# Identifying AC30 Master Volume Mod



## HolttChris (Aug 10, 2020)

I have a 20yr old Korg era AC30TB with a master volume mod I’m curious about 

Can someone more technically inclined explain to me what kind of mod this would be exactly? It cuts down a little bit of volume & not overly effective. Not a PPIMV?


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## Latole (Aug 11, 2020)

Wires seem to go to power supply. Look more a Variable Voltage Regulator ( VVR ) or Power Scaling.

Scroll mid screen;






Tube Amp Info – Bluetone Custom Amplifiers







www.bluetone.fi






*VVR (Power scaling etc.), Variac, PPIMV*
VVR = Variable Voltage Regulator/Reduction is a method to control amp output power using modern mosfet technology.
This control can be used different ways. Here are some examples of different approaches:
– VVR controls power tubes plate and grid (bias) voltages when it is a matter of fixed bias amp. In a cathode bias amp only plate voltages are regulated. In this case the best master volume is PPIMV (Post Phase Inverter Master Volume) to prevent power tubes from too much overdriving. If power tube plate voltages are set really low (30-50 V) and there is a high guitar signal level (30-50 V) coming from PI without any master volume, the result is really ugly sounding distortion.

– VVR controls power tubes + phase inverter voltages. In this case it is good practise to have a master volume before phase inverter, to prevent power tubes from too much overdriving as mentioned above.

-VVR controls all tubes plates voltages. In this case there is no need for a dedicated master volume, because VVR acts like a master volume to reduce noise level.
VVR is a good way to drop noise level even to bedfroom level. To our mind it works sonically best in cathode biased amps, but works technically well also in fixed bias amps when needed. VVR/Power scale does not overload power tubes compared to attenuators an dummy loads. These methods shorten power tube life cycle radically.

VVR works best in one channel amps where there is a need for power amp distortion. If you have a multi channel amp where the biggest part of distortion is generated in preamp, VVR is not the best way to go.


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## nonreverb (Sep 19, 2006)

From what I can see, (and there may be some things i can't) it looks like typical PPIMV. I see two intercept wires coming from the PI tube and a couple of caps behind that barnicle board with the pot 'n switch. What surprises me is the other mods that should to be done on those amps to take the hum/buzz away. is it quiet as is? I had to do both of mine as Marshall cut a couple of corners in their board design.


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## Latole (Aug 11, 2020)

Some unsafe mods;


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## nonreverb (Sep 19, 2006)

Latole said:


> Some unsafe mods;
> 
> View attachment 336459


Those aren't mods....they're factory.
There's so much clearance they decided to do it that way. Besides, I've never seen a failure from that set up in these amps and I've done many of them. There are other things that usually need attention.


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## Latole (Aug 11, 2020)

nonreverb said:


> Those aren't mods....they're factory.
> There's so much clearance they decided to do it that way. Besides, I've never seen a failure from that set up in these amps and I've done many of them. There are other things that usually need attention.


Very poor job, not professional.


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## nonreverb (Sep 19, 2006)

Latole said:


> Very poor job, not professional.


The power tubes are off the board and mounted to the chassis. Marshall has been doing this style mount for their grid resistors their entire existence and no one's complaining about them being unprofessional. BTW these were made under licence my Marshall...


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## Latole (Aug 11, 2020)

No one complaining ! 

1- How can you be sure ? 
2- 99.99999% people don't know electronics.


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## nonreverb (Sep 19, 2006)

Latole said:


> No one complaining !
> 
> 1- How can you be sure ?
> 2- 99.99999% people don't know electronics.


True...but I do. Tell you what, cast your discerning eye across that board and see if you can pick out two of the three real problems this amp suffers from... all are factory oversights that usually need immediate attention. I'll give you a clue for each. one is for hum, the other is to prevent the output from distorting or shutting down.


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## Latole (Aug 11, 2020)

I just say I will not build a amp this way.


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## nonreverb (Sep 19, 2006)

Latole said:


> I just say I will not build a amp this way.


They're actually a pretty good amp but I agree, they did cut corners here and there on them. Once they're "upgraded" they're fine.


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## greco (Jul 15, 2007)

nonreverb said:


> ....one is for hum


Is this approach to wire dressing (i.e., black wire wire tied to red wire loops) the one for hum? 
(I'm trying to learn through these threads)


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## nonreverb (Sep 19, 2006)

greco said:


> Is this approach to wire dressing (i.e., black wire wire tied to red wire loops) the one for hum?
> (I'm trying to learn through these threads)
> View attachment 336507


Indeed it is greco! Although it usually requires much more on the first loop from the power supply. There's a sensitive part of the circuit there and I usually have to twist them tight and locate them at a particular angle to suppress the hum. It's a PITA.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Indeed it is greco! Although it usually requires much more on the first loop from the power supply. There's a sensitive part of the circuit there and I usually have to twist them tight and locate them at a particular angle to suppress the hum. It's a PITA.


Those black & red wires @greco referred to, they look like they might be the heater wires for the pre-amp tubes?? Funny way to run them for sure.

That brown wire from the board to the speaker jack, is that negative feedback? Is it ok to run it right over top of the rectifier socket like that? Would that arrangement ever pickup any noise?


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## nonreverb (Sep 19, 2006)

Lincoln said:


> Those black & red wires @greco referred to, they look like they might be the heater wires for the pre-amp tubes?? Funny way to run them for sure.
> 
> That brown wire from the board to the speaker jack, is that negative feedback? Is it ok to run it right over top of the rectifier socket like that? Would that arrangement ever pickup any noise?


Good eye Lincoln. I was wondering about that myself. It shouldn't have negative feedback being an AC30


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## Gordie (Mar 22, 2014)

Isn't the brown wire the B+ coming from pin 8 of the rectifier? But it looks like it leaves the socket in both directions...


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## nonreverb (Sep 19, 2006)

After looking at it more carefully and referring to the schematic, the brown wire is the B+ for the output transformer. It looks like it might land at the speaker socket but it doesn't. It's hiding in the wire loom behind that mod assembly.


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