SVT 3-PRO

[bsco]

Quick question..some amps have fans that run all the time while others only have them running when there is a problem or the amp is heating up......due to being pushed hard..... With the SVT 3 pro, the fan is running all the time and appears to be running fairly fast.....Amp plays fine but I did notice that when you turn down the tube gain control to minimum quickly, there is a fairly loud hum which lasts for a few seconds then goes away....however, I don't hear any difference in the sound of the amp when the tube gain control is adjusted...so there is a problem there for sure but what about the fan?? Is it supposed to be running all the time in this amp or do I have two individual problems here...
Cheers,
Bernie

[The Dude]

There's a thread HERE about fan operation. Though fraught with peril, there is some useful information in the thread.

[soundguruman]

Quick question..some amps have fans that run all the time while others only have them running when there is a problem or the amp is heating up......due to being pushed hard..... With the SVT 3 pro, the fan is running all the time and appears to be running fairly fast.....Amp plays fine but I did notice that when you turn down the tube gain control to minimum quickly, there is a fairly loud hum which lasts for a few seconds then goes away....however, I don't hear any difference in the sound of the amp when the tube gain control is adjusted...so there is a problem there for sure but what about the fan?? Is it supposed to be running all the time in this amp or do I have two individual problems here...
Cheers,
Bernie

The fan is not supposed to run all the time, it has a temperature sensor that makes it speed up and slow down, or stop.
The fan control could have been modified to run all the time (and I wouldn't blame the person who did it).
OR the fan control may have a bad temperature sensor, or a shorted transistor...
As I have said before, the LM35 temp sensors go bad, and should be replaced when (ever) the amp is serviced.

But it is not unusual for somebody to modify, or bypass the fan control. Simply because the amp really runs too hot when it's pushed hard.
Also because these type fan controls are not remarkably reliable. (LM 35 fails, and the fan stops, and the amp blows up, etc...) (OR the cheap fan wears out...)
So, taking out the 24 volt fan, and installing a 120V fan, is a common practice. It's done all the time, because players get fed up with the amp getting too hot.
And from bass player point of view (I'm a bass player too) I can't blame them a bit for doing it.

So yes, you have 2 problems there, without question.

ampeg svt 3 pro power amp.pdf

But as a bass player and an amp tech, I would RATHER that the fan runs all the time.
Regardless, if it speeds up and slows down, which I regard as a rather silly gimmick.

In a REAL SVT (which this "svt 3 pro" amp is not)
The power is on and the fan is running, full speed. It's built that way.
So I don't know "how" the REAL SVT could ever get by, without a temperature sensor (?) I was being sarcastic...because obviously, you don't need a temperature sensor, at all.

[bsco]

Thanks Dude...I just had a quick look....and you're right....I never seen that thread there before....even though I searched...anyway, I'll dig deeper to see what I can sort out...Thanks for the info.....
Cheers.....

[Jazz P Bass]

The SVT 3 Pro addresses this "Gain Control 10 to 0" item.
Page 16.
http://www.ampeg.com/pdf/svt-3pro.pdf

The fan runs all the time.
Looking at the schematic, the +65 Vdc goes through the preamp tube heaters, through the fan winding & then to ground.
There will be about 16Vdc on the fan when the amp is cold.
That is enough to run the fan at all times.

The LM35 ramps the fan speed up faster when the amp gets hot.

[drewl]

Actually page 6....
Hey this sounds familiar!

But yes, the fan should run slowly when amp is powered up with around 12-18v across fan plug and as the amp warms up the fan speeds up.

[Jazz P Bass]

Thanks for correcting the typo.

[bsco]

Thanks Jazz P and drewl.....I came upstairs to download the owners manual but you guys beat me to it....I was thinking that the fan in this unit must run all the time seeing that there is 16V applied to the fan when the amp is cold....so there is no problem there...I measured the voltage variation at the emitter of TIP50 and the plate voltage varies from about 100 volts up to the max plate voltage of somewhere around the 300 volt mark...so that is working ok...and all the tubes have the proper plate voltages going to them....This amp is working by the way but the guy who owns it said that when he was using it last, there was a funny burning smell coming from it and he also said some smoke came from it as well.....I could not find any signs of anything burnt anywhere...however......this unit uses a 12AU7 tube.....all the rest are 12AX7A's.....I found the 12AU7 tube installed in the first tube location in the pre-amp right next to the input jack.....The V2 location on the power amp board had a 12AX7 there....when the tube gain control was turned down fairly quick, there would be a loud hum through the speaker for a few seconds....until the circuit settled down....when I switched these two tube socket locations with the proper tubes, this hum has pretty much disappeared.... I do get a very small touch of hum there and I noticed that when I spray the 47uf 450V cap with cold spray,(C5), the hum goes altogether...So I will have to replace the cap...but as far as everything else goes, the unit is working fine...it was stated in a previous reply that the fan was not supposed to run until the amp ran hot...but after sizing up the schematic I came to the conclusion that it has to run with 16 volts across it...and you guys just confirmed it...I am going to read through the owners manual and dig a little deeper into the tube gain control...Thank you for the link...It is much appreciated.....
Cheers,
Bernie

P.S. Just took a look at the owners manual...it turns out that this hum issue Is not an issue at all.....this is the way it is supposed to work if the control is turned down too quickly....Why the tubes were in their wrong locations I have no idea....I'll ask the owner but I already know what he is going to say......anyway, thanks once again for the link to the owners manual.....
Cheers.....

Ahhhh...just talked to the owner...he doesn't know how the tubes could have been installed in the wrong locations.....he has never had it repaired before.....so there you have it...