Preparing for power scaling

[guitarmike2107]

if you have changes in tone with your circuit, perhaps you over looked employing keeping the screen at AC ground in your circuit.... -g

Gary , Can you expand a bit on this for those a less informed. (me!) what are the requirements here, I am about to start building an amp where I only control the screens, So do I need to place a Filter cap after the mosfet to maintain AC Ground for the screens?

[mooreamps]

Electronic Power Brake

I'll do you one better.. This is a drawing of one half of the power amp from my 18 watt SE amplifier ; with the actual component values used... I consider this layout down-rev now, but it you build it this way, you won't need a FET...

-g

[mooreamps]

Hi Guys

My actual comments in TUT regarding EL-34s had nothing to do with how they sounded, rather about their reliability.


Have fun
Kevin O'Connor

Sorry... I stand corrected....

-g

[guitarmike2107]

Thanks Gary, I was actually trying to understand the application of AC grounding with the screens, and how the mosfet would affect it if I was driving the screens straight off the mosfet.

I take it your using a 2w pot for your power control, whats the 1 ohm resistor for?

[Steve Conner]

I don't see the need for a capacitor after the MOSFET. If you put the capacitor on the MOSFET gate instead, you get a multiplying effect that allows it to be much smaller for the same amount of smoothing. The general circuit concept is called a "capacitance multiplier".

If you assume the capacitor to be a short circuit at the frequency of interest, then the output impedance at the MOSFET source is 1/gm, less than 1 ohm for most TO-220 FETs.

Here's how I did mine.
http://scopeboy.com/scopeblog/wp-con...etteHybrid.pdf

The resistor between pot wiper and MOSFET is a bit too small, it should be at least 100-250k for best hum rejection. I intended it to stop the pot burning if the MOSFET shorted, and only thought about the capacitance multiplier side of things later.

For higher powered amps, the idea of putting the full B+ across a pot track gets increasingly shaky. I think the modern ones are rated 250V. I used the biggest old pot I could find in the components store at work.

PS: I said above that the bass thinned out as the idle current increased. Not decreased.

[guitarmike2107]

Thanks Steve, thats pretty much what I was planning on using, same Mosfet too. I will probably use a 500v 2W pot for this next build.

[d95err]

For higher powered amps, the idea of putting the full B+ across a pot track gets increasingly shaky. I think the modern ones are rated 250V. I used the biggest old pot I could find in the components store at work.

The "DC Power Scaling" kits from London Power use a low control voltage (something like 12V) regardless of how high the B+ is. So you can use any kind of pot and it is no noise problem to keep the Power Scaling pot near sensitive preamp stuff.

[Steve Conner]

I made a point of never buying any of Kevin's kits or books, so I can say that I know nothing about what he did, and therefore "reinvented" his ideas rather than ripping them off. Does the DC power scaling look anything like this?

[mooreamps]

Thanks Gary, I was actually trying to understand the application of AC grounding with the screens, and how the mosfet would affect it if I was driving the screens straight off the mosfet.

I take it your using a 2w pot for your power control, whats the 1 ohm resistor for?

The resistor bleeds the charge from the cap when the dial control is set to zero. It's that resistor that makes the whole thing work....
That is a 20K 5 W pot I sourced when I first cobbled up the circuit... I still have a few left... But you could use either 50K or 100K at 2 W and should still be fine... But , tweek the value of the series feed resistor in the same proportion to keep the max screen voltage lower of the plate value, and within the recommended max value as listed on the data sheet......

-g

[mooreamps]

I made a point of never buying any of Kevin's kits or books, so I can say that I know nothing about what he did, and therefore "reinvented" his ideas rather than ripping them off. Does the DC power scaling look anything like this?

Perhaps the both of you are talented engineers.... Not the point... I have reviewed "all" of Kevin's publications and I would recommend any of his books or kits to anyone who wants to learn how to build these circuits...

-g

[guitarmike2107]

cool, I like the fact that it is simple and may try it on a simple amp if I can get the pot, if you dont mind of course, I take it the pot scratches a bit?

[guitarmike2107]

I made a point of never buying any of Kevin's kits or books, so I can say that I know nothing about what he did, and therefore "reinvented" his ideas rather than ripping them off.

I have only read his first book, no power scaling in that IIRC, but I don’t get this whole ripping things off thing with VVR / power scaling etc, if you publish a text book with technical information and ideas, and somebody buys that book and uses the technical information to further their product that’s not ripping someone off, that’s called doing your research and using what you find. Unless its patented or protected somehow, which it isn’t.

If I couldn't use the information in text books and product manuals at work and had to rely on everything being invented from first principles first time every time then I would get nothing done fast!

If you want your idea protected then patent it, I have a patent in my name, still standing 8 years later, the product is public now but its protected, if someone wants to use it they can pay to use it.. simples.

Rant over.. sorry

[GregS]

I have only read his first book, no power scaling in that IIRC, but I don’t get this whole ripping things off thing with VVR / power scaling etc, if you publish a text book with technical information and ideas, and somebody buys that book and uses the technical information to further their product that’s not ripping someone off, that’s called doing your research and using what you find. Unless its patented or protected somehow, which it isn’t.

If I couldn't use the information in text books and product manuals at work and had to rely on everything being invented from first principles first time every time then I would get nothing done fast!

The distinction is between using the information and ideas for your own personal purpose and using them in something your selling to make money. To be more clear there is no issue with using the ideas in the books for your own personal amplifier build but there might be if your using the ideas in a product your selling for profit.

[guitarmike2107]

There is no might be, either there is or there is not.

We use laws of physics established by famous scientists all the time without paying them dues, how many guitar amp builders have started building amps by looking at other peoples designs and learning from them... All of them!

[mooreamps]

cool, I like the fact that it is simple and may try it on a simple amp if I can get the pot, if you dont mind of course, I take it the pot scratches a bit?

Me mind ? Not at all. Like I said, this layout to me is down-rev...
Pot scratches ? Nope.. You shouldn't hear a thing when adjusting it.. Also, you can put the pot all the way to zero for both power up and power down, and it puts the power tube into a standby mode....

-g