HELP!! strange noise!!

[Rickey_Dee]

Hey folks!!

i have a problem with some strange noise coming from the amp... it's definitely not the tubes, i changed all pre and power tubes, and it's also not the speaker cabs. please listen to this and hopefully someone can tell me where this noise can come from. i just played an A on the low E string and let it ring out... take a listen and you'll see what i mean.

http://www.soundclick.com/bands/default ... ID=1293835" onclick="window.open(this.href);return false;

the file is called "noise", it's the only one anyway...

THANKS in advance, i hope someone can tell me what might be wrong in here...
Rickey

[Rickey_Dee]

what i should maybe add, is that the noise isn't that obvious when i switch to the 8ohm setting with a 2x12 cab... at least it seems so... but it's definitely there when i tried 2 different 4x12 cabs on the 16 ohm setting.

could it be the OT?

[demonufo]

Sounds unstable. Have you checked your NFB loop/presence thoroughly?

What are you running for NFB?

[Rickey_Dee]

i'll check it when i'm at home again, but i THINK i'm running 100k here... no, i'm pretty sure about the 100k. as far as i remember i built it after the 70 lead spec.

[neikeel]

I had a listen and thought 'speaker cab' 'microphonic output valve' in that order but of course you have done those things

I agree with Demonufo some thing sounds as if it is about to 'break away' in the power amp as opposed to preamp.

[demonufo]

Well check your work carefully and try a smaller value resistor/more negative feedback if you can't find anything.

[Rickey_Dee]

thanks guys!

i will check everything, solderjoints, voltages, etc., and i will replace the 100k, i still have the 47k that came with the kit... i hope i can find the fault.

thanks so far, i will post the results later!
Rickey

[Rickey_Dee]

ok, first of all, the NFB is already the 47k... so far so good...

BUT... i checked the voltages, and it seems i have a serious problem here.

Heaters: 3,28V AC (shoudn't that be 3,14V AC)
Bridge Rectifier + (standby off): 292V DC
B.R. + (standby on): 449V DC
B.R. AC Terminals: 323V AC

and here where the real problems start:

V1 Pin1: 443 Pin3: no reading Pin6: 444 Pin8: no reading
V2 Pin1: 444 Pin3: no reading Pin6: 448 Pin8: no reading
V3 Pin1: 445 Pin3: no reading Pin6: 444 Pin8: no reading

V4 Pin3: 448 Pin4: 448 Pin5: -35.8 Pin6: 448
V5 Pin3: 449 Pin4: 449 Pin5: -35.8 Pin6: 449

wall voltage is 231V

this was measured without the tubes installed, power and standby in on position.

could anyone tell me what causes these readings??? solderjoints look ok...

Rickey

[demonufo]

Think we'd best see some pics.

[Rickey_Dee]

i have measured the same with the tubes installed, so here are the new results:

V1 Pin1: 153 / Pin3: 0.83 / Pin6: 196 / Pin8: 1.48
V2 Pin1: 154 / Pin3: 0.93 / Pin6: 268 / Pin8: 154
V3 Pin1: 215 / Pin3: 31.9 / Pin6: 208 / Pin8: 31.9

V4 Pin3: 426 / Pin4: 421 / Pin6: 427
V5 Pin3: 426 / Pin4: 421 / Pin6: 427

so far, they read "ok", not exactly like in the instructions, but more or less in the ballpark...

i have taken some pics, but need to convert them first to be a bit smaller, i'll post them later...

thanks
Rickey

[Rickey_Dee]

is there a way to measure if the OT is ok?

[demonufo]

The OT wouldn't cause runaway feedback like that.

[Rickey_Dee]

ok, here are some pics... sorry, i noticed the shadow on the back when i have left home already. i can take better pics the next time, if desired. maybe someone can find something on these pics anyway...

[Rickey_Dee]

some more...

[neikeel]

Is that shield on your PPIMV not covered with shrink wrap? The wire that comes from the PI output caps should have shrink wrap on it as it is very close to arcing on the board mounting when the wire vibrates.

cannot see much else amiss