Thanks to all of you, I`m about to finish my 124 build.
I will post all the pictures as soon I am having proper voltages.
However, there an issue with a significant voltage drop from the rectifier.
First off, it's not really all that low IMHO. My 124 B+1 is 440V for mains of 122VAC through a Hammond Fender Twin replacement PT.
What you're seeing may be 'normal'. All transformers exhibit a voltage drop between standby and on because of the increase in loading. The amount of drop will depend on the output impedance of the transformer (a value generally not published, but can be measured and calculated). So this may be the expected behavior with your PT under this load. To get the whole picture, the other supply voltages are necessary since everything hanging off the power supply loads the PT HV secondary.
One thing you might do is reduce the bias current. You're running a little hotter than I normally run (try 30-35mA range) and that bias current represents a significant load on the PT as the voltage difference between the 50 and 100W operation measurements show.
"Let's face it, the non HRMs are easier to play, there, I've said it." - Gil Ayan... AND HE"S IN GOOD COMPANY!
odourboy wrote:First off, it's not really all that low IMHO. My 124 B+1 is 440V for mains of 122VAC through a Hammond Fender Twin replacement PT.
What you're seeing may be 'normal'. All transformers exhibit a voltage drop between standby and on because of the increase in loading. The amount of drop will depend on the output impedance of the transformer (a value generally not published, but can be measured and calculated). So this may be the expected behavior with your PT under this load. To get the whole picture, the other supply voltages are necessary since everything hanging off the power supply loads the PT HV secondary.
One thing you might do is reduce the bias current. You're running a little hotter than I normally run (try 30-35mA range) and that bias current represents a significant load on the PT as the voltage difference between the 50 and 100W operation measurements show.
Yes I agree. The loss from PT is 5V and could have been more in my opinion, but if I`m calculating the post rectifier voltage it should have been:
330V-1,5V(drop from diodes)x sqrt2=464Vdc...
The unloaded , rectified HV will be closer to the rectified peak than RMS, since there's little or no load on the capacitors. Add to that most EI core transfomers are spec'd at no load, and will have a drop as explained in the previous post
Arne Marius Kirkaune wrote:
I`m missing 10V at both V1 and V2. Assume that I have to lower the 22K resistor, but to what value?
PI voltage seems correct.
Different tubes can very by 10V or so, but I would drop it to an 18K.
Arne Marius Kirkaune wrote:
I`m missing 10V at both V1 and V2. Assume that I have to lower the 22K resistor, but to what value?
PI voltage seems correct.
Different tubes can very by 10V or so, but I would drop it to an 18K.
TM
Thanks!
Tested with two different sets of tubes las night. No difference.
I will first try the 230v wiring (now 240v), however my heater voltage is now 6.6V. Highest heater voltage for 12AX7 is 6.93V? (+/-10%).
If I measure the resistance at the PT output I am getting 110ohm. Is this normal or too much?
According to my father, RMS measurements in front of the diodes, will always be wrong due to the diode load caracteristic, and the simple RMS calculations in the fluke.
I ran your rectifier simulation and was only one volt off your B+ so the transformer is right on spec. The trick is getting the right load resistance in the model. Figuring 35ma per tube (at idle) plus 3ma for each 12AX7 you get 149ma. Divide 431 volts by that and you get approximately 2k9 in voltage drop. I used 110uf as the first cap (220uf in series). Redo the math for the 50 watt version and figure they are going to draw a bit more juice by themselves.
As these things go, you haven't got a lot of tweaking left
I ran your rectifier simulation and was only one volt off your B+ so the transformer is right on spec. The trick is getting the right load resistance in the model. Figuring 35ma per tube (at idle) plus 3ma for each 12AX7 you get 149ma. Divide 431 volts by that and you get approximately 2k9 in voltage drop. I used 110uf as the first cap (220uf in series). Redo the math for the 50 watt version and figure they are going to draw a bit more juice by themselves.
As these things go, you haven't got a lot of tweaking left
Skip
I downloaded it two days ago, but was too busy to test it properly.
Tested with 230V tap, and got 454V at plate. Rest of Voltages are OK.
Heater is 6.93v unloaded, and 6.8V loaded, so i guess it is OK.
I must say that the diode board without snubber caps makes a lot of noise.
