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Old 28th October 2009, 02:26 AM   #1
TheGimp is offline TheGimp  United States
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Default V-Reg, Gyrator, Low (relative) voltage tubes

I'm working on my 10W pp amp design which I have a baseline schematic and have run through simulation (LT spice), and have decided the next step is the power supply.

The issue at hand is availability of a suitable transformer.

The requirements are 200mA at 275V for the finals (50ma per tube, Push Pull, two channels 250V Plate to cathode Plus Cathode bias plus output transformer drop).

I've got several transformers on hand, but all have too high output.

Based on a 123.6V line voltage I get the following:

Hammond 270FX 448V no load, 385V at 200ma (rated at 170mA so it is probably overstressed in this application).

PT106 432V no load, 370 at 200mA

Stancor P6013 700VCT split into two separate 350V windings, 120mA each. 578V no load, 498 at 200mA load.

All transformers were tested by configuring them with full wave center tap rectification (1N4007) followed by a single 150uF 450V cap. The Stancor was an extrapolation from 93VAC in and 435V on the filter cap since I didn't want to configure new filter caps just for it.

For a load I used an active dummy load (ST W9NB80 on a massive heat sink). This had a weird oscillation that I had to fix to prevent it from blowing 0.75A fuses on the B+ line between the cap and FET. 10K pot with a 220K resistor from source to drain with the wiper to the gate and a 100 Ohm current set resistor.

I've searched for a better (off the shelf) fit in a transformer and the closest I've found is the Hammond 263CX ($63 from Digi-key which I'm hesitant to pay) it is rated at 360CT @ 287mA (287mA? where do they come up with these numbers?). 360CT will probably yield 360/2 *1.414=254.5V, and if I get a boost of 123/115 for line variation I will have 172.25V. This leaves too little for additional LC filtering and I'm below my desired 275V B+.

So what to do with what I have on hand.

I could use the Simple Shunt Regulator with any of the transformers I have on hand, but it would be wasting a lot of energy.

I got to thinking about CLC filters and Gyrators and am wondering if a low input capacitance around 5uF followed by a gyrator and then a large (150-330uF) final filter cap would allow me to drop the voltage without generating as much heat?

The Stancor is probably out of the question due to it's high output and best saved for another project, but the other two would require a drop of 100-110V. For better line regulation I was thinking of using a gyrator to drop the voltage to the 300V range and follow it with a simple pass regulator (FET with Zener ref) to drop the final 30V.

I've found a couple of references to using Gyrators in other circuits, but not much info on using them to drop voltage for B+ to avoid power dissipation (100V at 200ma is going to waste 20W).

At issue is whether I can use the Gyrator to translate voltage to current (what an inductor can be used for). If I use a gyrator to drop 100V, does it reduce the current draw from the transformer, such that I might be able to use the Hammond Transformer to get the 275V at 200mA.

I fried the current pass resistor in my dummy load and need to purchase a >4W 100 Ohm resistor tomorrow before I can make measurements to verify whether this will work or not.
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Old 28th October 2009, 02:54 AM   #2
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Quote:
I'm working on my 10W pp amp design ..... The issue at hand is availability of a suitable transformer.... The requirements are 200mA at 275V for the finals
I have designed a 6CW5 version of the Simple P-P with similar requirements. I need 250 or so volts but I have measured the total amp current at nearly 400 mA under normal operation and about 500 mA at hard clipping. Yes traditional transformers are a bit hard to find, but there are some toroidal solutions.

I am currently using the Antek 1T200 which costs $29. It works good and gives me 250 volts of B+ with 1N5408's (the surge current will blow 1N4007's) and a 180 uF - 1 HY 40 ohm choke - 470 uF CLC filter. It does not get hot after playing loud for hours.

http://www.antekinc.com/pdf/AN-1T200.pdf

The 2T230 is $35. It handles twice the power but will provide a bit too much B+ for my amp. I don't have one yet but I may get one and experiment with smaller input caps, and lossier chokes.

http://www.antekinc.com/pdf/AN-2T230.pdf
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Old 28th October 2009, 02:25 PM   #3
TheGimp is offline TheGimp  United States
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Thanks George.

I'll probably go with the AN-1T200 and put the iron I have aside for other projects that they are better suited for.

I was hoping a Gyrator would behave like a true inductor and store energy, not just provide a phase shift to simulate an inductor. However looking at the circuit I don't see how it could store the equivalent energy of say a 5H inductor dropping 75V in a 10uF capacitor with less than 30V applied to it. It would still have the same power dissipation as a resistor, and cost more.
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Old 2nd November 2009, 11:46 PM   #4
TheGimp is offline TheGimp  United States
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AnTek AN-1T200s arrived today (since it was only $10 for shipping flat rate I bought two).

I wired one up to a variac, thermistor, fuse then the two primaries in parallel to 123VAC line.

With both 200V outputs in parallel with fwb rectification 47uF cap, 7H inductor, 330uF cap I was getting 268V @ 200mA with only 1.35Vrms ripple.

I added a 8 ohm load on the two filament windings in series and got 12.9V on the load for 1.6A load on them.

The output only dropped to 265V at 200mA with 1.37Vrms ripple.

This ought to work out well.

Thanks.

ps. As an afterthought I loaded one 6.3V winding at 1.3A and wired the second in series with the HV winding. I got 275V at 200mA which would allow 10.5V for cathode bias and 15V to drop in the output transformer leaving me with 249.5V from plate to cathode.

Last edited by TheGimp; 3rd November 2009 at 12:01 AM. Reason: added ps.
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