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Ampeg SVT VR

W

WINGS012345

Dec 7, 2009
57
2
#1
Hey Everyone. Just bought a New Ampeg SVT VR. Sounds great. Did the Standby relay change that I read so much about on here. Question I have for anyone who is familar with the amp. Since the day I bought it, the BALANCE light on the back is yellow. I'm assuming it is suppose to be green. I read but could not totally understand in the manual what this balance is for and exactly how do I adjust. it. The manual talks about needing some specific signal. Anyone know how the balance affects the amp if it's off and how to adjust?

Thanks
Jeff
 
#2
Balance is the hum balance control. Unless you're getting a ton of buzzing, I wouldn't worry about it. As for the voltage-relay, I think I've read in comments on here that they changed the minimum or removed the circuit or something on newer models but I can't find where I read that so I can't back up that claim.
 
#3
Thanks. I'm not sure exactly how old the amp is. It was brand new from a Music store over the internet. Not sure how long they had it sitting. So I figured I wouldnt chance it. I emailed Ampeg and they told me to take it a Ampeg dealer service tech in my area. Never asked or informed that they fixed the issue. I actually asked them if they fixed this issue and they just told me to take it to a service tech for the warranty fix. So I'm guessing they didnt fix it. I dont believe they think this is a fault.
 
#4
WINGS012345 said:
I'm not sure exactly how old the amp is. It was brand new from a Music store over the internet. Not sure how long they had it sitting. So I figured I wouldnt chance it. I emailed Ampeg and they told me to take it a Ampeg dealer service tech in my area. Never asked or informed that they fixed the issue. I actually asked them if they fixed this issue and they just told me to take it to a service tech for the warranty fix. So I'm guessing they didnt fix it. I dont believe they think this is a fault.
Click to expand...

Here is a link describing how to determine the manufacturing date of your VR based on the serial number. See the botom of the page. For an VR intended for the North American market, the first four letters are "EFVD", next is the year (2008 is "U", 2009 is "V"), next is the month (1-9 for January to September, O for October, A for November, B for December), the rest is the unit number.

Yellow means it is balanced. It is not a green LED. I've found that it takes about 2 hours of the amp operating (off standby) for everything to stabilize (Ampeg says 20 minutes). A tech can apply a signal to the input and examine the output to optimize the balance setting. This is more accurate than the LED circuit. The amp could be set properly and the LED indicates that it isn't. If it isn't distorting at low volumes and the amp sounds good, you are fine. If you aren't sure and there are no obvious problems, it would be best to allow the tubes to burn in over the next couple of months and then take it in to have the bias and balance checked.

The procedure using the LED's is as follows: warm up the amp, with the amp operating and the volume controls down all the way, adjust each bias pot until the green LEDs are lit, then with a dummy load in place, apply at 40Hz since wave to the input, adjust the output to 25 VRMS, then adjust the balance pot until the yellow LED is lit.
 
#6
coreyfyfe said:
Balance is the hum balance control.
Click to expand...

actually the 'balance' control on the VR is for the two drive tubes that drive the power amp so that each side of the push/pull circuit drive each traid of 6550's equally. and you need a signal generator along with a DVM to set it properly. the early SVT's had a hum balance control.
 
#8
The tech ampeg reccomended in my area told me it would be a 3 week turn around to look at it for the standby relay mod. He's busy. I just followed John k's easy to do instructions (thanks John ) and it took all but an hour. I was just un-sure about the Balance led being yellow. Kinda throws you off because Green always means good! The manual makes no mention of this LED's color.
 
#9
I was thinking about the balance adjust.
Wouldn't it be possible to search on the net for a pure 40 Hz sine wave, load that on your mp3 and than feed it into the svt?
So you don't need a signal generator.
And is the dummy load nescessary? If you just hook it up to your cab and put the Volt meter on the parallel out on the cab?

Another question I had was the 25 VRMS, I assume you can get that by adjusting the volume knob?
And than adjust the balance pot until the led is on...
 
#10
So the standby mod is now recognised by Ampeg and fixed under warranty? I was totally unaware of this and at least it means you won't void the warranty by doing it yourself. It would have been nice of Ampeg to notify owners, or at least their agents....
Having said that, operating it at 240 volts downunder I have never had a problem.
 
#11
bass bart said:
I was thinking about the balance adjust.
Wouldn't it be possible to search on the net for a pure 40 Hz sine wave, load that on your mp3 and than feed it into the svt?
So you don't need a signal generator.
And is the dummy load nescessary? If you just hook it up to your cab and put the Volt meter on the parallel out on the cab?

Another question I had was the 25 VRMS, I assume you can get that by adjusting the volume knob?
And than adjust the balance pot until the led is on...
Click to expand...

Did we ever receive an answer to these questions?

It would seem that the answer would be a bunch of yes's, but I'm not an amp tech. Perhaps Jerrold Tiers or Johnk can chime in with their knowledge.
 
#12
I was also wondering about these questions. Further....

- An Open E is approximately 40 Hz. Can I get by with just plucking an open E as the input?

- Can anybody say, just roughly, what position the volume knob should be in to get roughly 25 VRMS output?

My objective is, in lieu of having equipment that I don't have or paying a tech, I should be able to do a rough approximation to set the Balance reasonably close.

Is this just totally off base? Does the input signal have to be a fixed, constant amplitude in order to get the output to be anywhere close to a stable 25 VRMS?
 
#13
I usually connect a signal generator to the power amp in jack so I can't tell you where the volume knob would be for 25VRMS​.

I can tell you that 35VRMS​ is around 300W into 4 ohms. 25VRMS​ into 4 ohms is about 160W. P= V2​/R so (25 VRMS​)2​/4 = 160 W. A low frequency sine wave into the amp with 25 VRMS​ into a 4 ohm cab, putting out about 160W, is going to be very loud. That's why they use dummy loads.

Just to provide some background. You set the balance because you want the positive going part of the output signal feeding the power tubes to be the same as the negative going signal. When they are the same, the waveform is balanced. Here are a couple of pics. The sine wave on the top is asymmetrical or unbalanced; the one below is balanced. So the balance pot is adjusted to provide a signal that has the same amplitude, positive and negative going. If it isn't adjusted properly, the amp has a higher harmonic distortion. It doesn't sound as clean. Some players like the dirt of an unbalanced amp.



To answer your other question about feeding in a low E signal from a bass. The frequency isn't so much of an issue. It doesn't need to be exactly 40Hz. This is a standard that Ampeg uses to ensure that things are done consistently. You want a low frequency, it can be 100 Hz, for example. It is best if you use a clean signal. A tech will use a signal generator that outputs a pure sine wave with low distortion to do this. If you have a clean input feeding the amp, then you can use what is called a harmonic distortion analyzer on the output to set the balance for minimal harmonic distortion. You don't need to be so fussy when setting the balance though. On the old SVT's you could use a voltmeter between the two test points (K1 and K2) on the back of the amp and adjust the balance to read 0 +/- 0.01 volts. You can still do this on the VR if you didn't want to use the balance LED but you would need to open the amp up to gain access. Instead of a signal generator, you can buy small boxes that run on a 9V battery that will output a sine wave. You can also download tone files from the internet and feed the output from your computer or a music player into your amp.

Can you simply use your bass? The signal from your bass isn't a clean sine wave, it also has harmonics. So it is a complicated waveform. This makes it very hard to set an accurate balance.
 

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