5150 Combo Chassis

[sweatyk]

If you pull a preamp tube, the heaters will go out on another tube as well, however the empty socket is a fine place to probe for voltages. Pins 1 and 6 ought to have B+ on them.

Forgive my noobness but are you suggesting that I pull one of the preamp tubes while it is powered on? And if so would it need to be in standby mode? I have never pulled a tube from a live amp before however I'm not afraid to do it.

[sweatyk]

[QUOTE=g1;504635
How about J21 & J22, on either side of standby switch?[/QUOTE]

J22 on = 495 V
off = 530 V

J21 on = 495 V
off = 0 V

[Enzo]

I am suggesting you have the amp on with a preamp tube or power tube (depending upon what we want to measure) not in the socket. I am not suggesting pulling the tube WHILE the circuit is running. Turn it off, pull the tube, turn it back on. Not in standby mode, it must be n full operate mode, because in standby the B+ voltages will not be present. Now you can probe the empty socket with a meter.


Clearly the circuit will not work this way, but the voltages are going to remain as they were largely. Once we know we don't have missing voltages, we power down, put the tube back, and power back up.

[sweatyk]

On V1 pin 1 = 380 V
Pin 6 = 372 V

That's pretty high going by the schematic. Pin 1 should be around 225 V and Pin 6 at 310 V

[sweatyk]

Due to an illness and death in my family I have had to shelf this repair until now. I recently pulled a tube one by one and read the voltages at the tube sockets. Physically V5 is mounted between V2 and V3 so I posted the results in the same order for the preamp tubes. Does this tell a tale?

V1 V2 V5 V3 V4
(1)460V (1)466V (1)497V (1)499V (1)514V
(2)1.7mv (2)3.5mv (2)6.6mv (2)8.2mv (2)16.5mv
(3)0 (3)0 (3)0 (3)0 (3)0
(4)27.5V (4)27.5V (4)40mv (4)32.9mv (4)26.7V
(5)-39mv (5)-39mv (5)-31V (5)-28.2V (5)26.6V
(6)439V (6)430V (6)482V (6)502V (6)520V
(7)1.7mv (7)0 (7)0 (7)6.1mv (7)-7.9mv
(8)0 (8)0 (8)0 (8)0 (8)0
(9)128.5mv (9)125mv (9)380mv (9)370mv (9)26.5V

I had all of these numbers separated and in columns however when I posted it this is what it came out like.

[Enzo]

The tube heaters on the small tubes are wired in series. If you remove any, the rest go dark. Readings taken with any tubes removed are meaningless, sorry.

[sweatyk]

The tube heaters on the small tubes are wired in series. If you remove any, the rest go dark. Readings taken with any tubes removed are meaningless, sorry.

Thanks Enzo. I'm not great at these things but willing to follow any task.




In your earlier post.............."I am suggesting you have the amp on with a preamp tube or power tube (depending upon what we want to measure) not in the socket. I am not suggesting pulling the tube WHILE the circuit is running. Turn it off, pull the tube, turn it back on. Not in standby mode, it must be n full operate mode, because in standby the B+ voltages will not be present. Now you can probe the empty socket with a meter.


Clearly the circuit will not work this way, but the voltages are going to remain as they were largely. Once we know we don't have missing voltages, we power down, put the tube back, and power back up."

you were mentioning pin 1 and 6 for the B+ so that is listed with the useless info I posted. So not a total loss.....lol. So pin 1 ranged from 460 to 514 and pin 6 from 439 to 520V. Pins 3 and 4 on the power tubes seemed on point also.

[TimmyP1955]

Dunno if this will help, but I once had a 5150 that was extra wimpy. All of the tubes tested OK (in an Orange tester - not good for matching, but it usually tells me if a tube is crap). But it wasn't OK in this amp - I swapped in another 12AX7 one at a time, and found the culprit.

[sweatyk]

Dunno if this will help, but I once had a 5150 that was extra wimpy. All of the tubes tested OK (in an Orange tester - not good for matching, but it usually tells me if a tube is crap). But it wasn't OK in this amp - I swapped in another 12AX7 one at a time, and found the culprit.

Yes I have done that to no avail. Here are the symptoms. I can get some low volume but it is only adjustable thru the pre Rhythm/Lead pots only. Both the post Rhythm/Lead controls pots has zero effect on the sound. As if they are not even connected.

[Enzo]

With tubes out, the voltages won't be remotely what they would be in an operating amp. I shouldn't say totally meaningless, because the really high voltages you found on pins 1 and 6 of the 12AX7s tells me the B+ is arriving at those tubes. SO if all we are trying to do is determine if B+ is present or absent, that will suffice, but all other readings, like at cathodes, won't happen.

I'd be thinking maybe a lost ground connection for those dead controls.