Constant Current uses

[Pedro]

Hi,
I'm getting very interested in trying constant current approaches in my amps.
What i'm finding is that there isnt a lot of "newbie" info out there via google.

Initially I just want to try using CC sources as loads or cathodes - whats a good "starter for ten" , say in biasing an input stage or using as a plate load in an input stage ?

I'd like to try it on a couple of my fairly simple guitar amps - as well as the "Dinosaurus" - the 6SN7/300B Fi Primer amp which is undergoing surgery and is rapidly becoming the test bed for tweaks while the PT15 is on the breadboard. ( and before it gets its c3m input stage to become a decent 300B amp ! )

Any good places to start experimenting with CCS ?

cheers all,
Rgds
Pete C

[Andrew]

Hi Pete

The CCS world is the mollusc of your choice.

For a simple CSS you could, as you probably know, just use a depleteion mode mosfet biased with a resistor. I think MJE 340/350 have been used with some success, next up IXYS do a CCS on-a-chip, which are pretty good for a £1 part and can do big voltages. Then there's the C4S see the Bottlehead's site, and there's of course pentodes and triodes and finally there's Gary Pimm....

cheers,

-- Andrew

[Pedro]

Andrew,

thankyou - but you overestimate my capabilities and existing know-how !

any basic reading on this - or something simple to try for starters ?

I'll search the topic in the interim

cheers
Pete

[Andrew]

Hi Pete,

Do have a look at Gary Pimm's site and Morgan Jones is a good place to start. Have you got a copy of MJ?

cheers,

-- Andrew

[Pedro]

Andrew,
thanks....
I now have MJ , and have read Gary Pimm's introduction to CCS. Gary's description is very straightforward but I need to find some example circuits to try. I get the concept completely now.
MJ is quite advanced for me but I will persevere...

I think I will start by trying to replace plate resistor loads in my 300B amp - on the input stages of the 6SN7s and see where I can get.

Also I can mess with simple 12ax7 input stages on a couple of guitar practice amps.

I need to read up on suitable MOSfets now and get my head round the V/I curves to determine right current draw.

Thanks for the tip.

If anyone has any ( simple ) real world examples they are using I'd like to study them.

cheers
Pete

[Andrew]

Hi Pete,

Work out what current your small signal valve is currently passing at idle then set the current on the CCS to the same value. I usually then set the size of the cathode resistor to get the right voltage at the anode. You have to work the sums differently, if you see what I mean, as the current is now fixed. So, instead of fixing the HT and load resistor, and varying the bias you're fixing the current, varying the bais to get your HT.

It is also worth thinking through all of the consequences of adding a CCS to an existing stage. Please don't let me put you off, but putting in a CCS does make quite a difference, electrically speaking. For example, you will find that solid state CCS is able to still operate and drop much less HT than, say the equivalent resistor, tho' some CCS will tolerate a large drop across them so are a drop-in replacement, pardon the pun.

cheers,

-- Andrew

[Primalsea]

Have a look a Rod Elliots ESP pages. he has a good article about current sources there as well