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potentially very stupid questions....
potentially very stupid questions....
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Old 20th August 2010, 10:48 AM   #1
dperry is offline dperry
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Default potentially very stupid questions....

would a valve light up at all if it was faulty, and should a power valve get so hot you can't touch it?
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Old 20th August 2010, 11:05 AM   #2
mnturner is offline mnturner  Australia
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That depends on the fault in the tube. If the filament/heater is still intact, the tube would probably still light up. Other faults may be present even if the valve still lights up.
And yes, power tubes will get so hot you can't touch them.
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Old 20th August 2010, 11:08 AM   #3
dperry is offline dperry
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Thanks. I know its not necessarily related to this forum, but its from a signal generator that takes it output from the anode of said tube, yet there is no output.

Would it still light up and potentially not give output?
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Old 20th August 2010, 12:14 PM   #4
Arnulf is offline Arnulf  Europe
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Yes.

If this is the output amplifier you shoudl check the input to its grid and make sure it's there. Chances are your tube is just fine and it's one of the passive components ahead of its grid that gave up.
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Old 20th August 2010, 12:55 PM   #5
dperry is offline dperry
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Quote:
Originally Posted by Arnulf View Post
Yes.

If this is the output amplifier you shoudl check the input to its grid and make sure it's there. Chances are your tube is just fine and it's one of the passive components ahead of its grid that gave up.

yeah..its an output amplifer. so i should check the tracks going to pins 2 & 7 right?

i am using this diagram... http://www.drtube.com/datasheets/12bh7a-rca1955.pdf

sorry...i know very little about tubes
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Old 20th August 2010, 01:49 PM   #6
Mr_Zenith is offline Mr_Zenith  United States
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potentially very stupid questions....
What mnturner and Arnulf said are correct. Any tube that handles a bit of power - rectifier tubes and output types like the 6L6 family, EL34, EL84, etc. - will get too hot to touch.

One fault that sometimes shows up on older amps (and can get the output tubes reeeeally hot) is a leaky capacitor between the plate of the driver tube and the control grid of the output tube. This can place a positive bias on the output tube and lead to redplating - definitely not a good thing!
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Old 20th August 2010, 02:26 PM   #7
Frank Berry is offline Frank Berry  United States
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Are you sure that the signal generator is actually oscillating. Perhaps the problem lies in an earlier stage. You should be able to sample the circuits preceeding the output tube to verify that there is a sinewave being generated. You may need to clean the switches and level control potentiometer to make sure that the unit is actually oscillating.
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Old 20th August 2010, 02:27 PM   #8
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by dperry View Post
yeah..its an output amplifer. so i should check the tracks going to pins 2 & 7 right?
That depends on the configuration of the tube. Either locate schematic diagram for your equipment or draw the section around (and preceding) output tube yourself, then post it here.

Without knowing the topology it is difficult to give specific advice (for instance, since your generator uses a dual triode at the output it could be that it is set up in cascode arrangement for HF signals, which means one section gets input at its cathode - this is just to illustrate the issues with guesswork, it doesn't necessarily mean your generator is built this way).

Quote:
i am using this diagram... http://www.drtube.com/datasheets/12bh7a-rca1955.pdf
This datasheet is fine, provided that it is really 12BH7 that is used in your circuit.

Quote:
sorry...i know very little about tubes
If you are familiar with amplifiers it doesn't matter whether you're unfamiliar with tubes or not. If however you're unfamiliar with electronics in general you'll have a giant mountain of an obstacle ahead of you to climb
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mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."
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Old 20th August 2010, 04:14 PM   #9
dperry is offline dperry
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Ok,

Here is the schematic.

WAYNE-KERR-Signal -generator.pdf

I am familair with electronics, so no worries there...

Where would be the best places to check for oscialltion?

There are burn marks around V2, R12 and R24 if that helps.
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Old 20th August 2010, 04:15 PM   #10
richwalters is offline richwalters
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Quote:
Originally Posted by dperry View Post
would a valve light up at all if it was faulty, and should a power valve get so hot you can't touch it?
A working power tube at near max ratings, the spec sheet may have quoted 250C max envelope. Too hot to touch ?

I have a faulty 6550, the heater works, the grids are at their proper working voltages but there is no electron flow between cathode / anode. It's duff.

richy
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