Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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  #1   Report Post  
rijo1
 
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Default 7.2 volt to 6.3 volt ?

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

  #2   Report Post  
James Sweet
 
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Default


"rijo1" wrote in message
...
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.


  #3   Report Post  
rijo1
 
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Thanks James , I can't read the amp load because this is a Radio Frequency
circuit. Any suggestions ? Thanks ,

James Sweet wrote:

"rijo1" wrote in message
...
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.


  #4   Report Post  
rijo1
 
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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 ,

James Sweet wrote:

"rijo1" wrote in message
...
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.


  #5   Report Post  
Ken Weitzel
 
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rijo1 wrote:
Thanks James , I can't read the amp load because this is a Radio Frequency
circuit. Any suggestions ? Thanks ,


RF on the filaments?

Ken



  #6   Report Post  
rijo1
 
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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
,

Ken Weitzel wrote:

rijo1 wrote:
Thanks James , I can't read the amp load because this is a Radio Frequency
circuit. Any suggestions ? Thanks ,


RF on the filaments?

Ken


  #7   Report Post  
NSM
 
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Default


"rijo1" wrote in message
...
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


  #8   Report Post  
JURB6006
 
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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
  #9   Report Post  
JURB6006
 
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Ooops, I misread part of that. I'll be back.

JURB
  #10   Report Post  
JURB6006
 
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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


  #11   Report Post  
Ken Weitzel
 
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Default



rijo1 wrote:
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

  #12   Report Post  
Thomas Tornblom
 
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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
  #13   Report Post  
Jerry G.
 
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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.
======


"rijo1" wrote in message
...
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


  #14   Report Post  
Jerry G.
 
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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.
======


"rijo1" wrote in message
...
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


  #15   Report Post  
Bob Shuman
 
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Default

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

"Thomas Tornblom" -to-reply wrote in message
...
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





  #16   Report Post  
Sam Goldwasser
 
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rijo1 writes:

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.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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  #17   Report Post  
Sam Goldwasser
 
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"Bob Shuman" writes:

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
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  #18   Report Post  
Bob Shuman
 
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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

"Sam Goldwasser" wrote in message
...
"Bob Shuman" writes:

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.



  #19   Report Post  
Andre
 
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Default

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 wrote:

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
,

Ken Weitzel wrote:


rijo1 wrote:

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


RF on the filaments?

Ken





--

Please change no_spam to a.lodwig when replying via email!
  #20   Report Post  
rijo1
 
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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

"Jerry G." wrote:

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.
======

"rijo1" wrote in message
...
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




  #21   Report Post  
Sam Goldwasser
 
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Default

rijo1 writes:

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 .


But was the voltage checked under load? Depending on the quality of the
transformer, a 0.9 V drop might not be out of the question when loaded
If this is the original transformers, failure with increased vcltage
is extremely unlikely.

The filament voltage on these types of tubes has virtually no effect on
operation as long as it's enough. That's not your problem. Current and
plate dissipation are determined by other factors.

--- 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.
  #22   Report Post  
drwxr-xr-x
 
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On Tue, 25 Jan 2005 15:53:29 GMT, rijo1 wrote:
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 .


Not even a possibility.
Your illegal CB amplifier has some other problem.
  #23   Report Post  
Bill Jeffrey
 
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What? When did resistors start generating harmonics?

Bill
==========

rijo1 wrote:
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 ,

James Sweet wrote:


  #24   Report Post  
Bill Jeffrey
 
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Why/how does RF load the filaments and change the filament current? Are
you sure you know what is going on in this circuit? Why don't you tell
us what the circuit is, and what the tubes are, and we'll calculate the
filament dropping resistor for you?

Bill
================

rijo1 wrote:

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 .


  #25   Report Post  
jakdedert
 
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First off, you 'have' measured the actual voltage with the tubes in-circuit,
correct? Just reading the voltage from the unloaded transformer will give
you an incorrect reading.

Secondly, the tube characteristics are published. Use those figures to
calculate the current draw, and Ohm's Law to figure the resistance value and
power rating for the dropping resistor. Most likely you'll need a wirewound
resistor of several watts rating.

