Master Volume I added. Now some questions.

[Papa Jim]

Attached is my 4-4-0 AX84 Studio stock Schem. of the Phase Inverter section and then the same schem. with London Power Master Vol. mod. I have a few questions concerning it if you could all help me it would be greatly appreciated. I am not an electronics guru but able to layout and build nice working amps. At least this amp turned out very nice but was just still too loud for a bedroom amp and I still had to use an overdrive pedal. I would like not to so I added the Master Volume. Thus the questions.
1) Will the the amp need to be re-biased after this?
2) Are the original values for the coupling caps C11 and C12 still o.k. with the mod.
3) I see a lot of amps where these caps vary by usually a factor of 10 and was wondering why mine were the same values.

[PRR]

Woah, those oversize pics are scary. Here's one that fits a screen.

Bias does not change.

Caps can be that size. Or C11 could be 0.01u. Makes little difference.

[Papa Jim]

Haha. Thanks for fixing the schematics and the answers. I really never use the prescence pot. Can it be replaced with some fixed parts, or totally removed. Actually I used the hole it occupied in the chassis for the Master Volume Pot. I think I set the prescence at 50%, put some very large heat shrink around it and stuffed it away inside the chassis but still connected. I would rather just eliminate it from the circuit though I think.

[Colas LeGrippa]

Adding a master volume will still be too loud and you won't get the crunch we like when playing at low volume in our appartment. In my house, I use a 12VDC  tube (12AU7) overdirve pedal that I made and I get plenty of crunch at really low volume. The trick to succeed in building these low voltage pedals is in the power supply. You got to use a REGULATED 12vdc POWER SUPPLY  or make one. It has to be regulated to get pure DC otherwise, welcome hum!!!! You can easily make one with a 7812 voltage regulator. More, the heaters are much more easy to hook up. Pin 4 : 12VDC, pin 5 to gnd, while pin 9 stays unconnected.


Colas

[PRR]

> Can it be replaced with some fixed parts

Yes. With the pot at the happy-spot, measure both sides, find similar resistors, use the same cap.

[Papa Jim]

> Can it be replaced with some fixed parts

Yes. With the pot at the happy-spot, measure both sides, find similar resistors, use the same cap.

Will do. Thanks.

[Papa Jim]

Adding a master volume will still be too loud and you won't get the crunch we like when playing at low volume in our appartment. In my house, I use a 12VDC  tube (12AU7) overdirve pedal that I made and I get plenty of crunch at really low volume. The trick to succeed in building these low voltage pedals is in the power supply. You got to use a REGULATED 12vdc POWER SUPPLY  or make one. It has to be regulated to get pure DC otherwise, welcome hum!!!! You can easily make one with a 7812 voltage regulator. More, the heaters are much more easy to hook up. Pin 4 : 12VDC, pin 5 to gnd, while pin 9 stays unconnected.
Colas

Colas, It supposedly the amp is only 2 watt amp. I have tested it with the Master Volume and for my purposes, it worked out good. Here's the description of the master volume from london power website.
london power.
This is the best master-volume on the planet! Truly transparent. No tone change with setting. And easy to install.

The majority of guitar and bass amps use a Schmitt splitter for the front-end of the power amp. The LP-MV becomes a part of the phase-inverter - not "pre-PI"; not "post-PI" - but embedded as one with the splitter, giving truly outstanding performance. Most MVs that are fitted to amps at the factory or by techs as after-market add-ons change the tone as you sweep the control from zero to maximum. The LP-MV does not. It is transparent in its function while reducing the signal that reaches the output.

[tubenit]

Setting aside KOC's description of the master volume, ...................   how do you think it sounds?

Compared to a typical pre-LTPI MV .......... or a LarMar  post-LTPI  master volume,  do you think it sounds better and more transparent?  or just different?

With respect, Tubenit

[ac427v]

I love the the sound changes that come with a good presence control. But I don't understand the design of this one. It looks like the "top" of the voltage divider is only 5.6k while the bottom is 25k. Did it work? Is the funky presence design related to your inability to get good sound at low volume?
Craig

[PRR]

Is the AX84 forum still active?

I have doubts about that NFB presence network, and especially that a turn-down at any stage (even where you put the MV) will cripple the amp enough to reduce maximum output before gain gets useless.

My morning-thought is that you want to reduce B+ to the output stage *very* significantly. Like from 340V down to 200V and probably 100V for bedroom work. The simple path would be a bunch of 5K 5W resistors switched in to the feed to the OT B+ tap. Probably with an added 10-20uFd cap there to hold the lowered B+ steady. But that is not elegant, and may have other shortcomings.

I'm wondering if others have been down this trail already.

[Papa Jim]

Here's a corrected schematic. Sorry for such a delayed post.

[Swampertech]

Papa Jim, this is the 10% power switching arrangement I'm using in my Mr. C-Verb SL.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#10%_Power_Switch

[Papa Jim]

Will you have this attenuator and a MV also?

[Swampertech]

PJ, yes I'll have both. I'm putting the 10% switching in for my grandson to be able to practice at a descent level, but still have the tone of the amp.

[Papa Jim]

cool. report back with tonal comparison between the two.