But im still far away from 'simulating' amp circuit.
I hope that graduately I will understand and learn more and more.
A breadboard will come for some simple stuff as I find it interesting and there is a lot on-line to look-at-and-copy.

After thoroughly examining this circuit I realised that  the reading is from left to right, and then I also realised that 'i knew that' but I was never aware of it. Somehow it was logical to me so I have never paid any attention to it.
However, motorbike and car elec circuits (that I know and understand) are also from left to right .. also never thought about that.

Ok ill report when I find the time to tweak this resistor with a pot,

Cheers

Lex