Active load output stage

[ThorstenL]

Konnichiwa,

Originally posted by tubelab.com
Next, build a 600 to 700 volt supply, depending on whatever transformer I can dig up. Then I want to experiment with screen drive. Then depending on the outcome, I will prototype the driver, then build the amp. The results will be posted here, and on the web site.

With screendrive you can be really neat. use the "Mu-Follower" output from an IXYS CCS loading (say) a 6SN7 to drive directly the screengrid of the lower KT88, then the upper valve also set up with mu-follower output (from the cathode) and with enough +B and anode current you should get an SE Amplifier with just one capacitor in the signal path (parallel feed output), very wide bandwidth and distortion and output impedance class leading for SE.... Except of course all stages would be push-pull, but without cancelling even harmonics... ;-)

Sayonara

[tubelab.com]

Now I think that you are reading my mind, except that I am using a 6EM7. I may try a different tube later though.

Now if you build 2 of these amps, drive them out of phase, and hang the OPT between the two output stages, there will be no caps in the signal path, but it will be a push pull amp. I believe that DC offset correction will be needed.

[croccodillo]

GREAT WORK GEORGE!!!!

Well, during the weekend I made some tests too:

First of all, I'm moving in a different direction, I will use a pure SS CCS, without any tube in it:

This for few reasons:
1. I have a lot of big, nice to see, heatsink (so heat dissipation in the CCS is not an issue for me).
I'm planning to use one of them as a part of the chassis, making a really good-looking (and strange) arrangement. You will see.
2. I have only 4 KT88 tubes (I'm still a newbie), so I want to use them to build a parallel SE output stage, so I have no tubes left to use for the CCS.
3. I think a SS CCS is faster and more precise than a tube one: but this is only my opinion.
4. I can dissipate more power and have higher voltage across it than using a KT88 tube.
5. The minimum voltage across my CCS is about 3.5V, so I can use a lower power supply and have a bigger swing (for example, using a 550V PS I can swing the KT88 voltage from 150V to 540V...)

Back to my tests:
Of course you're right, BU508A worked fine but for a really short time, due to the secondary breakdown effect (I knew it before, but completely forgot about it cause the enthusiasm).

I substituted it with a high voltage mosfet (IRFP460N), it worked really fine without any modification.
Such a Mosfet can work up to 500V; within the current I choose (150mA) I'm into its safe area.
I will change it with a higher voltage mosfet, like a IRFPG30 or SK2610 device.
Perhaps I can also put my hands to some IXFH6N100, IXYS mosfet: 6A@1000V.
I think I wll go with the IRFPG30: they have a much lower input capacuitance (easier to be driven).
I do not have them now, I have to order them.

I built a 450V power supply, connected the CCS to it and, then, used a power resistor with a value of 1200Ohm as a load between its output and the ground.
The circuit worked perfectly for the whole afternoon, until I switched everything off.
Due to thermal drift of the sense resistors (R2, R6) the current slowly increased from 150mA (when cold) to 152 mA after half an hour (standard carbon resistor have a negative temperature coefficient): this is not an issue, I only have to substitute the resistors with temperature stable ones.
This is the last version of the circuit.
The PCB is exactly the same as before (the mosfet has the "same" pinout than the bipolar transistor).
The current was set to 150 mA, so the load was dissipating about 27W, while the transistor about 40W.
The Mosfet can dissipate up to 125W, I mounted it insulated on a BIG heatsink and had no problem at all (at the end the mosfet was really cool).
Tomorrow I will post some picture of the circuit (you know, when you need it, your digital camera batteries are always discharged...).
Now I'm building the SE circuit I posted before, with two KT88 paralleled and para-feed outout.

But,

Next, build a 600 to 700 volt supply, depending on whatever transformer I can dig up. Then I want to experiment with screen drive. Then depending on the outcome, I will prototype the driver, then build the amp. The results will be posted here, and on the web site.

What is "screen drive"? can you drive a tube by its screen? How? Benefits?

