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Power Transformer Specs. for a Tube Type Rec.
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by WD8PHW on January 16, 2005
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Hello Everyone,
I'm going to have a power transformer made for a receiver, and I need some help.
The tube line - up will be:
1 X 45, 4 X 26, 1 X 27. I plan on using a solid state full wave bridge rectifier...not center tapped.
Here are my questions:
First, does the filament winding current specification need to be exact? In other words, the 26's filaments will draw 4.20A, can/should the filament winding have a 5A rating? Will the filament winding voltage rise due to the rated current not being drawn?
Next, the 45 demands the greatest plate potential...260V. A bleeder/divider string will be used to supply B+ to the others. What would you spec. the HV winding voltage rating for. I understand - there are a zillion power resistors available with different values to control this, but what is reasonable?
Thanks for your help,
WD8PHW
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WD8PHW on January 16, 2005
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Hello,
I forgot to mention, the power supply will have two chokes. One filter choke (105 ohms) and one speaker field coil (1100 ohms). The operating plate current will be 80mA. I've simulated this on Spice, and a 390V secondary looks good to me, but I need all of the help I can get.
Thanks again,
Greg
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WB6BYU on January 17, 2005
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Gee, I don't suppose very many such power supplies were
designed using SPICE before. Do they have a model for a
5U4 or 866 rectifier?
There is no problem specifying a current rating beyond
what you actually are going to draw. Well, within reason...
If the heater winding is rated at 5 amps, it means you can
draw that much current before the voltage drops below
the specified voltage and/or the power dissipated due
to the wire resistance in the transformer will overheat
the core. Or perhaps, combined with the current drawn
from the HV winding, the core has reached saturation.
As long as you operate below the rated current - even
at 1 amp - you should be fine. But, just in case, you
could specify a maximum no-load voltage and a minimum
voltage under the rated load when ordering the transformer.
Most tubes will run on a range of plate voltages. If
the lower voltage tubes will work at around half of the
higher plate voltage, then you can feed them from a
center tap on the transformer (without having to add
an additional rectifier), which will save the dropping
resistors and the associated heat dissipation.
Regarding the HV winding: I think you are using a choke
input filter, and it sounds like the calculations are
based on having at least a critical inductance in the
choke. I'm not in a position to check that part at the
moment, but without critical inductance and a minimum
bleeder current draw the output voltage will rise. So
the inductance of the chokes, the sequence, and what
values of capacitor are used with each one, and the
bleeder current (especially if there will be times that
the supply will be on without plate current in the
tubes - like while the heaters are warming up)
will all need to be specified. Otherwise the
filter will start to act more like a capacitive input
(if the minimum choke inductance isn't met) and the
output voltage will go up: it could be as high as 500
volts with a 390 volt transformer.
Just a few things to consider.. but perhaps you already
have done so.
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WD8PHW on January 17, 2005
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Hello Dale,
Thanks for your comments.
Well, the SPICE simulation was based on a "full wave CT" design using 1N4007 diodes as the rectifier. I intend on eliminating the rectifier tube. The PIV for 1N4007's will give me trouble, but I've found "beefier" diodes. I don't have a SPICE model for those.
The current ratings are understood now. I may just beef it up just a bit....just because?!
I understand the "minimum voltage under the rated load" (minimum operating plate voltage of the 45), but how can I determine the "maximum no-load voltage"? I guess once these values are determined - there will be a range to work with.
The supply will use a capacitve input filter with back to back "L's" (shunt cap 8uF., choke 5H, shunt cap 8uF., field coil unknown inductance). Additional filtering may be added at the output.
Thanks again and 73,
Greg
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WB6BYU on January 18, 2005
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With a capacitor input filter, the voltage across the
input cap can rise to 1.4 times the RMS voltage applied
to the rectifier. With a choke input filter it is more
like 0.8, but only if the choke has sufficient inductance
for the minimum current that will be drawn, otherwise it
will be higher.)
