Yes, but you have to measure the resistance between 250V and 0V, or just take off the AZ1 and measure the resistance there.
About motorboating the easy way is if you have scope measure all regs to see if are oscillating.
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Where does 14v come from?
Vrk(L channel)=9.35 V
Vrk(R ch.)= 10.3V
Vfil(pin 1-4) = 1.48V (Rchannel); 1.485V(Lchannel)
Vgrid L=0 VgridR=0
Coleman Supply voltage = 20.83V(R) and 21.86V(L)
Coleman Supply Out= 11.89V (L), and 10.88V(R)
If the filament voltage is 1.48 then the current would be around 1 amp, maybe a fraction less. For the resistor you have to add the current going through the tube - very low in this case like 5ma or so, so that can be ignored. So the variations in Vrk of 9.35 to 10.3 just look like variations in 26 filaments and the resistor values. All looks correct, apart from the supply voltage which should be 16.5v to 18v as Rod said (5v above 11.5v where the Reg is attached to the 26).
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Hi - no need for apology! Filament bias is new for most people and it can take a moment for it to "click". Once you get it you then have to do a lot of calculation and measurement, because everything is inter-related - the bias, the filament resistance, the resistor value, the current.... That's why I have a routine for testing filament bias and use a bench supply.
And a common mistake (which I've made when not thinking in the past) is to forget to put the cathode resistor in circuit when testing and just connect to the two sides of the filament or to a resistor to represent the load which turns out to be the wrong value. There's no substitute for testing on the bench and watching what actually happens.
And then leaving it for a short while to see if it's stable. I just today found out the importance of this. One of my filaments started drawing a lot of current a minute or two after turn-on. I scratched my head and asked myself if something was expanding with heat. It turns out there was a microscopic bit of metal stuck in one of the thermal pads under a transistor in the Reg. and it was shorting out. As my Dad used to say "these things are sent to try us..."
And a common mistake (which I've made when not thinking in the past) is to forget to put the cathode resistor in circuit when testing and just connect to the two sides of the filament or to a resistor to represent the load which turns out to be the wrong value. There's no substitute for testing on the bench and watching what actually happens.
And then leaving it for a short while to see if it's stable. I just today found out the importance of this. One of my filaments started drawing a lot of current a minute or two after turn-on. I scratched my head and asked myself if something was expanding with heat. It turns out there was a microscopic bit of metal stuck in one of the thermal pads under a transistor in the Reg. and it was shorting out. As my Dad used to say "these things are sent to try us..."
Hello Rod,
I installed a 56uFoil cap just before the CCS input to ground, no change.
I reduce the input voltage to the filament supplies from ~21V to 15.4V for each. The R channel still have 1.48 Vdc between pins 1 and 4 and can be adjusted by adjusting the filament current. However, the L channel drops from 1.48Vdc(supply of ~21V) to 1.30 Vdc (supply of 15.4V) and seems like it cannot be adjusted through changing the filament current. It stays the same. The voltage on the cathode resistor for each are 9.4V and 9.42V respectively.
Just for kicks, If I turn off the filament supply, the noise is gone of course as the tube is not conducting, after this I scope out the B+ and does not find any oscillations. I really think it is coming from the filament boards.
Abe
The Reg needs to supply 11.5v (10v + 1.5v). Add 5 to 7 volts as Rod says and you have 16.5v to 18.5v. By going down to 15.4v you risk going too low now. Did you specify to Rod that you were using filament bias to provide 11.5v to a 26? The resistors in the Regs vary with the supply voltage. The small potentiometer on the Reg should be working in its range, and if it isn't doing anything send Rod an email - I'm sure he'll help. Note also with Rod's regs that they don't give you a fixed voltage/current out. This varies a small amount with the supply voltage, which is why the adjustment is there. Not like a LM1084, for instance, which gives you a steady output irrespective of the supply voltage. Do you have a 1,000v cap on the input of the Reg, as suggested for outboard supplies with umbilicals?
I'd be surprised if it's the regulators. I've never had a problem like that, and if they are correctly specified and built they are very reliable - it's a high quality part. Did you check the grid stopper (up to 1K on the pin) and the grid leak (100K or whatever) on the input? If that's all correct I'd be looking at the B+.
