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
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!
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!
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.
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).
This datasheet is fine, provided that it is really 12BH7 that is used in your circuit.
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
Ok,
Here is the schematic.
View attachment 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.
Here is the schematic.
View attachment 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.
A working power tube at near max ratings, the spec sheet may have quoted 250°C 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
Looks like both sections are simply paralleled for larger current output so you can observe input with an oscilloscope at either grid (pins 2 and 7).
Check capacitor C17 to confirm it is not leaky (voltage at the R20 end of the capacitor should measure about 0V DC and whatever AC the V1B amplifier is feeding it while voltage on the other end will be way above 0V, more like 1/2 of supply voltage which is probably a couple hundred volts).
Also make sure that by some weird conincidence both R20 and R26 haven't gone open circuit (extremely unlikely but it would also result in symptoms you've described).
If you get no input at the V2 grids, trace the circuit backwards (through both stages of V1) and note where the signal appears, then let us know what you've found out and you'll be given further directions.
Remember: voltages in the circuit can be lethal to you and your equipment so be careful ! (few hundred volts)
WRONG !
As long as the chassis is hearted, no matter that it carries +HT or -HT.
Here you'll just find bothering measuring negative only voltages relative to the chassis
Anodes will be near zero volts while grids and cathodes will show some "minus 200" !
Yves.
Edit:
"Why on earth would someone do that? Other than to save a bit of money on wiring. "
Probably to save a link cap on output ! ? ! ?
But was a common way to power CRT in some o'scopes.
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To give you a little background, I have recently started a job at as a calibration technician, and due to my experience building 9v effect pedals, I am lumbered with the tag 'electronics guy' and now expected to know whats wrong with everything thats faulty I don't have any experience with anything like this., but have been able to diagnose a few simpler things.
So, it came in to be calibrated and worked, it was around 20% out of spec at the lower end, so while consulting my manager, manual and schematic, we agreed to dissasemble C1-C12 to measure them as the manual states that these are responsible for frequency control.
Upon reassembling, it no longer worked, and began to burn around the base of the 12bh7 tube. It also gets very hot around R12. I have a replacement 12bh7 btw, but am yet to fit it.
Regarding all the suggestions so far, thanks very kindly for taking the time to answer, its all greatly appreiciated
So, to elaborate on your questions....and sorry once again for my stupidity....
"Looks like both sections are simply paralleled for larger current output so you can observe input with an oscilloscope at either grid (pins 2 and 7)."
So you mean power it up and measure the voltage with an oscilloscope to ensure the oscillation is getting that far? Is that on both V1 and V2?
"Check R20 and R26 haven't gone open circuit"
By this you mean make sure they are still acting as resistors and haven't shorted?
Just to ensure.. the input at V2 should be what exactly?
Yves mentions leakage current...where exactly would this be measured?
Frank Berry....I am sorry once again, but i have no idea what you mean by B+ voltage
Finally, i have touched the chassis and haven't been electrocuted....
I have measured all the caps so far and some of them were way off their given value.....I will update this later..
Thanks once again everyone for your suggestions and tips.
I don't have any experience with anything like this., but have been able to diagnose a few simpler things. So, it came in to be calibrated and worked, it was around 20% out of spec at the lower end, so while consulting my manager, manual and schematic, we agreed to dissasemble C1-C12 to measure them as the manual states that these are responsible for frequency control.
Upon reassembling, it no longer worked, and began to burn around the base of the 12bh7 tube. It also gets very hot around R12. I have a replacement 12bh7 btw, but am yet to fit it.
Regarding all the suggestions so far, thanks very kindly for taking the time to answer, its all greatly appreiciated
So, to elaborate on your questions....and sorry once again for my stupidity....
"Looks like both sections are simply paralleled for larger current output so you can observe input with an oscilloscope at either grid (pins 2 and 7)."
So you mean power it up and measure the voltage with an oscilloscope to ensure the oscillation is getting that far? Is that on both V1 and V2?
"Check R20 and R26 haven't gone open circuit"
By this you mean make sure they are still acting as resistors and haven't shorted?
Just to ensure.. the input at V2 should be what exactly?
Yves mentions leakage current...where exactly would this be measured?
Frank Berry....I am sorry once again, but i have no idea what you mean by B+ voltage
Finally, i have touched the chassis and haven't been electrocuted....
I have measured all the caps so far and some of them were way off their given value.....I will update this later..
Thanks once again everyone for your suggestions and tips.
Update!!
So...looking at my notes, I got perculiar readings from the five variable capacitors...C1, C3, C7, C9 and C15.
Also C19 should have been 100uF and read 75uF,
C21 shoulda been 32uF and read 16uF,
C22 (500uF) read 525uF,
C23 (50uF) read 19uF
C24 (8uF) read 4.2uF
C25 seemed to move between 18uF or 36uF
All of these readings where taken with an RLC meter, which the power of the signal generator switched off.
So...looking at my notes, I got perculiar readings from the five variable capacitors...C1, C3, C7, C9 and C15.
Also C19 should have been 100uF and read 75uF,
C21 shoulda been 32uF and read 16uF,
C22 (500uF) read 525uF,
C23 (50uF) read 19uF
C24 (8uF) read 4.2uF
C25 seemed to move between 18uF or 36uF
All of these readings where taken with an RLC meter, which the power of the signal generator switched off.
The positive supply is referenced to chassis and the negative side of the supply delivers the power to the active circuitry. Take a very close look at the direct coupled output attenuator which forms the plate load for the output tube..
From a safety perspective it is no different from grounding the negative side of the supply, and the design in general seems to be very well thought out.
Whether or not there is any particular advantage to it I'm not sure, probably somewhat relaxes the insulation requirements on the switches since they are not connected to B+, other than that I am not sure. Probably a theory of operation somewhere explains the reasoning behind this design. There is no particular cost advantage I can see, and it probably wasn't primarily done as a cost cutting measure.
IMHO it is probably beyond time to replace all of the electrolytics in this oscillator. Endless trouble shooting time can be saved by so doing first, and it greatly reduces the likelihood that once having gotten it working again it will fail shortly thereafter. (from experience)
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