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Old 9th November 2005, 07:38 AM   #11
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Thanks for all your help.
I'm slightly begin to understand vacuum tubes...

George,
The preliminary schematic for the SMPS is ready, the magnetic core is ready with all the windings done, but I have no time at all to build the circuit.
Just to show it, THIS is the schematic.
It is really simple (keep it simple as you can).
THe core is an EPCOS ETD44/N67 core, and with the given oscillator frequency (35Khz) and the given topology (half bridge) it can output a theoretical continuous power of 700W!!!
Of course we do not need so much power: the mosfets are rated for a 300W continuous power (not so much powerful Mosfets -as IRF830A are- means they are easier to drive).
The output voltage, with the ratio reported, is of about 500V.
Higher voltage can easily be obtained with more turns on the secondary windong.
The core must be provided with an air gap (0.5 - 1 mm.), in manner to avoid histeresis shift of the core if drivers are not exactly balanced.
The C3 capacitor is used to block DC component in the core, in manner to make it more stable (Please don't ask me to explain it: it is too complicated in your language for me...).
The Jumper SJ1 can switch between 115V (when closed) and 230V main supply.
The SG3525 IC is used as the main oscillator/PWM controller; its PWM outputs are reported to IC1, an high-voltage Mosfet driver used to minimize the turn-on/off time of Mosfets.
The feedback is taken from the opto-coupler (they are quite linear in a certain range of current), driven by a Mosfet.
This past of the circuit is the one I'm not sure of: I never tried a feedback made in this way (my past SMPSs have current feedback), so it will need some serious tests to make it work fine.
I hope to build it in the next year (yes, year...).
Due to the lack of time, my idea is to build my tube amplifier with a traditional power supply (I could buy some cheap surplus transformers), and to exchange it when the SMPS will be ready.

Ciao,
Giovanni
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Old 9th November 2005, 10:37 AM   #12
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Looking at the schematic, which should be, in your opinion, the right supply voltage?
Was thinking something about 700V@100mA: with this value you should be able to drive the two KT88 about in the middle of their curve in triode connection (I hope you can understand what I mean, my language is not so correct).
Looking at the curve (this) we should be in the middle of the linear zone.
Using such a voltage and such a current (100mA), how can I calculate the output power?

Ciao,
Giovanni
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Old 9th November 2005, 11:36 AM   #13
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by croccodillo
Looking at the schematic, which should be, in your opinion, the right supply voltage?
First work out the lower valves operating conditions the normal way (load line).

Find the highest positive anodeswing point you want to have without clipping.

Then consider the SS CCS and it's minimum operation voltage (say 5V for the IXYS CCS).

Look at the valves curves and find a point where the current is the value have chosen and the gridbias equals your minimum voltage.

Read off the anode voltage, add the bias voltage and add to that derived earlier as the maximum positive swing for the lower valve. This gives your +B.

If using self bias for the lower valve add the selfbias voltage and if using a direct coupled driver stage add that stages anode voltage.

Here an example for what I call "optimised loop" amplifier (note, theoretical/experiomental ONLY)...

Click the image to open in full size.

In the simulator (with Valve models based on real triode curves from Sofia) it suggests 8W RMS @ 3% 2nd HD with a textbook behaviour for the other harmonics.

Frequency response prior to the output transformer is pretty much DC to light. The Sowter 8995 would probably the ticket for this circuit, they are 76 each, so 112 Euro or 132 $ US each, plus taxes which in the EU is 17.5% and shipping (I not normally recommend Sowter BTW).

The FET/Battery CCS can be substituted by a IXYS CCS (obviously) beware of thermal drift in the J-Fet CCS!

Sayonara
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Old 9th November 2005, 12:21 PM   #14
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Let's see if I've understood...

Click the image to open in full size.

I traced some load curves on the above graph.
I have a pair of very good toroidal transformer with a primary impedance of 2600Ohm (they are not audio equipment but industrial components, at least good as audio iron), I will use them for the first tests.

Following your suggestion:

Fixed the operating point of the lower tube at 350V, 100mA (blue point).
The load curve is the magenta one; it means that I can have some more that 400V swing and a current variation from a minimum of 20 to a maximum of 170 mA.
The load current, then, will vary from 0 (no signal) to a maximum of 150 mA (maximum swing).
That mean that, with a load of 2600 Ohm I can have a power of (Ohm's law?):
P=R * I^2 = 2300 * 0,150^2 = 50W

This should be the peak to peak power, the rms power should be:
50/2.86 = 17,5 W.

is it correct?

Let's say that the maximum voltage at maximum swing is 560V.
Let's use 5 Volt as minimum CCS voltage.
From the curve the point where we have 100 mA at such a minimum grid voltage is 150V.

So,

560V (max voltage@max swing) +
5V (min CCS voltage) +
150V (operating point@min grid voltage) =
-------------------------------------------------------
715V B+

Not so far from what I've proposed in the previous post... I'm lucky.

I do prefer to have a separate bias negative voltage for the lower tube, so the above voltage is my B+, and another bias voltage will be needed.

Your comments, please?

I'm beginning to have a lot of fun with tubes!!!!

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Old 9th November 2005, 02:47 PM   #15
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I think that your power output expectations are a bit optimistic. This is the way that I would approach this circuit. First I would consider the static (DC) conditions at the signal extremes. This is to make sure that you won't run out of voltage and clip unexpectedly.

