I bought a pair of used Cary cad50m mkii signature monoblocks amps on Audiogon. Guy had great feedback, seems like a good guy in emails. Paid $1000. Beautifull looking, In as new condition cosmetically.
When I got them today, 3 power tubes were crushed glass in there boxes. Cary double Factory boxes and styrofoam protection, so amps looked fine. He sent numerous driver and power tubes but none seemed to be matched. A hodge poge of 6550 and kt-88's and 4 jan phillips 6sl7 driver tubes.
I found 4 6550 tubes,(2 per amp) that werent crushed to death and put them in. Didn't have 4 good kt-88's. Put in a 6sl7 for each amp also. Some were Ruby, Chinese and other brand I never saw before.
Hooked them up in my system and turned them on. The right amps rear 6550 glowed then smoked. Powered off immediatly. Replaced the rear 6550 and after giving them several minutes noticed that neither the 6sl7 or front 6550 in each monoblock would light up. Only the rear 6550 in each amp lit. I switched power tubes front to rear,(Powered off first), and the problem stays with the front tubes. The 6sl7's still don't light either. So I suspect the problem is in the amp.
Could not using matched pairs of 6550's cause this ? But if so, why are the 6sl7's not lighting either? Cad50m mkii is a self biasing amp.
I have built a couple SE triode amps and am thinking maybe I can fix this myself with a little help on where to start looking.
I have seen these amps go for 1800 used. So I got an excellent price unless the output transformers are bad. Maybe worth keeping and fixing or paying Cary to fix.
I am hoping maybe just a purchase of a matched quad of 6550's will be all they need.
I contacted the seller but am still waiting to hear from him.
Any help greatly appreciated.
A couple of questions?
1. Did you test the tubes?
2. What do you mean by "6550 glowed then smoked"? Did the tube give off smoke, or something else?
3. What did you mean by "the problem stays with the front tubes"? Did the front tubes "light up" when you put them in back?
4. Did you check inside the chasis for obviously blown parts?
1.I don't own a tube tester, but I will be looking for one shortly due to this experience.
2. The rear 6550 glowed bright red, made a "pop" sound, and smoke rose up around it for 1 second as I dove for the power switch.
3. the 2 6550 tubes in the monoblock are mounted 1 in the front, and 1 in the rear with a 6sl7 driver tube in between the 2 power tubes.
Only the rear 6550 lights up on either monoblock.
I took both rear 6550's that I had watched light up in the amps, and inserted them into the sockets of 1 of the amps. Only the rear tube lit. Swapping them with each other still leads to only the rear 6550 lighting.
I tried this with both amps. Results were the same.
4. I will probablly remove the bottom covers tommorrow and look for damaged parts.
I have bought at least 10 tube amps from ebay and Audiogon over the years and never had a problem until now.
Do you have an owners manual?
I ask this because you made no reference to biasing the 6550 tubes when you put them in the amps. There is a bias FAQ here:
How long after you fired up the amps did it take before the rear 6550 glowed bright red?
Do not sweat, I am sure we will get there.
Thanks for the encouragement.
Yes they came with manuals.
These amps are auto-biasing, no adjustments necessary as per the manuals.
I am thinking then that maybe it requires matched pairs of output tubes. None of the tubes that survived shipment are matched. But then again they should still light up and maybe not sound great. Can't explain why the 6sl7's don't light either.
I would say less than 2-3 seconds after the high voltage indicator came on. I threw the filament switch, waited a few seconds and then threw the Plate voltage switch. This was outlined in the manual which I read before turning on the amps. Within 3 seconds, the tube glowed and then smoked and I immediatly threw both switches on the amp off.
Now when you threw on the "filament" switch did all tubes glow? I am trying to figure out what you mean by "glow," as the filaments are ultimately responsible for the glow of most tubes.
Did you have the amps hooked up to speakers? Did they make some sound before failure?
Did the tube show any signs of failure, white getter ect...?
We are getting there...
I had been looking at the picture of the amps at that Audioreview site for several days before the amps arrived.They are CAD50M MKII, but not signatures.
It clearly shows the 6sl7 in between the 6550's. I guess the signature models are layed out different. Never would have thought that.
Anyway when you said manual, I re-read it. I should have read past the section marked operation. On the last page of the manual, there was the tube layout for the amp. The 6sl7 is in the front socket, and the 2 6550's went in the remaining sockets behind it.
I had the driver tube and 1 power tube switched on both amps. No damn wonder those 2 didn't light.
I inserted the tubes in there correct socket, and the amps work great. Sound really nice even before a good warm-up. Albiet with mis-matched funky looking tubes. I am lucky I didn't really screw them up. Can buy some nice Svetlana KT-88's now.
Thanks for helping me out.
I am a total idiot.
I did learn something...
No worries. I have done similiar myself with results such as yours. However, I am still worried about the smoke. If you heated a resistor up and it began to smoke, you may have damaged it. For sake of mind, I would check under the hood and make sure nothing looks burnt. Indeed, I would put a voltmeter to the circuit and check the values.
