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 ! |
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... :-) |
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... |
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....
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............ :-)
To be continued............. |
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?
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............. |
Grannyring,
Thank you very much for your valuable contribution, I really appreciate.
I there a way of checking if the power transformer has 1 or 2 taps?
It's a toroidal transformer with 4 sections/steps or windings as I described in a previous post, and there are 8 cables coming out of it (4 colours, 2 cables of each colour, for a total of eight cables). Not sure this means anything... |
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 |
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). |
Hi Wolf, thenks for sharing your experience. interesting!
It's good to know that there are no known issues, but... I wouldn't like to be the first one... ;-)
What I'm trying to do is to determine from a technical standpoint if the amp can operate properly with this new tube fitted in, and within good comfortable %margin (without stretching too much neither the individual components or what the amp design can handle). |
Grannyring,
Not sure I understood your last message. What's the "good idea" you refer to? |
If power transformer is indeed rated at 600 VA (as indicated by Copland) I guess it would be more than enough... (??) |
Anyone knows if a 600VA power supply transformer i good enough for the current demand of a quad of KT120 "under-biased" at the kt88 level?
I am assuming plate current would remain unchanged and that the output transformers (said to be dimensioned for twice the nominal output of the amp) should be easily able to come with it (see my first posts in this thread for details)... |
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... |
Sorry, I meant that the 7A should bot be an isue (not the 6,3... These are volts...). :-) |
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.... |
Hi,
For all Copland owners that may be interested,
I just talked to the Official dealer and technical service, and the feedback was the following:
For the model CTA-405 they have formally asked the manufacturer, who confirmed that KT120 can be used without any change in this amplifier.
Apparently Copland even recommended such change as a good upgrade.
Bias adjustment must be at same level (0,45V) as for KT88.
The dealer also confirmed that they have mounted KT120's for several customers and that they are preparing several other KT120 sets to be mounted as an upgrade for their customers owning CTA-405's.
Voila... |
Hi again,
Just wanted to share with you that after 3 weeks now with the KT120 in my Copland I can only say 2 things:
- great sound, bigger, more refined and extended, more weight in the lower frequencies and no harahness in the treble. Better in my opinion than the KT88 and 6550 that I tried from EH, Tung-Sol, Gold Lion and Svetlana SED-C.
- no issues at all. Actually my perception is that the amplifier runs cooler. No hum at all.
Thanks everyone for your contributions. |
