Ampeg SVT-VR Balance Adjustment?

[Slpbass81]

So a few months ago I was posting that I was having issues with my SVT-CL amp. Took it to the shop a couple times and no solution. Well 8 months later and a bunch of money spent I figured out one of my Mogami patch cables was bad. The reason it never clicked in my head before was I change my pedalboard setup for each show so sometimes it would be before my tuner that mutes the rig and sometimes after.

Anyways, I decided to buy new power tubes for my backup SVT-VR. I took it to the shop and got everything set up but it was still making noise so I bought some new JJ's. I adjusted the bias but I don't know how to do the balance since I don't have a dummy load at home. What are the side effects if I don't adjust this for 6-12 months? Do I need to adjust it if the shop just did even though I changed my power tubes?

 

[B-string]

Yes changing the power tubes can change the balance even with same numbered replacements. Will it do harm, NO. It just makes it so both the "push" and "pull" sides of output tubes are at the same level in the output transformer. Some people even like the sound/feel of imbalance there, but slightly less power is the price. Bias is a must for tube life and safe operation, balance is more "gravy for the biskits" (if I don't get shot for being untechnical here).

 

[Slpbass81]

When biasing should you leave the amp in standby when warming up or full on?

Any recommendations for an easy/cheap way to build a dummy load?

 

[B-string]

Bias use the lights with a cab hooked up, amp in operate with NO signal for a 20 minute warm up before adjusting. The balance you need a dummy load, signal generator and true RMS volt meter. If you want to build a load two of these would be a minimum for balance test Invalid Link Removed wire in parallel, four and use the 2 ohm tap if you are going to power test. They will get HOT, that is normal, but burns or fire from laying them on paper or wood has been a concern. If you have the other test equipment then a load is a good investment, if not, have a tech do it (25Vrms of 40Hz is pretty darn loud with a speaker cab).

 

[beans-on-toast]

The amp should warm up with standby off, in playing mode. It helps it to warm up if you play it.

If the balance is not set properly, the THD will be higher.

Will post an image a bit latter of a dummy load using the resistors that B posted.

 

[coreyfyfe]

Will post an image a bit latter of a dummy load using the resistors that B posted.

You had posted them here, if it helps save you some digging.

Retubing my SVT II (non pro)

I use a similar setup, a pair of 8 ohm/200 watt non inductive loads mounted in an aluminum chassis with a switch for 8 (single) or 4 (both in parallel) ohms. I picked them up from weber a while back as they were half the price as the parts express ones, but I think they've stopped selling them.

 

[beans-on-toast]

You had posted them here, if it helps save you some digging.

Retubing my SVT II (non pro)

I use a similar setup, a pair of 8 ohm/200 watt non inductive loads mounted in an aluminum chassis with a switch for 8 (single) or 4 (both in parallel) ohms. I picked them up from weber a while back as they were half the price as the parts express ones, but I think they've stopped selling them.

Thanks Corey that's what I was thinking of. I found another post as well: Amp techs, what's on your bench?. You can see how it should be wired by looking at the images. On other dummy loads I also add a BNC connector in parallel, good for connecting test equipment. The BNC connector can be seen at the bottom in the last image.

I use Arcol resistors (mouser part number 284-NHS300-8.0F ) now, they are accurate but unfortunately, very expensive. Those are the gold ones in the SVT II thread. Build in an electrical box bought at a hardware store.

It isn't necessary to get this fancy though. You can slap together a couple of resistors on a board and wire it with a speakon cable to connect to the SVT.

For the 40Hz signal, there are phone apps that generate tones. There are also tone files available on the internet. The signal can be fed into the input of the amp.

 

[Slpbass81]

That is one heck of a contraption. I think I will let the amp roll the way it is (bias is done) since it is just my backup. Has anyone else compared the SVT-CL to VR and notice the VR has more noise/hum even when the volume is at 0 than the CL?

 

[B-string]

Neither one should have anything but a very very faint him if any at all. Is it a 60Hz or 120 Hz hum? Online Tone Generator - Free, Simple and Easy to Use.

 

[beans-on-toast]

Different tubes could be the reason for the hum. The small tubes that came with my VR were quite noisy. I changed them and it made a big difference. The amp also stopped picking up radio stations.

 

[beans-on-toast]

That is one heck of a contraption.

Certainly a lot more than is needed for you to tweak your amp. This is a bench tool that gets used often enough to make all the fuss worth while. A big part is making it safe which is why it might be best to have a tech do this next time it is on the bench.

Years ago, some amps had balance controls. Like a hum balance pot for the heater circuit, the power tube balance was done by ear. The pot was adjusted for lowest noise.

 

[Nikwood]

Hi guys, I'm asking for confirmation on what I read on all the discussions about it. Does the balance adjustment have to be precise and periodically checked by a technician or is it enough to check the bias lights every now and then if you have never changed the tubes? The head is about two years old and has never had a problem, but a slight background hum has always been a part of it. Thanks and sorry if I ask for things that may have already been said

 

[beans-on-toast]

Hi guys, I'm asking for confirmation on what I read on all the discussions about it. Does the balance adjustment have to be precise and periodically checked by a technician or is it enough to check the bias lights every now and then if you have never changed the tubes? The head is about two years old and has never had a problem, but a slight background hum has always been a part of it. Thanks and sorry if I ask for things that may have already been said
View attachment 4662568

Normally the balance is adjusted when you change the power tubes. It balances the two trios and helps minimize hum. If the amp is fine, leave it.

 

[Nikwood]

Normalmente il bilanciamento viene regolato quando si cambiano le valvole di potenza. Bilancia i due trii e aiuta a ridurre al minimo il ronzio. Se l'amplificatore va bene, lascialo.

ok so I'll do .... just that reading the misadventures that the users of the forum had with the 50th, I got the doubt that the balancing was not done as a certain thing at the time of purchase. But I know that a slight rattle is quite normal. I put my heart in peace thanks

 

[Wasnex]

@Nikwood it's not critical. In fact SVT classics don't even have a balance control.

Bias and Balance

The 6 output tubes are organized into 2 trios. Each trio of tubes works together as one tube. One trio is the push side of the output section, and the other trio is the pull side. Each bias control adjusts the current passing through 3 tubes.

The phase inverter breaks the signal down into two paths. One side is in phase, and the other side is out of phase. The balance control adjusts the signal strength of the in-phase output. The intent is not to balance the signal level at the phase inverter, it's to balance the signal level at the output.