Good advice for sure from Rodney. I'll bet you find a smoked resistor or two. Also, do you REALLY want to use Svetlana KT-88s? I just retubed a Cary with the EH KT-88s which sound really good according to the owner.
I just find the Svetlanas so rolled off on the top end...
About those eh kt-88's. I noticed on your web site you said the eh's sounded very good but did nothing for you in triode mode. My Carys are switchable from UL to triode. I love triode mode. What would you reccomended in kt-88 for triode mode?
Totally. I have backed off on my criticism of the Svetlana KT88 a bit because I was getting charred from the responses.... I still think it is OK in some amps but overall I find the Svetlana KT88 a bit flabby/bloated and quite rolled in the treble. Funny, because I REALLY like the Svetlana 6550... I mean REALLY!!! Their 6550 has great treble detail and extension, slammin' dynamics, and is a reasonable facsimile of the TungSol, much like pre 2003 Ei 12AX7 were a reasonable facsimile of a Telefunken 12AX7. I feel Svetlana tried to somehow "reverse engineer" a M-O Valve KT88 and actually multiplied the minor shortcomings of the M-O (as referenced against the TungSol 6550)slight lack of focus etc. The EH KT88 is exciting to listen to, like a good 6550, it is... engaging. Makes TONS of power too.
I opened up the amps today. There is a blue power resistor,(probably a metal oxide 2W), that is charred and has a chunk blown out of it.
I can see no other charred or unusual looking parts. The charred resistors match on the other amp looks fine. It measures 100 ohms,(the good one that is), the charred one measures open circuit.
This 100 ohm resistor connects to pin 4,(looking underneath), of the tube socket that had the problem. The other end connects to the triode/UL switch contacts.
This is obviously the screen gride 100 ohm resistor that gets connected when the switch is thrown to convert the amp to triode mode.
I don't have a metal oxide 2w 100 ohm in my parts bin. I will have to send for one. Rat Shack only sells 1w 100 ohm metal oxides.
How much would it change the sound if I used an AB carbon comp 2w 100 ohm in place?
The filament trans on both amps have discoloration on the chassis underneath them. The "bad" monoblock" filament trans discoloration is slightly greater. I hope I didn't damage that.
Also I am worried about the switch. If that much current blew a chunk out of the resistor which feeds to the little switch contacts, I may have ruined the switch also. I have soldered to similar switches on diy proects and have ruined some just by applying too much heat with the iron. They aren't very heavy duty.
Nice Kimber caps and Vishay resistors in this amp on an up note.
As soon as I get this thing straightened out and sounding wonderfull, I will get those EH's from you.
I am the guy that traded you that box of tubes for all those great
6dj8 bugle boys for my preamp. They are still sounding fantastic in my preamp.
Quick question, were the amps in triode mode when you had the failure?
Those metal oxides sure leave a nasty smell when they burn up. I had a similiar experience (due to my own stupidity), and it took weeks to get that stink out of the chasis.
Glad you are getting things sorted out,
Yes, I had them set to triode mode. It was an "acrid" smell. Hope I never smell it again. I ordered some Panasonic 100 ohm @ 2W 5% Metal Oxides from Digikey. Should have them next week. I am wrong about the filament transformer. I looked closer at the writing. Its a Magnetek Triad 1H @ 240 ma. choke in the power supply. Allied sells them for $8.00. Resistance is to be 50 ohms. I will measure it with an ohmeter and if its bad, get new ones. This may turn out ok. I hope.
Use 100 ohm, 2 watt metal oxide (high temp) resistors, in these positions. Keep the lead from the socket to the body as short as possible.
I ordered 10 from Digikey.
Panasonic 100 ohm 2 Watt @ 5% tolerance Metal Oxide power resistors.
Guess I'll be down until they arrive.
Suggestion: Measure all the grid stoppers. In fact, replace all grid stoppers, for peace of mind.
I can't count how many grid stopper resistors have popped and saved power tubes---and the idiotic fuse in the power stage remained intact.
If you have swamper resistors feeding the controller grid, you may want to measure these, too. Go with metal oxide, tight tolerance varieties, also. These help stabilize the amp and prevent oscillations.
Youre right the tube smoked underneath, but the ac line fuses never blew.
Is a grid stopper resistor the same as the resistor connected between the tube and opt of the amp that converts operation to triode. I thought grid stoppers were used in series with the signal input grid not g2. Duncan amp sheet shows pin 4 as g2 on the 6550.
My burnt 100 ohm resistor is connected to the switch on the amp that controls triode/UL operation.
Seem that resistor goes into effect when the amp is switched from "triode" to UL mode (super WAG, without schematic security blanket).
Replace the burnt resistors. And if they lay near hot regions---go for metal oxide. Ever see a flaming carbon resistor? Cool to look at, not so much fun replacing peripheral damage done (like the PCB board).
I have used EH EL-34'S and they were kind of a yawn compared to either JJ or Svetlanas. If you reccomend the kt-88's, I'll try them. I have to look under the hood first and try to straighten out my screw up first. I am not a great repairman on things I haven't built though. I am tempted to ship them to Cary and have them gone over and fixed right.
