First build sounds great/terrible -help!

[pbreyfogle]

I just powered up my first amp and it sounded great plugged into the current limiter -which had it only powered at about 75V.

When I plug it strait into the wall though it sounds like crap. Tinny, fading in and out, like a harpsichord, and distorted from the very lowest volume settings.

the schematic is largely based on the mission schematic modified by Sean Kilback with some other changes based on posts around here and other places:

-no tone stack other than the volume (I need to get those pots)
-22k only on the inputs (though I added 27k for a total of 49k and that didn't do much -maybe a little less distorted)
-Split cathode on the first stage with 1k5 on each side
-power tube screens decoupled with 350ohm resisters.
-381ohm output cathode resister
-other small value changes based on posts and local availability

Checking voltages I get

320/258/180 in the power supply

First stage:
1 - 116v
3 - 1.05
6 - 137
8 - 0.8

PI:
1 - 120
3 - 1.06
6 - 134
7 - 35.4
8 - 55.9

6V6 #1:
3 - 301
4 - 259 (with 10 mA)
8 - 37

6V6 #2:
3 - 320
4 - 262 (with no current!)
8 - 37


Any ideas why? What to do? Thanks,

Peter

[Satamax]

Sounds like you have a bad case of ultrasonic oscilation due certainly to bad lead dress.

[pbreyfogle]

Solutions?

I'm not familiar with the term 'bad dress' . Is that disorderly layout? As I posted before (How much layout change is OK?) my board layout is different -is it likely to be that or more the leads themselves?

What lead (or section of leads) is most likely the culpret?

Also, is the screen current in one 6v6 and not the other indicative of anything?

Cheers,

Peter

[Satamax]

Peter, could you show us the wiring?

Well, bad lead dress, is wires doing capacitive coupling and such things.

Are you aware of thoses?

http://www.geofex.com/ampdbug/ampdebug.htm

http://www.geofex.com/Article_Folder..._tube_amps.htm

http://www.geofex.com/tubeampfaq/taffram.htm

[pbreyfogle]

Thanks, problem solved

Satamax, thanks for the prompt reply. While you were writing I was looking around on the site and found the suggestion of poking around in the amp with a chopstick.

Turned out all it was was a bad solder joint between the 1k5 resister and pin 5 of the 6v6.

I've got to run out now; but I'll do another thurough voltage check this evening and see if that raises any more questions.

Anyway, thanks again.

-Peter

[Bruce / Mission Amps]

Bring that screen node voltage up to around 300vdc.
What value resistor is in between the first two filter caps?

[pbreyfogle]

new numbers

So, after being on for 5 minutes, here are the numbers:

Power supply:
368/323/224

First stage:
1 - 99v
3 - 1.97
6 - 99
8 - 1.89

PI:
1 - 117
3 - 1.64
6 - 152
7 - 41
8 - 68

6V6 #1:
3 - 368
4 - 327
8 - 37

6V6 #2:
3 - 365
4 - 326
8 - 23

So, if I do my math right, that looks like 19w for both tubes. How's that sound?

[teleamp]

I would try to get the V1 plate voltage up to 160, same with the PI.

Mike

[pbreyfogle]

True for 12AT7's

Is that true even if I have 12AT7's in there?

If so, should I adjust the load resisters of each plate or just bump down the 22k (already 20k) in th PS?

What's the cost of leaving it where it is?

-Peter