Monday, April 20, 2009

audio: tube servicing

After several years of light use, one of my monstrous VTL MB-450 power amplifiers died. I replaced its KTK-2 fuse (ordered from Fuses Unlimited), and it worked fine for a few listening sessions, then it stopped, I replaced the fuse again and it died an hour later.

The right thing to do is ship all my amps back to VTL in beautiful downtown Chino for service, tube replacement, and rebiasing; they could simultaneously install the the coupling caps and the extended tube life upgrades that they have developed for this model. Total cost about $2,000; a worthwhile economic stimulus to support a small company with top-notch service, but it will have to wait for my OBAMPO (Obama bailout amp owners) stimulus money. Bea Lam at VTL thought the most likely explanation of these symptoms is a bad tube, so with trepidation, I did the wrong thing and tried to fix it it myself.

I watched the videos of guitar amp tube replacement (!) on ExpertVillage and eHow and carefully removed all the tubes. At first they all looked OK, I envisioned dangling wires and rattling noises like a broken bulb.
6550C tubes, broken Svetlana second from left, replacement SED tube
Looking more closely I spotted a silvery-brown discoloration on one tube (second from left). I ordered two replacement tubes from Magic Parts for $70. I received a matched pair labeled by Ruby of the winged C rename of the original Svetlana version of the 6550C variant of the original 1955 Tung-Sol 6550 design, all the way from beautiful downtown St. Petersburg Russia. (The tube on the right.)
VTL MB-450 with the top off and three valves removed
Reinsert tubes, adjust the bias on each one in that row of holes (risking death from the high voltages and massive currents), insert another fuse and the music is back!

The history and complexity of vacuum tubes is impressive: hot clouds of electrons, filaments, plates, platinum grids, etc. refined over a century. They demand careful mechanical and electrical engineering, are hand assembled, and even then every one has different electrical properties and benefits from careful selection and matching to its partners. I don't get obsessive and mystical about it (the VTL amplifiers simply sounded much better than solid state amplification), but many audiophiles and musicians do: in a guitar mag interview John Mayer confessed to spending evenings auditioning and matching tubes for his amps. Enjoy your equipment, but love your music collection more.



  • Beautiful!
    6550's are well worth the effort. I've got a small cache of Tung-Sols on hand for emergencies, and must admit I also periodically have 'tube swapping nights' where I check out new glass for the amps here at home.

    Electroharmonix (EHX) has started selling some absolutely gorgeous 'old/new' reissue tubes manufactured in the original Russian factories, for what it's worth. I'm exclusively using their 12AX7 preamp tubes now in both bass and guitar amps (very warm and round, but IMHO a bit crisper than the originals, with much clearer high-end definition), but still rock Tung-Sol 6550s in the guitar amp power sections (at least until my vintage stash runs out)... :)

    By Anonymous Scott Fegette, at May 27, 2009 11:18 AM  

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