2. amps-preamps
  3. 1352857846

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
 
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...
yater OP
20 posts
 
PART 2: my in-house assumptions & conclusions:

Without being an expert, I assume from the toroidal power supply transformer specs posted above that it can supply up to a max of 7 Amp. current. This seeems to be the current drawn by the output tubes.

Correct me if I'm wrong, but as far as I understood, in push-pull operation, one tube's grid is driven until its plate current cuts off (stops) completely, then the other tube takes over and handles the power output.

If this is correct, and assuming the amp work with 2 pairs of output tube (2 per channel), I assumed that thefilament current in each channel is NOT the sum of the filament current of each of the tubes in this channel (since they actually don't work in paralel but in a sequential way).

Again, if my assumption is correct, it would mean that the TOTAl filament current requirement (drawn from the toroidal power supply transformer) would be:

- for a KT88 quad: 2 x 1,6 = 3,2 Amp
- for a KT120 quad: minimum of 2 x 1,7 = 3,4 Amp - maximum of 2 x 1,95 = 3,9 Amp.

Which in that case would mean that the current requirements are well below the 7 Amp that the toroidal transformer can supply without being overdriven.

Of course, if my assumption is not valid, it would mean that twice the current is needed: 4xKT88 require 6,4 Amp and 4xKT120 require between 6,8 and 7,8 Amp. This would be OK for KT88 when compared to the 7 Amp given by the transformer, but it would be risky (if not impossible) for the KT120.

However, this would go "against" the claim from the manufacturer stating that both the input and output transformers are over-dimensioned for 100 W (twice the power output of the amplifier).

This can lead to think that my assumption was correct, since the range of current required (from 3,2 to 3,9 Amp depending on the case) is approx. 50% of the potential current delivery (7 Amp) by the input transformer....

....
........
............ :-)

To be continued.............
yater OP
20 posts
 
PART 3: any relationship with the bias set-point ?

Apart from what I just posted above, I wonder if the filament current is related to the bias set point, or to put it otherwise, if a lower bias setting means a lower filament current in the tubes.

I think the "optimum" bias setting of the KT120 is more "aggressive" (higher) than for a KT88.

IF there is a relation btw bias setting and filament current, I guess that if I follow the instructions of setting the bias of the KT120 at exactly the same level recommended for KT88, I would be biasing the KT120 below their optimum set point then in turn I would have a lower filament current (???).

In that case, it would be again going in the right direction in terms of not over-driving the transformers, the whole set-up would be relatively conservative... isn't it?

..........
.............
yater OP
20 posts
 
Well... that's about it.... never thought that I would have to refresh my BASIC electrotech classes.... :-)

Thank you for your patiance if you took the time to read it.

Looking forward to hearing from your experience, and I would really appreciate if anyone could confirm:

- whether or not my assumptions / conclusions are correct or make any sense

- which of the official dealer (#1 and #2 vs. #3) was right... :-)

THANK YOU !
grannyring's avatar
grannyring
6,436 posts
 
As I understand it each power tube will require the rated filament juice. In this case KT120's will pull from the 6.2v 7 amp taps. Each KT120 requires a minimum of 1.7A. Four tubes x 1.7a = 6.8A. The tranny is rated for 7A so.....However, the KT120 can draw as much as 1.9A meaning the 7A tranny spec is not enough.

I think this is too close for comfort to be honest. I would consider installing a separate filament tranny rated for 8A and wire it in. Do you have room?

I purchased a great NOS filament tranny for $60.
More to discover