Biasing a tube amp

[shafesk]

Hi all!

I've recently tried to bias my Cayin tube amp which has bias ports right beside the tube for easy access. The problem is that I can't get an accurate reading, the meter reading never settles. Any advice?

Regards,

Shafin

 

[eggontoast]

When you say it doesn't settle how much is it fluctuating by ?

 

[shafesk]

what I mean is the reading is fluctuating from zero to infinity

 

[eggontoast]

For all the valves ?

 

[shafesk]

yes for all of them :?

 

[eggontoast]

Describe exactly what you are doing including what setting you have your DVM on, do you have the amp just idling etc.

 

[shafesk]

I'm pretty much following the manual, I put the amp on for 10 minutes without any signal then put the voltmeter probes into the bias port. I'm failing to get a stable and accurate reading, it is just varying too wildly. Also, do you have any idea how far I can push the bias safely? The manual says I should leave it between 0.35 to 0.4 volts, can I push it further safely if I don't care how long the tubes last?

 

[eggontoast]

I don't know why you are getting such a fluctuation when measuring the bias, if you have no input you should get a stable DC measurement. There must be a 10 ohm resistor in the cathode circuit though as 0.35 - 0.4V is way too high, measuring across a 10 ohm resistor it would make it 35 - 40mA which is more like it.

With regards to how high you can bias it, can't say without seeing the schematic really or at least knowing what the HT voltage is.