Thirdly, if the voltage is indeed too high, you might simply place a four
amp diode in series with the transformer lead to drop the voltage to almost
the exact value desired.

Last, but not least, make absolutely sure that some other fault is not
causing the problem in the first place.

jak


rijo1 wrote:
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 ,

Ken Weitzel wrote:

rijo1 wrote:
Thanks James , I can't read the amp load because this is a Radio
Frequency circuit. Any suggestions ? Thanks ,


RF on the filaments?

Ken





  #26   Report Post  
rijo1
 
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Wirewound resisters will induce harmonics .

Bill Jeffrey wrote:

What? When did resistors start generating harmonics?

Bill
==========

rijo1 wrote:
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 ,

James Sweet wrote:


  #27   Report Post  
Jamie
 
Posts: n/a
Default

rijo1 wrote:

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

the load that the heaters of the tube exhibit will most likely put the
voltage down to 6.3 or less..

  #28   Report Post  
Jamie
 
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Sam Goldwasser wrote:

"Bob Shuman" writes:


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.

sounds like a Tank miss match or not enought on the grid verses the
plate current
maybe even both!





  #29   Report Post  
James Sweet
 
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"rijo1" wrote in message
...
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

Can't you disconnect the plate current so you're just running the filaments,
then measure the current? You could always use trial and error, start with a
resistor you know is too many ohms and then tweak it until you get it right,
they even made wirewould adjustable resistors for tasks like this, not sure
if you can get them anymore though.


  #30   Report Post  
Heavy G
 
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Considering you said the tubes cost 93 bucks a piece, and you run the risk
of damage with as resistor, I think its easier to just replace it with the
correct transformer, than run the risk and expense of replacing a 90 dollar
tube.


"rijo1" wrote in message
...
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





  #31   Report Post  
Jamie
 
Posts: n/a
Default

James Sweet wrote:

"rijo1" wrote in message
...

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

Can't you disconnect the plate current so you're just running the filaments,
then measure the current? You could always use trial and error, start with a
resistor you know is too many ohms and then tweak it until you get it right,
they even made wirewould adjustable resistors for tasks like this, not sure
if you can get them anymore though.


must be a common cathod&heater type tube in which case the filament
transformer should have a point to ground via a Zener to regulate the
standing current. if this is not operating correctly, the tubes will
be glowing.!
my AL-80A using a 3-500z tube uses a 7.1 volt Zener stud mounted
to get the tube near the pinch off point.

  #32   Report Post  
Terry
 
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"Sam Goldwasser" wrote in message
...
He said the plates are glowing red. That's not a filament voltage
problem.

I agree with Sam; it is quite possible that the OP is chasing something that
is not a problem.
And resistors do not 'generate' harmonics!
While a wire wound resistor may have inductance in most heater circuits this
is unlikely to be problem.
Red plates in transmitting tubes is most likely to be a different problem;
if it's a transmitter it may be 'off-tune'?
Or the transmitter drive may be low, (or too high) depending on the
operating class of the final/s?
Reluctant to start the OP on that angle because of the high voltages etc.
involved. Must be fairly high power?
We really don't have enough info! But really wonder if heater voltage is
REALLY the problem?
Wonder what the application is if it's a radio amateur transmitter would
expect the operator to be a little more knowledgeable? If it's something
critical such as a paging transmitter or something like that
................. in say a hospital etc. should be maintained by a qualified
tech.
Voltage 7.2 compared to nominal 6.3 is a bit (14%) high but perhaps
believable depending on the voltage of the AC supply, 115 or 230 volts? Or a
mains transformer may be set on the wrong voltage tap; e.g. if it is on the
100 volt setting (which I believe is the Japanese AC supply?) and the supply
is 115 that is 15% high to start with.
On other hand OP may be measuring the 7.2 volts off load, with the tubes
removed or something? With the tubes plugged in it might be spot on 6.3 and
at what point in the circuit is the 7.2 being measured? It all sounds rather
dangerous!


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