Active load output stage

[ThorstenL]

Konnichiwa,

Originally posted by croccodillo
Here we are. This is the circuit I will try to realise.

While no doubt a nice enough idea, I do not think that the result will materially improve on either an optimised "ring of 2" BJT or on either a DN2540 Depletion FET or the IXYS CCS, all of which do not require all the added stuff, this is not even so much a question of cost, but one of complexity, less complexity, less to go wrong.

If you compare Gary Pimm's measurements of CCS's, then you find the simple 3-Pin Ixys part does quite well, both in terms of bandwidth and actual impedance.

Those who have tried it (without additional cascoding) felt it great (that is TubeLab, Pete Millet and some commercial concerns better left unmentioned) often in more demanding applications than what you have in mind.

Sayonara

[tubelab.com]

As I explained in another thread, experimentation is the forerunner of innovation, and we need some innovation in this hobby. Giovanni's hybrid CCS may be complex, but it may turn out to work great. No one will know until someone builds it. Any of my attempts to connect opamps to vacuum tubes generally resulted in fried opamps, but who knows until he tries it. If it doesn't work well, he has traded time (and maybe a few parts) for knowledge and experience. If it works great, then we all have one more new circuit to use. Each individual can choose to use it or ignore it.

I (and others) have stated that the IXYS chip works great, but it does have limitations. It handles 450 volts max, and 35 watts max. It also doesn't seem to work well at very low currents. I have applications that require more voltage and more power. IXYS has a new chip that claims 900 volt capabilities, but you can't get them anywhere. When my lab is back up and running, I intend to try a variation of his other circuit with an IXYS chip in the cathode circuit of a pentode to attempt a 100 watt capable CCS.

Giovanni is new to the vacuum tube hobby, but he is an accomplished SMPS designer. Once he gets good with tubes, maybe we can talk him into designing a dynamically modulated plate supply that tracks the peak music voltage. This would feed the CCS just the right voltage at the right time thus eliminating the power dissipation in the CCS. In fact the CCS wouldn't even be needed if the supply was clean enough. I have designed such a power supply for a mobile two way radio, but it worked at 28 volts.

[agent.5]

I agreed with tubelab. Try something different and see how it goes. Also, your idea of a 813 active loader for the big bottles sounds very tasty.

[croccodillo]

Sorry,


Just realized that my previous CCS schematic cannot work with tubes, but can work great (IMO) stand-alone.
I will think something different to drive the tube.

George, Agent.5:
Thanks for your support, you exactly centered my kind of thinking: always try to experiment something new, you'll go wrong a lot of time and, sometime, only sometime, will have great success.

Kuei Yang Wang:
Thanks for your feedback, I agree with you, but my mind is different from yours.
This should be our hobby, our fun: and, for me, the biggest part of the fun is to create something that nobody has ever seen.
I have no fun to use a ready-to-use solution: I have about 200 schematics of SE, PP, CF schematics collected on my HDD, but no one can be called "mine"...

I'm an R&D engineer, my job is to develop innovative Microwave Histoprocessors (try Google, Milestone is the company for which I work...): I simply cannot follow a solution somebody else has already drawn.

This is a my limitation.
Don't get me wrong if I do not follow your suggestion: I'm a little bit crazy...

Ciao,
Giovanni

[ThorstenL]

Konnichiwa,

Originally posted by croccodillo
This should be our hobby, our fun: and, for me, the biggest part of the fun is to create something that nobody has ever seen.

By all means, but your Op-AMp circuit (if made to work) will do nothing else than the on-chip stuff inside the IXYS CCS.

If you want something novel take a 5mA FET CCS and use that instead of the usual resistor to the base of the pass transistor in a ring of two CCS. This cascoded by the KT88 will make a very good CCS that is simple (7 parts including KT88 and it's gridstoppers) and easily build (which will get you a lot of followers).

Sayonara

[croccodillo]

Kuei Yang Wang:
I cannot find here where I live IXYS chips, and there's nothing else like such a thing (or, better, there's nothing else cheap enough for me); so, I have to build my own CCS.

So, again another one CCS.
This CCS is the simplest I can realise using a high-speed op-amp.
The CCS can handle up to 1000V and up to 1.4Amps.
I've selected the IRFBG20 Mosfet due to its really low input capacitance.
How it works:
Q1 is a CCS with a current of about 4 milliamps, feeding the rest of the circuit; together with the D1 zener forms a stable supply of 6V.
The LM311 low power op-amp drive the Q2 mosfet that is the "real" CCS.
The R2 resistor is used to set the current, that can be regulated from 50 to 200mA.
This circuit should really work fine, and its extremely easy to build.
One can use it without tubes, simply remove D2 and the "TUBE GRID" connector.
Its maximum ratings are: 1000V, 200mA, max. dissipation capacity 54W.
Minimum voltage across connectors: about 10V.

One can parallel more than one mosfets and, thus, multiply the output power.
This is what I will try to build in the next few days.

Ciao,
Giovanni

[ThorstenL]

Konnichiwa,

Originally posted by croccodillo
I cannot find here where I live IXYS chips,

You can buy the "a'la carte" from Mouser and other such places, with a credit card or in extreme case money transfer and they do ship to the EU.

Past that, you can buy a little PCB with DN2540 (depletion MosFet) from K&K Audio:

http://www.kandkaudio.com/accessories.html

And if you e-mail Brian Cherry from DIYHIFiSupply you may be able to get a set of parts and little PCB's from him including the IXYS Parts, i know he has a bunch he had made which need a slight mod to work reliably (I have some) otherwise very neat:

http://www.diyhifisupply.com/index.htm

Originally posted by croccodillo
So, again another one CCS.
This CCS is the simplest I can realise using a high-speed op-amp.

Does it HAVE TO HAVE an Op-Amp?

BTW, this looks exactly like the inside of an IXYS CCS ;-)



From: http://www.ixys.com/98703.pdf

BTW, are you sure that Q1 will work as CCS? I suspect the IRF part you have is an enhancement FET, not depletation (eg it wants it's grid...uuuhm...gate positive to conduct).

Originally posted by croccodillo
One can use it without tubes, simply remove D2 and the "TUBE GRID" connector.

A KT88 readily dissipates 35W static on the anode. 35W Static dissipation in a solid state device need a lo of heatsinking, which is why I have been toying with the "Tube Cascoded CCS" idea, it makes it easier to dissipate the heat.

Sayonara

[croccodillo]

How can I say?

You're right, absolutely right.
I was trying to replicate (without knowing it) an IXYS chip...
OK, I will evaluate the alternatives you listed.

Thanks,
Giovanni

[croccodillo]

Some questions:

1. If I use a KT88 as a triode CCS in the upper side of a para-feed output stage, do I need separate filament supplies for it and the low side and output drivers?
This is because I can have about 400-500V between the cathode of the upper triode and the cathode of the lower one.
I know the filament is insulated from the rest, but the maximum voltage between it and the cathode is reported as 250V.

2. Is it better to power the filament with AC or DC? I mean, does it worth to use DC in manner to avoid "hum"?

3. If I use separate filament supplies for "high side" in para-feed output stage (floating supply), and I use DC filament voltage, can I electrically connect the filament to the tube cathode?

Thanks,
Giovanni

[ThorstenL]

Konnichiwa,

Originally posted by croccodillo
1. If I use a KT88 as a triode CCS in the upper side of a para-feed output stage, do I need separate filament supplies for it and the low side and output drivers?

Yes, absolutely. I would use a seperate heater windings for each of the upper valves, referenced to the cathode.

Originally posted by croccodillo
does it worth to use DC in manner to avoid "hum"?

Not in this application.

Sayonara