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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
jmarcedo
1 posts
 
Hi,

I use the Tungsol KT120 in my Copland CTA-405 with the exact same bias values and they worked fantastic, as this valve is great!

So, go for it and enjoy your amp!

Jorge Marcedo
yater OP
20 posts
 
Thanks for your feedback Jorge. I actually saw your posts in a forum in Portugal (I'm based in Spain myself) but I had no way of asking you directly about your experience... :-)
yater OP
20 posts
 
Some additional info + some "in-house" assumptions / easy calculations / preliminary conclusions:

PART 1:

Copland CTA-405 characteristics:

- operation in push-pull ultra-linear, 50 W per channel

- manual bias (1 individual setting per tube)

- from Copland CTA-405 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." The transformers are "custom made" based on Copland's specs.

- Power supply toroidal transformer: model NORATEL RTD 250-30144 with 4 windings:
Sec.1 : 0 - 304 V / 0,7 A / VT-VT
Sec.2 : 0 - 55 V / 0,2 A / YE-YE
Sec.3 : 0 - 11,2 V / 2 A / BU-BU
Sec.4 : 6,2V / 7 A / WH-WH

- 2 output transformadores NORATEL SU 120A-30187 (no data available)

to be continued...
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