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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
grannyring's avatar
grannyring
6,436 posts
 
If you have only one 6.2 tap coming out of your tranny - meaning you only have that one winding for your power tube filaments, then follow the wires to the power tube it goes two. It should hook into pins 2 and 7 of one of the power tubes.

Usually there is a second 6.2V tap with another two wires going to the second set of power tubes. Without it, it would seem all 4 tubes are hooked to this one 7A tap. They must all be hooked in parallel with jumper wires going over to pins 2 and 7 on each power tube.

If this is the case, then I would not use KT120's as you don't have enough filament juice for all 4 power tubes.

Do you have another tranny powering the filaments to pins 2 and 7?
yater OP
20 posts
 
Not sure Grannyring. Don't know how to check that.

Here are a few pics from the web, maybe they help...

http://www.thf.fr/uSalons/ImagesSalons/ImagesMunich05_06/Monde/Copland.jpg

http://www.novomusica.com/sites/default/files/imagecache/ampliacion/Copland_CTA405_.jpg

http://www.canhifi.com/Resources/ProductImage/1182/CTA405_%28SATILDI%29_2___Selected.jpg

http://www.uraltone.com/blog/wp-content/uploads/2010/01/copland_cta405_3.jpg
grannyring's avatar
grannyring
6,436 posts
 
Ya, your amp is a PCB board. Can't tell from the pictures.
yater OP
20 posts
 
Some additional info:

I found in a good review of this amp that the chief designer confirms that the power supply toroidal transformer is rated at 600 VA...

Also, he says that there are 2 separate power supplies for pre and power amplifier (it's also mentioned in the product brochure).
wolf_garcia
6,602 posts
 
I asked this question, more or less, when considering KT120s for my Jolida 502p. Based on my reading posts, suggestions, tube articles, labels, tea leaves, talking to tube salespeople (and talking to the Jolida people), stopping strangers on the street, and reading any info I could find anywhere regarding being careful about overloading transformers with higher demand, it would seem that nobody has ever had an actual issue with that in the history of KT120s...nobody...ever.
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