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KT120 on Copland CTA-405 integrated amp

Hi everyone,

Looking for some guidance... I hope you can help!!
I own a Copland CTA-405 integrated amplifier (great gear btw). Runs as a standard with 4 x KT88 delivering 50 wpc.

I am considering replacing my set of KT88 with a KT120 quad.

Being aware of the precautions to be taken (it is generally advised to check with the manufacturer, due to potential impact on power/output transformers, considering the KT120 characteristica) and since it's not possible to contact Copland directly, I asked 3 different official Copland dealers in 3 different countries, which gave me the following feedback:

- dealer #1 & #2: checked with Copland and no problem as long as bias is set at the same level as for KT88 (0,45V). From what they explained, it's something they verified with the manufacturer, who gave "green light" and recommended same bias setting.

- dealer #3: stated that "main transformer will over run if KT120 are used and could create a mechanical noises. We do not recommend to use KT120 on this unit". Not sure if this comes from the manufacturer or if it's the dealer's own advice. Not sure neither if they reached such conclusion thinking on the typical / optimum bias setting for a KT120 or considering that they would be biased below optimum spec at the same level as KT88.

From Copland's product brochure:

"The current capacity of the CTA405 power supply and output transformers is fully sufficient for a 100 Watts output power. However, with operation conditions set for minimum variation of plate and screen current in the output stage, excellent performance is obtained with very low feedback at 2x 50 Watts per channel."

I am only aware of 2 people having changed to KT120 with this same amplifier: one good friend of mine (he biased at KT88 level) and he's having no problem at all so far since about 3 months, and another person I read about in another audio forum, for whom the swap seems to be working fine as well but no way of verifying... :-)

Anyone here tried this with this amp already, or can provide some guidance/advice? I would really appreciate... since I got some contradictory feedbacks from dealers and I would prefer to play it safe...

Thanks !!!

BTW I have no clue about the plate current, etc of the amp (not mentioned in the manual)
yater
yater OP
20 posts
 
If power transformer is indeed rated at 600 VA (as indicated by Copland) I guess it would be more than enough... (??)
wolf_garcia
6,602 posts
 
The "might fail down the road" is the sum of Failure Information to date. I am thusly comforted by my warranty, and will patiently wait for the possible mass transformer meltdown that could be sweeping Tubedom over the next couple of years. My experience so far is an imagined physical amp hum (turns out it does this very low level "stick your ear next to the amp to hear it" buzz regardless of tube type...overall a VERY low noise amp) being a non-issue, a really interesting self education regarding the sonic characteristics of various tube types, singed fingers, and hours and hours of glorious tubey music.
reynolds853
377 posts
 
I put the KT120s in my amps and didn't find them as holographic or organic as my Gold Lion KT88s. The120s were a little weightier in the bass but that alone wasn't sufficient for me to leave them in the system. The GL KT88s are twice as expensive as the Tung Sol 120s and one can certainly hear what the extra $$$ delivers. If Gold Lion ever comes out with some 120s then I may try them again.
wolf_garcia
6,602 posts
 
Gold Lions and KT120s are made at the same plant, so you may already have your wish, although "GL" KT120s sort of can't exist since the GLs are a new-ish reproduction of a classic tube...it is interesting that nobody (I've heard of anyway) is making a KT120 type in China or someplace since they're such a hit. I'm gonna buy a matched quad of GL KT88's because I'm curious about 'em, they look very cool, and they're only slightly more expensive (maybe 20 bucks) than 120s if you avoid "cryoed" or NOS originals. I already have a set of Sovtek 88s (Again, same plant as GLs...that sound great...really), along with the 120s and Tung Sol 6550s, so this might put my tube stash into the "serious compulsive tubehead" area, for which I may need therapy.
yater OP
20 posts
 
Anyone knows if a 600VA power supply transformer i good enough for the current demand of a quad of KT120 "under-biased" at the kt88 level?

I am assuming plate current would remain unchanged and that the output transformers (said to be dimensioned for twice the nominal output of the amp) should be easily able to come with it (see my first posts in this thread for details)...
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