I usually calculate it by using Vcc-Vt as the voltage, accounting for peak and not RMS volts. About double the wattage is a good place to be from here.
Wirewounds are a little better when it comes to thermal distortion on transient peaks.
A resistor dissipating about half its rated power gets pretty hot. My F5 came with a marginally undersized resistor - 6.5w as supplied in the techdiy kit as against a 9w peak dissipation - and it ran so hot I needed a small sink for it. I measured 80 degrees on the body of the resistor and I'm guessing internal temps would have been about 100C or more. Resistors are normally fine at those temperatures, but I don't like it
I have another use case with a series dropper in a tube B+ supply where a 20w resistor is dissipating almost exactly 10W. The body of the resistor is at 140C almost all day. More extreme, but worst case it fails and my B+ disappears.
I wouldn't use a resistor more than 30% continuous and 50% peak duty unless we have. a lot of sinks. My Turbo build, hopefully completed this winter, is using Caddock MP930 units with proper sinks.
Wirewounds are a little better when it comes to thermal distortion on transient peaks.
A resistor dissipating about half its rated power gets pretty hot. My F5 came with a marginally undersized resistor - 6.5w as supplied in the techdiy kit as against a 9w peak dissipation - and it ran so hot I needed a small sink for it. I measured 80 degrees on the body of the resistor and I'm guessing internal temps would have been about 100C or more. Resistors are normally fine at those temperatures, but I don't like it
I have another use case with a series dropper in a tube B+ supply where a 20w resistor is dissipating almost exactly 10W. The body of the resistor is at 140C almost all day. More extreme, but worst case it fails and my B+ disappears.
I wouldn't use a resistor more than 30% continuous and 50% peak duty unless we have. a lot of sinks. My Turbo build, hopefully completed this winter, is using Caddock MP930 units with proper sinks.
There are two resistors in series to form the feedback ladder divider.
2 off 100r to give an effective 50ohms for the upper leg of the divider.
1 off 10r to form the lower leg of the divider. The 50r and 10r give the standard gain of 6times (+15.56dB) for an F5.
The total resistance of these two parts of the NFB divider is 60ohms.
That is used to determine the maximum CURRENT through the feedback loop.
Then the CURRENT is used with P=I²R to determine the power dissipated in each part of the divider.
Both parts should be checked for dissipation.
Adjust the maximum voltage and the actual NFB divider resistances for your build.
Now using a 12 VAC transformer with a 7812 regulator. Now operating the relays with 12 VDC. This is a much better design -- much more modular.
Thanks!
The temp gain on the 7812 ( with small heatsink ) is about 30 deg C, so it is currently operating at 51 deg C. The temp gain on the pass transistors is 7 deg C, and the temp gain on the transformer is about 9 deg C. All seems to be working nicely.
It is time to start laying out the back panel and soldering parts.
It is time to start laying out the back panel and soldering parts.
400mA through the 7812 at ~16Vrms to 12Vdc is 1.6W, so it is expected to heat up to >100C without heatsink. I used heatsinks with solderable pins, they make the whole thing more sturdy.
Good luck with your project.
I'm waiting on my last shipment from mouser to start building the actual amplifier.
I have a little control circuit like yours, too, which I have been working on for a few weeks.
Really takes time for a novice like me to design something.
Good luck with your project.
I'm waiting on my last shipment from mouser to start building the actual amplifier.
I have a little control circuit like yours, too, which I have been working on for a few weeks.
Really takes time for a novice like me to design something.
I finally received by parts from Mouser and the PS caps I ordered 3 months ago to update the SA/1s to 568000 mf and then I can reused the caps from the SA/1s for the F5 turbo, 236,000mf. Once I finish the machine work and andonize of the new heat sinks and cover plate for the SA/1 I will have even left over chassis parts for the F5 turbo, I just hope the heat sinks can handle the F5, they were marginal in the SA/1s.
Hope you build goes well.
John
Hope you build goes well.
John
What wattage amp are you saying the 1/4 watt resistor is OK for? 10 Watts?
We are in the Turbo Thread, with turbo power. Andrew did the math a few posts back on the F5 thread: Feed back: 50 ohms, Source: 10 ohm = 60 ohms from output to ground.
Do the math with your rated output.
Rush
I tried to power up the FE board without anything else connected, and no jfets in place.
The voltage from Drain to Source of the jfet on the front end was full rail voltage ( 45 volts before I shut it down. Rail voltage is supposed to be +/-47.2 VDC when power supply has finished charging) This does not seem correct.
R5/6 are 1K, R27/28 are 5K6 and R25/R26 are 12K.
The calculation for the cascode voltage gives 47.2/(12,000 + 5600) * 5600 = 15 VDC
I am reluctant to solder in the jfets at this time....
I've checked and rechecked the circuit. Is it possible that the cascode transistors are reversed? I assumed that the square pad is pin 1 for the cascode transistors which seems to agree with the board traces.
The Caps, C1,2,5,6 are 22 uF, as that is what was sent with the kit.
Any help would be very appreciated
The voltage from Drain to Source of the jfet on the front end was full rail voltage ( 45 volts before I shut it down. Rail voltage is supposed to be +/-47.2 VDC when power supply has finished charging) This does not seem correct.
R5/6 are 1K, R27/28 are 5K6 and R25/R26 are 12K.
The calculation for the cascode voltage gives 47.2/(12,000 + 5600) * 5600 = 15 VDC
I am reluctant to solder in the jfets at this time....
I've checked and rechecked the circuit. Is it possible that the cascode transistors are reversed? I assumed that the square pad is pin 1 for the cascode transistors which seems to agree with the board traces.
The Caps, C1,2,5,6 are 22 uF, as that is what was sent with the kit.
Any help would be very appreciated