Tubes for VTL MB-450 Series II

I have no experience with your amps, I would ask the maker,

a quick search, no mention of KT88’s popped up, this comment about Auto Bias I think is worth posting:

"Further, I found that the VTLs lived up to Luke Manley’s stated goal of producing tubed components that are as easy to live with as their solid-state compeers. They auto-bias the output tubes, and even turn on sequentially after the preamplifier when you connect them using VTL’s bidirectional RS-232 interface. When you don’t have to think about the gear, you tend to concentrate on the music."

from this

https://www.stereophile.com/tubepoweramps/108vtl/index.html

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Bias: I would love auto bias, or at least external adjusters and meter. But, would it make an AUDIBLE difference? (I need 16 ohm taps for my 16 ohm speakers drivers made in 1958, my Cayin A88T MK1 has internal bias adjusters).

I have read bias effects the life of the tubes, the power measurements, heat output a lot (all more important for you than me or most), how much if any of bias adjustment is audible? I’ve only had bias adjusted once, by someone else, with no way to listen/compare directly. I am unaware of an audible improvement.

After about a year, I did change my used Cayin A88T to KT88’s (from the 6550’s it came with) (I had a nice set someone had given me), preferred/stuck with the KT88’s, but the bias was set for the 6550’s it came with.

Note: I also prefer my amp’s ultra-linear to Triode. My guess, IF you prefer your’s in Triode mode, you might prefer the 6550’s anyway. If now using ultra-linear, I also suppose you will prefer KT88’s to 6550’s.

guess .... suppose .... 

Next, to ’do it right’ I had to drive it 35 mins away to VAS (former Cayin USA Rep) and have them change the internal bias adjusters for the KT88’s, then go back a few weeks later and pick it up. I used the Fisher 80AZ’s with no remote volume in the meantime. The Cayin sounded the same to me after adjusted, but no timely comparison (of pre/post bias change), just compared favorably once again to the Fisher 80AZ’s

I never adjusted bias on any tube I used in my prior Fisher 80AZ Mono Blocks made in 1958. I didn’t know bias was a thing back then. I used them quite a lot from 1973 up to 2023 (minor repairs over the years). I never used covers, glowing tubes always visible, none ever glowed too brightly to my eye. Perhaps the pro's working on them routinely adjusted the bias, but I doubt it, they never mentioned it that I recall, and wasn't it the screw with the red paint on it?

It had (presumably originally biased for) 15 year old EL37’s that tested weak when I inherited them from my uncle:

https://www.audiogon.com/systems/11420

I never heard the magic of ’good’ EL37’s. Over those 50 years I scavenged and was given a LOT of used tubes, and bought new if needed.

In those amps I used 6L6 variations, 7581, 5881 ...... and the others all sounded better than anything else I or my friends ever had. I checked all the tubes every 6 months, finally got my own tube tester. Changes usually involved a simultaneous replacement in another component.

Steve at VAS bought the pair of 80AZ when I mentioned selling them after he adjusted my Cayin for me.

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Btw, you might discuss with Brent Jesse, he may know the auto-bias answer, or let you try a set of KT88’s, and change to 6550’s if you don’t love them. He’s closing for vacation soon

https://www.audiotubes.com/

Think Long. You might consider buying two full sets, hearing them, breaking them in 60 hours (just on, no signal needed), keeping one, returning the other, or, if they both sound great, it’s many a good nights sleep having a matched set broken in ready to go.

 

 

 

+1 email VTL to confirm but 6550 and KT-88 have “similar” bias requirements.  The Series III can run either per website. Beast of an amp, btw.  If KT-88 equivalent, consider looking at Shuguang Nature KT-88-T (not -Z).

I have a Dennis Had Inspire KT-88 that is designed to run 6L6, 6550, KT-88, etc.  Stock tube was GL KT-88, good tube, clean, transparent, linear, up and down frequencies. The Shuguang KT88-T has a bit more heft in bass and depth in mid-range. Treble extension is equivalent. Kind of like the difference between a Porsche  Carrera and Carrera S.  The S has a little more grrr at every turn.

You can use other 6550 brands as well. Winged C's are good but not the best avail. 

trivema

 

"6550 and KT-88 have “similar” bias requirements."

I have to wonder, my Cayin A88T mk1, when new, was possibly biased for KT88’s. The seller might have put 6550’s in without altering the bias, and when I went to KT88’s, it probably didn’t need a bias alteration. Internal adjusters, no meter, it's being blind that bothers me.

however, as tubes age, does the bias need to be checked/adjusted?

unless they are running hot and will thereby have a short life, why bother?

I used to have these amps. Never ran 6550s in them only GL KT88s. Stay away from the Chinese KT88s unless you like tube fireworks. They will not hold up to the B+ voltages on the 450. If you want a warmer sound check out the Mullard re-issue KT88s. If your preamp is fully differential, use the balanced inputs on the 450. It will be more dynamic and greater imaging.