F4 power amplifier

[albertNL]

If you are using GR Jfets, you can get a little more bias on them by reducing
R3 and R4 to 1 ohm instead of 10. Actually, you can just short R3 and R4,
but 1 ohm lets you sense the current if you want.

😎

Thanks Nelson, I will try to get this done next week. Hope to hear some magic when it is done. Albert

 

[Triodethom]

If you are using GR Jfets, you can get a little more bias on them by reducing
R3 and R4 to 1 ohm instead of 10. Actually, you can just short R3 and R4,
but 1 ohm lets you sense the current if you want.

😎

So 22.1 ohm originally used is for idss in the 9 to 10 ma range is that correct ?

 

[Marra]

Would a preamp with 12db of gain give enough output from the F4 to drive speakers of 90db efficiency?

 

[Zen Mod]

2Vrms will give 8Vrms

that's 8W/8R

99db

 

[6L6]

If you are not a devotee of Black Sabbath at continuous concert levels, that's more than enough. 🙂

 

[Marra]

Thanks guys.
I think that I will have to try one of these.My valve pre sounds really good with the Aleph J but the volume pot barely gets more than a few clicks to reach loud.

 

[albertNL]

Hi guys,

It all took a bit longer than I had hoped, but I have some more information.

As I explained I have two identical F4's. One is at my home, the other back at my friend who built them for me.
I was pretty sure that the JFets used were GR grade so I followed up on Nelson's advice and did some quick and dirty soldering: I added 1R2 resistors parallel to the original 10R (R3 & R4) to give about 1R overall but this has not helped the sound.

I only just re-read a lot of posts in this thread and had not realised I should have measured the voltage drop across R3/R4.

My friend just emailed that he has checked the JFets in the second F4 and has confirmed the grade is GR. I have asked him to measure the voltage drop across R3 and R4 (with original 10R in place) and hope to get an answer in the next day or so.

In order to decide what I should do, I have a few questions:

1. What can I expect with GR grade JFets?
2. Can the GR supply the current the circuit requires?
3. Is the sound going to be as good as with BL (i.e. is it just a matter of finding the right resistance value)?
4. Or is the BL better suited and would replacing the JFets be the best option?

I am afraid I have a similar problem coming up, my F5.
When I ordered kits from tech-diy for the 2 F4's I included an F5 kit. My friend is still working on the F5 but has checked and confirms the F5 kit also contained GR grade JFets.

Same questions as above for the F5.
5. Should I try to get it to work with the GR's or is replacement with BL the best option?

Your help is appreciated.

Thanks,

Albert

 

[6L6]

F4 - with the change in the source resistors the GR should work fine. I would suspect something else, likely bias. Is the F4 to proper bias? Also, what preamp are you driving the F4 with?

F5 - GR are no problem, (My F5 uses them) just increase resistance in series with the bias pots until they bias properly.

 

[albertNL]

F4 - with the change in the source resistors the GR should work fine. I would suspect something else, likely bias. Is the F4 to proper bias? Also, what preamp are you driving the F4 with?

F5 - GR are no problem, (My F5 uses them) just increase resistance in series with the bias pots until they bias properly.

My friend biased the F4's to 250mv. DC offset is close to 0. The sinks get hot but not too hot.

I don't think the preamp is the problem.
In my study I listened to one of the F4's using an old Yamaha CX-1000 preamp (I hope to replace it with a DIY Aleph P1.7 some day).
My brother tried the other F4 using a Metaxas preamp (using >95dB Lowther based speakers).
When I compare the sound of the F4 to a diy Aleph J (same source, same Yamaha CX-1000, same old speakers) it sounds boxed-in, no midrange and harsh. Plenty of bass but even that does not sound right.
My brother found that the soundstage collapsed (compared to the Metaxas power amps he normally uses): He could hear speakers rather than a soundstage. Mid-fi at best. Not at all what it should sound like.

 

[Zen Mod]

whatever you do , F4 will transfer character of preceding stage , adding 2% of it's own

so - same preamp is idling with normal gain amps , while screaming full throttle , while feeding F4 ; hardly same conditions

take proper and muscular (swing wise) preamp stage* , then you'll hear that you can't really hear F4 itself

*BA3 , Impasse , , few else

 

[6L6]

whatever you do , F4 will transfer character of preceding stage (your preamp) , adding 2% of it's own

This can be an issue with the F4 - it's practically invisible. Really. It sounds like the preamp driving it.

The Aleph J sounds good because it 1) needs little or no gain from your preamp, and 2) is an amp full of pretty and gorgeous sounds, although perhaps not the most accurate thing in the world, it always sounds fantastic.

 

[albertNL]

It is too bad I cannot let you hear it. This is not a preamp issue. Something is wrong with my F4 and I want to find out how to fix it.

 

[Zen Mod]

pics

 

[albertNL]

pics

These are all I have at the moment.

 

[Zen Mod]

IEC filter is sometimes bad thing
worth trying without

 

[cviller]

Have you double checked the bias - on all mosfets?

 

[biju55]

IEC filter is sometimes bad thing
worth trying without

What is the bad thing with the IEC filter ? . It will influence the dynamic range of sound ?

 

[Zen Mod]

yup

 

[Triodethom]

yup

I have found that if it bigger say 30 amp the dynamic are far less compressed .

 

[Itsmee]

In the second picture, it looks like the middle two mounting screws could be shorting the output track, I would recommend fitting some nylon washers under the screws.