We could fry and eat Pop, to collectively assimilate his knowledge.
Just an idea (as long as i get the doggy jammies).
Read the PDF to fully understand the circuit. Once that's done, for NOT using the thermistor, just put a wire where the thermistor should be.
Hello,
As Mr. Nelson Pass, says
"Probably even geico customers could build one."
Probably I am a geico customer. I am taking the first steps in these matters, but would like to build the F5 and have some questions. Among other I would like to know if is possible build it using a 230V toroidal transformer.
I would appreciate your comments.
Regards
jpedro
As Mr. Nelson Pass, says
"Probably even geico customers could build one."
Probably I am a geico customer. I am taking the first steps in these matters, but would like to build the F5 and have some questions. Among other I would like to know if is possible build it using a 230V toroidal transformer.
I would appreciate your comments.
Regards
jpedro
Have you read Zen's post?
Which thermistors are you referring to?
Do not jumper the tempco Thermistor.
Simply omit the tempco Thermistors. You can also omit the tempco series resistor as well. The spare holes will then allow a trimming resistor to be fitted neatly to replace the variable resistor or allow the variable resistor to be changed to a much higher value.
Which thermistors are you referring to?
Do not jumper the tempco Thermistor.
Simply omit the tempco Thermistors. You can also omit the tempco series resistor as well. The spare holes will then allow a trimming resistor to be fitted neatly to replace the variable resistor or allow the variable resistor to be changed to a much higher value.
Andrew, referring to the thermistor next to R15 and 16, like I said.
Ok, I have omitted the thermistor next to R15 and R16 and the R's are omitted also, but I used the 5k variable resistor as used in the original article.
Should the pot be a higher value in watt rating or resistance? Read that the pot sees more current when R15 and 16 are removed?
Thanks,
V~
Ok, I have omitted the thermistor next to R15 and R16 and the R's are omitted also, but I used the 5k variable resistor as used in the original article.
Should the pot be a higher value in watt rating or resistance? Read that the pot sees more current when R15 and 16 are removed?
Thanks,
V~
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From the supply rail to the gate of the MOSFET you should get a bit over 4.0V. Easier to do this without the MOSFET soldered in.
I do the initial adjusting with a current limited power supply (Tektronix PS5010 @400mA). Spares the expense of fuses. Once you get to 400mA it's just a matter of time and patience adjusting to 1.3A with or without thermistor.
I do the initial adjusting with a current limited power supply (Tektronix PS5010 @400mA). Spares the expense of fuses. Once you get to 400mA it's just a matter of time and patience adjusting to 1.3A with or without thermistor.
I would read too the huge F5 thread from this forum, as it will solve lots of questions. A long job though, but it's a very good amp indeed
If you put a wire, you should put the resistors too.
You can skip all (wire/thermistor and resistors), but the adjusting turn of the trimmer will be different.
My preferred idea is substituting the thermistor with an equivalent resistor, more or less the same ohms.
In know r3 and r4 will alter the pot, how do 15 and 16? Not doubting, just wondering. My current F-5 build isnt going to use the current limiting, which gets rid of the thermistors on the mosfets, R16,R17,R18,R19, R20 and I believe R21 and R22, (I will have to look) and was wondering how this would affect the trim pot. I'm using the Toshiba Mosfets and I've seen a few that needed different R3 and R4 to get enough range on the pot. I've also seen builds with the same Toshiba output mosfets that used the standard 2.2K for R3 and R4 and they biased up fine. Now I'm wondering what effect this may have on the pot...
Russellc
Let's do some example numbers.
For Vgs = 4V @ 1.3A output bias.
Vdrop across the 0r47 resistor is 0V611
Total Vdrop across the jFET collector load is 4V611
Assume that the jFET is drawing 8mA = 0.008A
The resistance required to generate 4V611 @ 0.008 A is 4.611/0.008 = 576r375
This is made up (cold) with the 2k2 // 4k7kthermistor+2k2 // Vr = 1668r13//Vr
Set Vr to 880r66 This requires the Vr to be moved from 0r0 to ~17.6% of travel.
