troubleshooting: preamp tube voltages

[neikeel]

If you look at the NFB resistor on the schematic it is hooked up to a PT secondary tap - which is continuous and goes to ground at the speaker jack. It also has a connection at the other end to the presence pot which is the tail of the PI and ultimately goes to ground so you will get a 'short'.

[Elad E]

I guessed the NFB must send the oscillating harmonics somewhere like a tone pot (low-pass filter) sends the highs to ground. it's just that I wasn't sure the impedance selector terminals also go to ground. I wish I could solidify my understanding of the circuit - turns out building one doesn't help much in that department.

anyway, I worked on the board a little a bit today - checked continuity on all jumpers and under board wiring, filled all the turrets with solder (I didn't fill them initially - only enough for a solid mechanical connection), replaced the green output tube grid wire, shortened some wires such as the yellow wire from pin 6 of V4 to the board and the purple wire from the 16ohm tap to the NFB resistor and changed the purple wires to blue (for that extra bluesy tone - doh!).

I also reversed the 47Pf silver mica cap between the 82K and 100K resistors as I saw that even though it doesn't seem to be directional no one oriented it like I did which is between the resistors instead of facing outward of the board with the values pointing upward (I didn't like the overhang so I soldered it pointing inside as can be seen in my previous pics).

will test it again tomorrow morning, wish me luck!

[Elad E]

I restarted the testing and on the first phase, when checking the voltage across pins 2 and 8 of the rectifier socket (with no tube inside, on Standby) the instructions mention a voltage above 5V AC while I'm getting a voltage reading that keeps on jumping between 4 and 5 Volts.
is that normal? could it be bad solder joints on the terminals or the PT may be shoot?

[Elad E]

ok, some news:

checked the voltages with all tubes except the KT66 - got -44 on pin 5, heaters were alright and 500 on pins 3 and 6.
thats a bit high.

I decided to go on and try with the power tubes in - immediately got red plating on V5 (thats new).
I decided to check the bias resistors anyway and got a wonderful glowing spark shooting from the pin 8 to the air. god only knows if that didn't hurt any of the components...

needless to say I turned everything off.
I'm almost ready to give up on this build, I've gone through the instructions maybe a dozen times and nothing seems faulty.

[Elad E]

swapped output tubes again - now the redplating is on V4, it's probably the tube then though I still have no clue why I got that spark when I tried to check bias current on the bias resistors of V5.

[neikeel]

Even new KT66s should be ok with 500v. Of course we do not know if one was damaged in testing.
What make are they?
Silly thing to check is that you have your output tubes wired correctly (not a pin out - ie 1 to 8 or the other way 1 to 2 all out of synch. ??
Most likely will be a duff KT66.

Don't give up (yet!) you will get there

[Elad E]

they are Valve Arts.
I'm feeling like I'm chasing ghosts...hope I can sort this out, it's frustrating running back and forth.

see my post about the mains voltage - I'm thinking maybe I should've wired the PT to operate in 230 since my VAC readings are closer to 230 than 220 (228V AC).

[Elad E]

rewired the PT for 230V.

these are the reading I get with all tubes in except the KT66s:

---pin1 pin3 pin6 pin8
V1: 225 1.9 239 1.9
V2: 197 1.2 347 207
V3 254 47 254 47

---pin3 pin 4 pin 5 pin 6
V4: 489 490 -43 490
V5: 490 490 -43 490

with the output tubes in I get red plating again on the same tube, I'm beginning to think it's the tube that is faulty.

[neikeel]

rewired the PT for 230V.

these are the reading I get with all tubes in except the KT66s:

---pin1 pin3 pin6 pin8
V1: 225 1.9 239 1.9
V2: 197 1.2 347 207
V3 254 47 254 47

---pin3 pin 4 pin 5 pin 6
V4: 489 490 -43 490
V5: 490 490 -43 490

with the output tubes in I get red plating again on the same tube, I'm beginning to think it's the tube that is faulty.

I agree. Try some Gold Lions or the TAD ones are supposed to be ok.

[Elad E]

actually, the high voltages I measured got me thinking, I checked AC voltage across pins 4 and 6 of the rectifier tube (with no tubes installed) and got a 740V reading when the data sheets show there should be 690V on these.
I'm afraid this has something to do with the very high voltages I'm recording on all sockets.

what do you think about those voltage readings?

[neikeel]

Your last readings are absolutely fine (490 and -43)

Beg/ borrow /steal some more octal output tubes of any type to a test.

[Elad E]

thanks, I really appreciate your patience.

any octal tube you say? 6L6s would do too? wouldn't the plate voltage red plate them?

[stef]

The 6L6GC should not rade plate with something like -30 -40VDC on stand by

[Elad E]

update: couldn't find an octal tube set for loan, ended up ordering a pair of Gold Lions which should be here sometime next week.
will report back when they are here.

thanks for the help guys!

[Elad E]

Gold Lions arrived today, installed them, plugged the amp to my Aguilar DB112 (a 12" 300W 8 Ohm bass cab).
let the amp warm up, switched the standby on after a few minutes, waited for a couple of minutes to see there is no red plating.
plugged in my LP, raised the volume...it's alive!
I continued to play it, checked all tone knobs work, played with both channels, jumpered the channels - everything seems to work.
I do however get a low volume high pitched static when the presence is above 8 and one of the volumes is above 8 too.
I guess it could stem from the cold bias or the components not burned in yet though. I will wait and see how it goes after I bias the thing, I'll have to do some reading prior to that as last time I tried I got sparks flying, I hope I can do it right now.

thanks again for the support, I'll post some pics and clips when I finish biasing.