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Old 7th November 2005, 09:46 AM   #1
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Default Active load output stage

I found this site:

Link

It shows an active-load amplifier.
It is simply an SRPP output stage.
I never saw an output stage like this one before, so my question is:
Is this one a good solution?
I have 4 KT88, I could realise an amplifer wih this topology, but why nobody has already built it?
My idea was to build an SRPP input stage using two 12AX7 (paralleled) and then drive with it an SRPP output stage made with KT88.
It seems to be a good solution: a perfectly balanced PP output (if you make the right calculation...), more (twice) power than SE, pure class A, more speed and dynamics (at least this is the theory).
Why it is not used at all?
What are your feelings?

Ciao,
Giovanni
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Old 7th November 2005, 01:36 PM   #2
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I saw several schematics for amps loke this about 5 years ago. It seems that this fad never took off. I built one like this, and tried several variations. The basic concepts are on my page:

http://www.tubelab.com/SEoutput.htm


I did all of these experiments about 5 or 6 years ago, and unfortunately did not document them well. I remember that the EL34 was the best tube that I tried and that 6AS7's were the worst due to bias drift. I did not try KT88's but some of them are known to drift over time. You will need to make at least one of the cathode resistors adjustable, and adjust it for symetrical clipping. If this changes too much after a few hours of operation, try different tubes.

I have since found that replacing the top tube with a CCS IC works the best for me. That winds up being a CCS loaded active parafeed amp which wastes a lot of power but sounds good. I then began experimenting with hybrid tube - transistor output stages and never returned to finish up the SRPP output.

http://www.tubelab.com/SuperTubeSE.htm

Since drawing those schematics, I discovered the IXYS 10M45 CCS IC. That makes Constant Current Sources easy. Just make sure that you use a gate stopper resistor (1K) otherwise they will ocsillate!

I am still experimenting with this concept and will put the results on the web site when done, but the recent hurricane has set all Tubelab experimentation back for months.

I was beginning to re - visit the Totem Pole output topology since Triode Electronics came out with a driver transformer with 4 independent secondaries. I want to build an H bridge output stage using tubes.

http://www.tubelab.com/TotemPole.htm
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Old 7th November 2005, 04:28 PM   #3
Previously: Kuei Yang Wang
 
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Default Re: Active load output stage

Konnichiwa,

Quote:
Originally posted by croccodillo
I found this site:

Link

It shows an active-load amplifier.
It is simply an SRPP output stage.
I never saw an output stage like this one before, so my question is:
Is this one a good solution?
Depends. I have been messing with active load solutions on papaer for a while.

The key drawback is that you need nearly double the +B to choke loaded parallelfeed or series feed.

Here a little trick for you.

Combine a KT-88 and the IXYS CCS Tubelab mentions (replace the cathode R of the upper SRPP device with the CCS). This way dissipation in the Solid State CCS is limited, most is dissipated in the KT88. You can take the output from the lower Valve Anode (basically traditional SE parallelfeed) or from the upper KT-88 Cathode for basically a Power Mu-Follower.

Here some more active load SE Amp's for you:

http://www.studioerosbarone.it/diego/MAD.htm

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

Sayonara
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Old 7th November 2005, 09:55 PM   #4
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As I type this, I'm listening to an amp inspired by Peter Millett's design you linked to. I ended up using a pair of 12e1s both triode connected.

more details here

Have been listening to various different versions of this circuit for over a year and not bored with it yet which is fairly unusual for me.

Worth experimenting with I reckon
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Old 8th November 2005, 02:10 PM   #5
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Quote:
Combine a KT-88 and the IXYS CCS Tubelab mentions (replace the cathode R of the upper SRPP device with the CCS). This way dissipation in the Solid State CCS is limited, most is dissipated in the KT88. You can take the output from the lower Valve Anode (basically traditional SE parallelfeed) or from the upper KT-88 Cathode for basically a Power Mu-Follower.
You mean something like THIS ?

I cannot find the IXYS CCS here in Italy, but I can realize a good discrete CCS (not the one in the schematic, perhaps the Gary Pimm's one).

The idea is really good, the KT88 will handle all the job, controlled in a precise way by the CCS. I like it!

With the value of R2 in the schematic the current through the KT88s is 100mA, more than enough to have 8-10W output power.

My question is: how can I bias correctly the lower tube? Do I have to bias it with a negative voltage and, then, use a trimmer to regulate its anode voltage to 350V (half the power supply voltage)?

Again: A KT88 connected in this way could be driven by a SRPP made with two 12AX (I have lot of them...) or is it better to use two EL84 as driver (I have some EL84 and some ECL84 tube).
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Old 8th November 2005, 02:11 PM   #6
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Could worth the try to add a CCS also to the driver section?
I mean, between EL84 anode and the +400V?

