IRF510 push/pull again

[WA1GFZ]

YES

[ssbothwell KJ6RSG]

YES

great

i installed the irf630s but some weird thing are happening with the bias circuit.

1. whenever i power on the circuit the solder melts off the anode end of one of the bias circuit diodes. 
2. when i check the voltage across the bias pots, the voltage is 0v for almost the full rotation of the pot and then jumps to 5V when i have rotated completely. when i power off the circuit and check the resistance of the pots it goes smoothly from 0 to 5k across the full rotation of the pot.
3. this is likely related to #2 but adjusting the bias does not change the output waveform at all until suddenly it cuts out when i have rotated the pot completely to the 5v bias position.

any ideas?

[WA1GFZ]

sounds like a wiring error or one of the FETs is leaky drain to gate. It could have taken an eSD hit.
Try running the stage without bias and measure the gate voltage. If there is DC voltage on a gate the fET is dameged. fc

[ssbothwell KJ6RSG]

the weird desoldering of the bias diode stopped happening. maybe something was sitting on the board causing a short that i did not notice.

i didnt measure any voltage between gate and ground when the bias is disconnected. the IRF630s seem to be working fine, its just that adjusting the bias only a small portion of the bias pot's rotation seems to impact the bias voltage and i cant seem to get rid of the crossover distortion for the life of me.

i wonder if i am not properly driving the IRF630s? i didnt make any changes to the circuit to accommodate the switch 630s.

i wish i had some 510s with which to verify the circuit. --edit: i just checked my mail and my order of IRF510s arrived!

[ssbothwell KJ6RSG]

i installed IRF510s finally

here is the output: http://i.imgur.com/5yLW7.jpg

it still has really nasty crossover distortion which i cant get rid of. bias adjustments are behaving weird as i described earlier.

is it possible that using a 7805 rather then a 78L05 the source of my bias problems? when i rebuilt the circuit i chose 7805 because 78L05 is rated at 20V input and i am using a 28V power supply.

other then the bias and crossover related issues this new board layout seems good. using 2x 1ohm 5w power resistors in parallel for source resistance seems to improve the stability but it is still very slowly dropping in voltage. maybe a fan will help with this.

[ssbothwell KJ6RSG]

well it turned out that all the latest problem was the same as all other problems: i didn't notice something obvious. when i rebuilt the board i forgot to include the R1,R2,C1, and C2. oops.

now the amp is working great! when i first turn it on it produces 47VRMS and over ten minutes it dropped down to 38VRMS. i wonder if this is actually the amp putting out less power or my dummy load overheating and having its resistance change. i am using an oil filled load that is rated for 20W so i am going way over its specs here.

i am going to consider this project 90% complete. i would like to test it with a higher rated load and see if this power dropping issue is real or not.

wa1gfz, do you have any final comments, suggestions, or tests i should run?

[WA1GFZ]

Go back to the higher voltage 78XX regulator of reduce the input voltage.
I've seen them take a real beating but the one thing that takes them out is high input voltage.
Your output voltage would go up if the load resistance was increasing.
I suspect the FETs or source resistors are warming up.
I would think 50 watts RMS is a good place to be with 2 TO220 devices. I do know IRF840s last forever at 100W PEP/FET in class E. Do the FETs seem hot after 10 minutes?
You could try going lower with the source resistor or add another pair of FETs.
If you add more FETs you could add a turn or 2 to the output  transformer secondary. That core should easily do 100 watts.

[ssbothwell KJ6RSG]

i'm using 7805 (35V Max input).

i just remembered i have a temperature probe!
after ten minutes the output is down to 37.5VRMS and the the fets are 140C and the source resistors are 100C! i measured one of the heatsink fins and it is 60C.

i guess i need to put a fan on the transistors?

edit: i aimed an 80mm fan at the fets and the voltage is slowly creeping up. its at 41VRMS right now. i am going to get a giant fan to mount to the bottom of the heatsink and maybe i second one as an intake to the enclosure i eventually put this amp inside.

edit2: with one 80mm fan aimed at the fets and one 80mm fan aimed at the heatsink fins the fets are at 55C and output is 39-40VRMS.

[WA1GFZ]

140 degrees C is way too hot for a case temperature. The junction is hanging by a thread. You need a better heat sink. I monitor case temperature of one FET in the 160 meter rig and they never get above about 50 degrees c on a hot day without a FAN.
As the junction gets hot the RDS on must be going up. 

[ssbothwell KJ6RSG]

but when you are using your 160m rig it is not left on peak output for extended periods of time. i would imagine that when i modulate the power supply it wont always be reaching peak output so it will have more of a chance to cool down, right?

with fans the transistors are stable at 50-55C. is even that too high a temperature?

[WA1GFZ]

Fan is ok if you don't mind the noise.
My 14 FETs are running about 300 watts carrier and about 1500 watts peak

[ssbothwell KJ6RSG]

so this brings up something i have been wondering about. when i am driving my amp with an unmodulated VFO and the amp is putting out ~35W, that is my peak output right?

when i setup a modulator circuit i want to set it so that it reaches 35W at 100% modulation, right?

[WA1GFZ]

It is your peak power for that drive level and VCC voltage you are running.
You can low level modulate it if you take the output of the VFO and use a double balanced mixer as a modulator. You can't run more than 0 dBM into most mixers. VFO drives LO port and output is the RF port. Audio goes in the IF port with a slight DC offset to set the carrier level. All three ports want to see 50 ohms. you will need a little gain after the DBM to make up for circuit losses. go to mini-circuits site and look at their app notes.

[ssbothwell KJ6RSG]

cool, i assumed it was peak power but i wanted to make sure.

if you low level modulate a signal and then input it into a class AB amp, wont you get distortion? i thought that AB is not completely linear.

i was planning to modulate the power supply ( is that called series modulation?), but i already have a working diode ring mixer from a previous thread. i guess i should try it out.

[WA1GFZ]

You could run class B if you series modulate the power supply. It will take more drive though. Class AB will work as a linear amplifier if biased properly and has some RF feedbach which you have.

[ssbothwell KJ6RSG]

oh interesting. how can i tell if the amp operating linearly? is there a way i can verify it precisely using my scope?

also, in the SS rig challenge thread, W1FVB recommended a modulation circuit. i am attaching it here. do you think it would be suitable for my amp?

[WA1GFZ]

That circuit will not go up to 30 volts output peak. Maybe 28 volts. I would simulate it and check the voltage swing. I like using a PNP to drive an NPN. Also need a big heat sink to dissiapate the heat. I don't like to run much more than 3 amps through a 2N3055.
I'm not sure if there will be distortion due to the AC terminated op amp - input but see the need to have it float.

[ssbothwell KJ6RSG]

doesnt it only need to reach 28V? thats the voltage of the power supply i am currently using.

[WA1GFZ]

yes but it will have a little extra loss

[ssbothwell KJ6RSG]

why is that?

[WA1GFZ]

VBE of both transistors and the maximum voltage from the op amp.
Not a big deal at this power level but it will limit the maximum output