SVT 3 Pro - Bias adjustment - weird value!

[kidney]

Hi guys,

As I've just got an old SVT 3 Pro head which was distorting at low gain, I went through a tubes change and bias adjustment (on my own following the advices of JT and Foz ).
Sounding is far much better now, thanks!

Yet, I've never been able to get too close to 25mV average because of a very low one ; here are my measures:
14.4-----32
13.9-----4.6
20 ------5.5
0.3 ------7.2
So, whereas one is already near the top limit (i.e. 32mV), there is one to only 0.3mV

Is this something I shall be concerned about?

Actually, even if the sound is now great, I find this head a little bit "under powered" - I mean I guess I will have to turn it on to the max to play with a drummer.
Could this be related to the great discrepancy between Mosfets voltage measures?

 

[Jerrold Tiers]

You say an OLD one, and if old enough it may actually have been made without the newer matched mosfets..... That would account for the differences.

Since it has lasted this long (dunno HOW "old" your "old" really is) it should be OK, but that does have a larger difference than most any newer one probably ought to..

As for volume, no, it should have no effect on volume. once any serious signal is present, all mosfets contribute. The bias really affects only low volumes/low power.

Causes for low volume:

Not running gain high enough to get the preamp light flashing regularly.

having the "tube drive" set too low

Using the "ultra-low" when you want volume.

any other type of EQ that tends to boost lows/highs and leave out the midrange

using an 8 ohm speaker when amp is rated higher power for 4 ohms.

 

[kidney]

Thanks for your reactivity Jerrold!

Don't really know when this one came out... serial is T3PWH10088, can this help to get an idea about how "old" is it?

But you are right, there are no color on Mosfets, so they are certainly not matched.
Is it something that can be changed "easily"? Is this costly? Actually would this be usefull some how?

By the way, regarding the lack of power feeling, I'm gonna get a cab in 4 ohms to get the 450 Watts from the head and check if it feels better.

FYI, I played it in my flat with an SVT410 pro; the gain was set to 2 o'clock and the master to 3 o'clock (and tube gain to 0). The sound was loud, but not that loud, I could play one meter before it and the walls weren't not shacking (neither were my neighbours ). I'll try to use it next week with my drummer...

 

[Jerrold Tiers]

Thanks for your reactivity Jerrold!

Don't really know when this one came out... serial is T3PWH10088, can this help to get an idea about how "old" is it?

But you are right, there are no color on Mosfets, so they are certainly not matched.
Is it something that can be changed "easily"? Is this costly? Actually would this be usefull some how?

Looks like January 1995..... probably correct for not matched, I had not beat management over the head long enough at that time.

Color dot is up "above" the mounting screw, in the corner.

Matching will give no particular advantage, yours are matched enough to evidently not give any operational problems... I'ts lasted 15 years just as it is.

 

[dmrogers]

I had a late '90's SVT 3Pro, and it was not clean at lower volumes. After talking Jerrold and reading all the post on the problem, I adjusted my bias, and had a similar problem to yours. One of the pots wouldn't get close. They were matched too.

I adjusted as best as I could, and it improved the sound a great deal. That was a couple of years ago. I have since sold it to my father that is still gigging with the head. And it still sounds great.

I would suggest operating that head a a 4 ohm load, it sound much better.

 

[kidney]

1995!?! Wow, I didn't expect it to be that "old".... anyway, so far so good!

I'm gonna leave it this way and check how it sounds like when I'll get my GK 212MBE (in 4 ohms) in a couple of weeks.

Hope the head will get on well with this cab...

Thanks for your input guys; cheers from France!