Distortion channel trouble, B-52 AT-212

[ManRay]

Hi,

My B-52 AT-212 combo is giving me a weak signal on its dirty channels. The clean is still there and strong but when I click to dirty the sound gets thin and tinny without any bass response and an all around lower output on both gain channels.

I swapped the preamp tubes in Valves 2,3,4, and 7 without any luck. I have also run a patch cable from the send to the return with no change. All 3 rectifier modes respond equally, which leads me to believe that the problem lies elsewhere.

The amp was just returned from the shop where it had a screen grid resistor replaced because a power tube short which put it out of commission. While there the rectifier tube was shown to be bad as well and was replaced and modded with a couple of diodes slapped across it to help ease the load.

If anyone else has any ideas about where to turn from here, I am taking suggestions.

Thanks,
Mr.

[52 Bill]

The sound that you describe would seem to be that of an open/dirty connection in the signal path, or an open cap in the signal path.

Check any and all connectors and all of the coupling caps in the distortion channels.

Love the photographs too!

[Enzo]

I woud go down the row of preamp tubes and chek pins 1 and 6 of each for B+ voltage, then pins 3 and 8 for cathode voltages. Looking for a dead stage.

[ManRay]

Preamp tube voltages

Ok,

No loose connectors as far as I can tell and I couldn't turn up any obviously burnt out caps anywhere.

I went ahead and checked the pinouts for all of the preamp tubes per Enzo's suggestion and got at least some sort of reading for each (which I guess means that the gain stages are not totally dead).

Below are the results, but I am not really sure how to interpret them. This amp was a bottom end monster before the screen resistor trouble and now it just sounds so thin and brittle. I appreciate the time guys, anything else you can tell me to get her back will be greatly appreciated.

Thanks,
Mr.

Preamp tubes: All 7, in order from input to PI

V1:
pin 1=198
pin 6=240
pin 3=1.3
pin 8=2.0

V2:
pin 1=199
pin 6=196
pin 3=1.4
pin 8=1.4

V3:
pin 1=217
pin 6=187
pin 3=2.0
pin 8=1.6

V4:
pin 1=303
pin 6=303
pin 3=189 (seems high?)
pin 8=81

V10: (The last 3 preamp tubes jump up in # designation)
pin 1=396
pin 6=93
pin 3=5.5
pin 8=1.1

V11:
pin 1=406
pin 6=264
pin 3=99
pin 8=2.0

V5: Phase Inverter?
pin 1=268
pin 6=256
pin 3=47
pin 8=47

[Enzo]

NO glaring problems offhand. Get the schematic and start tracing a signal through the channel stage by stage.

I think Bill is right. And the tinny, bottomless sound is really crosstalk. The actual signal is missing.

[Gtr_tech]

I had the head version in with the same complaint a while back. My records say it was the buffer stage (V11) that was causing the problem. Turned out to be that damn ROHS lead free solder that brought it down. I'd advise removing that shit from *all* the tube sockets and resolder with regular solder. Try it on V11 first just to make sure yer chasing the right gremlin.

[ManRay]

Update

Thanks for all of the help!

I went back through that rat nest of connections in the amp and found nothing out of the ordinary.

Under the advice of a friend I plugged the speaker out into a different cab and lo and behold SOUND IS BACK!

The speakers in the combo were what was sucking the life out of the amp!

Again thank you for all of the help you guys offered, I appreciate it.

Gratefully,
Mr.