Hello, hope you are all well and safe, I've been listening with pleasure to my amplifier and now it is time to put him back on the workbench.
I am not fully driving the output tubes, the pentode pre-amp output goes easily to a Vpp of 150v but as soon as I hit Vpp input of around 34V (+/-17V) on the phase splitter it clips.
Does anybody have some suggestions on how to modify this ?
The Anode of the Phase splitter Triode is at 322V and the Cathode at 16.4V
It is a 6U8A tube and I tested with the negative feedback disconnected.
I have attached a diagram.
I am not fully driving the output tubes, the pentode pre-amp output goes easily to a Vpp of 150v but as soon as I hit Vpp input of around 34V (+/-17V) on the phase splitter it clips.
Does anybody have some suggestions on how to modify this ?
The Anode of the Phase splitter Triode is at 322V and the Cathode at 16.4V
It is a 6U8A tube and I tested with the negative feedback disconnected.
I have attached a diagram.
Attachments
The grid of your splitter has to be at 80 ish volts and the plate and anode resistors changed accordingly.
Eg. 27k would yield roughly 2.9 ma current.
V p-p will be about double the dc cathode voltage.
Eg. 27k would yield roughly 2.9 ma current.
V p-p will be about double the dc cathode voltage.
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Hi Lampie519, the board I bought does not match exactly the diagram, in reality there is a coupling capacitor and the grounding resistor on the board.
Please remove them both and measure again. To protect the grid you can use a diode between the grid and cathode. In case the voltage is positive on the grid the diode will conduct.
Attachments
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Do I understand it correct that in the current design it is cathode biassed so the screen is (virtually) 16.4 volt biassed since that is the cathode voltage ? Also the reason why it clips around that value ?
Why do you modified original DC coupled (good) schematic?
This type is working.
Using the Pentode/Triode Tube as the Amplifier/Phase Splitter.
This type is working.
Using the Pentode/Triode Tube as the Amplifier/Phase Splitter.
With a coupling capacitor, there must be another biasing resistor from the grid to B+
or the phase splitter would not work at all. Look for a 470k or so resistor to the B+ from the grid.
Measure the DC voltage on the grid to check. It should be at about 110VDC.
or the phase splitter would not work at all. Look for a 470k or so resistor to the B+ from the grid.
Measure the DC voltage on the grid to check. It should be at about 110VDC.
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I will measure again but pretty sure the DC GRID voltage is 0
I'll be back , Thx Guys
Euro21, I did not modify it the board came with those on, I'll triple check and have a look at your link.
I'll be back , Thx Guys
Euro21, I did not modify it the board came with those on, I'll triple check and have a look at your link.
If the grid is at 0VDC, is there a -100VDC negative rail that the 39k cathode resistor goes to?
Maybe you are looking at the pentode screen bypass capacitor instead, and the grid does directly connect
to the previous plate, and so is at an elevated DC voltage.
Maybe you are looking at the pentode screen bypass capacitor instead, and the grid does directly connect
to the previous plate, and so is at an elevated DC voltage.
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With my limited understanding and your input (Thx !) it seems the splitter is cathode biassed at ~16V that is why it only works till 16V input (Do you agree ?) Because it does play fine (until that voltage)
I'm going (carefully try) for the direct coupled option, would it matter a lot that then it is biassed at 133V instead of 110 ? Can I leave the Anode and Cathode resistors at 39k ?
I'm going (carefully try) for the direct coupled option, would it matter a lot that then it is biassed at 133V instead of 110 ? Can I leave the Anode and Cathode resistors at 39k ?
If it behaves that way, then the circuit design is bad. A linear circuit should amplify signals
by an amount that is independent of their magnitude. Both a 0.1V and a 1V signal should be
amplified by the same factor.
The circuit may or may not be optimized with the present DC voltages and resistance values,
but if direct coupled, it will work a lot better than the way it works now.
Please be certain that you are not mistaking some other capacitor for a coupling capacitor
to the triode grid. Measure the DC voltage at the triode grid before changing anything.
If the grid voltage is at the same DC voltage as the pentode plate, then it is directly coupled.
by an amount that is independent of their magnitude. Both a 0.1V and a 1V signal should be
amplified by the same factor.
The circuit may or may not be optimized with the present DC voltages and resistance values,
but if direct coupled, it will work a lot better than the way it works now.
Please be certain that you are not mistaking some other capacitor for a coupling capacitor
to the triode grid. Measure the DC voltage at the triode grid before changing anything.
If the grid voltage is at the same DC voltage as the pentode plate, then it is directly coupled.
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Thx rayma, I'll make certain and then bias correctly. But it really seems that way there is the coupling capacitor and the triode grid is not connected to the pentode. Triode grid is ar 0V , as a test I'll add a resistor to the grid to create a voltage divider with the existing 220K to ground. If that works I'll make the direct connection with diode limiter as earlier suggested.
The help is appreciated. I'll feedback soon. (Have to go to work now)
The help is appreciated. I'll feedback soon. (Have to go to work now)
This could be made to work well direct coupled , no need for capacitors and modifications ...
Like somebody said the DC voltage should be 80-90V in anode of pentode for maximum output swing of the triode cathodyne . For this I would raise a little the anode resistor maybe to 220K .
39K anode and cathode resistors for triode are OK .
With this circuit and tube in my experiments the max output swing undistorded on the scope was 2x43Vrms at 350V ... as a rule of thumb usually for any cathodyne the usable max AC swing if you tune it right is about B+/10 , so 2x20Vrms at 200V , 2x30Vrms at 300V and so on .
You can fine tune it by replacing the cathode resistor 1K with a pot , so optimum resistance could be 1K2 or so ... then you use fixed 1K + 200ohm as part of the negative feedback to ground
Like somebody said the DC voltage should be 80-90V in anode of pentode for maximum output swing of the triode cathodyne . For this I would raise a little the anode resistor maybe to 220K .
39K anode and cathode resistors for triode are OK .
With this circuit and tube in my experiments the max output swing undistorded on the scope was 2x43Vrms at 350V ... as a rule of thumb usually for any cathodyne the usable max AC swing if you tune it right is about B+/10 , so 2x20Vrms at 200V , 2x30Vrms at 300V and so on .
You can fine tune it by replacing the cathode resistor 1K with a pot , so optimum resistance could be 1K2 or so ... then you use fixed 1K + 200ohm as part of the negative feedback to ground
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Grid should be at a quarter of the supply voltage so take out (A) as that really messes things up.
At grid quarter supply voltage, anode swings from 1/2 b+ to b+ and cathode swings from 1/2 b+ to B-.
At grid quarter supply voltage, anode swings from 1/2 b+ to b+ and cathode swings from 1/2 b+ to B-.
Hi guys and girls success as an easy test I biassed the phase splitter Grid at 86 v with a voltage divider to the 350 V (left the coupling capacitor) and re-used the existing 220K to ground on the board. Pin 1 is now 257 V and Pin 8 92 V DC ~ 2.5 mA through the tridode.
Max output swing is now somewhere between +-33 and +-36 V on pin 1 before clipping 🙂 Following Depanatoru rule of thumb this looks quite ok. Very happy.
A 2 cent resistor brought my output power from 5 W RMS to 25W RMS (little dance here)
Thank you all! Feedback and hints still welcome, going to test later today the direct coupling.
Max output swing is now somewhere between +-33 and +-36 V on pin 1 before clipping 🙂 Following Depanatoru rule of thumb this looks quite ok. Very happy.
A 2 cent resistor brought my output power from 5 W RMS to 25W RMS (little dance here)
Thank you all! Feedback and hints still welcome, going to test later today the direct coupling.