Matching tubes? (SVT content)

[smo]

Hi guys,

I'm getting my SVT2 (non-pro) serviced right now, and the amp has a few issues getting fixed. The amp was shutting itself into standby on a regular basis whenever I played a few songs in a row. This might be explained by quite a few of the high-voltage caps being bloated and ugly lookin.....
I've not been to this fella before, so I'm keeping an open mind, but I did get one little surprise.

The fella working on my amp said;
"...one thing I need you to decide is the power tubes.
I would have them replaced because when I rebiased power tubes, one side had to be biased hotter than the other - it's evidence that the tubes are not matched.
That could also be the reason for the shutdown. Regardless, the imbalance in the circuit is not healthy for an amp of this size and wattage which pulls alot of current through its transformers."

Now I understand that whereas most amps work in pairs, the SVT's work in groups of three.
We always want each bank of three to be matched.
But all six?
Is this guy just trying to get more money out of me?, is this problem an issue I should take seriously?, or is it just a small thing I shouldn't worry about?

Cheers

[jellymax]

how about testing the tubes?
if you replace them, make sure to ask for the old tubes back---they belong to you
can always chk them later

[JimmyM]

Personally, I think he's trying to sell you a full set that you don't need. I have yet to hear of a compelling reason why both sides need to be identically matched, for the exact reason you said...they're grouped in triplet sets, each with their own bias, so matching 3 per side is all that's needed.

[B-string]

The separate bias controls allow you to match triplets. It could be an indication of one tube beginning to fail or weak but further testing would be needed. I don't personally believe this is the cause of the fault you have or a contributing factor.

[Sid Fang]

I can only speak for the original SVT design, which I own, but in that context what the tech said makes a certain amount sense, but there's an element of BS. If, by the luck of the draw, all three tubes on one side are much "hotter" than all three on the other, the bias adjustment between the two phases may be extreme, or even impossible, but in that case, what he *should* be able to do is to migrate one of the weak tubes to the hotter side, to even things out a bit and make the bias adjustment reasonable. If all he's saying is that one side is a little hotter than the other and he had to tweak the bias to account for it, then, yeah, he's right, the tubes aren't all the same strength, but, hey, that's why there's a bias control.

There *is* a catch in all this, though, where your tech *may* actually be right about wanting to replace some tubes (though probably not all), and that is that there's no bias adjustment between the three tubes on each phase, just a resistive network that tries to equalize the loading a bit. If, as a result of the migration of tubes to even out the phases, you have one side with both super-strong and super-weak tubes, that circuit can be stressed, *maybe* to the point of impacting sound and component lifetime. In that case, I'd think that the best thing to do is to buy a single matched 6550 pair to replace the strongest and the weakest in your current set, reducing the extremes. If your tech has access to a tube tester, this should be easy. If his only way of figuring out the gain of the tubes is to rotate them around on your chassis, then it's going to get expensive in labor time.

[smo]

Thanks fella's.

Yeah what he said got my alarm bells going. Why I'm here.

When I asked him to tell me the differences in the bias, as in a measurement, he couldn't tell me. So I asked him, well was it a large difference or a small one. Again, he couldn't remember....

I think i'll pass on new tubes for now.
Ta fella's, thanks for the advice.

[T-Bird]

Hi.

IIRC You gig?

What's the cost of having an amp crap out on You during a gig vs. a new set of reputable tubes and a clean bill of health from a tech?

While replacing all the power tubes is always the best option, testing them and matching as pairs, or triplets in the case of a SVT is very viable option as well.

If the banks bias differently, that tells absolutely nothing about the individual tubes condition.

It's very surprising that the tech didn't take notes while benching and biasing the amp.
I can't remember a s*** when it comes down to measuring results (I do a different sort of measuring for a living), so I always write it down.
That way I can check the results from my own notes a decade later if needed, and not have to rely on the possibly falsified documents I have given to the customer.


Without a possibility to test the individual tubes, my vote would go for a new set of tubes.
If You can get the old ones tested, then 3 matched new ones + 3 old ones is an alternative.

I wouldn't let go of the old ones without compensation though.
SE amps have created a market for used individual tubes.

Regards
Sam