My reluctance to sell 6550A tubes

[Randall]

I have a Peavey Classic 400 in, and as mentioned in another thread, needs a new octet of 6550A. This amp runs them at almost 700v! A quick google search comes up with options for a matched set few and far between. On the low end ($200 + ship) there is a vendor I don't know, Viva Tubes, selling at retail a brand I don't know Psvane. There are some eBay sellers, and there is CED selling TAD STRs at $300 wholesale before shipping.

The problem I see here is if this tube torture machine eats one or more of this expensive set of modern 6550s, the customer is not going to be happy at all, and that could put me on the hook for replacements. I don't want to be in that position. So I explained all this to the music store I am doing this job for, and told them the customer needs to purchase the set himself and bring them to me, he can see prices for himself and choose a vendor, and I will install, bias, etc. That way if one or more tubes fail in the not so distant future, it will be between himself and whomever he chose as a vendor, and not me. This was agreed to, and now this huge beast sits waiting for tubes to arrive.

If I have to replace a 6L6 once in a while, or a rectifier once a year, it isn't the end of the world. I hate losing the profit I would have made from selling him a set, but in this case I really just don't want to take that risk, considering the state of affairs that modern tubes are in. I also told him I wouldn't proceed with this unless he brought a matched set from a reputable source, preferably CED or TAD.

[Enzo]

Yes, 685v on the plates, but only 385v on the screens. Dissipation kills tubes, not just voltage.

I looked at a spec sheet for the 6550, which had 660v plate and 440v screen ratings. So not pushing the ratings really.

42 watts plate dissipation, and screen dissipation 6 watts continuous or 10 watts for music.

[Leo_Gnardo]

Viva is the "new" outfit in Easthampton MA. They've been open a couple years. I have yet to hear any feedback on them good bad or indifferent.

PSVane - AFAIK another brand under which Shuguang tubes are sold.

Good on you for getting the crustomer to take on the responsibility of dealing with the vendor if his tube purchase goes wally. 8 tubes at a time, heck I'd buy a match set of 10 figuring a couple will check out early, why not have spares on hand. Maybe even 12 if he's going to haul that mammoth amp around on the road. My position is, be prepared, have extras, why waste time mailing dead tubes back & waiting for new ones to arrive. Big tube amps = an expensive game. Kind of like owning old sports cars.

[g1]

Like Enzo said, screen voltage is much lower so not stressing the tubes so much. Knowing Peavey it will be biased fairly cold. I've never heard of any Peavey amp being especially hard on tubes.
The schematics and bias info are available here: http://www.hartsafire.com/classic400/
I'd be sure to follow the note of no more positive than -53V grid volts with 120V line voltage. Then I would look on the scope and see if I could even go colder than that.

This is the one with the weirdo output transformer which means you can't get full power unless you mis-match the load:
https://music-electronics-forum.com/...l=1#post472278
See also post #51 there for tube suggestion (JJ KT88)

[Randall]

Well, it came in with the screen fuses blown, and one dead tube. Do not know which caused which.

"I'd be sure to follow the note of no more positive than -53V grid volts with 120V line voltage. Then I would look on the scope and see if I could even go colder than that."

That's pretty narrow bias target. I would/am tempted to tweak it back a little from that. I'm all for a cold bias in this situation.

[SoulFetish]

It's crazy what NOS 6550's are going for on ebay (I know, because I'm selling some right now).
But, yeah, like Leo says - not a bad idea to have your cust. deal with the tube vendor so you're not on the hook if any fail. Every time I put in a new set in an SVT it scares the shit out of me when I shock test 'em for failures. In fact, last SVT that needed a new sextet the customer ordered new JJ's. I usually warm up the tubes, and bring up the B+ on my variac, watch the mains ameter with my had on standby watching for shorts. This last time, while shock testing one of the 6550s, I saw the needle jump indicating a short and was able to cut the B+ before it took out any plate resistors/fuse/screen resistors. I wanted to double check so I went through the procedure again and the damned'st thing, the tube was fine after that. I put it through the paces, left it on, checked the bias and drop across the 5Ω plate resistors and everything was fine. I could not get it to fail again.

[Enzo]

Well, it came in with the screen fuses blown, and one dead tube. Do not know which caused which.

Pretty sure the tube failed and blew the fuse. If the screen fuse opened for some other reason, it removes screen voltage from the tube, and that causes the tube to essentially turn off and stop conducting. Hard to damage a tube by not conducting. For all the world, removing screen voltage is turning it to standby.

[J M Fahey]

For a few years now I never supply tubes any more ... (unless Iīm building a custom amp that is), but have customers buy and bring them to me.
Doubly so because we donīt have much choice , an importer brings a container and thatīs whatīs on selves until they run out, rinse and repeat.
Plus their business is buying cheap and selling expensive, so we often get bottom of the barrel stuff, I am quite certain a few buy cheap rejects or offspec ones, because failure rate is statistically way higher than what I see here or in other Forums.
So many times I send customers to buy "guitar brand" (say, Fender or Marshall or Mesa) tubes, under the assumption that being pre screened by those resellers "they canīt be the worst". So far, never had problems with those.

[Chuck H]

So many times I send customers to buy "guitar brand" (say, Fender or Marshall or Mesa) tubes, under the assumption that being pre screened by those resellers "they canīt be the worst". So far, never had problems with those.

THIS^^^^^^^^^^^^^^^^^^^^^^^

I've been thinking of doing this myself here in the US. It's long been my opinion that the good tubes and reverb tanks get swallowed up by the major users and what the DIY and internet storefront community is left with is more likely to be further out of spec, less QC or possibly even rejects from the big users that met the criteria of the manufacturer and so are resold. If Fender rejects a couple of crates of tubes because too many are out of spec you don't think the guys they send them back to are throwing them away, do you? Same for a crate of reverb tanks sent back with a note that says "These sound like shit." I'd bet, based on my experience, those are the tanks we get get to choose from. And then not even choose. Some online vendor gets that crate (with the note from Fender removed ) and they just take one out and mail it to you. That crate of tubes? Some on line Vendor takes that high failure crate and tests those tubes to find any mates that aren't grossly bad and sells them to you for high $$$ as select. If Fender gets a crate of tubes that might compromise their products reputation they won't use them at all. And the new Fender tubes at the retailer are almost certainly the same tubes they put into their amps at the factory. In this light I'm seriously considering abandoning internet storefront tube vendors. Not sure what to do about reverb tanks though.