7.2 volt to 6.3 volt ?

[rijo1]

Hi group , I have a project that has a transformer with an output
voltage of 7.3 volts AC that drives 5 tubes . The correct voltage to
drive the tubes is 6.3 volts AC. This is used in a RF circuit and I need
to step down or reduce the voltage from 7.2 volts down to 6.3 volts
directly from the transformer lead .
What would be a simple way to reduce the voltage to 6.3 volts without
replacing the transformer ?
Please give details as clear as possible . Thanks for any good help
in advance .
Rick

 

[James Sweet]

Hi group , I have a project that has a transformer with an output
voltage of 7.3 volts AC that drives 5 tubes . The correct voltage to
drive the tubes is 6.3 volts AC. This is used in a RF circuit and I need
to step down or reduce the voltage from 7.2 volts down to 6.3 volts
directly from the transformer lead .
What would be a simple way to reduce the voltage to 6.3 volts without
replacing the transformer ?
Please give details as clear as possible . Thanks for any good help
in advance .
Rick

Just use a resistor, calculate it with ohms law.

 

[rijo1]

Thanks James , I can't read the amp load because this is a Radio Frequency
circuit. Any suggestions ? Thanks ,

 

[rijo1]

Hi James , another thing is I can't use a wire wound resistor in this circuit
because it will emit harmonics that will be unfovorable . Thanks ,

 

[Ken Weitzel]

Thanks James , I can't read the amp load because this is a Radio Frequency
circuit. Any suggestions ? Thanks ,

RF on the filaments?

Ken

 

[rijo1]

No Ken , the tubes are used in a RF circuit . I can't take a amp reading off the
circuit without loading the circuit with RF . That is the problem .
The voltage is too high and is driving the tubes too hard and turning the plates
RED in a matter of seconds . The tubes cost $ 93.00 each and I don't want to
kill them . There is no RF on the filaments but only on the plate caps . Thanks
,

 

[NSM]

Hi group , I have a project that has a transformer with an output
voltage of 7.3 volts AC that drives 5 tubes . The correct voltage to
drive the tubes is 6.3 volts AC. This is used in a RF circuit and I need
to step down or reduce the voltage from 7.2 volts down to 6.3 volts
directly from the transformer lead .
What would be a simple way to reduce the voltage to 6.3 volts without
replacing the transformer ?
Please give details as clear as possible . Thanks for any good help
in advance .

Use a separate filament transformer with the right voltage.

N

 

[JURB6006]

You can put DC on those filaments and read the current drain, after a suitable
warmup time. With that you can use Ohm's law. You need the resistance once
heated.

Of course a resistor will increase warmup time, so keep that in mind if you
want to avoid cathode stripping.

A resistor is not the best method because when plate current rises so does the
cathode temperature. The resistor abates the decrease in current which is
figured in by the designer. Delta temperature is bad for cathodes. Whether this
is significant in your case or not, I don't know.

Actually at this point I'm wondering just how you know the filament voltage is
high. Y'know you could get the supply running into a resistor, well a series of
resistors and just use a coil around one of the feed wires. Picking it up
inductvely should avoid most RF drain.

At any rate if you can get insulated wires through the core of the transformer
you can wind a "bucking" winding. You'll have to see which phase it is, and
then it goes in series with the original winding, but out of phase. This will
give you a solid voltage at a lower level.

Good luck with it.

JURB

 

[JURB6006]

Ooops, I misread part of that. I'll be back.

JURB

 

[JURB6006]

On third thought, after reading the thread again, don't use a resistor.

If these tubes' characteristics are that dependent on cathode temerature the
circuit will not be stable.

Is it possible something else is wrong ?

JURB

 

[Ken Weitzel]

No Ken , the tubes are used in a RF circuit . I can't take a amp reading off the
circuit without loading the circuit with RF . That is the problem .
The voltage is too high and is driving the tubes too hard and turning the plates
RED in a matter of seconds . The tubes cost $ 93.00 each and I don't want to
kill them . There is no RF on the filaments but only on the plate caps . Thanks

Hi...

Then NSM gave you the answer... after an investment
of about 500 bucks in tubes, invest a little more in
the correct filament transformer...

But like JURB suggested, I suspect you have something
more than over-voltage on the filaments if you have
the plates glowing after a few seconds. Load, perhaps?

Ken

 

[Thomas Tornblom]

Can you unwind a few turns of the filament winding from the
transformer? If not, then perhaps you can add a few turns and connect
in series with the current winding? Depending on how you connect the
two windings you will either add or subtract the voltages.

Another possibility would be to connect two or four diodes in an
anti-parallel connection in series with the filament circuit, that
would subtract one or two forward drops, but would perhaps introduce
some unwanted harmonics into the circuit.