So the no-load voltage with a 390V RMS winding would be
close to 550 volts. This would drop somewhat under load,
and the actual voltage delivered to the tubes has to
account for the voltage drop through the resistance of
the chokes. But one of the problems with replacing tube
rectifiers with solid state is that the resulting voltages
are higher, both due to reduced voltage drop across
the diode and the fact that the high voltage is produced
before the heaters warm up enough to get the rest of
the tubes conducting, so the power supply is very lightly
loaded at power-on (only the bleeder resistor - and the
initial charging current for the capacitors.) SPICE
should be able to model transient response - do the
models include the warm-up time for the tubes?
But if you have a good SPICE model, you should be able to
verify this - it should tell you the voltage across the
first capacitor.
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WD8PHW on January 20, 2005
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Hi Dale,
Well, I simulated this on SPICE and PSU Designer II. I'm getting the same results from both simulations. I revised the secondary to 335 V, and the output is roughly 325 V. The no-load is at about 450V. I'm being a real WIMP on making a final decision for the secondary voltage rating of the new transformer. I want to run the 45 at maximum plate voltage, yet I don't want to heat the house with power resistors.
Anyway, I have not considered adding a "warm - up" time to the simulation. I'll look at the transient analysis and see. Five out of six of these tubes are directly heated, so I don't think warm up will be that long.
Thanks for your comments/help and 73,
Greg
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WB5YYX on January 21, 2005
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According to my 1961 RCA receiving tube manual, the specifications for the tubes you are using are:
45- plate voltage max 275
2.5 volts dc @ 1.5A filament
26- plate voltage 180VDC max
filament 1.5vdc @ 1.05A
27 plate voltage 250vdc max
filament 2.5vdc @ 1.75A
Electron tubes are not the scentific or mil spec quality of today's solid state devices. They are certainly very forgiving. For the 45 I would recommend a plate voltage of 220-240VDC to allow for initial voltage surge when it's applied. For the 26 I would recommend 140-150VDC for the same reason. The 27 can be operated at 220VDC also for the same reason. Even the discrete components are not of the precision of today's. For example I worked for Electronic Parts Company in Albuquerque for sixteen years. Resistors originally had a tolerance of plus or minus twenty percent. The Sprague electrolytic capacitors including the TVA "Atom" axial lead series and the TVL "Twistlok" into a chassis series had tolerances of plus 80 percent and minus twenty percent. Getting the capacitance reasonably close works just fine. 5 or 6 amps for the power transformer is acceptable. I also collect and repair antique radios so if you need anything else just ask.
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WB6BYU on January 25, 2005
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Warm-up time may not be very long, but many common
capacitors are rated at 450 volts. That may seem like
plenty of safety factor on the 325 volt line, but in
this case they would be stressed to the limit during
warm-up. That is my concern about the no load voltage
more than the tubes themselves.
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WD8PHW on January 25, 2005
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Hi Dale,
Yes, I see your point. 450 volt caps. will be pushed to the limit in this case. A certain over site on my part! A couple of 16uF/450V caps. in series should do the trick though. I should have plenty of room -since the power supply is on a separate a chassis.
Thanks again for your help and 73,
Greg
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RE: Power Transformer Specs. for a Tube Type Rec.
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by WD8PHW on January 26, 2005
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Hi Bob,
Thanks for your input. I wanted to "push" the 45 in order to achieve maximum power output. It will be used as a single ended audio amplifier, and it's only capable of 2 watts. Anyway, I'll back off on the plate voltages for the reasons you've explained and tolerate the lower audio out.
Anyway, I finally ordered the power transformer, and here are the specs.
325-0-325 @ 100 mA
2.5 V @ 1.5 A for the 45
1.5 V @ 4.2 A for the 4 - 26's
2.5 V @ 1.75 A for the 27.
The high voltage rating may be a little high, but I'll deal with that. Too much is better than not enough!
Thanks again and 73,
Greg
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