F5Turbo Illustrated Build Guide

[joesch1999]

I'll suggest that you read Jim's instructions on how to make the initial adjustments, be patient with it, it takes time to bring it up to steady state or thermal equilibrium. You do best if you have 3 volt meters, one for each board (N and P) and one to check DC offset at the speaker output. It's exciting to see the amp working but you really have to take the time to make sure all is good so all the work pays off. Don't connect anything to it, you could ruin your speakers. Build a light bulb cord, do the grounding isolation for your power supply. Do not mix the audio/signal ground with the chassis ground.

It’s not supposed to show any sort of millivolts? Always zero on both n and p?

 

[Keremito]

Don't change them. 47.5 ohm is too low a value for gatestoppers on this amp, I suggest closer to 200, and have used as much as 680 with no issues. Keep the 100ohm in place.

Hello Jim, could explain why 47.5 ohms is too low of a value for the gate stoppers? Too big of load for the FE board?
I have 47.5 in my amplifier, the FE boards were built to drive V3 monoblocks, I am using 2 pairs of cascoded jfets per channel but in a V2 stereo configuration.
Any disadvantages on this set up?
Thanks

 

[joesch1999]

Help please

Don't change them. 47.5 ohm is too low a value for gatestoppers on this amp, I suggest closer to 200, and have used as much as 680 with no issues. Keep the 100ohm in place.




Single-ended = RCA jacks, Balanced = XLR

The center of the RCA should go to the "in" pad on the front-end board, the outside of the RCA attached to the "G" next to "in", the metal of the RCA should not touch the chassis, and the "G" and "Link" should be connected as in the above photo.

Ok got the amp up and running had the bias up to 200 my both n&p then I think one of the so called matched j170s went out, I got them from a place in China 2 years ago when I first planned to build the amp, will the A rated jfets they sell in the diy store work? They are out of matched pairs

 

[AndrewT]

High A grade are the same as low B grade.

So order up a batch of A grade and measure the Idss. Select the highest Idss and use them in your F5.

Where do you live?
Put some background info into your profile. (nothing personal that criminals can use against you).

 

[joesch1999]

High A grade are the same as low B grade.

So order up a batch of A grade and measure the Idss. Select the highest Idss and use them in your F5.

Where do you live?
Put some background info into your profile. (nothing personal that criminals can use against you).

Please explain how to on the idss, I live in southern illinois

 

[6L6]

Don't order bulk from the store to try to match. You won't get good mates picking from K170 loose bags and J74 loose bags. Trust me, I've handled essentially every Jfet in the store.

Order a new Matched pair for both channels, if you got your Jfets in China, the chance that they are genuine is quite slim.

This is a trusted seller Toshiba 2SJ74 + 2SK170 MATCHED PAIR 0.03mA AND 4mV 6.5mA-7.9mA Idss Range | eBay

 

[6L6]

Hello Jim, could explain why 47.5 ohms is too low of a value for the gate stoppers? Too big of load for the FE board?
I have 47.5 in my amplifier, the FE boards were built to drive V3 monoblocks, I am using 2 pairs of cascoded jfets per channel but in a V2 stereo configuration.
Any disadvantages on this set up?

The gatestoppers work essentially like a shock absorber at high frequency (way above audio) and damp the Mosfet from oscillation. The F5T is a pretty darn high bandwidth design and depending on a ton of variables it can oscillate. Higher value gatestoppers help damp that tendency.

BUT, if your amp works, don't worry about it. If it was going to break out in oscillation it would already have done it and you'd know because your Mosfet would be fried.

 

[joesch1999]

Thanks

Don't order bulk from the store to try to match. You won't get good mates picking from K170 loose bags and J74 loose bags. Trust me, I've handled essentially every Jfet in the store.

Order a new Matched pair for both channels, if you got your Jfets in China, the chance that they are genuine is quite slim.

This is a trusted seller Toshiba 2SJ74 + 2SK170 MATCHED PAIR 0.03mA AND 4mV 6.5mA-7.9mA Idss Range | eBay

Seems he sells them for 45 too 😂 I ordered from him at 45 for the matched set before you showed them for 30

 

[joesch1999]

Ne

Never mind I got the matched quad for 45 from him! Good buy, I just wanted to say thank you for your patience with nebs like myself, been a great learning experience.

 

[6L6]

I think the $45 are a different Idss range. Will still work wonderfully

 

[joesch1999]

Millivolts across source resistor

I think the $45 are a different Idss range. Will still work wonderfully

Is 390 mv the target?

 

[alexkosha]

Hi Jim, can you please clarify about 47.5R gatesstoper resistors. Are you gays discussed gate resistors for JFET or 475R for MOSFETs? Thank you.

 

[joesch1999]

Hi Jim, can you please clarify about 47.5R gatesstoper resistors. Are you gays discussed gate resistors for JFET or 475R for MOSFETs? Thank you.

We were discussing mosfets, Jim suggests 100 to 200 ohm 47.5 is to low.

 

[alexkosha]

Even for V3 monoblock version and NOT cascaded JFETs?

 

[6L6]

Alex - has your amp blown up? If not, don't worry about it.

 

[alexkosha]

Alex - has your amp blown up? If not, don't worry about it.

Not yet.... It is behaving very nice, so far.
No oscillations observed, but I use Toshiba MOSFETs. Could not find BOM advised Fairchild made MOSFETs, back then.
Can you please describe again (or direct me to the relevant thread) all confederations behind usage of cascoded JFETs?

 

[dewdrop]

And better yet, mount the FE on the back wall to get the input wiring really short and free up floorspace for everything else. Similar to this -

Hi Jim,
Is it worthwhile to use the Clarity Caps for better audio performance?

 

[6L6]

I couldn’t tell you... I put a soldering iron into one of those and killed it, and had to remove the pair and replace it with something else....

Generally speaking, coupling caps are not anywhere near as important as many people make than out to be. Use what you have. Experiment with different caps, it’s fun! And remember that expectation bias is a real thing.

 

[dewdrop]

I couldn’t tell you... I put a soldering iron into one of those and killed it, and had to remove the pair and replace it with something else....

Generally speaking, coupling caps are not anywhere near as important as many people make than out to be. Use what you have. Experiment with different caps, it’s fun! And remember that expectation bias is a real thing.

LOL - thanks for the advice. I am just thinking that soldering and desoldering will finally rip one of the pads up and that's it, end of the road for the PCB.

 

[dewdrop]

Don't change them. 47.5 ohm is too low a value for gatestoppers on this amp, I suggest closer to 200, and have used as much as 680 with no issues. Keep the 100ohm in place.




Single-ended = RCA jacks, Balanced = XLR

The center of the RCA should go to the "in" pad on the front-end board, the outside of the RCA attached to the "G" next to "in", the metal of the RCA should not touch the chassis, and the "G" and "Link" should be connected as in the above photo.

Hi Jim,
The 47.5 ohm resistors which you are saying its values are too low, are they R13, R14, R15 and R16? These are recommended in the F5T BOM list.

Reason for asking is I can't afford to replace the rare Toshiba MOSFETs.