retube VTL amp

It's time to retube my VTL MB250. The unit is 5 years old w/ the original tubes. I just lost one tube and so its time to retube.

This will be my first to completely retube this amplifier. In fact, it will be my first time to retube an amplifier with so many tubes, 10 tubes per side.

To me, it seems the easiest thing is to purchase the complete set from VTL? If you have purchased tubes from VTL, I would like to read your thoughts, pros/cons.

Any advise with purchasing tubes from another vendor? Any recommendations?
Purchasing tubesets from the amp manufacturer will generally mean overpaying, which can amount to a lot with amps having as many tubes as these.

The SED 6550C is VTL's stock output tube for these amps and a fine choice, though I've also liked the results from using the drop-in compatible Electro-Harmonix KT-88EH for a less-'tubey', cleaner sort of sound (I haven't yet tried SED's own well-regarded version of the KT-88, nor the JJ/Tesla some have spoken highly of). Based on past experience with other power tubes I'd avoid the cheaper Sovtek or Chinese versions, but haven't heard those 6550's/KT-88's specifically.

The 6350 drivers are normally available made only by US Sylvania and are not costly.

Those are the tubes you should replace at 3-5 year intervals due to wear (depending upon use). The input amplifier 12AT7's may last longer than the driver and power tubes, but here it can pay to experiment with premium NOS varieties, as none of the newly manufactured examples are as good as what used to be made back in the day, and this tube is very deterministic regarding the overall character of the amps' sound.

But I won't open that can of worms here (you should do a forum search on the subject), and if you just want an inexpensive 12AT7 to drop in and see if the performance of your existing input tubes can be improved for the time being, I would recommend the commonly available US JAN Philips/Sylvania/GE varieties or Yugoslovian EI's over the current Russian, Czech, or Chinese versions.

The best pricing I've found on the more costly and numerous power tubes is at Triode Electronics Online, while the EI input tubes are sold at BTW, this is just my experience, but I haven't found it cost-effective or failure-preventative to pay a premium for new power tubes from a reseller charging extra to burn-in, pre-test, and match the power tubes beyond what the Russian tube factories already do as standard. (Close output tube matching is not needed with the VTL's individually-adjustable bias trimpots, though your input tubes should be bought as matched pairs to ensure an even gain balance between channels.)

Before installing your new tubes, remember to turn the bias pots on the output tubes all the way down first, then turn on the amps and let them warm for a few minutes, then gradually bring tubes up to spec on the meter as the they continue to warm, rebias them again after 10 more minutes, and then again 30 minutes later, plus check them again after 3 days and once more at 2 weeks, by which time they should have stabilized enough to only need a check every 2-3 months or so.
Zaikesman covered everything very, well.

I just got my first tube amp last year, and started rolling.
Both the places he mentioned were at the top of everyone's
list for good service at a fair price.

So far I've only ordered at the TubeDeopot, one power tube
blew at about 5 weeks, I sent it back and they exchanged it.
Joe was nice to deal with.

You will probably save a few $ on power tubes at Triode if you don't need them bias matched ( I think they charge $1.25 to match), TubeDepot matches all power tubes standard, but you will pay a bit more.
I have Manley Reference 350's.

Be glad you ain't got KT90's Schieza. The good one's are no longer made, but Kevin Deal tells me the new EH version are a little more lively, hmm what does that mean.

Tube dealers, Kevin Deal, or jim McShane are good choices, you know what your getting and they stand behind them.

I am toob rolling again. The current NOS Phillips @ about 5-10 each depending upon whom you buy them from are pretty good. Better than stock VTL.

The Sylvania's the old one's, I don't like as much as the newer ones. I am finding that more and more. I am not a fan of the Chineese tubes I've heard yet.

Mullard as new, as in new 6550's out. They are new on the market and pricey. If you can hold out a couple of more months those will drop in price like a rock.

I have a new set of 83 vinatge Mullard 12at7's that came in yesterday. $13 each. I will roll those, and give me a day or two to listen and I'll tell you what I think.

I made a rather large omission when I forgot to point out that although the 12AT7 input tubes may not wear out in the electrical sense as quickly as the driver and power tubes do, they are subject to deterioration in the area of microphonics, a problem which if it develops can severely compromise the overall sound of the amps. More than once has been the time my VTL's began to sound vaguely disappointing compared with memory or a solid state reference amp, but still it might take several days to dawn on me that the input tubes had started to exhibit audible resonances - D'oh! Replacement with a quiet pair immediately tightens everything back up and eliminates the creeping sensation that the presentation is beginning to sound confusingly frayed rather than solidly wholistic.

You can check these tubes simply by tapping gently on the glass envelope in various spots with something like a pencil. (Please feel free to cheerfully ignore those who admonish that you 'shouldn't' do this - it won't do any harm as long as you don't go crazy, and it is positively diagnostic, though it's true that a negative test result can't be taken to mean there still couldn't be problems). Any ringing or rattliing heard through the speakers means time to replace, and the louder and more lingering the audible results, the sooner the better. A good tube should display near silence when tapped. I recommend doing this test routinely every time you check the bias settings on the output tubes, since the problem can develop slowly and the deleterious effects are insidious but subtle at first. I would also do this test once a week for about 6 weeks after first installing a fresh pair, because you will want to replace new tubes under warranty if they show any incipient microphony during the break-in period. And although they're much less prone to developing an audible problem in this area, go ahead and do the same test on the driver tubes while you're at it, as they can occasionally become physically noisy too.

