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Early 90's Ampeg SVT-2 PRO problem! Help?!


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I think my ampeg is early 90's? It says 'svt 2 pro' in all blue writing except the 'v' which is red. The serial number simply says 'D'.
I normally play this amp through an Ampeg 6x10 4Ohm cab live, but at smaller shows and rehearsals I use an SWR goliath junior III rated at 8ohms unfortunately. I got advice from a tech guy and he say it should be fine to run through an 8Ohm cab instead of 4Ohms, and it does work albeit a strange distortion when bass guitar is turned to half volume.
BUT a couple of nights ago half way through a number a huge volume drop occurred. I thought the amp had shutoff, so I turned it off and on again to no avail, only being able to get enough volume with the gain and master much higher than normal.
I should mention, a few days before this happened, a loud static noise introduced itself, going up and down in volume randomly. Even with the input gain at zero and the mute switch on... the only thing that affects it is the master volume or putting the amp into standby.

I have replaced all the power valves about four months ago. I read that it could be a loose/dirty pin in one of the valves so i took them all out, inspected them and wiggled them. The master volume pot is also dirty as loud pops happen if you dont twist the knob alot to clean before switching the amp on. Upon reassembling the amp, there seemed to be no change.

Anyone got and ideas?! Help will be much appreciated!!

Thanks.

Edited by rageingtelephone
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[quote name='rageingtelephone' post='1012223' date='Nov 4 2010, 04:02 PM']I think my ampeg is early 90's? It says 'svt 2 pro' in all blue writing except the 'v' which is red. The serial number simply says 'D'.
I normally play this amp through an Ampeg 6x10 4Ohm cab live, but at smaller shows and rehearsals I use an SWR goliath junior III rated at 8ohms unfortunately. I got advice from a tech guy and he say it should be fine to run through an 8Ohm cab instead of 4Ohms, and it does work albeit a strange distortion when bass guitar is turned to half volume.
BUT a couple of nights ago half way through a number a huge volume drop occurred. I thought the amp had shutoff, so I turned it off and on again to no avail, only being able to get enough volume with the gain and master much higher than normal.
I should mention, a few days before this happened, a loud static noise introduced itself, going up and down in volume randomly. Even with the input gain at zero and the mute switch on... the only thing that affects it is the master volume or putting the amp into standby.

I have replaced all the power valves about four months ago. I read that it could be a loose/dirty pin in one of the valves so i took them all out, inspected them and wiggled them. The master volume pot is also dirty as loud pops happen if you dont twist the knob alot to clean before switching the amp on. Upon reassembling the amp, there seemed to be no change.

Anyone got and ideas?! Help will be much appreciated!!

Thanks.[/quote]


Might not be the problem but very bad advice from your tech guy. With valve amps it is important to have the correct speaker impedance connected as this sets the reflected load on the power tubes via the output transformer.

I think the power tubes might well be blown.

When you changed the valves it should have been re-biased as well.

sorry,

Dave

Edited by DHA
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[quote name='gareth' post='1013847' date='Nov 5 2010, 11:06 PM']I have never quite understood that

I had an SVT II Pro and there are controls on the back that allow you to self-bias - admittedly only in two groups of three a side.

So are you saying that there are further ways of biasing the tubes that presuambly must be inside the amp and only known to techies?[/quote]


All valve amps need to have matching output valves that will need biasing. If the SVT has controls to self bias then that's great but it must be done when valves are changed.

Most amps have a pre-set inside the amp that allows the techie to set the bias.

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[quote name='DHA' post='1012377' date='Nov 4 2010, 06:08 PM']Might not be the problem but very bad advice from your tech guy. With valve amps it is important to have the correct speaker impedance connected as this sets the reflected load on the power tubes via the output transformer.

I think the power tubes might well be blown.

When you changed the valves it should have been re-biased as well.

sorry,

Dave[/quote]


Thanks Dave, yeah I did change all six of the power valves not too long ago and used the bias controls on the back. I've also taken them all out to check and all seem fine (no cloudyness etc), and also all glow as normal when amp is turmed on. So i dont think the tubes are blown... could a dirty switch cause any of the trouble I mentioned before?

I was aware of the dangers of not matching the impedance correctly, but as far as I knew I thought it was only if the impedance of the speaker was lower than the amp that damage could occur? Cheers, Thom.

I should also mention, whilst playing another show the other day it started out fine and normal, then the same volume drop happened after a couple tunes. I was using the correct impedance cab this time.....!

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[quote name='rageingtelephone' post='1017018' date='Nov 8 2010, 11:05 PM']Thanks Dave, yeah I did change all six of the power valves not too long ago and used the bias controls on the back. I've also taken them all out to check and all seem fine (no cloudyness etc), and also all glow as normal when amp is turmed on. So i dont think the tubes are blown... could a dirty switch cause any of the trouble I mentioned before?

I was aware of the dangers of not matching the impedance correctly, but as far as I knew I thought it was only if the impedance of the speaker was lower than the amp that damage could occur? Cheers, Thom.

I should also mention, whilst playing another show the other day it started out fine and normal, then the same volume drop happened after a couple tunes. I was using the correct impedance cab this time.....![/quote]


The impedance does need to be match but damage is more likely if a lower impedance cab is used. The valves are expecting to see a load of a few 1000ohms depending on the type. The speakers are very low impedance so an output transformer is used to match the valves to the speakers. If half the impedance speakers are used then twice the current goes via the tube (but not for long!) so in your case only half the required current was drawn by the valves so I think it would not have sounded very good, thin and weak?

The cloudyness would be if there was a leak and the glowing is just the heaters so not a good way to see if they are working.

But saying that looks like it may be something else, so it's time for a techie.


Dave

Edited by DHA
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I had this problem many years back... and ended up selling my amp. :)

Here's some advice from my experience...

1) Never use it with an 8ohm cab. Make sure it always sees at least 4ohms!
2) Have a word with a good tech about disabling/reducing the sensitivity of the 'protection circuitry'. It's MUCH too sensitive and shuts down the amp at the slightest thing...

Hope this helps!

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[quote name='rageingtelephone' post='1017018' date='Nov 8 2010, 11:05 PM']Thanks Dave, yeah I did change all six of the power valves not too long ago and used the bias controls on the back. I've also taken them all out to check and all seem fine (no cloudyness etc), and also all glow as normal when amp is turmed on. So i dont think the tubes are blown... could a dirty switch cause any of the trouble I mentioned before?

I was aware of the dangers of not matching the impedance correctly, but as far as I knew I thought it was only if the impedance of the speaker was lower than the amp that damage could occur? Cheers, Thom.

I should also mention, whilst playing another show the other day it started out fine and normal, then the same volume drop happened after a couple tunes. I was using the correct impedance cab this time.....![/quote]

Did you set the bias with a 4 or 8ohm cab attached?

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[quote name='rageingtelephone' post='1017018' date='Nov 8 2010, 11:05 PM']I was aware of the dangers of not matching the impedance correctly, but as far as I knew I thought it was only if the impedance of the speaker was lower than the amp that damage could occur? Cheers, Thom.[/quote]

Only true of solid state amps I'm afraid. Valve amps shoud be impedance matched. [b][u]I may be wrong [/b][/u](in which case someone please correct me) but if anything it's better to present a load with lower impedance than higher to a valve amp.

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