F5 Turbo is posted

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lhquam

Member
Joined 2010
Roots of the F5 Turbo

I just ran across this. Perhaps it has been mentioned before in this thread, but I have not seen it.

http://www.diyaudio.com/forums/soli...-interview-bjt-vs-mosfet-248.html#post1294203

Nelson says:
BTW, there is a pair of diodes - I also used a trick that I got from
Barney Oliver of using power diodes to shunt across emitter
resistance so as to limit the maximum voltage at high current.
It made the job easier for my (admittedly crude) bias circuit.
​
 
Pointed

Pointed

Member
Joined 2010
lhquam said:
In the F5 Turbo, would you say that the goal of the diodes was to increase transconductance by decreasing the effective source resistance, or to increase the output voltage swing at high currents, or something else?
Click to expand...

In user point of view, may I consider this diode trick as a kind of power saving solution for class A schematic ;)
We don't need to set highly idling current (Id) but we still can get large, intact output power out signal (without clipping at negative half cycle)

Not too hoot, not bad at all. Sorry, if may I wrong!
 
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WithTarragon

Member
Joined 2007
WithTarragon said:
On the original F5, the voltage gain was about 6 (or 15 dB). What is the voltage gain on the Turbo F5 versions 1, 2 & 3?
Click to expand...

Let me ask my question a different way.

The required votage input on the original F5 was about 2.4 volts to get the nominal 25 watts. Is the required input voltage the same for the Turbo version(s) (with nominal outputs of 50 to 100 watts)?
 
6L6

6L6

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Joined 2010
Paid Member
If you have the potentiometer, just put it in place of R2, and take the output from the wiper.

Try it, as it will take only a few min to try and you can hear for yourself of there is an adverse effect. (Which I bet, if there is, will be very minimal.)
 
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nyxflier

Member
Joined 2013
I've just sourced 4ea 2SJ74 and 2SK170 (hopefully reliably sourced..) - now looking for someone who's got experience with a working example replacing FQA12P20 and FQA19N20?

Of course, these are discontinued / obsolete but possibly still available (just like the JFETS). It might be possible to get the exact P/N but I think I'd rather substitute in this case. I've read here in this thread about some possible replacements but would still like to know if someone's got a life F5T running with them. I was planning on using the diy-store heatsink chassis and pcb - not sure what impact the substitution would have?
 
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nyxflier

Member
Joined 2013
I used the source 6L6 mentioned some time back - hoping they are still as reliable as he found them to be. Their service so far has been impeccable (fast process, cheap shipping, arrived well packed etc).

OK thanks - I'm going to build a tester. Mr NP seems to favor these JFET devices and I wouldn't mind having a 'screening' tool since I doubt this is my last amp. Do you know offhand what values I should be looking for? I'll post all my measurements and take any advice you have on:
Whether to use these parts.
Which ones to match up. It might be obvious but I'm not sure.
 
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AndrewT

R.I.P.
Joined 2004
Look at the 170/74 datasheets.
The graphs in there give a good guide to the Vp for different Idss.

Expect a low Idss of 6mA to give a Vp~500mV and a high Idss of 12mA to give a Vp~800mV.
Borbely and others give a formula for Vp & gm that should confirm you have the correct devices.
 
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