F5 Turbo Builders Thread

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cubicincher

Member
Joined 2016
to thompsontechs #5763

Hello Thompsontechs,


I've built my F5T - Monoblocks circa 2 years ago with the IRF240PBF and
IRF9240PBF. I matched them carefully.

My F5T is running on a very high rail voltage +/- 42 V. I have double pair of JFETs and cascoding on the frontendboard.
I can't bias higher than 350mV over the bias resistors. These babies heat! :redhot:


I drive my electrostatic speakers (Audiostatic ES 300) with the F5Ts -
I love this combination!
It sounds very detailed and clear. But without any harshness.


IRF240PBF and IRF9240PBF are available at different suppliers. I have bought my last ones at MOUSER (few weeks ago).


Enjoy the sound! :D

Greets Dirk
 
JeffYoung

JeffYoung

Member
Joined 2017
Hi JT,

Cascoding is (primarily) to protect the front-end JFETs from high voltages. You can run a high bias without high rail voltages (for a larger class A envelope) and not need cascoding. You can also run a moderate bias with high rail voltages (for higher class AB power delivery) and the cascodes would then be advisable.

Granted, most of the time people do both at the same time (for more class A power delivery), so they are related in that respect.

Cheers,
Jeff.
 
Sangram

Sangram

Moderator
Joined 2002
Paid Member
bambadoo said:
Whats the "breaking point" for this F5T V2 regarding cascoding or not?
Initially planned running with a CRC (diyaudiostore) and 2*24VAC secondaries transformers.
Click to expand...

25V rails + ~4v on the mosfets, so anything over 29V rails needs cascodes.

However on first run entire rail voltage appears across JFET (if you've followed the instructions properly), so the F5 (non-T) is running them at maximum capacity already.

The 2SJ74 parts have a breakdown of 25V, and Nelson has found that the specs are conservative.

The parts are also very expensive and rare. Whether you need cascodes or not comes down to your risk appetite, depth of pocket and availability of many spare sets - of everything, as a failure of a JFET is usually catastrophic.

Oh, and the Toshiba parts enter a so-called 'low-noise' region below 12V D-S, so I don't see the benefit of running them at higher rails. They only need to swing a few volts anyway and have very low self-bias voltages, so even 8-10V will get you excellent results.

Running them at low D-S voltages also means you can run higher currents through them with impunity - and you can lower source resistances appropriately.
 
BigE

BigE

Member
Joined 2008
My cascode failed but the jfets actually survived 40V rails. I cannot guarantee all jfets are so robust.

Therefore, it is advisable to check if your cascode is actually working before you insert the Jfets.

And as Nelson suggested, consider putting a 100 Ohm resistor in series with the Jfet drains to avoid oscillation.
 
thompsontechs

thompsontechs

Member
Joined 2019
JeffYoung said:
Hi JT,

Cascoding is (primarily) to protect the front-end JFETs from high voltages. You can run a high bias without high rail voltages (for a larger class A envelope) and not need cascoding. You can also run a moderate bias with high rail voltages (for higher class AB power delivery) and the cascodes would then be advisable.

Granted, most of the time people do both at the same time (for more class A power delivery), so they are related in that respect.

Cheers,
Jeff.
Click to expand...


Hmm, well V2 will have 32v rails, and the build guide, to me, read like cascode wasn't needed at that point, but if the fets are rated at 25v and I think Papa said those particular ones failed about 40v. so it looks like if I do not cascode, I am operating out of spec of the fet, but if teh don't live up to the 40v, or at least 32v, I will have an issue which could take out I don't know what.



The boards are mounted an all is ready, so it's more of a pain in the *** to go back and do it now, but I'm leaning toward doing that very thing. This amp, if/when it works properly will be a gift for a dear friend for Christmas. That said, I would like to have something very stable.



What would you do Jeff?


JT
 
thompsontechs

thompsontechs

Member
Joined 2019
sangram said:
25V rails + ~4v on the mosfets, so anything over 29V rails needs cascodes.

However on first run entire rail voltage appears across JFET (if you've followed the instructions properly), so the F5 (non-T) is running them at maximum capacity already.

The 2SJ74 parts have a breakdown of 25V, and Nelson has found that the specs are conservative.

The parts are also very expensive and rare. Whether you need cascodes or not comes down to your risk appetite, depth of pocket and availability of many spare sets - of everything, as a failure of a JFET is usually catastrophic.

Oh, and the Toshiba parts enter a so-called 'low-noise' region below 12V D-S, so I don't see the benefit of running them at higher rails. They only need to swing a few volts anyway and have very low self-bias voltages, so even 8-10V will get you excellent results.

Running them at low D-S voltages also means you can run higher currents through them with impunity - and you can lower source resistances appropriately.
Click to expand...


Most of that did a crop duster on me, but the part about needed cascode with rails over 24v... it didn't read that way in the guide, but I will be running 32 with V2. I guess I need to think about pulling it back apart and adding the cascode.
 
thompsontechs

thompsontechs

Member
Joined 2019
@ takitaj, thanks for the link have downloaded and printed to read.


@ AudioSan, I, in fact, pulled the first board out and have the parts list for cascode.


@ Anyone, Looking at the FE board I need to fill R26-30, remove jumpers on Q7 & Q 8 install 2SC4793 & 2SA1837 and C5, C6 with the 10uf 100v caps. Have I missed anything?



The guide: I in no way want to imply the guide lead me off path. Without 6L6, I probably would not have taken on this project and I don't want to appear I'm blaming him for my lack of knowledge, or ability to interpret information properly. Thanks for your work 6.


JT
 
B

bambadoo

Member
Joined 2003
From the "Illustrated build guide" that is linked to from diyaudiostore.
Please remember that this build is showing the cascode circuit being used. It is not strictly necessary for 32V rails, but is required if you choose to use higher voltages. If building a v2, I would suggest not cascoding, simpler is better.
Click to expand...
Think I have some 2sa1837/2sc4793 at hand anyway.
Q1.1 and Q2.2 are those not to be used?
 
thompsontechs

thompsontechs

Member
Joined 2019
bambadoo said:
From the "Illustrated build guide" that is linked to from diyaudiostore.

Think I have some 2sa1837/2sc4793 at hand anyway.
Q1.1 and Q2.2 are those not to be used?
Click to expand...


From the F5T build thread: The Q1.1 and Q2.1 position are left empty, they are there if you are going to make a huge F5T with more than 4 pair of output transistors, so you can parallel the Jfets and have more driver current."




JT
 
thompsontechs

thompsontechs

Member
Joined 2019
BigE said:
My cascode failed but the jfets actually survived 40V rails. I cannot guarantee all jfets are so robust.

Therefore, it is advisable to check if your cascode is actually working before you insert the Jfets.

And as Nelson suggested, consider putting a 100ohm resistor in series with the Jfet drains to avoid oscillation.
Click to expand...


That's a good point, what would be a good test procedure to make sure the cascode is working correctly.



Also, if we were to add the 100R, where would we put it on the V3 board?



JT
 
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