Thomas

 

[Jerry G.]

Since these tubes are so expensive, and you stability is necessary, I would
definitely invest in the proper filament transformer. Using a series
resistor, would lead to some instability, because the resistance of the
heaters at the particular temperature will effect the current pull, and thus
introduce some instability in the actual heater voltage. If one tube has a
heater that goes opened or changes characteristics for some reason, for
example, the rest of them will have an increased heater voltage.

Get the right type of transformer, and be done with the problem!

--

Jerry G.
======


Hi group , I have a project that has a transformer with an output
voltage of 7.3 volts AC that drives 5 tubes . The correct voltage to
drive the tubes is 6.3 volts AC. This is used in a RF circuit and I need
to step down or reduce the voltage from 7.2 volts down to 6.3 volts
directly from the transformer lead .
What would be a simple way to reduce the voltage to 6.3 volts without
replacing the transformer ?
Please give details as clear as possible . Thanks for any good help
in advance .
Rick

 

[Jerry G.]

A thing to also consider, is that the heater transformer has some internal
resistance, and there is some resistance in the wiring from the transformer
to each of the tube heaters. You must not read the transformer output with
no load. In the engineering of the system, assuming it is properly designed,
there should be compensation for the transformer winding resistance, and for
the resistance in the tube heater wiring.

In a properly designed system, the transformer heater output should come to
a main terminal, and each of the heater's wires pairs should be spidered out
from that point.

In the lower cost daisy chain type of wiring, as what has been done on low
cost consumer tube equipment, they were daisy chaining a single pair of if
wires around the chassis to distribute the heater supply. This is not a good
practice, especially in high a design where the loading is of high current,
or where the best possible heater supply stability is required.

--

Jerry G.
======


Hi group , I have a project that has a transformer with an output
voltage of 7.3 volts AC that drives 5 tubes . The correct voltage to
drive the tubes is 6.3 volts AC. This is used in a RF circuit and I need
to step down or reduce the voltage from 7.2 volts down to 6.3 volts
directly from the transformer lead .
What would be a simple way to reduce the voltage to 6.3 volts without
replacing the transformer ?
Please give details as clear as possible . Thanks for any good help
in advance .
Rick

 

[Bob Shuman]

The diodes were my thought as well, but this may not give the necessary
control since voltage drop must be a multiple of the bias voltage. But this
will certainly get you closer and help determine if you have another problem
that is causing the filaments to glow so red hot.

Bob

 

[Sam Goldwasser]

No Ken , the tubes are used in a RF circuit . I can't take a amp reading off the
circuit without loading the circuit with RF . That is the problem .
The voltage is too high and is driving the tubes too hard and turning the plates
RED in a matter of seconds . The tubes cost $ 93.00 each and I don't want to
kill them . There is no RF on the filaments but only on the plate caps . Thanks
,

That's not a filament voltage problem. There is a wide range of voltages
over which the system should work properly.

You are reducing the life of the tubes but not affecting operation
significantly. Fix the voltage problem, preferably by obtaining a
proper transformer, but there is most definitely something else wrong.

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[Sam Goldwasser]

The diodes were my thought as well, but this may not give the necessary
control since voltage drop must be a multiple of the bias voltage. But this
will certainly get you closer and help determine if you have another problem
that is causing the filaments to glow so red hot.

He said the plates are glowing red. That's not a filament voltage
problem.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[Bob Shuman]

Agreed. Thanks for the clarification since I missed it was the plates
glowing and assumed it was the filaments. Sounds like he has a short
loading the output of the tubes in question.

Bob

 

[Andre]

First of all, check all capacitors that are in touch with the gates.
If the anode gets red, the anode current is too high. Usually a gate cap
is leaky or a negative gate voltage is missing.

If it is a transmitter, it may also need the correct load, otherwise the
power that was meant to be transmitted heats up the tubes.

Be carefull because of the anode voltages!!!!!!! If your tubes cost this
much, I guess you deal with kilovolts.

Best regards,

Andre

 

[rijo1]

Thanks everyone for participating in this disscussion . This problem is in a
liniar amplifier . The plate that glows red is the driver tube . The tube is a
6LX6 and is not that expensive . The 4 driven tubes are 8908 tubes and are$
93.00 each . Anyway all the tubes are designed to have 6.3 v. to power the
filaments . Like I said , the voltage coming from the transformer is 7.2v. These
voltages were all checked on the bottom of the tube sockets .
I feel that being the voltage is so high on the filament that it is making the
tubes over conduct and saturating the plate .
I will buy a filament transformer and see if this clears the problem .
Thanks to everyone who participated in this discussion , Rick