Hallo to All FetZillas.
Thanks to Hugh for quick reply to my Question.
I've send these moment a E-Mail to Patrick in Belgium.
I'm curious about it how good it Sound if i finnished my diy construction.
If the Amp is running i will send my personally impressions.
Many Thanks for Answering and Possibility to buy some Boards.
Best Regards Hagen. Bangla H.
Thanks to Hugh for quick reply to my Question.
I've send these moment a E-Mail to Patrick in Belgium.
I'm curious about it how good it Sound if i finnished my diy construction.
If the Amp is running i will send my personally impressions.
Many Thanks for Answering and Possibility to buy some Boards.
Best Regards Hagen. Bangla H.
Well, thank you kindly for the answer!
My question was prompted by the observation that most other schematics generally seem to stick with one value of gate stopper across the board, and the value seems fairly arbitrary. I have no theoretical background to hang this observation on though, so until now it's been one of those things I just accepted. I clearly need to find some sort of reading to explain why the stopper prevents oscillation, and how the best value is derived...
If I may ask a couple of other questions about the Fetzy component choices?
LED2 is spec'd as a 3mm Grn, yet the lead pitch is rather larger than is the norm for a 3mm. Whats the go here? and is a green needed specifically - or just any LED with a larger voltage drop?
C10 is spec'd as a 100nF 160V MKP - any reason not to use a 100nF 63V MKT? Or have heaps of greencaps in the right size and value...
Silver Mica Caps - any serious reason not to use ceramics? I have heaps on hand is all...
Thanks for your help by the way - there does appear now to be a rather large number of Fetzilla threads that may well contain the answers I'm looking for, so your help does indeed speed things up somewhat!
My question was prompted by the observation that most other schematics generally seem to stick with one value of gate stopper across the board, and the value seems fairly arbitrary. I have no theoretical background to hang this observation on though, so until now it's been one of those things I just accepted. I clearly need to find some sort of reading to explain why the stopper prevents oscillation, and how the best value is derived...
If I may ask a couple of other questions about the Fetzy component choices?
LED2 is spec'd as a 3mm Grn, yet the lead pitch is rather larger than is the norm for a 3mm. Whats the go here? and is a green needed specifically - or just any LED with a larger voltage drop?
C10 is spec'd as a 100nF 160V MKP - any reason not to use a 100nF 63V MKT? Or have heaps of greencaps in the right size and value...
Silver Mica Caps - any serious reason not to use ceramics? I have heaps on hand is all...
Thanks for your help by the way - there does appear now to be a rather large number of Fetzilla threads that may well contain the answers I'm looking for, so your help does indeed speed things up somewhat!
No problem.
The size of the gate stopper, particularly with AB amplifiers, affects the bass from the amp. Some asymmetry is a refinement, but not actually essential. Pass uses 220R on the hexfets, but laterals are a bit more sensitive in my experience.
5mm or 3mm LED is just fine. The original primitive for Protel was made for 5mm designs some years ago, so it's a design artefact. Any LED is specifically green, as it will have at 1.95V drop at 3.5mA. This is needed for this bias generator.
C10 should not be less than 100V, since the peak voltage from a 36V rail is around 32V. Double this to 64V and this is on the limit. You could use a 80V 100nF filmcap but preferably use a 100V or more that suits the pin space of 7.5mm.
Ceramics have poor sonics for some reason. I also suggest that silver micas be used here, this is the MOST critical cap in the circuit and determines the Bode-Nyquist stability AND the sonics.
Good luck with your FetZilla. It is a lovely amp which punches well over it's weight.........
Cheers,
Hugh
The size of the gate stopper, particularly with AB amplifiers, affects the bass from the amp. Some asymmetry is a refinement, but not actually essential. Pass uses 220R on the hexfets, but laterals are a bit more sensitive in my experience.
5mm or 3mm LED is just fine. The original primitive for Protel was made for 5mm designs some years ago, so it's a design artefact. Any LED is specifically green, as it will have at 1.95V drop at 3.5mA. This is needed for this bias generator.
C10 should not be less than 100V, since the peak voltage from a 36V rail is around 32V. Double this to 64V and this is on the limit. You could use a 80V 100nF filmcap but preferably use a 100V or more that suits the pin space of 7.5mm.
Ceramics have poor sonics for some reason. I also suggest that silver micas be used here, this is the MOST critical cap in the circuit and determines the Bode-Nyquist stability AND the sonics.
Good luck with your FetZilla. It is a lovely amp which punches well over it's weight.........
Cheers,
Hugh
Nice one! Thanks again for the very informative reply.
Sounds like the greencap is in and the ceramics are out. I'm doing a few odd things with my build in that I'll be using a smps and a bf862 jfet purely in the name of experimentation.
It seems like not many folk have documented the process so I'll be putting up a few pics as I go.
Thanks again!
Sounds like the greencap is in and the ceramics are out. I'm doing a few odd things with my build in that I'll be using a smps and a bf862 jfet purely in the name of experimentation.
It seems like not many folk have documented the process so I'll be putting up a few pics as I go.
Thanks again!
Anyone with better knowledge than me know if we should decrease R2 and/or R6 if we use BF862 for T1? Most designs feature a 470 Ohm grid stopper when using the BF862. However, instinctively i sense this may alter the way P2 adjusts offset.
And finally, how would one go about to increase current(slightly) through T1? Decreasing R8 seems the logical choice, but what about R9 etc? Is it possible just by inserting one trimmer somewhere?
Thanks for your consideration.
And finally, how would one go about to increase current(slightly) through T1? Decreasing R8 seems the logical choice, but what about R9 etc? Is it possible just by inserting one trimmer somewhere?
Thanks for your consideration.
Hi Jacuza, you naughty Japanese crim fellow,
You are right. 470R would be a great idea. Ensure the Idss is around 8mA for this application. It won't affect the bias as the gate drive is around 10nA.
T1 should be set up to give roughtly 2/3 of the Idss from the datasheet. This is a very good drain/source current in this role.
Could you please give the schemat you are using? I want to be sure I have the correct designations.......
Cheers,
Hugh
You are right. 470R would be a great idea. Ensure the Idss is around 8mA for this application. It won't affect the bias as the gate drive is around 10nA.
T1 should be set up to give roughtly 2/3 of the Idss from the datasheet. This is a very good drain/source current in this role.
Could you please give the schemat you are using? I want to be sure I have the correct designations.......
Cheers,
Hugh
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