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woody 12th October 2003 01:24 AM

kt88 subsitute?
I just picked up a Antique Sound labs labs amp it can be run in triode mode. Well one of the kt88,s disn,t survive shipping
and I was wondering if a EL34 could be subsituted for a kt88?

Oh I wil have a replacement tube in a few days but I shure would like to hear this amp now.

Thanks in advance,


NickC 12th October 2003 01:49 AM

a kt 88 has a peak dissapation of 40-45 watts while el34 is only 25 or 30 with its screen tied to plate. and the operating curves are different to el34. THe closest alternative in the power dissapation is the 6550. Can your amp adjust bias aka fixed bias method? if so all the tubes can be swap say for el34 or kt 88 or 6550 same pin out .

fdegrove 12th October 2003 01:59 AM



Oh I wil have a replacement tube in a few days but I shure would like to hear this amp now.
As Nick advises correctly you need to rebias at the very best.

Replacements for KT88s are indeed the 6550A, KT90 and KT100 ( is the latter stil around?).

Unless you want to see cherry red EL34s and feel like relacing blown cathode resistors and accompanying bypass caps I'd wait 'till the replacement tube has arrived.

I'm not too sure but I think the ASL use cathode bias.


woody 12th October 2003 02:33 AM

Thanks for the advice ... seeing a plate go cherry red is a real downer, I would much perfer seeing a power tube show a little
soft blue glow.


NickC 12th October 2003 02:55 AM

for the soft blue effect
use el34 screen and suppressor tied to plate gives a internal soft blue and quite visible with the lights out. I know my svets have that effect have to check whether JJ have that effect too

fdegrove 12th October 2003 03:02 AM



for the soft blue effect
This is often an indication of residual gasses and often disappears over time. It gets burned away by the getter.

I see this far more often on newly manufactured tubes than on NOS ones...still more often than not, nothing to lose sleep over.

If any of you would like to know more about I probably could give you a link where this is discussed.


SY 12th October 2003 05:16 AM

Yes, I would like to see some references, if you've got them to hand. I had thought that there were a few sources for the blue glow, gas being one and photon emission by the glass from stray accelerated electrons being another, but I got the latter idea from hearsay.

LaMa 12th October 2003 11:14 AM

Hope this helps


C. Blue Glow -- what causes it?

Glass tubes have visible glow inside them. Most audio types use oxide-coated cathodes, which glow a cheery warm orange color. And thoriated-filament tubes, such as the SV811 and SV572 triodes, show both a white-hot glow from their filaments and (in some amplifiers) a slight orange glow from their plates. All of these are normal effects. Some newcomers to the tube-audio world have also noticed that some of their tubes emit a bluish-colored glow. There are TWO causes for this glow in audio power tubes; one of them is normal and harmless, the other occurs only in a bad audio tube.

1) Most Svetlana glass power tubes show FLUORESCENCE GLOW. This is a very deep blue color. It can appear wherever the electrons from the cathode can strike a solid object. It is caused by minor impurities, such as cobalt, in the object. The fast-moving electrons strike the impurity molecules, excite them, and produce photons of light of a characteristic color. This is usually observed on the interior of the plate, on the surface of the mica spacers, or on the inside of the glass envelope. THIS GLOW IS HARMLESS. It is normal and does not indicate a tube failure. Enjoy it. Many people feel it improves the appearance of the tube while in operation.
2) Occasionally a tube will develop a small leak. When air gets into the tube, AND when the high plate voltage is applied, the air molecules can ionize. The glow of ionized air is quite different from the fluorescence glow above--ionized air is a strong purple color, almost pink. This color usually appears INSIDE the plate of the tube (though not always). It does not cling to surfaces, like fluorescence, but appears in the spaces BETWEEN elements. A tube showing this glow should be replaced right away, since the gas can cause the plate current to run away and (possibly) damage the amplifier.
PLEASE NOTE: some older hi-fi and guitar amplifiers, and a very few modern amplifiers, use special tubes that DEPEND on ionized gas for their normal operation.

-Some amps use mercury vapor rectifiers, such as types 83, 816, 866 or 872. These tubes glow a strong blue-purple color in normal use. They turn AC power into DC to run the other tubes.
-And occasionally, vintage and modern amplifiers use gas-discharge regulator tubes, such as types 0A2, 0B2, 0C2, 0A3, 0B3, 0C3 or 0D3.
These tubes rely on ionized gas to control a voltage tightly, and normally glow either blue-purple or pink when in normal operation. If you are unsure if these special tubes are used in your amplifier, consult with an experienced technican before replacing them.

ALSO NOTE: these light sources cannot be seen in metal-ceramic tubes, because their parts are opaque. As we said above, it is difficult to tell if a ceramic tube has become gassy. Usually, in a large radio transmitter, a gassy tube will arc over internally. (This does not damage the transmitter
From the svetlana site

fdegrove 12th October 2003 12:26 PM


Here are some other links I found on the topic, of which some are contradicting.





I had thought that there were a few sources for the blue glow, gas being one and photon emission by the glass from stray accelerated electrons being another, but I got the latter idea from hearsay.
I think both of the statements are correct.

What has me a little worried though is that people with less experience start to consider the phenomenon as "normal".

I don't feel it should be viewed as such although it won't prevent the tube from operating correctly, I don't view it as an indicator of good craftmanship of the manufacturer.

While some small amounts of blue light emission on power tubes around the top of the tube can be acceptable, others may indicate that the tube wasn't manufactured in a clean enough environment and as such contains gasses that shouldn't be there, has either too low or too high a vacuum or is operated in a range that it not too happy with.

Those conditions may well reduce life expectancy for those tubes.
I don't want to start a panic here, it's not that problematic; the thing is that it takes you years of experience to tell what's acceptable and what not.

When in doubt a good tubetester is a handy piece of equipment to have as it will sift the good from the bad within a few minutes of testing.


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