Originally posted by Treena Foster
All-tube power amps, or all tube amps per se, are great, but heavy on maintenance
Not true. Once a tube amp is up and running there is NO maintainence involved, period, end of story. When its power tubes wear out after about 25 years, it needs a cap job and a rebias, just like a ss amp would need a cap job, new output and driver transistors, and a rebias.
The power stage can have anything from 4 to 8 tubes, like KT66s (or 88s, I forget) and these cost some ... plus there's the need to get them matched and biased, so it takes a qualified tech to do the job.
Any reputable tube dealer should be able to supply a set of matched tubes and a bias job is not that difficult to perform. As I stated earlier, ss amps require biasing as well, but no one has gone plastering that in transistor ads in guitar magazines so most folks aren't even aware of it. It's just "something that the amp repair guy takes care of."
Also, power amp tubes don't last as long, especially if you place the amp head on the cab and play - the vibrations, when the tubes are hot, are the quickest way to shorten tube-life. As I stated before mine are rack mounted in a SKB, it has a shock mount system in it so I get a lot of time out of my tubes.
Dirty little secret here: if vibrations are strong enough to cause microphony in power tubes, they're also strong enough to crack solder joints and unseat IC's in ss amps. Vibration is bad, period, not just for tube amps.
Tubes are probably the single most sought-after part for old Ampeg amplifiers. Some of the tubes used have since become very rare and expensive. Ampegs have used 7027a, 12DW7, 7199, 7591, 6BK11, to name a few off the top of my head, that command premium prices. These tubes are still available, if you look hard enough. Don't expect to just walk into your local guitar shack and find them. Keep an eye on the internet newsgroup rec.audio.tubes. Fortunately, interest in tube circuitry is on the rise, and factories outside the United States have begun to add some of these tubes to their production lines. The 7199, for instance, is being manufactured in Russia and is marketed by Sovtek, although it's been reported that the Sovtek 7199 is not a true 7199, but is merely pin compatible. (Supposedly their 5Y3 isn't a true 5Y3, either, and I've read the same of their 5881; it seems that many old Soviet tubes are similar to desired types, but not quite the same, but they number them as such anyway.) I have also seen the 6BK11 listed in Sovtek advertisements. They've been receiving requests for a run of the 7591, so it wouldn't surprise me if that was to appear soon as well.
Treena
You forgot to give Aspen Pittman credit for that. That last paragraph is lifted from
The Tube Amp Book, which he wrote. Aside from the VERY useful schematics, most of the book is just pretty pictures and total Groove Tubes ad BS.
Besides, most of the "info" in that statement is outdated. Sovtek has been marketing a 7591 and 7199 for the past two years (not that ANY Ampeg bass amps ever used them or the 7199 either). A 7027 is nothing more than a 6L6GC with its grid connections pinned out an extra pin. The 6L6GC is a
direct substitute for
ALL Ampeg amps. A 5Y3 is low powered tube rectifier and not used in bass power supply circuits either and is in VERY strong NOS supply. SVT's use 12DW7's and 12BH7's (currently made by both EI and JJ electronics) and V4's and B15's use 6SL7's (currently made by Sovtek). B15's also use a 5AR4 rectifier which is currently made by several manufacturers. Modern SLM Cratepegs don't use ANY "weird" tubes, so if you've got an SVT CL or V4BH, you've got nothing to worry about.
If you're going to use a line, credit the author so that it can be put into proper context. Pittman was a salesman for Acoustic amps in the 70's pushing their s/s amps for years before he began cashing in on the emerging tube craze in the 80's. Many of his technical claims are ridiculous and ring more of sales pitch that true information (see my above post). Part of the reason that people think tubes need replacing every five minutes is because of his advertising which is designed to sell his tubes, not tell the truth.
You've been climbing around those 300 Watt + monsters too much and forgotten about the low-powered jewels.
