[Leadman]

Hello,
What components would be deleted for build the TT LX18 without master volume? Maybe C6, c7, P4 and r28?

Thank you in advanced.

[tuxele]

Well, just delete C7 and P4 and connect the free pin of C8 with C6. R28 is needed.

[Striker52]

Hi, I think it would be better to keep all components at their place and only replace the volume pot with a voltage divider (like R10 and R11) to avoid a too strong signal at the input of the phase invertier.

[Leadman]

Thanks for the replies,

Why r28 is needed? What is the funtion of r28 resistor without another resistor for a voltage divider? I thought R28 is in parallel with the master volumen pot for softening the volumen control curve.

Best Regards!

[Striker52]

R28 together with C6 build a frequency filter (high pass). R28 is not in parallel with P4; the both are seperated by C7. As I wrote before you can use a voltage divider (2 Rs in series) to replace P4. Use P4 first to determine the signal level you want to have at the input of th PI, then measure the resistances between input/taper and taper/ground of P4, and replace P4 with the according Rs.
Good luck!

[Leadman]

I got it now!
Thank you so much for the help


Best Regards!

[tuxele]

R28 together with C6 build a frequency filter (high pass). R28 is not in parallel with P4; the both are seperated by C7.

But surely, of course, R28 is in parallel with P4 in AC terms! I guess that C7 is even needless due to its big value.
Heaven knows why Dirk implemented this additional capacitor ...

[Dirk]

I guess that C7 is even needless due to its big value.
Heaven knows why Dirk implemented this additional capacitor ...

Give it a try and you will see (hear) why C7 is needed 

Best regads, Dirk

[tuxele]

Give it a try and you will see (hear) why C7 is needed 

Best regads, Dirk

Unfortunately, I haven't built this amp, so there is no chance for me to try it out.
I'd like to learn more about designing guitar amps. So what does this mystery cap do? Please give us a hint.

[Dirk]

oh, my mistake, I did not read the full thread. You want to remove the Master. In this case you do not need the C7, C8 and R28 but you must replace P4 with a voltage divider as Striker52 described earlier. I highly recommend to simulate the circuit with SPICE so you can test and simulate different modifications to get an idea if they will work or not and when you have a good working circuit then you can build the amp.

[tuxele]

In my opinion, the coupling cap C8 should be left in place when a voltage divider instead of pot P4 is going to be used in order to block the DC component (grid voltage of about 45 Volts) of the phase inverter. i agree with Striker52 that C6 and R28 are used to do a highpass job and should be also left in.  As far as the "mystery cap" C7 is concerned - I cannot explain the use of it ...

[Striker52]

I fully agree that c28 has to be kept in place. The PI Grid gets its ground reference from the bottom of R16.

[Leadman]

I´m still thinking about C7 cap funtion.
What is the purpose of the c6 r28 c7 node before the master volume?
What will happend if I change de c7 cap value. Could I put a smaller cap like 100nF?

Thank you in advance for the help.

[Dirk]

Why do you want to remove the master if you do not know how it sounds or how it works ? This makes no sense for me.

My feeling is that some people think that they must make some changes just to make some changes without any idea of what they are doing or what the result will sound like. I have see this some many times now...  and when they run into problems, the frustration is going up because they do not know how to solve the problem and at the end they have an amp with some cool mods but also without any function.

The LX18 (or any other of our kits) are well tested and known to work fine if you build them correctly. I highly recommend that you build it first and make sure that the amp works staple and after this you can think about some changes.

You don't have to remove the master. Just turn dial it up and keep it there so you do not have to change anything and it works like a none-master.

Best regards, Dirk

[Leadman]

Hello, the amp has been built like the real one, it goes fine and sound superb.
But I want to know more a more about electronics, and I don´t find nothing alike that Cap, I only wanted to know how the master (and that cap) works. It´s only about curiosity ....

Best Regards.