F3 Builders Thread

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csample

Member
Joined 2017
I don't think that any of the active devices need to be matched, but I did read a post suggesting matching of the devices could help with similar distortion profiles between channels if you don't have the equipment to measure and adjust the distortion. I did not match any of the IRFP240's or the ZTX450's and I was very pleased with the sound.
 
C

csample

Member
Joined 2017
Matching R5 and Q1

Looking for confirmation if I did this right or not:

I tried measuring the Vgs using the circuit in image below and then choosing R5 based on the chart, but taking the Vgs measurement was difficult for be because the it was constantly changing with the jfet temperature. So I took my best guess at the Vgs value before the jfet got too hot and selected R5 from the chart. This didn't work well at all, my voltage at the drain of Q1 was ~2.7V (should be ~3.5).

So this is what I did (and am looking for confirmation on). I built up the amp, and put a 10ohm 5W potentiometer in place of R5 with the initial resistance set per Vgs and the chart below. P1 and P2 set to mid position by counting turns. After approximately 5 minutes after power up begin to periodically adjust P1 and the "R5" potentiometer as the amp warms up to keep the voltages at Q1 drain and R9-C1 junction at the values on the F3 schematic (3.5V and 21V respectively). Let the amp run for 1 hour make final adjustments. Remove "R5" potentiometer and measure resistance. Solder in real resistor for R5 of closest value I could make up from resistors on hand.

This worked well to obtain the voltage values on the schematic is there anything I am missing with this approach?

Thanks,
Chas
 

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  • R5 to Q1 Chart.JPG
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chede

chede

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Joined 2016
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Well, there's also Epcos, Vishay, Kemet ... if you shop around a bit and are not in a hurry, you can find some amazing bargains, as these are now standard industrial items on power electronics like industrial SMPS or power inverters.

I paid 38 EUR for two boxes with 27 caps each of these, surplus ...


I've built the B+ power supply for my EL84PP tube amp by paralleling a few of this type of caps for each channel ...
 

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  • Box of Epcos DC-Link Caps.jpg
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Zen Mod

Zen Mod

Official Court Jester
Joined 2003
zenmod.in.rs/
Paid Member
chede said:
Well, there's also Epcos, Vishay, Kemet ... if you shop around a bit and are not in a hurry, you can find some amazing bargains, as these are now standard industrial items on power electronics like industrial SMPS or power inverters.

I paid 38 EUR for two boxes with 27 caps each of these, surplus ...


I've built the B+ power supply for my EL84PP tube amp by paralleling a few of this type of caps for each channel ...
Click to expand...

you, Lucky Predator Bstrd

:clown:
 
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