F6 Amplifier

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Semisouthfan

Member
Joined 2012
hkminn said:
Hi,

Problem.
After playing music for several hours suddenly one channel distorted and showed about 500mv of DC offset. One of the Jfets was almost turned off, and the other one was conducting more than initial bias current. Heat sink fins were about 55 C according to the "skin test". Amp works normally again after cooling it down and the problem has not shown up again yet.

Since I am not using the same components as in original F6 I have some concerns.
1. transformer with higher winding resistance(150ohm).
2. single-end buffer(2x2ks170BL like in B1) that might have higher output impedance than push-pull buffer in original F6.

Do you think this problem might be related to the gate leakage issue?
Or is it 'Thermal runaway' that Semishouthfan mentioned in previous post?

Thank you.

By the way, this Amp sounds too good to give up!
This is my second Firstwatt clone project. I thought the first one(BA3+F4) sounded really good but when I listened to F6 I liked it more by a big margin.
Click to expand...

I haven't built an F6 (thinkin about it, but transformers, ugh!) so I don't have direct experience. But my many hours playing around with the F2J produced instances like yours which I related to gate resistor loading at temperature, perhaps helped by a particularly significant thermal event from the ac modulation. That experience is what I based my thermal runaway observations on. The fact that when it cooled down it went back to where it was is a pretty good hint.

Personally, I find it hard to believe that R7 and R8 are large enough to ballast the DC bias of the JFET into cutoff. R3/P1/R5 or R4/P2/R6 are the culprits if any are to be found. R7 or R8 damp feedback through C1 and C2 resonating with the leakage inductance of the transformer windings. Those are in an ac loop, not a dc loop.
 
generg

generg

Member
Joined 2009
Paid Member
hkminn said:
Yes, they are bolted to heatsink with mica and greese.
Jfet pins are not soldered yet. I am using molex sockets temporarily.

You mean, R5 or R7?
Click to expand...

I recommend to measure nevertheless the temperature of the SS cases or nearby on the heatsink. Possibly your DMM can do it.

I often had differences of over 10 degrees although I had the impression to have screwed them all the same. Doing it again often solved the problem.

But I never had the effect you describe neither with my three J2 clones nor with two different F6 and my friend building the same amps also never spoke of such a problem.

So I do not really know.....interesting that Mike knows this effect too....

Mike you should better screw your SSs.....:):)::D:D
 
generg

generg

Member
Joined 2009
Paid Member
flocchini said:
Semisouthfan said:
I haven't built an F6 (thinkin about it, but transformers, ugh!) so I don't have direct experience.

Mike

You have to hear a F6 !!!! I think it comes from another world :D

Best

Bob
Click to expand...

Yes it sounds from very far away.....:D:D

Once accustomed to the sound the F6 produces, it is great, for me it was a long journey.....

And I prefer the Jensen over the Cinemag, because with the latter it sounds like coming from this world.

It seems the sound of the F6 is less the sound of the J-FETs and the SS but the sound of the x-former.
Click to expand...
 
Nelson Pass

Nelson Pass

The one and only
Joined 2001
Paid Member
hkminn said:
Do you think this problem might be related to the gate leakage issue? Or is it 'Thermal runaway' that Semishouthfan mentioned in previous post?
Click to expand...

Not very likely the transformer. Somebody is getting hot and misbehaving. You
could try some method of cooling individual components (like freeze spray) and
see if you can localize it.

:cool:
 
H

hkminn

Member
Joined 2007
generg said:
I recommend to measure nevertheless the temperature of the SS cases or nearby on the heatsink. Possibly your DMM can do it.

I often had differences of over 10 degrees although I had the impression to have screwed them all the same. Doing it again often solved the problem.

But I never had the effect you describe neither with my three J2 clones nor with two different F6 and my friend building the same amps also never spoke of such a problem.

So I do not really know.....interesting that Mike knows this effect too....

Mike you should better screw your SSs.....:):)::D:D
Click to expand...

Thank you for your suggestions. I 'skin test'ed Jfets and could put my fingers on them directly for more than a couple of seconds. I think they are within a reasonable range. I paid extra caution when I put them on. I even sliced mica to make it thinner.

I am sure you would not have any problem if you use the same component as in the original schematics. But I happen to like the sound of particular transformer and am trying to adopt it into a carefully calibrated circuit. The problem is self-generated by deviating from a well paved road.

Thank you
 
H

hkminn

Member
Joined 2007
An interesting observation!

As Semisouthfan and Nelson suspected I am observing a heat related phenomenon. Trying to replicate the problem I stuck the Amp into a rack with not much space for cooling. I have not had the same problem yet, but DC offset creeped up several mili-volts toward negative side on one channel during about an hour without playing music. When I pulled it out of the rack and opened top cover the DC offset came back to zero within 10 minutes.

When I pulled it out from rack I checked the temperature of Jfets the first thing. I could put my fingers for more than a couple of seconds on all jfets.

I will play around during weekend and will report back.

Thank you.
 
H

hkminn

Member
Joined 2007
Semisouthfan said:
I haven't built an F6 (thinkin about it, but transformers, ugh!) so I don't have direct experience. But my many hours playing around with the F2J produced instances like yours which I related to gate resistor loading at temperature, perhaps helped by a particularly significant thermal event from the ac modulation. That experience is what I based my thermal runaway observations on. The fact that when it cooled down it went back to where it was is a pretty good hint.

Personally, I find it hard to believe that R7 and R8 are large enough to ballast the DC bias of the JFET into cutoff. R3/P1/R5 or R4/P2/R6 are the culprits if any are to be found. R7 or R8 damp feedback through C1 and C2 resonating with the leakage inductance of the transformer windings. Those are in an ac loop, not a dc loop.
Click to expand...

Hi Semisouthfan,

Do you remember how hot the Jfets were in your F2J when it had thermal runaway? Mine happened when it was not very hot. I could still touch the Jfets without burning my fingers right after it happened. Well, the feed-back resister R1 was really hot due to the offset I quess.

Would you please explain more about "thermal event from the AC m odulation" for me? The problem has not replicated yet without AC signals. I will fix up some dummy load and will test with AC signals during weekend.

Do you think lowering R5 and R6 will help? At least to compensate the differences btw winding resistances of Jensen and Lundahl?

Thank you.
 
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