boyz
sometimes we have an obligation of being serious
Safety first as usual ?
Funny tnx ZM
One of the reasons I like the KOA Speer SPR / Kiwame carbon film resistors as even after all these years I can't think in stripes..... ;-)
Just enough experience to be dangerous.
Mostly rely on the great dudes on this site. Unusual, to say the least. Have done many amps etc. and this was a once in my lifetime, so far, occurrence.
I have to order parts for other projects along with a higher voltage transformer and store case for the VFET amp, so it will be awhile longer before I get to hear it's triodeness
Thanks to all for the entertainment.
Speaking of legs, safety or resistors out of spec?
Now more people are aware of the ICHIBAN !!!
Just a lurker here, nothing to see.
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Hi all,
Just finished building my Vfet amp but I am experiencing some problems. I don't seem to have had any problems with setting up the output stage being able to get 100mV across R32 together with 0mV across the output, the heatsinks getting nicely warm as one would expect with 1 amp flowing. The main problem is with the front end, whilst I can get 1.5 volts across both R5 and R6 the output voltage at T18 is over 200mV. I can get this voltage down to 0 but at the expense of totally imbalanced voltages across R5 and 6. Around 800mV across R5 and around 2.1 volts across R6. Furthermore, with voltages across R5 and R6 correctly set at 1.5 but still with over 200mV at T18 I tried connecting speakers and inputting some audio from my BA3 preamp. This was most disappointing with low gain and very distorted sound. This is the bit that concerns me the most.
A few differences from the original circuit, I used the Fairchild mosfets and used 2n5551/5401 instead of ZTX450/550 and 1N4007 for D1/2 instead of 1N4004.
All the resistor values when tested without power were the same or very close to those listed in the article.
This is the first time that I have experienced a problem when building amps so I am at a complete loss now as to what to try next so I would like to ask if anyone here can throw any light on this or make some useful suggestions.
Thanks, John
Just finished building my Vfet amp but I am experiencing some problems. I don't seem to have had any problems with setting up the output stage being able to get 100mV across R32 together with 0mV across the output, the heatsinks getting nicely warm as one would expect with 1 amp flowing. The main problem is with the front end, whilst I can get 1.5 volts across both R5 and R6 the output voltage at T18 is over 200mV. I can get this voltage down to 0 but at the expense of totally imbalanced voltages across R5 and 6. Around 800mV across R5 and around 2.1 volts across R6. Furthermore, with voltages across R5 and R6 correctly set at 1.5 but still with over 200mV at T18 I tried connecting speakers and inputting some audio from my BA3 preamp. This was most disappointing with low gain and very distorted sound. This is the bit that concerns me the most.
A few differences from the original circuit, I used the Fairchild mosfets and used 2n5551/5401 instead of ZTX450/550 and 1N4007 for D1/2 instead of 1N4004.
All the resistor values when tested without power were the same or very close to those listed in the article.
This is the first time that I have experienced a problem when building amps so I am at a complete loss now as to what to try next so I would like to ask if anyone here can throw any light on this or make some useful suggestions.
Thanks, John
There is almost the same, John!
I even was able to get to steady state my F6 with 0.1R source resistors and IRFP250, when I tested the circuit before putting in precious SemiSouth parts.
Now, I can't able to reach a "reasonably stable" DC offset with my Sony VFET.
(VFETs in their box yet.)
I even was able to get to steady state my F6 with 0.1R source resistors and IRFP250, when I tested the circuit before putting in precious SemiSouth parts.
Now, I can't able to reach a "reasonably stable" DC offset with my Sony VFET.
(VFETs in their box yet.)
Not sounding right even with dc offset of 200mv tells me there is a problem somewhere. Having high dc offset would not in itself make the amplifier sound bad. Try simple things first, bad solder joint, correct wiring etc. Is the problem on both channels or just one? Are the bjt's in correctly? The pinout could be different from what the boards say. Nelson used ZTX from the detective work of another member and the board is designed for it's pinout.
Are you running out of adjustment on your pots? If not you should achieve your voltage of 1.5V and low dc offset. It is very sensitive and a balancing act for sure. Multiple meters helps. I also had problems at my first try. With practice I learned how. I was not following Nelson's instructions precisely.
I think, this is the problem.
This does not change the voltage on R5/R6, it remains very near at 1.5V.
There are plenty of possibilities to change any of the trimpots settings, in any directions.
You may not have any problem. I thought I had problems and started changing parts and all I needed was more practice setting the front end bias and DC offset. My suggestion is to start over at a lower bias and zeroing the offset. I was unable to do it with just one meter. Using 2 meters with one on T18 I finally saw what was happening more clearly and then it became much easier. Practice here will make setting bias for the V-fets much easier.
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