Which high power CCS for output stage?

[andyjevans]

Hello guys - I already use various CCS on my input stages taken from Morgan Jones' book. I now want to try a CCS on the output stage. What I want to use is:
1) a pair of 6AH4 in PP
2) two pairs of 6AH4 in PPP
I'm guessing I need a CCS for each pair - this was recommended by Allen Wright for instance. Requirements are up to 30mA per 6AH4. If this works well I'll try it on 6CK4, 12b4, 6S4A and 1626 as well as 6V6 in triode, so could be up to 40mA for some of those.
So what should I use? Any Schematics or guides I can use to construct one? Many thanks for your opinions - Andy Evans

[Sherman]

If you aren't opposed to sand try the simplest CCS I've ever seen. Use an LM317 voltage regulator, between the ADJ pin and the OUT pin there will always be 1.25 volts. Place an appropriately sized resistor between ADJ and OUT. Bingo, you are done.

In your case you want 60mA so a 20 ohm resistor will give you 62.5mA. Probably close enough for government work.

[Sherman]

Oops, forgot to note that an LM317 doesn't like more than about 40V across it. Make sure the voltage you have across it isn't more than that. Should be OK for use in the cathode resistor position of most amps.

[Bryan]

Andy,

Have you thought about using Gary Pimms CCS. I've used both the battery biased and self biased CCS on the plate of my 12b4a linestage with GREAT results.

Just a thought.

Bryan

[tubelab.com]

I use the IXYS 10M45 integrated circuit. I have used them in preamp stages, and output stages. To use any CCS in an output stage you will need to increase the supply voltage considerably to provide headroom for the output to swing above the original supply voltage. This is provided by the output transformers inductance in a conventional design. I have a SE 45 amp using the IXYS chip and a supply voltage of 500 volts. Quiescent plate voltage is 275 volts. The limiting factor is the chips maximum voltage rating of 450 volts. Max current is 100mA and max dissipation is 40 watts, and a good heat sink is required. IXYS lists a 900 volt version on their web site but they are not available.

Gary Pimms web site has several CCS designs, but I haven't tried any of them.

http://home.pacifier.com/~gpimm/

There is another thread here where several of us have discussed some simpler hybrid solutions, but I don't think anyone has built anything yet. I am planning a really big hybrid CCS that can power an SE 845. I will probably use an 813, just bought some on Ebay, haven't got them yet.

Active load output stage

[andyjevans]

I use the IXYS 10M45 integrated circuit>>

That seems a good road to go down - how do I get some? I'm in London UK.

Also, am I right that for multiple pairs of tubes in PP, you'd put a CCS under each pair?

- thanks, Andy

[tubelab.com]

I have seem circuits that use a CCS for each pair, and circuits that use a CCS for each tube. I haven't tried either so I can't comment on the merits of either design.

The statement I made about increasing the supply voltage only applies if you are using the CCS as the plate load (chokeless parafeed).

I get my IXYS chips from DigiKey in the US. I believe that they ship internationally, but it may cost a lot for a small order. They also carry some good Panasonic electrolytics in 500 volt sizes, and many types of metal film resistors.

http://www.digikey.com/


I went to their web site, click on the UK flag at the top. The banner says "free shipping on orders over 100 pounds sterling". Put 10M45 in the search box and hit enter. Then choose the IC.

[gingertube]

Andy,
It is a fact that the CCS for an output stage does not have to be quite as good (as high an AC impedance) as one used on an input diff amp.

I have an EL84 Ultarlinear Class AB1 PP Amp in which I use "Ring of Two" transistor current sources in each tube cathode as shown in the morgan Jones book. I run these at 40mA (using BC547B and MJE340 for the bottom and top transistors respectively). I feel confident that this arrangemet could handle 60mA for a Class A push pull differential output pair.

When it comes to the 120mA for a PPP arrangement of higher current you may wish to try this simple circuit which is for a 300mA CCS. Its not a GREAT CCS (like the Gary Pimm jobs and similar circuits) but its good enough for the application. Caution - max of 36 volts across the TL431. If necessary use some fixed bias to the grids as well. To change its current change that 8R2 resistor - you want 2.5 volts across that resistor so for 120mA CCS use 20.83 Ohms, 47R parallel 39R will be close enough. That "Bias" label means to a positive supply such that there is 1mA (minimum) flowing through the TL431

Cheers,
Ian

[SY]

I've not had good experience with 317s as output CCS. Bad overload recovery. Maybe there's a circuit trick to get around this, but if it were my job to do, I'd be looking at discrete.

[Sherman]

Originally posted by SY
I've not had good experience with 317s as output CCS. Bad overload recovery. Maybe there's a circuit trick to get around this, but if it were my job to do, I'd be looking at discrete.

I am definitely not an expert and I've only personally used an LM-317 as a CCS once. I have a single-ended EL84 stereo amp and decided to do this to one channel as a test. I do not have "golden ears" (perhaps they would be more accurately described as "iron" ) but I could hear a definite positive improvement in that channel. Describing the change in words is difficult though. Maybe "clearer" or "more transparent"?

However this isn't exactly a high-power amp and being single-ended I think it would be pretty unlikely to overload the 317 so it probably isn't the best test. I've not done any testing other than listening at this point.