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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
6,479 posts
 
Your transformer is made up of several sections or windings. The 6.2v winding for the power tube filaments is what is suspect for the KT120 tubes. The windings for the B+ supply are fine.

Will the situation with the filament draw over-taxing your tranny ever really be an issue? I don't know, but that winding is not ideal for the filament draw by your 120 tubes.
yater OP
20 posts
 
D you think it could be possible that these values are actually given PER CHANNEL?

I ask that for 2 reasons:

1- as you can see in the pics (links previously posted) there are 2 cables going out of the PSU and seem to feed each 1 channel of the pcb power section. These are the purple cables, looking at the pics do you think it ciuld be arranged that way?

2- if the values were per channel, it would be in line with the designer claimng the psu to be rated at 600VA:

Adding the VA for each stage:
(304V x 0,7A)+(55V x 0,2A)+(11,2V x 2A)+(6,2V x 7A) =289,6 VA.
Roughly 300VA, half of the 600VA claimed

If the values were per channel, the 6,2 would not be an issue considering as it should feed half the heater current we discussed in earlier posts...
yater OP
20 posts
 
Sorry, I meant that the 7A should bot be an isue (not the 6,3... These are volts...). :-)
grannyring's avatar
grannyring
6,479 posts
 
Each tap has two wires, not one.....I still think you have 7a total for the filaments. Just call or email your builder for the specific answer to this question to be 100% sure.
yater OP
20 posts
 
But it that case it just doesn't make sense as compared to the 600VA claimed by the manufacturer (see previous calculation, it would be 289,6VA instead).

I have asked the question to an official dealer to be sure.

My feeling at this stage is that somehow the sticker on the psu (from where I got the info) is not very clear. It just doesn't make sense that when you add all the stages of a (in theory) 600VA transformer, it adds up to 289....
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