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Practical question: toroidal transformer / heater current / kt120

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Hi everyone!

I need some help with a technical question related to power supply transformers and heater current. I hope you can provide some guidance / advice that I would really appreciate... :)

Based on some good experiences of some friends having the same amplifier, I was planning to change the 4xKT88 of my Copland CTA-405 and put 4xKT120 instead. They did it and they're very happy with the change, and never experienced any sort of problem. Let me tell you that I'm not looking for more power output (I have more than enough), I just want to try the sound with these KT120,which apparently is quite good...

I asked my local official dealer & tech. service and they said the change was OK, no problem, and Bias setting recommended was exactly the same as for KT88.

However, I saw some comments in the web saying that one should carefully check the power supply and output capabilities of his amplifier before going for that change, so that's what I'm trying to do, and here's where I need some help with a couple of technical questions:

Copland CTA-405 characteristics:

- operation in push-pull ultra-linear, 50 W per channel. Comes with 4x6550/KT88 + 1x12AX7 + 2x12BH7

- manual bias (1 individual setting per power 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.
Manufacturer/Designer claims it's a 600 VA transformer.
According to the manufacturer, the amplifier has 2 separate power supplies for pre and power amplifier (it's also mentioned in the product brochure).

I had a look at this toroidal transformer, and there's a sticker on it with the following info (see picture attached below):

PRI: 0-115V, 0-115V 50-60Hz OG-BN, GY-RD
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

>>> COMMENT: transformers are not my core-expertise, but there's something I don't understand:
- on one hand the manufacturer claims this is a 600 VA transformer
- on the other hand, if I add the Sec.1...Sec.4 I obtain:
(304V x 0,7A)+(55V x 0,2A)+(11,2V x 2A)+(6,2V x 7A) =289,6 VA.
- So I wonder if these values in the transformer's sticker are "per channel"? or am I missing something? where did the 600 VA go? :) actually, 289,6VA is roughly 50% of 600VA, and the manufacturer states that the PSU is dimensioned for twice the power requirements of the amplifier... ???? any idea??

- 2 output transformers: NORATEL SU 120A-30187 (no data available apart from the manufacturer's claim of being dimensioned for 100W)

Both PSU and output transformers are supposed to be custom-built according to Copland's specs.

Current requirement with 4xKT120:

From what I understood, the "limiting factor" and the thing to watch for, is specially heater current drawn by the quad of kt120, and the ability of the PSU to supply it within its design limits.

1xKT120 has between 1,7 to 1,95 A heater/filament current
thus 4xKT120 will mean 6,8 to 7,8 A to be supplied by the toroidal transformer. Let's take the reference of 7,8 A as a worst case.

(current KT88 have 4x1,6 A = 6,4 A, for comparison purposes)

Assumptions:

- let's assume the output transformers are OK and sufficient. They are supposed to be 2x dimensioned.

- let's remember that the official dealer recommended to set the quad of KT120 at exactly the same level as the KT88, meaning 0,45V. So I guess the KT120 won't be running at full spec, they will be "under-used"...

"The question":

Can my power toroidal transformer safely supply the amount of current drawn by the 4xKT120? (let's say 7,8 A in a worst case).

thank you in advance for your support !!!
:)

Some pics:

An externally hosted image should be here but it was not working when we last tested it.


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An externally hosted image should be here but it was not working when we last tested it.


Uploaded with ImageShack.us

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.



copland_cta405_3.jpg
 
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It's very simple - as you say, the transformer manufacturer has marked them...

Hi everyone!
- on one hand the manufacturer claims this is a 600 VA transformer
- on the other hand, if I add the Sec.1...Sec.4 I obtain:
(304V x 0,7A)+(55V x 0,2A)+(11,2V x 2A)+(6,2V x 7A) =289,6 VA.

Any claim that this is a 600VA transformer seems rather suspect - possibly some creative marketing by the amplifier manufacturer.

You could measure the size of the transformer and then compare that to other 300VA and 600VA parts available in the market (for a toroidal, 600VA will be about 15 to 20% larger in all dimensions). An example from a Multi-comp transformer data sheet - 300VA is 125mm dia by 54 high; 625VA is 139mm dia by 71 high.

What size is your example?

To address your specific question - taking a little more current from the heater winding... You might get away with it, but measure the voltages carefully, monitor the temperature. Also, if the transformer buzzes then that's another sign of over loading.
 
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Thanks for the answer.

Mine makes approx. 125mm x 60mm

I will try to resize the pics, I took a big one of the transformer precisely because I am not sure that I am interpreting correctly the specs...
In my book, that's basically a 300VA transformer. It has quite a few windings and that will bulk it out a little compared to the Multi-comp examples that I quoted.

But like I said, given that you are mainly concerned about a small loading increase on only one winding I think I would try it (perhaps changing the valves on just one channel at first) and see how it goes.

If you watch the voltages and temperatures then you'll be able to switch off and revert to the original configuration before any harm is done.
 
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