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Old 4th April 2009, 10:24 PM  
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These are the beasts I have.
The guts of your tubes look the same as some Sovtek KT88's that I have. The date codes on them are 99 10 but they are in the "Tung Sol" shaped glass envelope. If they are the same guts they aren't bad tubes. They don't have the extra heat dissipating fins that the EH tubes have but sound similar and don't blow up when leaned on.
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Old 5th April 2009, 03:26 AM  
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I wonder if the tubes shown are "real" Sevetlanas or some of the new ones made by Reflector? I've heard that the real ones are still available in the =C= brand is that true?

A long time ago I used a set of the Sevetlana 6550's in an Ampeg SVT that I repaired and fixed up for a guy and they were really nice tubes. Biasing was easy and they have been in that amp for a LONG time.
Old 5th April 2009, 03:43 AM  
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It says with moderate grid current which does not apply to the more common amp designs. I noticed that you're into more exotic amps, but the average amp should not have G1 grid current unless there is a problem as I see it.
Gold plated grids should be able to handle some grid current, otherwise the 5881 would not have AB2 suggested operating points. Just don't crank it to 11 with sine wave input. I cranked mine to 11 playing music for quite a while and didn't have problems.

Also, the drivers of yesteryear may not have been able to inflict nearly the punishment that source followers can.
Old 5th April 2009, 03:44 AM  
rknize is offline rknize  United States
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Those all look like Reflektor plate construction to me (smooth plates, round holes).
Old 5th April 2009, 04:04 AM  
rknize is offline rknize  United States
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Older EH KT90 (Reflektor):

Click the image to open in full size.

Tung-Sol reissues (Reflektor):

Click the image to open in full size.


Click the image to open in full size.
Old 18th April 2009, 03:02 AM  
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I've had poor experiences with Ruby Tubes KT88s. Unfortunately I had purchased 28 of them in matched quartets over a couple years prior to actually using any of them. I would learn the hard way that 11 of the 28 had major failures very early in their life. Two provided a sparking light show right out of the box, while many others just quit conducting current after about 10 hours of usage punctuated with a few on/off cycles. Though the filament remained lit, plate voltage was still present at it's pin, and fixed bias voltage was still present in it's pin, the cathode current dropped to zero instantly, accompanied by a loud pop in the speakers. I run a 10 ohm cathode resistor on each of four tubes in PPP and monitoring current flow through each resistor, the voltage drop across the 10 ohm went instantly to zero for the affected tube.

When the amp is turned on from a cold start, everything powers up fine and the amp is working magnificently for awhile. Then suddenly a loud pop and one of the four tubes just stops conducting. This problem follows the specific tube, regardless of which amp I place it in. I have even retouched the solder inside the pin ... the problem persists.

After exhaustive testing, I believe the spot welds (or lack of them)between the anode plate material to the electrode wire are no good. Whatever spotwelding current is (might be) used during manufacturing, it may be adequate for small signal tubes with small plates and fine electrode wires, but is inadequate for the big plates and wires. OK, maybe I could accept an occcasional failure every couple years, but repeated failures inside of 10 operating hours, with multiple tubes having identical failure modes ... no .

Ruby's may have a replacement guarantee, but after experiencing the identical failure modes in an inordinately large percentage of the same tube numbers, NO, I am not interested in a replacement.

I am now purchasing Gold Lions.


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