#1
anyone got any instructions for something along the lines of power scaling?

apparently it sounds better than attenuation right down to whisper volumes.
Thank you please.
#2
There is a mod that cuts the power in half and gives you a marshall(ish) tone but it also makes it run with more background noise. In background noise going to be ok?
Not taking any online orders.
#3
is that the pentode/triode switch?

if so, i am planning on doing that too. i want some attenuation but i heard that power scaling is a better way to do it as the sound remains more intact at lower volumes. but the kits are quite expensive so i thought i'd try and find out how to do it.
Thank you please.
#4
From what I can understand, power scaling is just a means of lowering B+ voltages while adjusting tube bias accordingly - with a VJ it's not the easiest way of lowering the output.
Look into a L-pad (Kerry uses these as a attenuator in his amps) - basically it's just a huge pot that will lower the output volume without changing the resistive load on the amp. Othewise, do a google search and see what you can come up with for attenuators. A couple of big wire wound resistors will do the job, or you could even go as far as to create a reactive load and using a tapped auto-transformer to lower the output.
"Everybody, one day will die and be forgotten. Act and behave in a way that will make life interesting and fun. Find a passion, form relationships, don't be afraid to get out there and fuck what everyone else thinks."
#5
i heard a light bulb can be wired in there as an attenuator and giving a bit of compression. that true?
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


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#6
Quote by ECistheBest
i heard a light bulb can be wired in there as an attenuator and giving a bit of compression. that true?

yah thats true heres a picture and description of it, its near the top , judging by the scematic it doesnt look to difficult either, you could just install another jack and rig it for whichever ohmage you normall use then put a dpdt (on/off/on) wire one side no non bulb on side to bulb...i tihnk i just found my next vj mod ( once i get the fuse back in there)

http://www.valvejunior.com/
Last edited by mercinariesgtr at Nov 15, 2007,
#7
niiiiiiiice. that looks so dope. u need to put a socket on the top though. custom illumination.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#8
if you were really cool you could put a tinted thing ( not sure what to call it ) over the bulb then make it so the cloth i nthe front of the amp glowed ( red blue green )
#9
mmmm. u can swap the colors according to the season of the gig. hahahahaha.

brown/orange/yellow for now. green+red season coming up.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#10
yah i was just looking at radio shack for a bulb i found tons of 12v lights but none said their wattage(only had their amp draw), isnt wattage voltagexamperage? if so what ma would i need, i think i d need a 1.7 amp bulb hmm
#11
Quote by deadlydunc
anyone got any instructions for something along the lines of power scaling?

apparently it sounds better than attenuation right down to whisper volumes.

Powerscaling is just lowering the supply voltage and adjusting the bias to limit the output power. That allows you to get into a region where distortion begins to occur at lower volumes. It sounds more "natural" than using an attenuator, because the speaker is still connected directly to the output transformer, rather than through a resistor.

It's more valuable in a push-pull circuit, than in a single ended output stage like the VJ. But still I think it will be worth your while doing it.

There are 2 different techniques you should explore.

1 - adding a resistor between the rectifier and the first filter cap.

This will provide a "sag". Your maximum power will still be the same at the beginning of a note, but when the power is required in the output stage, the power supply voltage will "droop". Attack will be strong, but the note will sustain at a lower level and be distorted.


2 - adding a zener diode between the rectifier and the first filter cap.

some of the voltage will be dropped across the diode. The caps will never charge to the full voltage coming from the rectifier. The voltage drop across the diode, unlike a resistor is determined by the characteristics of the diode, not by how much current is flowing through it. You won't have the maximum power available at any time. This will act as if you have a lower voltage transformer.

In the case of the VJ, It's a self biased output stage. You should be able to greatly reduce the power supply voltage without changing the value of the cathode resistor.

Worst case, it will simply cause the output stage to go into cutoff sooner than saturation. Cutoff is not the best sounding distortion, it's more edgy than saturation. But it won't hurt anything. You'll not be allowing too much current to flow in the output tube. Rather, it will be too little.


Good luck,
SYK
Meadows
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#12
Quote by SomeoneYouKnew
1 - adding a resistor between the rectifier and the first filter cap.

