Possible bad bias pot?

hwaitung wrote:My VTA-ST70 is developing some random pops on left channel recently (It doesn't go away if I put in another power tube). It also produce a pop one or twice when I was making adjustment with front left trim pot. Today I found that the front left power tube has no bias value (0v) regardless of the adjustments. The amp is producing sound, even though I got no bias value. Is that a bad trim pot or I should look deeper for some other troubles?

Any advice will be appreciated!

hwaitung wrote:That resistor does have a value around 10 ohm and I don't see any unusual burn marks around that as well.

Bob Latino wrote:

hwaitung wrote:That resistor does have a value around 10 ohm and I don't see any unusual burn marks around that as well.

OK good - next with the amp ON but no rectifier in the amp and the bias pot for that tube socket centered > Check the negative DC voltage to chassis ground on pins 5 and 6 on that socket. Black probe on the chassis and red probe on pin 5 and then on pin 6. You should get about -25 to -45 VDC on both pins 5 and 6. If you do, then the bias system is working for that tube. If you don't get any voltage or some other strange voltage, then the bias system (on the driver board) has a problem.

Bob[/size]

hwaitung wrote:

Bob Latino wrote:

hwaitung wrote:That resistor does have a value around 10 ohm and I don't see any unusual burn marks around that as well.

OK good - next with the amp ON but no rectifier in the amp and the bias pot for that tube socket centered > Check the negative DC voltage to chassis ground on pins 5 and 6 on that socket. Black probe on the chassis and red probe on pin 5 and then on pin 6. You should get about -25 to -45 VDC on both pins 5 and 6. If you do, then the bias system is working for that tube. If you don't get any voltage or some other strange voltage, then the bias system (on the driver board) has a problem.

Bob[/size]

I guess there is no need to connect the speaker when doing this test?

Bob Latino wrote:
OK good - next with the amp ON but no rectifier in the amp and the bias pot for that tube socket centered > Check the negative DC voltage to chassis ground on pins 5 and 6 on that socket. Black probe on the chassis and red probe on pin 5 and then on pin 6. You should get about -25 to -45 VDC on both pins 5 and 6. If you do, then the bias system is working for that tube. If you don't get any voltage or some other strange voltage, then the bias system (on the driver board) has a problem.
Bob[/size]

hwaitung wrote:

I did that on all 4 sockets. They all have values around -35 VDC.

Bob Latino wrote:

hwaitung wrote:

I did that on all 4 sockets. They all have values around -35 VDC.

That reading of -35 VDC on all four output tube sockets with the bias pots centered is right on ... The bias system is working properly. There has to be something wrong on that tube socket. Either ..

1. A bad solder connection - Try resoldering all 8 socket pins

or (more likely)

2. Bad contact between the tube's pins with the socket pins.

Try cleaning and then "retensioning" the tube socket pins. To retension the socket pins you take a small flat head screwdriver and push each pin opening to make the opening in the socket pins a little smaller. Clean any dust off the socket pins first.

Bob[/size]