What besides maybe cathode resistors would go up in smoke?
The EH EL34 is a good solid tube, but I think it's pretty system dependent. I really like their KT-88, FWIW. I even can use/like them in the Citation II amps, which is my "acid test". Do be aware the EH KT-88s are pretty bass-shy for the first hours of use, then they get much better balanced. At first they'll seem a bit "hot" on the top end...
Check all the resistors around the tube that smoked, including screen resistors, grid stoppers, grid resistors, etc. Use your best powers of observation - and I'd use an ohmmeter to check the value vs. spec.
It couldn't hurt to do the same around the other tubes. It's too bad you aren't close by, I'd be happy to help you out. But go slow, if you get stuck drop a note and I'll try to help if I can.
I realize my descriptions aren't the clearest.
Here is a picture of them at this link:
The amps look very similar to this. Only difference is the 6sl7 driver is smack dab between the 2 power tubes instead of off to the right as shown in the picture.
They have gold faceplate and chrome chassis instead of all black but otherwise identical.
I do not wish to be critical here as I'm sure this is a most vexing and frustrating experience. I just have a few questions-- why on EARTH did you power up amps that obviously had a MAJOR MAJOR MAJOR problem? Whether it was the sellers' fault, or the shippers', plugging them in would have been WAY down on my list, after making contact with the seller and the shipper. I hope to God you had them insured; Angels may look out for drunks and... well... This whole thing has a kinda funky feel... he shipped the amps with the tubes IN THE SOCKETS???? I have heard of this, but it is ASKING for trouble. Anyway, I'll skip the rest and get to the point. Contact the Tube Store.Com or CARY audio and get a quote on a complete tube set. Get an invoice for what the broken tubes were worth from the seller (don't ask him to lie, but ask him to be generous) and collect that from the insurance. Inform the seller of the difference of what a new tubeset will cost, and request that he help you out with the cost.. TO PRESERVE HIS FEEDBACK. Don't threaten; the few stories I've heard here about pinheads starting communication with a seller with whom they have a dispute by making ridiculous legal threats are painful-- counterproductive and credibility destroying. Many will disagree, but you will have a hard time getting dime one by being nasty...TRUST ME. So, collect on the insurance for the broken tubes and split the cost of the remainder for a good set of tubes with the seller. I hate to say it, but if you blew a tranny you may be stuck. It just isn't "normal prudence and care" to turn on an amp that has obviously been damaged in shipping. This will be REAL tough for you... you may be able to collect by claiming that the tranny fried as a result of shipping damage, but there is the small matter of proving that. Your only hope is collecting insurance on tubes which you can physically take into the claims office, and working with the seller in a good faith arrangement. Good Luck-- these things can get ugly. One last piece of advice; take it slow; it is the holiday weekend. He may be out of town. Until you hear from him, keep it friendly. Be 100% honest; don't exaggerate ANYTHING to try to "lock down" your case. Don't piss this guy off; he may just tell you to piss off; you have some legal recourse at that point... but it'll be a pain in the ass for MONTHS.
Thanks you for your advice. Let clarify though. The tubes were all in there boxes. They were not in the amps. They amps were shipped as they would be shipped by Cary factory. The tubes weren't. They were stacked in and around 1 of the monoblocks.
The amps were in Cary factory double boxes. There were heavy factory styrofoam corners, supportting the inner Cary boxes from the outer,suspending them . The amps looked packed perfectly and safe as babys in the Cary factory original cartons and packing. Only 3 top tubes were broken inside there tube boxes. About 6 or 7 were fine. No cracks etc. There was absolutly no marks or damage to the Cary boxes they were shipped in. Having 6 good output tubes and 4 good driver tubes, I figured ok so he didn't pack the tubes in the best manner, but there are enough good ones here to make sure the amps work ok.
I have learned as have you that getting mad and pissed with someone only puts them on the defensive. I have no plans to fly off the handle. I will try new tubes and try to collect on the broken ones. But I don't mind the tube loss, I can eat that and buy new. I just want a pair of working amps.
The seller sent 12 tubes with the amps I bought.
There were 4 6550's and 4 kt-88's along with 4 6sl7 driver tubes.
The monoblocks came in 2 seperate boxes. A factory double box for each amp. The 12 tubes were placed in 1 of the monoblock amp boxes, along with that monoblock. They were kinda placed on top of the amp,(in there tube boxes), and down the inside.
Everything was sent via Fedex. The 3 top most tube boxes were full of broken glass. I guess something was stacked on top of that particular monoblock box, and the top 3 tubes took the brunt of the pressure and were crushed. I always ship tubes seperatly and cushion the tube boxes containing tubes with bubble wrap, and styrofoam peanuts.
These tubes were in there boxes then placed in the amp box, which was then sealed with strapping tape.
Should have metioned that this amp is a switchabe amp from 25 watt triode mode to 50 watt ultralinear mode. I had them switched in triode mode. Trying them in ul mode makes no difference.
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