SVT Classic Biasing Problem

[mlove]

I have a problem with the user biasing section of my SVT Classic and was unable to find another thread with my specific problem so here it is:

Amp is on and not in standby mode, no signal present

Bias Section 1
1. starting hard left the green and red lights are off
2. turning the dial right some and the green light comes on but is dime
3. turn it right some more and the red light comes on brit but the green stays dime so i dial it back so only the green light is on but it is dim

Bias Section 2
1. starting hard left the green and red lights are off
2. turning the dial right some and the green light comes on but is dim
3. turning right a little more and the green light comes on bright
4. turning right a little more and the green and red lights are on bright so i back it off until only the green light is on bright.

I had the amp go into fault mode on my a couple times yesterday so i was doing some troubleshooting today and found the biasing problem above, good thing is it did not go into fault mode today - there was to much plugged into a power bar which i think had something to do with the fault mode yesterday.

Also I get a static noise when the amp is on, the volume of the static is the same regardless of the gain or master level.

Any help would be great, I plan on taking it to a tech but would like to have a clue what the problem is in the mean time.

[Jerrold Tiers]

I can't quite decide if it is a possible tube or tube circuit issue, or a problem in the bias measuring circuit.

Either way, I do not trust the somewhat flaky readings you are getting from the "user bias circuit", so it's good it is going to a tech who can (hopefully) see and fix the real problem.

[mlove]

So i dropped if off at the tech today - he won't get a chance to look at it until tomorrow.
He had little faith in the user biasing system, he said he serviced and SVT a while back with troubles in the user bias. After checking out numerous things it turned out the tubes were not factory matched close enough to bias properly.

We shall see...

[Jerrold Tiers]

To be fair, the user bias is really to prevent gross issues...... it isn't intended to be the last word.....

personally, I would have left it off in favor of something else, or just left it off. The protection is iffy... it has prevented some damage, for sure, but is also more "stuff" that can go bad.

[mlove]

here is the verdict:
2 blow 6550 (replaced all 6)
1 blow 12ax7 in the power amp stage (replaced)
3 blow resistors in the power amp stage (replaced)

the tech was amazed the amp would still work at all with these problems.
my wallet hurts.

[JimmyM]

Were the two that blew out of the same triplet set? And did the second triplet set match if the tubes were still good? If so, he could have replaced 3 power tubes instead and saved you a little. In any event though, you did the right thing and probably saved money in the long run, especially with burnt resistors, since throwing tubes at it probably wouldn't have worked.

[mlove]

the tech used to do ampeg warranty work and had a factory matched sixtet of 6550's someone never showed up for (plus they were the only ones in stock) so he gave me a decent deal.