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 #26 pre amp
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diyAudio Member

Join Date: Sep 2009
Location: Catalonia - Europe
Quote:
 Originally Posted by amandarae We are talking about the resistance of the HV winding of the power transformer secondary connected to the plates of the AZ1 right? Abe
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.

Last edited by merlin el mago; 23rd March 2013 at 10:27 AM.

 23rd March 2013, 10:44 AM #2882 diyAudio Member     Join Date: Mar 2013 Location: Portsmouth Amandarae, what Filament voltage do you have on the 26 Tube....the 10R dropping resistor by my calculations should be 13.33 Ohms......(14 volts divided by 1.05 amps (filament current draw of 26)= 13.33 Ohms)
diyAudio Member

Join Date: Sep 2003
Location: London
Quote:
 Originally Posted by CV6045 Amandarae, what Filament voltage do you have on the 26 Tube....the 10R dropping resistor by my calculations should be 13.33 Ohms......(14 volts divided by 1.05 amps (filament current draw of 26)= 13.33 Ohms)
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).

Last edited by andyjevans; 23rd March 2013 at 11:14 AM.

 23rd March 2013, 11:13 AM #2884 diyAudio Member     Join Date: Mar 2013 Location: Portsmouth 26 The 14 volts comes from my assumption that the Filament Supply is delivering 15.5 volts out ? The 26 Tubes takes 1.5 volts @ 1.05 amps....... I apologize if I have misunderstood.
 23rd March 2013, 11:28 AM #2885 diyAudio Member   Join Date: Sep 2003 Location: London 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..."
diyAudio Member

Join Date: Oct 2004
Location: Diamond Bar, CA
Quote:
 Originally Posted by Rod Coleman Hello Abe, I think the quickest item to try is B+ regulator stability. Please add some HV capacitance located near the input circuit of the B+ CCS/Shunt regulator. 22uF or more for preference, but even 10uF may be enough to stabilise it. Before using the amp for a long period, the input-output voltage difference for the filament regulators (10V as indicated) must be reduced to 5 .. 7V, or the transistors may suffer from overheating.
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

 23rd March 2013, 06:31 PM #2887 diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe Have you measured looking for oscillation with the scope at all regs: B+ and Rod heaters?
diyAudio Member

Join Date: Sep 2003
Location: London
Quote:
 Originally Posted by amandarae 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+.

Last edited by andyjevans; 23rd March 2013 at 07:52 PM.

 23rd March 2013, 07:53 PM #2889 работник diyAudio Member     Join Date: Jul 2004 Location: Warwickshire UK 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.
 23rd March 2013, 07:55 PM #2890 diyAudio Member     Join Date: Oct 2004 Location: Diamond Bar, CA 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 Last edited by amandarae; 23rd March 2013 at 08:03 PM. Reason: text

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