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)
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)
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....
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?
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?
One last comment. It may "work" fine and you may think all is well....however, over time you may bring your output tranny to a quicker then usual end. The fact that your output trannys can handle 100 watts has nothing to do with your power tranny rating.
Does the power tranny get very hot to the touch? Hot enough to burn your hand after a few seconds or so? Check this with the 120's in play. Then check how hot it gets with your kt88 tubes in play after an hour or two.
Yater, I assumed 7A max spec on your tranny. I may be mistaken as your tranny may well have TWO 6.2volt/7A taps meaning you have a total of 7A per side (two tubes). If this is the way your amp is wired, then you are all set.
Just make sure you have two 6.2A/7A taps.
I am learning about tube amps and love to tinker with them. I am no expert, but just trying to help based on my little knowledge.
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...
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?
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.
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).
Looking into it.......in general. Based on what you told me, your pushing it. Again as Wolf said, it may work fine, but I would check for overheating as you don't want the unintended consequence of a tranny failure in a year or two...
The "might fail down the road" is the sum of Failure Information to date. I am thusly comforted by my warranty, and will patiently wait for the possible mass transformer meltdown that could be sweeping Tubedom over the next couple of years. My experience so far is an imagined physical amp hum (turns out it does this very low level "stick your ear next to the amp to hear it" buzz regardless of tube type...overall a VERY low noise amp) being a non-issue, a really interesting self education regarding the sonic characteristics of various tube types, singed fingers, and hours and hours of glorious tubey music.
I put the KT120s in my amps and didn't find them as holographic or organic as my Gold Lion KT88s. The120s were a little weightier in the bass but that alone wasn't sufficient for me to leave them in the system. The GL KT88s are twice as expensive as the Tung Sol 120s and one can certainly hear what the extra $$$ delivers. If Gold Lion ever comes out with some 120s then I may try them again.
Gold Lions and KT120s are made at the same plant, so you may already have your wish, although "GL" KT120s sort of can't exist since the GLs are a new-ish reproduction of a classic tube...it is interesting that nobody (I've heard of anyway) is making a KT120 type in China or someplace since they're such a hit. I'm gonna buy a matched quad of GL KT88's because I'm curious about 'em, they look very cool, and they're only slightly more expensive (maybe 20 bucks) than 120s if you avoid "cryoed" or NOS originals. I already have a set of Sovtek 88s (Again, same plant as GLs...that sound great...really), along with the 120s and Tung Sol 6550s, so this might put my tube stash into the "serious compulsive tubehead" area, for which I may need therapy.
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)...
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.
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...
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.
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....
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.
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.
I was going to order some GL KT88s for my Jolida and decided "meh"...the KT120s are doing such a great job I'm leaving them in there for now (note: Yhey hold their bias settings amazingly...I don't need to check that really but I do anyway out of curiosity and to justify having a meter) besides, do I really need 4 sets of output tubes for that amp? (rhetorical question...please do not respond)...
I just bought a used 405 and would like to try KT120's...
Copland does have an upgrade kit but seems very expensive for what it is...my question to you is did you BIAS at 45mv or something different? Also, did you do any other kind of modification?
I am Looking to MODIFY My Amp Which has * X EL34 to Take *x KT88 On its OUTPUT Stage and Modify My Input Stage which has Got 5X 6NI Input Tubes to Take ECC85 TUBEs to Improve The Sound Quality, I an after 2 X Main Transformers @ 2X Power Output Transformers WHO Makes Them or Where can I get Them to SUIT My MUSIC ANGEL XDSE AMP Made in CHINA. Please Help me out. I need The Output Transformers at around 560 Volts to Operate The KT88 , and Capable of Giving me around 90 to 100 Watts Per Channel in LINEA Mode and around 45 to 60 Watts Per Channel in Triode MODE., Your Help will be Gratefully Appreciated. KInd Regards Francis Jansz
I tried to replace the original electric harmonic 6550 to goldlion kt88. The minimum value of the bias current at kt88 was at least 74 mA. Therefore, it would be nice to know exactly which lamps are suitable for such a low bias current threshold.
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