• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Ampeg SVT6 PRO bias adjustment needed?

M

MC1970

Aug 25, 2011
97
99
Roma - Italia
#1
Hi All,

I'm the happy owner of a MIA Ampeg SVT-6 PRO head, which I usually run connected to a Furman Power Conditioner and drive two SVT 410 HE cabs with....
During last weeks I've started experiencing loud cooling fan noise and a lot of "hiss" through the tweeters.
As I turn the head on, with cabs connected, after 3-4 minutes the fan starts runnig at full speed, even when I'm not playing at all, and a lot of hiss comes out of the tweeters, just like would happen with the master volume cranked high.
A friend of mine, which unfortunately lives far from where I do, told me that the hiss and the fan "problem" could be caused by an offset in the power section, and also told me that a bias adjustment might be needed.

here's my question; does anyone know what the measurements on power section mosfets would need to be?
Somewhere here on TB I read that the power section on the 6pro is a sort of 2 x SVT3PRO bridged; are there known common issues?

Thank You very much,

Marco
 
#2
From the schematic, it kind of looks like the power section of a 4-pro to me, with a 1+ and 1- side instead of channel A and B.
For the 4 pro, the bias is adjusted by tweeking (ever so slightly) your trim pot (in this case AP1 and AP2) to obtain an average of 25mV across the the 5 watt/ .47 ohm resistors coming off of the source of each mosfet.
The problem *could* be a tube getting squirrely. It could also be caused by a cold/cracked solder joint. I just fixed a 4 pro last week that had a fan running full-power and the cause was a cracked solder joint on the fan speed control transistor (there are 2 of these in the 6-pro).
 
#3
The 6Pro is an awesome amp, my favorite of ALL the Pro series heads. If it were mine, I'd take it in to a good, competent amp tech, for a thorough once over. This way, any issues the amp has are all taken care of. It requires alot of testing equipment, and specific amp knowledge, most of us don;t have. good luck, that amp rocks.
 
#4
It is pretty similar to the 3PRO. But also a bit different.

However, the fan isn't directly associated with the bias....

If the bias were too high, the amp might get hot and the fans would speed up. That is true.

So.... the question is whether the amp really IS getting hot, or if the fan circuit maybe is misbehaving and causing the problems.

If you check the air coming out, is it very hot? If the amp is really getting hot, the air should be very hot also.

Has the amplifier shut down? That obviously can be caused by overheating.

If you can, check if there is any DC voltage on the output of the amp. If there is that would indicate a problem.

If the air is not hot, and the amp does not shut down, it is possible that the temperature sensing circuit has a problem.

The noise you hear is another thing altogether.... I am not sure that bias would directly cause that, although LOW bias can cause a similar problem in some other amps. The 3PRO and 6 PRO do not typically have that problem, they just sound bad if the bias is low.

Because the 6PRO is slightly different from the 3pro, and my notes on 6PRO are not as good as they should be, I have asked the designer of the amp for the exact bias, and will post it.. That possibility may as well be checked out if you have the means to do the check.
 
#5
As I suspected, same setup for 6PRO as for 3 PRO. Just as Hodgy said.... 25 mV average on each set of the 0.47 ohm resistors (there are 4 sets). Do this with no speaker, and no signal, and let the amp stabilize for at least 5 or 20 minutes after turning on before final adjustment.

BTW, Hodgy said "4 pro", he may have been thinking of the 3 PRO.......... ;)
 
#9
I was just reading this thread (and I understand its from a few months ago) but I was wondering if any solution had been found??

The reason that I ask is because I own a SVT 3 Pro bought new 9 years ago (give or take) and has a really noisy fan also. Truth be told, I've stopped gigging the amp for fear of it overheating (which hasn't yet happened). Not sure if it makes a difference, but the amp has been re-tubed twice during my ownership (2007 & 2012).

Anyway...just curious to know if there was a solution to quiet the amp or at least a way to know if these fans are just "noisy by nature". With more scheduled gigs on the calendar I'd like to get more use out of this amp if possible.
 
#10
Fans do wear out, and are pretty easy to replace. The 6-Pro has 2 fans, and they have to work very hard to cool off those mosfets pumping out 1100 watts of bassy joy.

And yes, the bias procedure for the 6-Pro is the same as the 3-Pro. I just did mine the other day.
 
#11
Another response in addition to the bias procedure and a possible solution:

The ST24B3 Sprint DC fan by Comair Rotron is apparently noisy by design, however this alternative has also been discussed here on TB.

By removing the fan attachment screws and switching on the fan separate from the metal cabinet the fan noise is normal but when it makes contact with the metal brackets the noise returns due to reverberation. The noise maybe eliminated by dampening the brackets carefully without blocking air-flow thus minimizing the noise.
 
#12
I just finished doing a complete rebuild of a svt6-pro. Does anyone know the best way to set the bias on the lower amp. This is channel A (+) half.

Do I need to use jumpers and measure it upside down outside of the case? Is it safe to measure the bias without the heat sink? This seems risky.

The top section or (-) half is not a problem as the .47 ohm 5W resistors are accessible.

Any advise on testing the bottom section would be greatly appreciated.

Thanks
 

Latest posts

Top Bottom
Back