Ciao,
Giovanni
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Old 8th November 2005, 02:36 PM   #7
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The lower KT-88 can be biased as shown with negative grid voltage, or it could be done with a cathode resistor. Either one should work. You can start by adjusting the plate voltage to half supply, but if you have a scope, adjust for symetrical clipping when slightly overdriven.

First set up the CCS to the desired operating current. I usually put a current meter in place of the bottom tube (plate to cathode) and set the current. Then install the bottom tube. The cathode resistor (or grid voltage) sets the plate voltage on the bottom triode when fed by a CCS.

If you use a CCS on the input tube you will need to raise the supply voltage to allow for the needed voltage across the CCS.

Which driver tube to use? That is your call, I am sure both would work.

I also think that this concept is a good idea, and I would like to experiment further, but I am not able to experiment right now, so let us all know how it works!

Are you using Eagle for your schematics? If so, where did you get your Symbols for KT-88 and EL-84, do they have PCB sockets?

Also the value for the grid resistor for the KT-88 is too high. They will go into a runaway condition if this resistor is too high. Check the data sheet for the brand tube that you are using. Some of the Chinese ones don't like much over 75K.
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Old 8th November 2005, 02:58 PM   #8
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Thanks, Tubelab (may I call you George?) it sound strange to me to call people with nicknames...
Well, I visited your site, I've seen the damages done by the hurricane... Really impressive!!!
All my best wishes for the next future, I hope you (and all of your compatriots ) can solve all the related problems soon.

Yes, I'm using Eagle, the symbols are from an add-in library you can find under the "Download->libraries" section of the CadSoft site,
HERE

The library is, of course, "tubes.zip",
THIS

In the library you can find also PCB sockets, yes.

HERE a screenshot of the library from Eagle.

I'm still collecting all the material I need to build the amplifier, I miss the two output transformers (my previous intention was to build a KT88 PSE amp with suitable O/T, but now I need to change tipology of O/T) and power transformer.
In short term, I miss all the iron.

Ciao,
Giovanni
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Old 8th November 2005, 05:55 PM   #9
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by croccodillo
You mean something like THIS ?
Yes, something like that.

I would make the following changes:

1) R7 = 68....100KOhm (tubelab remarked on that too).

2) R6 = 1K

3) Remove C3

4) Bypass R1 with a suitable capacitor

5) Use the EL84 as Pentode, Ra = 12KOhm, Rg2 adjusted for 150V screengrid and 250V Anode with a R1 = 470R, big film capacitor on screengrid.

6) For kicks use a 300B as lower valve, not a KT88.

7) Try taking the output both from the Top KT88 Cathode or the Bottom KT88 (or 300B) Anode.

Quote:
Originally posted by croccodillo
I cannot find the IXYS CCS here in Italy, but I can realize a good discrete CCS (not the one in the schematic, perhaps the Gary Pimm's one).
The one in the picture will not be all THAT bad either.

Quote:
Originally posted by croccodillo
My question is: how can I bias correctly the lower tube? Do I have to bias it with a negative voltage and, then, use a trimmer to regulate its anode voltage to 350V (half the power supply voltage)?
As Tubelab says, first set the current, then set the lower valve anode voltage. Done.

Quote:
Originally posted by croccodillo
Again: A KT88 connected in this way could be driven by a SRPP made with two 12AX (I have lot of them...) or is it better to use two EL84 as driver (I have some EL84 and some ECL84 tube).
KT88 (or even 300B) needs a good deal of swing, better use the EL84 as pentode or use a 6SL7/ECC83 SRPP or something.

Sayonara
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Old 8th November 2005, 06:41 PM   #10
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Giovanni:
Yes, you can call me George. Thanks for the info on Eagle. I downloaded that library when it first appeared (at least 2 years ago) and never looked back. It has been updated with many more tubes since I downloaded it. I got the new version.

As to the hurricane damage, we finally got our electricity restored this weekend after two weeks in the dark. There are still many people with no power. We were not able to go to work for 1 week, but my project deadlines did not change, so now I go to work for at least 10 hours per day, so no time for tubes. I will get back to the experiments in a month or two.

Any progress on the SMPS?

I would agree with the parts value changes proposed by Kuei Yang Wang (numbers 1 - 4), but use a KT-88 if you have them. After you have everything working, then you can experiment with the 300B. If you need good 300B's for a reasonable price, the new Chinese ones made by Shuguang (marketed by many companies) work pretty good. Driver in pentode mode? Depends on how much signal you have.
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