This will provide a "sag". Your maximum power will still be the same at the beginning of a note, but when the power is required in the output stage, the power supply voltage will "droop". Attack will be strong, but the note will sustain at a lower level and be distorted.

isn't that like how van halen used his Marshall? feeding it 90V and such?
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


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#13
Quote by ECistheBest
isn't that like how van halen used his Marshall? feeding it 90V and such?
lol, no. Eddie's method was simpler and far more sinister.

By using a Variac set to 90 volts (some rumored he went even more extreme, at times), he reduced the B+ voltage and also the grid bias voltage for the output tubes. (Most amps use a negative voltage applied to the grids of the output tubes, rather than a cathode resistor.) This had the idling power on the output tubes still within reason (because the B+ voltage was lower) , but the idle current was high. Compounding this, was the fact that the heaters were now underpowered. Starved heaters + excessive current from cathode to plate = extremely short tube life and often resulted in catastrophic failure.

This is similar to how powerscaling works. But the whole idea behind scaling is to reduce the maximum output power in a safe and predictable manner.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#14
Quote by SomeoneYouKnew
lol, no. Eddie's method was simpler and far more sinister.

By using a Variac set to 90 volts (some rumored he went even more extreme, at times), he reduced the B+ voltage and also the grid bias voltage for the output tubes. (Most amps use a negative voltage applied to the grids of the output tubes, rather than a cathode resistor.) This had the idling power on the output tubes still within reason (because the B+ voltage was lower) , but the idle current was high. Compounding this, was the fact that the heaters were now underpowered. Starved heaters + excessive current from cathode to plate = extremely short tube life and often resulted in catastrophic failure.

This is similar to how powerscaling works. But the whole idea behind scaling is to reduce the maximum output power in a safe and predictable manner.

i think i got it. so Ed was running the tubes biased 'hot' right? if he ran the idle current low (somehow) would it result similar to the stock amp with a bit less headroom and volume? is that kind of the concept of what powerscaling is?
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#15
Quote by ECistheBest
i think i got it. so Ed was running the tubes biased 'hot' right? if he ran the idle current low (somehow) would it result similar to the stock amp with a bit less headroom and volume? is that kind of the concept of what powerscaling is?

It's kinda complicated, but that's the gist of it. You want to keep the idle current in a normal range, keep the filaments heated normally, and reduce the voltage to the plates.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#16
Quote by SomeoneYouKnew
It's kinda complicated, but that's the gist of it. You want to keep the idle current in a normal range, keep the filaments heated normally, and reduce the voltage to the plates.


SYK you have been astounding me for days with your practical knowledge.

once again. many thanks. this is exactly the type of info i was after.

you dont have any diagrams or schematics do you?
Thank you please.
#17
For power scaling? Sorry, no diagrams on that. I suspect Kevin O'Conner keeps the schematics off the net. He does make the techniques available in book form from his London Power Press company. The books don't come cheap, though.

You might get some useful info at a new forum dedicated to this topic: http://www.powerscaling.com/

You'll need a real e-mail address. If you try to register using a hotmail or yahoo account, you get instabanned. I tried. InstaFail. lol When I get a real e-mail account, I'll go back and see what they're on about over there.

Don't be too impressed with me. Knowing the basic principles, and actually implementing these techniques in a sophisticated manner, are two different things.

Just adding a resistor or diode in series is simple.

Do you have a schematic for the mods you're already planning to use?
I could add the info on the simple version to that, if you like.

I'll tell you how to do the math, and let you figure out the rest.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#18
TBH i think building a power scaling cct is possibly beyond my basic knowledge/technique. plus i just saw ampmaker is selling a 15watt L-pad for £4.99.

i think thats going to be the way to go for me as its a sealed unit already prepped. all i have to do is break the speaker wire and run a common/earth and glue the knob into place. that seems more inline with what i can probably achieve.

thanks for everyones input though.

duncan
Thank you please.
#19
does anyone have any instructions for the pentode/triode switch? i cant seem to find them anywhere although i am sure i have seen them...
Thank you please.
#21
Quote by deadlydunc
anyone got any instructions for something along the lines of power scaling?

apparently it sounds better than attenuation right down to whisper volumes.


yes I do. In fact, I prototyped my original electronic power brake on a VJ.

-g