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Yes, Andy makes a good point. The value of R8 is tailored to the incoming supply voltage. And for filament-bias the voltage is higher, so the 3.3K resistors for R8 are the ones to use.
And as Andy reminds us, 1000uF/35v is needed at the PCB input, where an umbilical is in use.
Both these parts are required to guarantee stability.
And as Andy reminds us, 1000uF/35v is needed at the PCB input, where an umbilical is in use.
Both these parts are required to guarantee stability.
Hello guys,
The first pic is the output of the L channel at Vplate=145V, Vpin1-pin4=1.48V and filament supply voltage is 17V. As you can see, it is about 3.4Vpp oscillation.
The second pic is with 1000uF connected across the Supply Input for the regulators and it lowers the oscillation down to 0.8~ but still too high! I tried to adjust the supply voltage lower but as I mentioned, I cannot adjust the current when supply voltage reach 16V.
Right now, I cannot adjust the current on the L channel anymore! Something happened
I now see 1.72 to 2.02 volts at Vpin1-4 when supply voltage were 17 and 20 volts respectively and cannot adjust the current anymore.
Abe
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
The first pic is the output of the L channel at Vplate=145V, Vpin1-pin4=1.48V and filament supply voltage is 17V. As you can see, it is about 3.4Vpp oscillation.
The second pic is with 1000uF connected across the Supply Input for the regulators and it lowers the oscillation down to 0.8~ but still too high! I tried to adjust the supply voltage lower but as I mentioned, I cannot adjust the current when supply voltage reach 16V.
Right now, I cannot adjust the current on the L channel anymore! Something happened
I now see 1.72 to 2.02 volts at Vpin1-4 when supply voltage were 17 and 20 volts respectively and cannot adjust the current anymore.Abe
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Hello Andy,
I have, from the input RCA, a 100k volume pot, then 390 ohms to the grid of the 26. Not enough?
I checked the parts that Rod sent me, R8 is 3.3k.
Abe
That should work as long as the volume pot is all hooked up correctly. It's what I have myself, anyway!
It's looking like something isn't right in one filament supply if the trimmer isn't adjusting properly. You could measure all the voltages in each Reg and compare one with the other. That's a bit laborious, but may reveal a part not working or a dry joint or something. You can easily spend hours looking for faults - just being very methodical and measuring voltages and resistances at each point. It's useful if you have one fully working part.
At times like these I love my bench supplies.....
Hello guys,
Here's the latest of what I got. These measurements were with: B+=207Vdc input to shunt reg, then reg voltage at ~150vdc. Plate voltage on the tube is ~145V, with 1.45V on the filament. Supply voltage to the Rod Coleman reg is 17.2Vdc, through umbilical, 1000uF, input of regulator. Output of regulator is 1.45V + 9.5V(Vrk).
The two pics above are measured from Plate of the 26(1st pic), and Cathode of the 26.
The bottom pics are measured from the regulated B+(AC content of the output of the HV Shunt Regulator), and the regulated DC output.
I do not see anything significant on the B+(regulated output) above.
regards,
Abe
Here's the latest of what I got. These measurements were with: B+=207Vdc input to shunt reg, then reg voltage at ~150vdc. Plate voltage on the tube is ~145V, with 1.45V on the filament. Supply voltage to the Rod Coleman reg is 17.2Vdc, through umbilical, 1000uF, input of regulator. Output of regulator is 1.45V + 9.5V(Vrk).
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
The two pics above are measured from Plate of the 26(1st pic), and Cathode of the 26.
The bottom pics are measured from the regulated B+(AC content of the output of the HV Shunt Regulator), and the regulated DC output.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I do not see anything significant on the B+(regulated output) above.
regards,
Abe
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All looks as you would expect....if it was me I would be tempted to try the 26 on 1.5 volts AC and do away with the Rod Coleman regulator....see what happens !!!
Do you have a copy of the RCA TT3 Tube Data Book....if not I can email you a copy....page 75 to 77 is worth a read, and it may point you in different direction