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Tube turned black and rainbow. Cause?

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#41
I do remember something, change out the screen resistors for something more reliable. 🙂

Check current measurement resistors in cathode circuit and probably replace them too.

I don't remember having problems with leaky coupling caps, but the amps were not old at the time.

I saw a number of these amplifiers and the JA-80 (and larger) - even new they were not always reliable and were (are) sensitive to output tube quality.
 
#43
Amp

kevinkr said:
While it has been 20+ years since I serviced one (as you know), each tube has an individual bias resistor and if you reduce them to reasonable values for the 6550, the driver stage will not be able to drive the output stage properly.

Some of these amplifiers had fuses in the cathode circuit, presumably yours doesn't.

Whatever charm these amplifiers have is in the funky circuit design and output transformers, I would not modify them other than perhaps to reduce the raw AC supply to no more than 117 using a buck transformer.

I would make a jig to measure grid current and check each tube under the existing operating conditions for grid current. Select those with lowest grid current for use in the amplifier. (Get some extras)

FWIW most modern KT88 use the same internal structures as modern 6550 (there are probably exceptions) and there really isn't a large difference between them unlike MO KT88 and a GE 6550 4 - 5 decades ago..

The SV6550C if you can find NOS are probably about as good as you are going to find for these amps now.
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Mine has fuses. I will follow your advice. I have Svetlana 6550c in there, of which one went black and rainbow, and why I started this thread. I put an Electro Harmonics KT88 to try. In row with other two 6550c it draws half the current in circuit biased for 6550
 
#45
Buck transformer

marknoir said:
Mine has fuses. I will follow your advice. I have Svetlana 6550c in there, of which one went black and rainbow, and why I started this thread. I put an Electro Harmonics KT88 to try. In row with other two 6550c it draws half the current in circuit biased for 6550
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Hi
I’m not familiar with “buck transformer”. Found some online/ebay but not sure which one to get or how it works. I wonder if I could increase the 4.7 ohm resistor between reservoir caps and smoothing caps to drop the voltage. When I got the amp that resistor was MISSING completely. Someone just soldered smoothing caps to the same diode that supplies the reservour caps. Amazing.
 
#46
Amp

maxhifi said:
With respect to the other posters, I think it is premature to suggest modifying the amplifier, or replacing parts, without some concrete evidence of what is wrong. For one thing, modifications will ruin the value of an expensive audiophile amp. I could say sell it and buy an ARC, but that is also a useless response 🙂

If this is a defy 7, the schematic says the cathode of each output tube is fused with a 160mA fuse. Make sure these fuses are installed and are the correct value. If so, they will prevent chernobyl of the the output stage regardless of what happens.

I think the best course here is to put in the new tube, set bias, and just use it for a while under observation. If all is well after a few days, keep using it and the eventually buy some matched tubes once you are absolutely satisfied it is reliable and have saved the money for them (or maybe never?).

Another thing to check is bad solders on the board. I don't say to touch it up if it is good, just inspect all the solder around the suspect tube for cracks or cold/dry joints, and fix if required. This step is optional but may make you sleep better at night.

Yes you can use radio Shack contact cleaner, just a little bit in each hole, and then insert and withdraw the tube a few times, and wipe away the excess (if any) from around the socket with a rag.

The evidence here tells me that one tube went bad, so the owner threw in a used KT88 which was close enough, and got on with life. This gives me the impression that nothing significant is wrong with the amp.
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Yes, one tube went bad and previous owner threw in a used chinese KT88. He gave me a spare Svetlana 6550C, identical to the rest, which for some reason he did NOT install, but used that KT88. I pulled out the JT88 and put the 6550 in (tested nearly new on my BK747). It promptly went black and blue. I measured the bias on the KT88: when it is installed as a third tube in s triplet (Defy 7 pot supplies bias to each row of three), chinese KT88 draws 1/3 current of 6550. Russian KT88 draws 1/2 of current of 6550. They are barely working in the circuit, guaranteeing they will not run away, overhead or blow-up. I have to examine that #1 position carefully, actually all in that row. Perhaps there is a bad connection or another problem on #2 or #3, which sends all current to #1. Stupid design, if you ask me. If it fries a newly bought tube in that same spot again - ciao, Defy. I will sell it as is, and right-off my loss as experience...
 
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#47
The tubes just need to be burned in and closely matched, and all of those resistors good. I've always regarded these amps with something akin to morbid curiosity. I remember repairing some JA-80s and putting KT90s in them which were a bit more rugged than the then available KT88, and delivering them to the customer. I heard nothing more for 10 years or so, and ran into the owner at some audio meet who informed me that they had worked fine over all of those years. <relief> 😛
 
#48
Amp

kevinkr said:
The tubes just need to be burned in and closely matched, and all of those resistors good. I've always regarded these amps with something akin to morbid curiosity. I remember repairing some JA-80s and putting KT90s in them which were a bit more rugged than the then available KT88, and delivering them to the customer. I heard nothing more for 10 years or so, and ran into the owner at some audio meet who informed me that they had worked fine over all of those years. <relief> 😛
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Thank you
Well, this certainly gives me hope. Did you only repair those amps? With no mods? May be I should buy some KT90. Even though tubes in this one are very closely spaced, big bottles may have rough time.
 
#50
The sockets are these:
PLE08-0 Omron Automation and Safety | Relays | DigiKey
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Those are -plastic- relay sockets!! They are not rated for the high temps tubes produce. And someone already mentioned that that position gets poor air circulation. You would do well to change all the sockets to ceramic ones. The plastic may be near semi-conducting by now.
 
#58
New tube update

Hi all
So the new Svetlana 6550C that I was waiting for finally arrived today. Aaaaand... it’s a totally different tube. My old tubes are all serial numbers 96XX, which places them in 1996 production year. This one is 18XX so it was produced in 2018. Type is silver not gold, bottle is larger and taller, getters Fire directly up instead of 45 degree angle like mine, and the worse part - while mine are let’s say at 1v bias (between ground and fuse, Jadis instructions, I’m bringing it up on Variac), this one gets to only 150mV. Another words it will require a lot more voltage to get it to proper bias current. Even my Electro Harmonics KT88 was getting to half voltage of the old 6550s. What is going on? Did they change the tube this much??? I’m sending it back.
 
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