The bottom tube has its cathode grounded so the grid voltage is not limited to the minimum CCS voltage, it could even be driven positive if desired. From my experience triode wired pentodes don't like to have the grid driven positive. DHT's (300B, 845, etc) can be operated in this manner (class A2) if you have a good driver. So for the peak positive going extreme we will take the grid of the bottom KT-88 to zero volts. The distortion would begin to rise if driven above this value. At a fixed current of 100 mA (from your curves) it looks like the tube will want 100 volts across it. This is the minimum voltage across the tube when driven by a positive going peak.

The quiescent point was set arbitrarily at 350 volts, 100 mA. This gives a bias voltage of about -30 volts. Since the amp is designed with symetrical sine waves we must assume that the maximum negative excursion at the grid would be -60 volts. This implies a 60 volt peak to peak drive signal centered around - 30 volts. From the load line this would indicate a maximum plate excursion of 500 volts. This is the maximum voltage across the tube when driven by a negative going peak. This gives a peak to peak value of 400 volts. This gives a power output of about 8 watts RMS. At this point the top tube will need 150 volts plus 5 volts for the CCS giving a total B+ requirement of 655 volts.

Since you have already considered a 700 volt supply voltage, You might try raising the operating point of the bottom tube to about 400 volts. You will have to lower the operating current to stay within the dissipation limits for the tube. Redraw the load line and repeat the above steps. Avoid minimum tube currents much below 10 mA, they become non-linear. After a few itterations you might be able to find a point that gives about 10 watts.

I find that it becomes easier to just build it and adjust for best sound since there is a sweet spot for every tube, and you won't find it on a set of curves!

I agree with the simple linear power supply first. Get all to work and then build your SMPS. I looked at your circuit and understand most of it. I may not know exactly why C3 is required but I know that the switch fets have a short unhappy life without it.
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Old 9th November 2005, 06:53 PM   #16
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I built just such an amp.
I started out with the "RH807" partial feedback amp design, using an 807 as the output tube. I had 700V to use up so I put in a TT21 (KT88 with a top cap) at the top as a CCS. I used a resistor to set the CCS and then placed another in the cathode with a 1K value (I think) to stiffen the CCS action. The TT21 is running very lean at about 250V across it, but it seems happy enough. Took the output from the anode of the 807. Sounds very nice and works great. Can't claim to have optimised it much. Tried a Gary Pimm Pentode CCS, but the voltage across the TT21 collapsed.
My understanding is that taking the ouput as a Mu- stage may lead to problems, because a SRPP likes to see a constant fixed load, and most speakers aren't.

Certainly worth a try if you have plenty of volts to use up.

Shoog
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Old 9th November 2005, 10:10 PM   #17
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Will there be any difference in PSRR if one uses a triode or a strapped pentode as the CCS (the upper tube in the beta follower)?
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Old 10th November 2005, 12:16 AM   #18
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A CCS inherently has a high PSRR by its very nature. If a CCS (of any topology) exhibited a perfectly constant current, it would be totally immune to power supply variations. Unfortunately nothing is perfect. When I was developing the PowerDrive circuit, I tested several different CCS topologies and settled on the IXYS 10M45 chip as the best compromize between performance and simplicity.

Of the tube and hybrid topologies that I tried, those that used a true pentode worked the best. The curves of a pentode are closer than a triode to a CCS. I did not try the topology that has been proposed in this thread. I will do so as soon as I can get my lab operational, it became a storage shed during the hurricane. I am thinking 845 on the bottom, 813 on the top, 2KV of B+. If it didn't blow up it would sound awesome.

If I had to guess, I would think that you would see just as much variation between two different triodes, as between a triode and a triode strapped pentode.
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Old 12th November 2005, 03:33 PM   #19
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I have a different idea for the CCS:

I have some LM833 ics that I can use for this project: these are differential op-amp specifically designed for audio (THIS is its datasheet).
They have extremely low noise and distorsion, an extremely wide bandwidth and an extreme wide dynamic range.
If I can design a little driver circuit between them and a mosfet (I'm thinking at a class-A little driver) I should be able to have a perfect CCS, with almost no distorsion at all.
Of course my idea is to use a floating power supply for each CCS: the LM833 works at its best at a dual voltage of 15V, so I'll have to provide a little transformer and a stabilizer circuit for each CCS.
But we'll have only two CCS per each stereo amplifier, so the expense will be not high at all.

Ciao,
Giovanni
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Old 14th November 2005, 01:14 PM   #20
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Default A brand new CCS

Here we are.

This is the circuit I will try to realise.
TR1 is a tiny transformer, about 3-4VA.
Unfortunately we will need a separate supply for each CCS.
But, due the small size and the small cost of the trafo there are no problems at all.
With the given values of R2, R3, R4 and R5 the CCS current can be set from a minimum of 40mA to a maximum of 150 mA using R5 trimmer.
The CCS can work as it is (without tubes), but with the limitation of the maximum voltage across Q1 (Vceo=100V).
Using transistors with higher Vceo could raise the maximum operating voltage across LPS3 and LPS4 up to 600-800V (Remember to remove the D2 zener).
The R1-C5 network is used to soft-start the CCS: the C5 capacitor will be charged up to 5 Volts in about 1 minute and an half, increasing in the same way the output current of the CCS, that will start from 0 to the desidered value.
Remove the R1 resistor to have immediate nominal output current.
The whole thing can be mounted onto a really small PCB.
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