VTA ST120 Bias Adjustment

Soft kitty, warm kitty
Little ball of fur

Lazy kitty, sleepy kitty
Purr, purr, purr


Nighty night!

Hang in there ... good to hear you got a good backup ...

PS ... make sure your builder lets you know what was wrong ... and make sure you post up with the results. Enquiring minds need to know!

" . . . and probably install 10 ohm 3 watt resistors."


personally I like to use 1 watt resistors for those cathode resistors.
If the tubes are biased properly, there will only be .025w across that resistor (based on 50ma).
So even a 1/4w resistor would be 10X as big as needed.
I'd much rather burn out a 20c resistor than a $40 tube !!!!

sKiZo wrote:Soft kitty, warm kitty
Little ball of fur

Lazy kitty, sleepy kitty
Purr, purr, purr


Nighty night!

Hang in there ... good to hear you got a good backup ...

PS ... make sure your builder lets you know what was wrong ... and make sure you post up with the results. Enquiring minds need to know!

you're in my spot!!

Thanks to all who have been following this thread and providing input.

UPDATE: What the problem was I can't say. What I can say is that after installing screen grid protection resistors on the output tubes, upgrading from KT88 to to KT120 output tubes and installing a Weber SS rectifier, all is wonderful. I also discovered my SP12 preamp came with a faulty 24 stepped volume control pot. That I replaced with a Blue Velvet volume pot. I can't beleive how good everything sounds and bias measurements are stable. This is what I expected when I purchased this gear but it took several months to work out the bugs. Probably not what Bob and his crew want to hear but such is my case. I have absolutely no regrets with my VTA stereo. It performs as it should now and I am loving it. I don't know what my problem was but I do beleive dcgillespie is on to something and really appreciate his guidance.

The ST-120 and SP-12, when working propery, rock!

John

on right side i can set to .500vdc but left side lowest I Can set is 1.3 and .700 what could be wrong?

ok Bob, thanks.

Nice sêeing you still active. I got this amp prolly 4-5 years barely used. I will post the result.