Now remove the thermistor. The Vr needs to be set to 780r98 or ~ 15.6% of travel. Note just 2% of travel difference.
Now back to the removed thermistor holes. You can insert a fixed trimming resistor here.
You need 576r when set cold. set the amp up using no thermistor. Switch off. Discharge all the caps. Measure the resistance across the group of jFet collector load resistors, checking both the top and the bottom loads.
Let's guess you measure 630r for top and 510r for bottom. Yes they could be very different because the mosFET Vgs can be very different.
Use the existing 2k2 by jumpering across the thermistor location. You now have 2k2//2k2//Vr in top and bottom.
Set top Vr to 1k474, set bottom Vr to 1k211. Note these new Vr values are much higher than previously, needing ~30% and 24% travel to get the 1.3A bias.
If you use a lower fixed resistor in place of the 2k2 you will find that the Vr needs to be moved even further making bias setting a little less sensitive to small movements.
Although these numbers are pure guesswork they are an indication of the sort of Vr values you might need.
Let's look at Vr current. When set to 0r0 the Vr wiper current is 8mA. This gives a maximum dissipation across the whole Vr when set to maximum and passing 8mA of ~ 320mW. This is quite high for a 400mW Vr. A higher Vgs and/or a higher Idss could take the Vr beyond maximum current capability when set to 0r0.
The Vr current when set to 780r is ~5.9mA. The equivalent maximum dissipation is 174mW or ~43% of maximum dissipation. This should be OK long term.
Now look at Vr dissipations when the low value extra parallel resistor is put in place. Vr is set to 1k2 or 1k4.
I'll use the 1k2 since this is a more demanding situation.
the 5k Vr set to 1k2 will pass ~ 3.84mA and the equivalent dissipation is 73mW or about ~18% of maximum rating. Well OK for long term use.
For Vgs = 4V @ 1.3A output bias.
Vdrop across the 0r47 resistor is 0V611
Total Vdrop across the jFET collector load is 4V611
Assume that the jFET is drawing 8mA = 0.008A
The resistance required to generate 4V611 @ 0.008 A is 4.611/0.008 = 576r375
This is made up (cold) with the 2k2 // 4k7kthermistor+2k2 // Vr = 1668r13//Vr
Set Vr to 880r66 This requires the Vr to be moved from 0r0 to ~17.6% of travel.
Now remove the thermistor. The Vr needs to be set to 780r98 or ~ 15.6% of travel. Note just 2% of travel difference.
Now back to the removed thermistor holes. You can insert a fixed trimming resistor here.
You need 576r when set cold. set the amp up using no thermistor. Switch off. Discharge all the caps. Measure the resistance across the group of jFet collector load resistors, checking both the top and the bottom loads.
Let's guess you measure 630r for top and 510r for bottom. Yes they could be very different because the mosFET Vgs can be very different.
Use the existing 2k2 by jumpering across the thermistor location. You now have 2k2//2k2//Vr in top and bottom.
Set top Vr to 1k474, set bottom Vr to 1k211. Note these new Vr values are much higher than previously, needing ~30% and 24% travel to get the 1.3A bias.
If you use a lower fixed resistor in place of the 2k2 you will find that the Vr needs to be moved even further making bias setting a little less sensitive to small movements.
Although these numbers are pure guesswork they are an indication of the sort of Vr values you might need.
Let's look at Vr current. When set to 0r0 the Vr wiper current is 8mA. This gives a maximum dissipation across the whole Vr when set to maximum and passing 8mA of ~ 320mW. This is quite high for a 400mW Vr. A higher Vgs and/or a higher Idss could take the Vr beyond maximum current capability when set to 0r0.
The Vr current when set to 780r is ~5.9mA. The equivalent maximum dissipation is 174mW or ~43% of maximum dissipation. This should be OK long term.
Now look at Vr dissipations when the low value extra parallel resistor is put in place. Vr is set to 1k2 or 1k4.
I'll use the 1k2 since this is a more demanding situation.
the 5k Vr set to 1k2 will pass ~ 3.84mA and the equivalent dissipation is 73mW or about ~18% of maximum rating. Well OK for long term use.
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