Bias adjustment holes/pot on Colonial not working?

[andyjames]

I've just purchased a new set of el34's to put into my Colonial, as I wasn't a fan of how the 6l6's in it sounded. I sprayed contact cleaner on the tubes, inserted the tubes, adjusted the bias pot to as low as it goes and set the amp on standby for 5 minutes. Once I took it off standby and insterted multi-meter prongs into the bias adjustment holes and adjusted the pot, I couldn't get a reading and the multimeter still read 0mv's. I tried both sag and punch settings and nothing.... I also inserted an instrument cable to insure that it was indeed off of standby and it was. Also I am 100% that the multimeter works.....Pretty confused here. Please help.

 

 

Is there another way to get a bias reading from this amp by taking out the chassis? Who knows maybe its a loose part inside of the amp or something....

[andyjames]

Actually just tried the previous set of tubes and they are working just fine and adjustments can be made with them in, but not the EL34's. So its a tube problem....

[kbp810]

Sounds like the amp may not have been setup to accommodate both 6L6's and EL34's; they have a different cathode out pin (pin 1 vs pin 8), in amps wired for both, pins 1 and 8 and jumpered together.

 

One way you can check - power the amp off and remove the tube; with the meter set to read ohms (continuity), check between pins 1 and 8 - if it reads open, there is your problem. If not, maybe go one step further and check pin 1 to ground and pin 8 to ground (chassis should do), you should read 1 ohm at each.

[andyjames]

Sounds like the amp may not have been setup to accommodate both 6L6's and EL34's; they have a different cathode out pin (pin 1 vs pin 8), in amps wired for both, pins 1 and 8 and jumpered together.

 

One way you can check - power the amp off and remove the tube; with the meter set to read ohms (continuity), check between pins 1 and 8 - if it reads open, there is your problem. If not, maybe go one step further and check pin 1 to ground and pin 8 to ground (chassis should do), you should read 1 ohm at each.

 

It reads anywhere from 0 - .03 ohms when measuring between pin 1 and 8 on the tube sockets. That would be an open reading correct?

 

I have always heard of these amps being able to fit 6l6's and El34's, in fact I bought it for that reason. The person I bought it from also gave me the option of running either 6l6 or El's when I bought it. Looked under the chassis and there is a rod connecting both pins 1 and 8 with grounds connected, running to the same place. This is puzzling. Maybe this amp has been modded in the past or just malfunctioning?

[kbp810]

It reads anywhere from 0 - .03 ohms when measuring between pin 1 and 8 on the tube sockets. That would be an open reading correct?

 

I have always heard of these amps being able to fit 6l6's and El34's, in fact I bought it for that reason. The person I bought it from also gave me the option of running either 6l6 or El's when I bought it. Looked under the chassis and there is a rod connecting both pins 1 and 8 with grounds connected, running to the same place. This is puzzling. Maybe this amp has been modded in the past or just malfunctioning?

 

Nope, that sounds like they are indeed wired together as they should for tube swapping; those grounds should make a stop off at the bias test point first, and between the red and black bias test points there should be a 1 ohm resistor.

[andyjames]

Would I be able to get around this by using a bias probe set up to read MilliAmps? even though the manual states to use MilliVolts when biasing these amps.

 

EDIT:

 

Also keep in mind, that it did read voltage for the original set up tubes that I had in it. So it should all be connected.