Beepre voltage question

[Jamus]

2 terminals have voltages out of spec, not sure if this is something to be concerned about...
Terminal 17: should be 90-110v. Mine is 125v
Terminal B2: should be 90-110v. Mine is 135v
All others terminals were good

Thanks!
Jamus

 

[Paul Birkeland]

17 and B2 are connected by a piece of wire and cannot be different by 10V.

If you swap 300B's between channels, does this voltage move to A2?

 

[Jamus]

Before switching tubes I rechecked voltages and this time they were equal (142v each).
I swapped tubes and the error followed. Now terminal 2 and A2 are both 135v. Terminal 17 and B2 are both 99v. Bad tube?

 

[Paul Birkeland]

Before switching tubes I rechecked voltages and this time they were equal (142v each).

That's a 300B that's not conducting.

I swapped tubes and the error followed. Now terminal 2 and A2 are both 135v. Terminal 17 and B2 are both 99v. Bad tube?

I would take your soldering iron and reheat the pins on the 300B that's acting up, with a special emphasis on pin 2.  If you have enough heat to get the solder to flow, there's a small hole on the end of each pin, and a little extra solder can be added.

If these are old 300B's with years and years of use, this may not do you any good.

 

[Jamus]

Thanks PB, I'll try that tonight. This is a brand new Sophia Electric mesh plate, still under warranty. In my shoes would you just send it back or is this a pretty minor but permanent fix? I just don't want to end up right back here after warranty lapses in 30 days.

 

[Paul Birkeland]

Oh, I would send it back and not solder on the pins.  If the factory has any questions about the operating point of the 300B, let us know and we can help with that. 

 

[Jamus]

Thanks again, this is helpful information.
Jamus

 

[Jamus]

I got replacement tubes today and rechecked voltages. Now all 4 relevant terminals (2, A2, 17, B2) all measure 121v. This is only 10% over spec (90-110v) so I assume this is okay (?). All other voltages were within spec.

Next I hooked up CD player and BH crack to listen. All sounded great at first, listened for about 5 minutes. I left it playing for about 30 minutes to continue warming up. When I came back to listen there was almost no volume.

I took the BeePre out of the chain and the Crack works fine. I let everything cool down for a few minutes (top plate on BeePre was very hot). When I fired everything back up it sounded normal for about 5 seconds, then volume dropped down again.

Any suggestions on where to start with troubleshooting?

 

[Paul Birkeland]

Recheck all the voltages after the sound drops out.
-PB

 

[Jamus]

Now those same four terminals (2, B2, 17, A2) measure 142v each. All other spots are still in spec.

For what it's worth when I sent the last tubes back to Sophia, they said they remeasured both and they worked fine.

 

[Paul Birkeland]

You have 142V and still ~5V DC or so on the ungrounded end of each parallel pair of 8 Ohm resistors?

 

[Jamus]

Correct. 5.11 and 5.16

 

[Paul Birkeland]

Can you very, very carefully take a measurement of the DC voltage at pin 3 on each 4 pin socket? 

This is sounding a lot like a missed or flaky solder joint somewhere, but it's strange that it happens in both channels.

You have plate voltage, heater voltage, and bias voltage, so it is likely a mechanical connection that isn't allowing current to flow properly. 

 

[Jamus]

0V for both

 

[Jamus]

I reheated all terminals around this area (1-20, 4 pin sockets, C4S boards). No change.

Going back to the resistance checks these 4 terminals only reached 75k ohms. Could that be a clue to what is going on here?

 

[Paul Birkeland]

As long as you have 0V on pin 3 of the 4 pin sockets, then there isn't any concern for unusual operation conditions due to grid voltage variations.

It might also be good to ensure that the tubes are making good contact with the socket pins.

-PB

 

[Jamus]

Tubes do seem to be making good contact but I'm not really sure how to confirm it.

I noticed a new problem that wasn't there yesterday. The two LEDs on the C4S board, B side, near the front of the chassis light up on start up but then quickly go out. (Edit: perhaps not completely out but very faint)

 

[Paul Birkeland]

You might try reheating the center legs of all the MJE5731A's.  These can be tough to get a good solder joint on, and you practically cannot overheat these transistors when soldering them, especially on the center legs.

 

[Jamus]

Now that you mention it a few of those did look a little suspect. I reflowed them all but no change in the voltages or the LEDs dimming on the B side.

 

[Paul Birkeland]

You can also try poking around with a wooden chopstick to see if you can find the suspect connection that is causing this problem.

-PB