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For the most part, the output tubes made today may sound good but they just don't hold up. Amps that loose circuits, even just the bias/grid resistors when output tubes blow, are not good enough!!! Who makes amps that consistently loose nothing more then their B+/rail fuses, or, if they only have mains fuses, loose only the mains fuse, while completely protecting the electonics???????
-Jerry O.
Follow Ups:
Come on guys! There must be some other brands that can hold up??? If not, the state of tube amps is kinda sad......
-Jerry O.
tube amps are like boats and you're bound to have some downtime.I've got some Cary SLAM 100s that keep blowing coupling caps left and right and I'm about at wit's end with these amps. Presently, I'm running some nice & easy Wright Mono 8s with some '74 Cornwalls and I don't have that ticking time bomb floating around in the back of my mind.
Chris,What kinda coupling caps does this Cary carry? Maybe, a little under-rated, voltage-wise?
Hello Steve,I had Cary do the Jensen oil cap upgrade ($200 for .22uF/630vdc x two per amp) when I first picked up these 2 yr. old (were built in '98) used amps and had one die. Less than a year later, running nice Tung Sol smooth plates, one died while playing some music. I waited 16 months to finally send the one in and it was a Jensen cap that had shorted, so for a total of $175, Kirk replaced the cap and shipped it back to me.
Nine days later, the amp died at turn on. I opened her up and fiddled, finding that one of the two Jensens (don't know if it was the recently replaced one) had a short in it and since I can solder slightly, Kirk send me another for free. I put it in and for the past 2-3 months, the amps have been sounding great. I check the bias every few days, noting the AC wall voltage, etc. and I rematched up the Tung Sols after the long stretch of down time. No shorts, decent gm, etc.
About two weeks ago, after puttling the left amp (not the recently dead one) into standby for a couple of minutes, I heard motorboating and turned it off. The SLAMS use 4 tubes per amp and what usually happens is that one pair goes ballistic and the other pair goes dead or stays cooler.
I figured that since Kirk stated to me that nothing can really short out the Jensens in the amps, short of bad tubes, the problem is just in the caps themselves going bad. Some have claimed that oil caps are not the most reliable, but people like Ron Welborne and Kirk have stated that they haven't had any major returns on the Jensens. I opened this amp up and found a shorted Jensen cap.
At this point, I figured that a new brand was in order. I ordered four Jupiter beeswax caps (.22uF @ 600v) for $75 from Parts Connection. I got them in and soldered one into the bad spot, just to see if it would work and no dice. I then started throwing them all in and but for one 15 min. period where I was able to actually bias up the amp before the fuse blew, I killed them all.
I checked resistors, etc., and thought I saw a Dale that was out of spec by 22% (55k vs. 68k ohms) but it tested fine when the amp had cooled the next morning, so I'm at a loss.
I can't keep shipping these amps into Cary, so I'm looking for somebody who is local and then I plan on selling these boat anchors.
When I originally sent them in, Cary said that they fixed them and updated them with parts and the newer switches for 4/8 ohms and triode/UL and I realize that they don't rebuild them, but short of the Tung Sols being the cause, I don't know what to think?
I love the sound and they work well my the Dyn Contour 3.0s, but it's frustrating. If it was just the Jensen brand, I'd be a happy camper, but a fellow inmate has the same amps w/Jensens and he had two caps short out just like mine did, after his upgrade, so either Cary runs them too hard, or the values are too light for the job.
Any thoughts would be appreciated.
Chris,If you are blowing Jensens, rated at 630VDC---there may be something else going on (bad-wise) with your amp. Like some kinda bad instability. A friend had a 6550 driven Cary which needed new swamper resistors and grid stoppers. These should be metal oxide type, 2 watt rated (to withstand the heat).
And soldered so that the body of the resistors are right up against the socket lugs.
Check the resistors, in the pi-filter. Heck, just replace them. With metal oxide boys. Any drift in these can cause your B+ and C+ to be off whack. When were the filter caps last replaced?
Check the voltage point of the bias power supply. It may be off, from what the nominal value should be. I'd replace the diode(s), electrolytic caps, and resistors (use metal oxide) in this circuit. May be your idle bias is drifting up and up, as the unit is heating. So what looks like a good bias, at 15 minutes, may be creeping up in one hour.
Clean the sockets and the idle bias pot(s) with contact cleaner. Tighten the socket lugs, if need be. They may be loosening up with heat---and causing a poor contact.
Replace the feedback resistor and bypass cap off the secondary transformer. Especially, if these are close to heat. Metal oxide resistor and I use 1000 or 1600 Volts rated disc bypass cap.
The coupling caps from the phase inverter to the power tubes may also contribute to some instability. Try a plain-Jane SBE 715P 0.1 mfd/600 volt Orange Drop in place of the 0.22 mfd/630 volts mega-bucks caps. It's just a cheap empirical test---to see if the cap is contributing to the problem.Hopefully, something may help. These are the cheaper "tries" I can think of. Email me, if you are still having problems.
I got your email and unfortunately, a lot of it goes over my head due to the labels that you use. I know what the coupling caps are, etc., because Kirk told me what to look for. I may snap a small picture of the area and send it your way just for kicks.Jose Korneluk builds amps and is an EE and is up in Palm Bch Cty., not too far from me and I may ask him to take a look as he has offered his services to other locals nearby.
A local dealer here stated that these amps had some issues and Sherlock has had some of the same caps blow on his SLAM 100s, so maybe it's just a twitchy design? I don't really run anything at the limits and when I was installing the Jupiters, I broke out the Svetlana 6550Cs and dropped them in just to rule out a bad Tung Sol, so I'm about beat.
Take care and thanks for the advice.
Atma-Sphere M-60mkII.2 simply bulletproof...
potentially allow DC to pass thru to your speakers thus endangering your drivers?
Because it's basically two single ended amplifiers wired together bridged (differentially) the DC components cancel, by and large, with the help of a DC offset trimpot. I monitor this roughly monthly, or as the mood hits, or if I see or hear a need to do so. After experiencing a few tube section failures with NO audible results (NONE) I'm convincing they're bulletproof. After a complete retubing, I find no failures in over a year of daily usage, with 16 output tubes, a pretty good record. Ralph describes the basic circuit best.
nt
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