Along those same lines, I also recommend that anyone with these or similar amps make a small investment in some kind of resonance-damping rings for the input tubes, at least the inexpensive silicone types if not something more elaborate. They're cheap, they help, and many amp makers now install them as standard at the factory. Anything you can do to cut down on resonance in the input amplifier tubes will likely result in cleaner transient reproduction, more even frequency response with less tendency toward increasing brightness and forwardness under dynamic conditions, and more stable and coherent soundstaging and imaging with an overall greater sense of 'calm' and ease.
groove tubes does a good job at additional quality control on their tubes compared to the factories but you'll pay a premium. Still a better deal than getting hit with the factory markup. I'm not sure they do any additional tube QC or not.

Zaikesman, you seem to do a lot of experimenting with tubes for the VTL...have you (or anyone else for that matter) heard, or heard anything about the groove tube Mullard knock off 12ax-7s. A number of guitar amp guys have really liked them but i've not heard from the audiophile side of things
Piezo: Just as I opined above (and it is only my opinion, folks) concerning retailers who charge a premium for additional burn-in and testing/matching beyond what is done at the tube factory itself (obviously speaking only of new production tubes now), I have not found that paying extra for Groove Tubes-labeled tubes is any guarantee that small-signal tubes will show problems at any lower a rate than just buying them from the low-cost tube sellers who offer matching (sometimes at a nominal fee, sometimes not). The same goes for RAM in my experience; just like buying your tubes from an amplifier manufacturer, I haven't found that the extra cost signified in the relabeling is reflected in any greater reliability.

Andy at Vintage Tube Services indicated to me that this is because (paraphrasing now) the burn-in and testing regimens of most of these outfits is procedurally insufficient to predict meaningful outcomes, and that the cost involved to do it right would be prohibitive applied to new production tubes, which he feels are in themselves typically too inherently problematic or low-performing to justify it anyway.

To what degree that viewpoint is correct or not I don't know, but my best strategy so far has just been to buy new tubes at the lowest cost, watch for any problems to develop, and deal with replacing them under warranty if any do. Whenever I've paid more for supposedly superior burn-in and testing/matching (and relabeling if applicable), I've experienced problems after installation at rates seemingly no lower than when I buy from a discount source or when I walk into a guitar store.

To your question about a "Groove Tubes Mullard knock-off 12AX7", Groove Tubes is not a tube manufacturer but a tester/relabeler/reseller; Mullard is just a legacy trademark name these days, not an actual tube manufacturing company; and VTLs don't use 12AX7's, so whatever the tube you refer to actually is, I'm sorry but I haven't heard it. About your last sentence, I believe that most of the tube amp manufacturers who sell their own relabeled tubes do say they perform some burn-in and testing on them, but again I haven't personally found this claim to correlate with any decrease in defect or failure rates that might justify the often-substantial extra cost.
hmmm, sounds like i may have bought the spheel and just got lucky with the groove tube thing. cheap is always good and your experience level with retubing is a lot higher than mine. thanks for the input.

as to the 12ax7 mullard knock offs are the only ones i've seen marketing them that's why i applied the name. I'm pretty sure they are russian made. The ax7 is a very common preamp tube with a bit more gain than an AU7 and those two are interchangeable. I assumed that there is interchangeability with the at7s as well wich may be a bad assumption. thanks
Zaikesman - thanks for all the tips with tube install & debugging. it will come in handy.

can u elaborate on "less tubey" about using KT88 instead of 6550. i hadn't considered anything but 6550. i recall reading somewhere Luke Manley recommends only 6550s for his VTL amps.

i will experiment with the input tubes. very good idea.

thanks again.
VTL has fitted SED KT-88's to new amps on special request (it is a drop-in substitute for this application and will bias normally without modification, unlike the KT-90 according to VTL). From to what they related to me, they like this tube as much or better than the 6550C in the upper range, but still prefer the latter from the lower mids through the bass. I feel the Electro-Harmonix KT-88EH is more extended, tighter, and has more specific tonality in the bass than the SED 6550C's, but is not voiced as richly from the mids on down, and is at the same time more extended and cleaner from the mids on up, so the overall balance is on the leaner side compared to the 6550C's. (I haven't heard the SED KT-88's.) I personally would find a tonal balance somewhere between these two tubes to be nearly ideal, but given the choice, I favor the clearer, quicker, and more harmonically open presentation of the KT-88EH with most material, even if its basic character might want somewhat for richness of the fundamentals in direct comparison. But I'm something of a clarity, palpability, and openess freak, and others will gravitate toward the plusher balance of the 6550C and not care as much about giving up a little in the way of fine textural detail, 'floaty-free' imaging, or soundstage 'air'. (I prefer to adjust tonal balance at the input tubes when push comes to shove, and I also find I particularly like triode mode on my MB-185's better with the KT-88EH's than I do with the 6550C's, which seem not to thrive as much in this mode.) The phrase "less tubey" refers mainly to less added harmonic warmth and less treble roll-off along with more control in the bass - or put another way, more 'solid-state'-ish attributes, if you will.

Piezo: Although a 12AX7 might function if substituted for the 12AT7 in this position, I don't have plans to find out, because performance would probably suffer since the two tubes don't have the same electrical parameters (12AX7 is higher-gain) and the circuit was designed around the 12AT7.
Zaikesman - thank you. the mb250 runs in triode only, so it is worth considering & also to get a different perspective. "tubes rule"

btw ... it will cost about 33% more to get the tube set from vtl for the mb250s.