Ciao,
Giovanni

[tubelab.com]

Glad to see that your CCS works. I may try to copy it later and see how well it works. I am also going to try substituting the tube with a mosfet in the circuit that I am currently using. I see no reason why it shouldn't work. I like to use the Toshiba 2SK2700. It has an insulated case, and very low gate capacitance, but only 40 watts dissipation rating.

Screen drive is something that was derived by the audio community. Most tubes were never designed to be driven from the screen grid. Some tubes are linear in this respect, and some are not. The best candidates seem to be sweep tubes, since they have good screen grid sensitivity.

The advantage is the fact that the driving voltage is centered around a relatively high voltage potential, allowing direct coupling. This can eliminate the coupling capacitor. I will try using a mosfet follower with the gate tied to the plate of the driver, and the source tied to the screen. G1 is tied to the cathode, or a fixed bias source.

The disadvantage is that screen drive curves look like pentode curves, with the associated pentode characteristics, including a higher output impedance than a triode connection.

it might be possible to implement a "dual drive" circuit to get the best of both worlds.

[croccodillo]

Sounds good...

OK, I will try to do something more classical, with a simple g1 driver...
I'm not so skilled to implement screen driver, I'm afraid.
About 2SK700: mosfets have a safe operating area too, like the bipolar transistors, and I'm afraid you cannot use such a mosfet with high currents (let's say not more than 100 mA).
If you want to use higher currents (like me) you should use something like IRFPG40, that has a bigger safe area, at the expense of a higher input capacitance.
But I think my circuit should have no problem driving it: I drive the mosfet with a maximum current of about 20 mA, more enough to drive it well above the audio band.
I can rise the current, however, up to 60-100 mA (changing the value of R1 and R5 rewsistors).

[tubelab.com]

I have been using these mosfets as drivers, in follower configuration, to drive the grid of large power tubes (845, 833A) directly. Here the average current is low, and the capacitance is an issue. I will look through my parts collection to see what bigger ones that I have. I tested a lot of different fets when developing the PowerDrive circuit before I figured out that bigger is not always better.

Has anyone tried IGBT's? I got some but haven't tried them yet.

[ThorstenL]

Konnichiwa,

Originally posted by tubelab.com
Has anyone tried IGBT's? I got some but haven't tried them yet.

Yeah, a good while ago, when they where still hot, cute and virginal. Not only did they have the bipolar 2ndary breakdown, they also had the "mosfet fog" input capacitance. But I'm sure in the last 20 years "they" have come up with something better, I just wish "they" would tell me....

Sayonara

[ThorstenL]

Konnichiwa

Originally posted by croccodillo
I'm not so skilled to implement screen driver, I'm afraid.

Then please stay away from solid state stufff...

Sayonara

[croccodillo]

What do you mean, Kuei Yang Wang?
In my post I mean I do not know well tubes...

Well, bad news from my side:
I tried the circuit I drawn, this and, of course, the whole thing oscillates.
I connected a digital ammeter in series with the CCS, and a digital voltmeter between tubes anode and ground: both the meters got crazy, both show me out-of-range measurements (ammeter set to 2A, voltmeter to 600V).
I connected an old speaker to the output, I can ear a loud main hum.

So, time to make some serious tests.

First of all, can I substitute the CCS with a fixed resistor (let's say 2000 Ohm), connected between the B+ and anode, of course eliminating the load? In this way the amp will see a fixed load and, thus, I can see if the amp oscillate itself.

Ciao,
Giovanni

[ThorstenL]

Konnichiwa,

Originally posted by croccodillo
What do you mean, Kuei Yang Wang?
In my post I mean I do not know well tubes...

An allusion to the fact that screendrive mode makes a pentode into an "enhancement" device, in other words, a lot like a transistor....

Originally posted by croccodillo
Well, bad news from my side:
I tried the circuit I drawn, this and, of course, the whole thing oscillates.

Try gridstoppers (1K) on the Valve Grids.

Also, you may wish to bypass the cathode resistors with capacitors of suitable value.

And yes, you can use a big old power resistor as anode load for testing...

Sayonara