Change 1 tube - how to not buy 6 new tubes for SVT?

[matthewlein]

My SVT went all blinky light on me yesterday, probably a shot power tube. I don't want to replace all 6 if just 1 went bad, but I have no idea how to find a tube that matches the other 5.

Anyone know how I can do that?

Anyone swap their own tubes in an SVT?
I've done it in a fender and it was as easy as changing a light bulb...

[RickenBoogie]

Unless you're qualified, which it doesn't sound like you are, take the amp to an amp tech, and have the job done properly. You may need to replace 2 tubes anyway, I think they run in pairs. A good amp tech is your best friend.

[coreyfyfe]

The SVT runs in sets of three. Easiest way to do it without a tech would be to replace the three and bias the set, assuming you can at least bias the amp on your own. Otherwise, the only way to replace a single would be to match them, which requires a tube tester and some know how.

[matthewlein]

I have no official qualifications persay...but I'm not an idiot, and I like to fix my own problems when I can.

3 in group would make sense, since there are the 2 biasing screws. Biasing is easy on the svt, just need a screwdriver.

Thanks, I'll think about it.

[BassmanPaul]

Originally Posted by matthewlein

I've done it in a fender and it was as easy as changing a light bulb...

Unfortunately output tubes need to be biased properly while light bulbs don't. Even a Fender needs to run with a correct bias setting.

[BassmanPaul]

Originally Posted by matthewlein

Biasing is easy on the svt, just need a screwdriver.

Yes indeed, that's why so many techs make a living from SVTs

[JimmyM]

i'm going to say something people might consider pretty controversial...

i have changed power tubes in amps without biasing or matching them before, and i've never really had any problems. i don't recommend this, but i've done it.

however, there is a bit more to biasing an svt than using a screwdriver and watching the glow. but it ain't brain surgery and can be done with a common voltmeter. there are instructions on here for it somewhere. just do a search on svt biasing.

[Bevo1995]

So... if you don't bias the tubes maybe they don't last as long, and maybe you have a little hum, but tube amps, (vintage ones at least) are pretty tough. Maybe your ampeg will blow up, but I've operated both hi-fi and instrument amps with all sorts of mismatched tubes with no ill effects.

[DanRJBrasil]

if you can test the tubes, you can buy only one that match the other two.
there are several tube stores that can sell you only one that match your requirement

[Goodlawdy]

This guy's pretty helpful:
dougstubes.com

[T-Bird]

Hi.

This question seems to pop up very frequently.

There's no magical way to match any tube with another without having them both in a tester. Preferably at the same time.

Like Jimmy said, it can be done, and I have played a SVT CL with a mismatched tubes (it doesn't bias with the LED's though, obviously) since that's how it came to me, but why would anyone buy a SVT, have it limping instead of running?
Any SS amp will probably give the same, if not better tone.

OP, You do know that if something gives because of the bias mismatch, its highly likely that the repair bill will be a lot higher than just a trio of tubes? And obviously You still have to buy the trio of tubes.

Regards
Sam

[Alex1984]

Has anyone ever bought factory seconds from tubedepot?

[Scrappersa]

What about if I have two pairs of matched stock tubes and one pair of Tung-Sol 6550s? Would that work or do all three pairs have to be the same brand?

[JimmyM]

no, they don't have to be the same brand. but they should be matched in triplet sets for best results.

[Scrappersa]

Thanks Jimmy. What if I wanted to replace only one pair with a KT88? (Until I can afford to replace all six).

[JimmyM]

think less in pairs, think more in triplets. svt's group power tubes in sets of 3. don't know much about kt88's...my svt doesn't use 6550's.

[Gort]

Originally Posted by JimmyM

i'm going to say something people might consider pretty controversial...

i have changed power tubes in amps without biasing or matching them before, and i've never really had any problems.

Yup, that's the way it was done here for decades. Nobody matched or biased anything.

We'd just go to the local electronics supply shop checked the suspect tube out on the tester and if it read "Bad" we bought a new/surplus tube, installed it and went on our merry way. We had a local electronics shop that carried surplus parts including tubes so that's where everyone went.

[beans-on-toast]

With a tester, a tech can measure plate current and transconductance of your other tubes and select a new (or used) one to match.

Without a tester, a tech can measure the B+ and plate voltage and calculate the plate current and select a tube that at least matches that.

In a pinch, you can always just pop in a new tube, re-bias, and see how the amp sounds. It would be best if the new tube was the same type as the old tube. The worst that can happen is that the other two tubes in the trio will wear down a little faster. If you have NOS tubes, there is a better chance that you can just pop in a new NOS tube of the same type and the tubes are going to match.

If you have an old SVT, the 6 plate and two 1 ohm bias measuring resistors that are inside the amp can be so out of spec that if you drop in a set of matched tubes, your amp is not going to see the tubes as matched. These are parts that need to be checked on old amps. Then again, maybe the out of spec parts and subsequent imbalance in the power tubes is part of
the magic of that particular amp. This all gets confusing.

[Seans]

Originally Posted by Scrappersa

Thanks Jimmy. What if I wanted to replace only one pair with a KT88? (Until I can afford to replace all six).

That is not a good idea, KT88's have different plate voltages I think.

[Jerrold Tiers]

Originally Posted by Gort

Yup, that's the way it was done here for decades. Nobody matched or biased anything.
We'd just go to the local electronics supply shop checked the suspect tube out on the tester and if it read "Bad" we bought a new/surplus tube, installed it and went on our merry way. We had a local electronics shop that carried surplus parts including tubes so that's where everyone went.

The difference between then and now is significant......

Then:

1) Tubes such as 6550 were made by a limited number of good companies, who had really VERY good quality control. "Bad" tubes were rare, modern techs notwithstanding. Tubes often lasted the life of the equipment.

2) Tubes were closely controlled for characteristics, and were made in high volume. Equipment manufacturers bought tubes all of the same range, right up the middle of the performance distribution. If you bought replacements from them, you got the same selection, and could often plug them right in and have them match (unless the old ones were aged out of tolerance). if you bought drugstore replacements, that was "everything else" that was NOT in the center of the distribution, and might or might not match. Good repair places bought manufacturer tubes.

Now:

1) Tubes are made by companies who got into it fairly recently. Their engineers may not have even been alive when tubes were "it", much less have ever worked in a 'real" tube factory.

2) Tube manufacturing is a smaller volume specialty deal. So the economies of scale don't work the way they did when every new unit had a bunch of tubes in it. It may seem like a lot of tubes, but the volumes are small, and the investment in production cannot be anything like what it was before.

3)As a result they don't have the attention to close control that the old high volume makers did. Tubes are more widely distributed in characteristics.

4) QC is often not the same..... they don't want to scrap many tubes, so they ship even the "outliers" that are marginal at best. Tubes that GE, Sylvania or RCA would have tossed in the scrap bin. You get, not the old manufacturer type, but the "drugstore versions" with "whatever" characteristics. While they can be, and are, sold as matched sets, there is much more variability between sets. The "wide spray" of characteristics means that any random replacement unit is that much less likely to be anywhere close to what you have.

So....

What happens if the tubes are NOT matched?

OK, what happens is that one or two carry proportionately more current than the other(s). Power dissipation is higher, heating is more, tube life is less.

yes, if the "odd tube" is a LOT different, it might really hog current and have a serious problem. Just depends what the range is, and where the others are versus the new one.

More likely, the odd one will be close enough not to turn red hot, or do real damage.

But you never know. It's the luck of the draw.