7.2 volt to 6.3 volt ?

[Sam Goldwasser]

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.

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[drwxr-xr-x]

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.

 

[Bill Jeffrey]

What? When did resistors start generating harmonics?

Bill
==========

 

[Bill Jeffrey]

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
================

 

[jakdedert]

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]

Wirewound resisters will induce harmonics .

 

[Jamie]

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..

 

[Jamie]

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!

 

[James Sweet]

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.

 

[Heavy G]

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.

 

[Jamie]

off the


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.

 

[Terry]

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!