Tube Amps / Music Electronics
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|12/13/1998 9:13 PM|
||Ampeg amps: Reissue -vs- Vintage|
Forgive me for the length of this... A recent cross country move forced the sale of my vintage Ampeg amps, which I'm now needing to replace. I'm primarily a bass player versed on an early SVT-II coupled with the 8x10, and I occasionally hacked away at a guitar through a Reverberocket.
I'd appreciate any general Ampeg-related comments on the "vintage -vs- reissue" debate, specifically from players who have used these amps in both their original and re-born forms. Before I go diving into the morass of used gear buying, and all the pitfalls and anomalies associated with that task, I thought I should asked whether the reissues are perhaps a better choice.
A few specific questions:
SVT-II Pro: First off, how much do these current "Pro" models differ from the original SVT-IIs in terms of circuitry, components and assembly? Who supplies the tubes? And how many variations of the earlier II were there? -- I saw one the other day with a foot switch, which mine didn't have.
New SVT Cabinets: Time to downsize -- I don't want to lug that fridge around any more. Are any of the current, smaller configurations a reasonable exchange of lower heft while maintaining decent sound? Much has been made of the old models' two inch voice coils, shallow cones and sealed cabinets. How true to that design is the new stuff? And what about the new "Pro Series" cabs?
Reverberocket: What's up with the two speaker reissue version? I'm not aware that Ampeg originally made one. Anybody want to make a case for the new Superjet over the Reverberocket? Is the latter the only model w/ factory Groove Tubes?
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|12/14/1998 6:11 AM|
Can't comment on the sonic aspects of new Ampegs as this is so subjective. But, I do SLM warrantly work and I must admit that the current vintage of amps branded as "Ampeg" probably won't be around in 20+ years as the NJ made ones are. Unfortunately, having seen a few newer Fenders and Marshalls, I am not confident that we'll have that many of them to repair in the "next millenium."
The steel chassis, point to point wiring amps seem to hold up better than the pc board amps and most of the warranty work I do is resoldering connections to front panel controls. So, if you get some good feedback from musicians who have used both new and old amps and the new one are "only" "equal" in sonic quality and not superior, make your technician happy and go out and buy the vintage gear.
Holler at me if you want specific circuit advice on the newer Ampegs. As to the tubes, they are the dreaded Sovteks and a round of NOS or better quality current production tubes will greatly improve the tone quality.
|12/19/1998 1:14 AM|
Are there any particular years for the SVT-II that Dorothy should avoid? Or did Ampeg work out all of the bugs in the original SVT amps? (With the high wattage output and all of those tubes, I figure that they must have had a few problems with the original SVT's...)
|12/19/1998 6:31 AM|
Fortunately, or unfortunately, the bass players in my area don't seem to go in for tube amps and I've yet to work on the newer SVT's - have casually glanced at the schematics from time to time.
As to the older ones, the Magnavox/Tennessee made ones had/have a bad habit of eating transformers and I remember replacing quite a few in the 1970s - the factory was near Morristown, TN, and I journeyed there once to pick up a tranny and got a quick peek at the assembly line. The other vexing problem with the Maggotbox made Ampeg is shorted "protection" diodes from tube plate to ground. These mimic a shorted output tranny or tube socket and you can't replace them w/o pulling the pc board. I used to recommend either putting three in series (really don't under stand how two diodes in series can short) or just removing them. Supposedly they protected the output tubes but providing a dead short for B+ seems a funny way to do it.
PS: Have been too lazy to play with the Strat pickups yet but I'll get to it.
|12/19/1998 6:40 AM|
Actually, those diodes protect the output tubes by shunting inductive flyback from the output transformer. Fast recovery diodes with a high PIV work best. A regular rectifier diode like a 1N4007 will tend to short out in that kind of service, and I advise against using them for that job despite what some "guru" books say.
|12/19/1998 6:51 AM|
Am aware of the theory and, unfortunately, ther is no real way to determine if the diodes have done their job. I've seen older high B+ 1970s Fenders, designed sans diodes, with the original tubes that still function (and a recent 1959 Altec-Lansing PA with the original 7027s and 525 on the plates). Unfortunately, I've seen many more Maggotbox Ampegs with shorted diodes.
So, do they work, I don't know - but, can you tell me why they short instead of open up (original parts, not 1N4007 replacements) and why two in series short?
|12/19/1998 7:31 AM|
Hi, GW. I see you're also an early riser. I have no idea what I'm doing up at this hour on a Saturday
Some amps seem more likely to blow their output tubes and transformers and I can't always say why that is. For instance, I very, very rarely see an old Fender with a blown OT, but I see blown Marshall OTs and outputs every week! I suppose that underdesigned OTs, the use of a large amount of negative feedback, the lack of shorting contacts on the speaker jack, and the use of less reliable tubes (EL34s instead of 6L6GCs) might account for the higher failure rate of Marshalls. At any rate, I usually don't bother with adding the flyback shunt diodes to Fenders, but I've found that they do wonders for the reliability of Marshalls, particularly newer models with the really wimpy transformers.
Why do diodes short? Shorting seems to be the usual failure mode for PN junctions, and I'm no physics expert, so I can only take a guess. When the diode is forward-biased and conducting, the depletion region becomes very small. It seems that when the maximum safe current-handling of the junction is exceeded, the depletion region breaks down altogether. When diodes are in series, the current passes through ALL of them, so the whole string of them can very well short out. The best safeguard against this is to use fast recovery diodes with the highest PIV and current ratings you can get. The fast recovery characteristic is important because you want the diode to stop conducting as soon as possible when it ceases to be forward-biased. For what it's worth, I've only seen a fast-recovery diode used as a flyback shunt short out once, whereas 1N4007s or some such used in such service--even three in series--seem to short out fairly often. If you've ever worked on a few of those Crate "Blue Voodoo" amps, you know what I mean.
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