Hi Dennis,
That is the first thing I would try, it is a simple amp so reflowing won't be hard. I did have a similar experience with my F6 after a couple weeks after completion. First one channel went out, just started buzzing after I turned the amp on. I rebuild the entire channel, but in the end it was the Jfets that went bad. At first I blamed it on a tube preamp spiking the F6 on startup so I changed to a SS pre and was very careful to let the pre settle well before turning on the F6. Within hours of restarting the other unrebuilt channel went out the same way. This time I just replaced the jfets and all was well. At this point I suspect that the problem was that the jfets had quality control issues and suffered infant mortality. The amp has been back in constant service for several months now without any problems.
PJN
Note for friends; if you guys use valve preamp, first turn pre on, 10 seconds later, turn F6 on. First turn F6 of then valve pre. Till to learn, i lost jfets one time.
No one has mentioned a stumble in the BOM re: transistors: Q1,2 are correct, then Q2 and Q3 are listed. Indications are that the JFET pair should be Q3 (J170) and Q4 (J74)... Yes?
Yes
The Jfets are Q3, Q4.
If in doubt follow the schematic.
If creating an order consider using
LM329DZ. 6.9V As Zenners with
R7, R8 3k3 resistors.
Read through the build guide for the theory.
The Jfets are Q3, Q4.
If in doubt follow the schematic.
If creating an order consider using
LM329DZ. 6.9V As Zenners with
R7, R8 3k3 resistors.
Read through the build guide for the theory.
A diy shop item.
Certainly Not inexpensive, but Matched pairs and the easiest path to the parts.
Just Pay , then receive 😉
Oh yes, I have read twice this entire thread plus reviews and all that Nelson Pass has posted about this amp. That's just basic. And yes, holding the multi layer PCB up to the light and analyzing/translating the schematic to the traces, it appears as though what is marked Q3 on the board is intended for the J170... I just like everything to fall into confirmed alignment before I solder stuff in place. Having the BOM match the board is that confirmation I was hoping for but right now it doesn't...so I ask: Q3 is for J170?
From the 1st photo on the 1st page of this thread, you should make sure that the drain of the transistor at hand is connected to the resistor labeled R7 - if so then that transistor is a J170. Why? Because the drain of the other transistor, Q4, is connected to the zener, Z2.
Double check with the schematic and a multimeter if you have to. That's the only real way.
Best,
Anand.
there is no part there labeled as J170
JFets , as most other parts are having clear names - here Q3 being 2SK170BL and Q4 being 2SJ74BL
JFets , as most other parts are having clear names - here Q3 being 2SK170BL and Q4 being 2SJ74BL
nycavsr2000, your insight is a kindness. I have my confirmation. 🙂
Zen, you are so right. May I meekly suggest that the BOM be corrected to reflect that Q3 Is in fact: 2SK170BL (and add Q4) ?
Zen, you are so right. May I meekly suggest that the BOM be corrected to reflect that Q3 Is in fact: 2SK170BL (and add Q4) ?
Bit of a silly question: Is it possible for the turn-on/off thump of the preamp
to be so large and lasting so long to actually damage the input jfets,
even when the proper sequencing is used? Has anyone heard of instances
like that?
Thanks,
Dennis
if that is the case , something is seriously wrong with preamp
F6 being Off or On , irrelevant - that's only question - will spike from faulty preamp kill just input JFets on F6 , or speakers too .......
F6 being Off or On , irrelevant - that's only question - will spike from faulty preamp kill just input JFets on F6 , or speakers too .......
Speaking of seriously wrong...
I got my PSU working fine (25.6VDC with no load) checked my meter, checked the pot for P2 and checked every part for value as I put them in but I can get no reading for Bias.
The only thing that didn't really sink in when reading this thread is the Z1, Z2 discussion. I selected 1N4734 as a 5.6V zener, everything else is by the BOM.
Any ideas about my complete lack of bias?😕
I got my PSU working fine (25.6VDC with no load) checked my meter, checked the pot for P2 and checked every part for value as I put them in but I can get no reading for Bias.
The only thing that didn't really sink in when reading this thread is the Z1, Z2 discussion. I selected 1N4734 as a 5.6V zener, everything else is by the BOM.
Any ideas about my complete lack of bias?😕
for start , try to disconnect output mosfets ; if nothing else , lift one end of source and gate resistors
then , measure what voltage you have across zenners
if that's in ballpark , check what voltage you have between bottom pin and wiper of bias pots ,varying it with screwdriver
if you can set anything between 0 and zener voltage (ignore polarity for that) , you're good ..... and then we can proceed further
then , measure what voltage you have across zenners
if that's in ballpark , check what voltage you have between bottom pin and wiper of bias pots ,varying it with screwdriver
if you can set anything between 0 and zener voltage (ignore polarity for that) , you're good ..... and then we can proceed further
Well I did as instructed got the expected 5.4 V across the Zeaner and checked from the low pin to the wiper of P2 for voltage : I got 3 V and I was able to just at the 3.2 V I don't know if that's the right kinds of values...
My apologies for the delay in response I'm having massive computer problems had to switch to my phone 😑
My apologies for the delay in response I'm having massive computer problems had to switch to my phone 😑
you must be able to set voltage from 0V to Zener voltage , with probes between bottom pin and wiper
it's same for P1 and P2
if not , something is wrong
post pictures
it's same for P1 and P2
if not , something is wrong
post pictures