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Question for Cary SLI-80 users: I recently replaced the Mullard EL-34 with GEC KT-88 tubes. The liked the sound with the EL-34 tubes but I just wanted to see if I get more dynamics and sounstage with the GEC KT-88 tubes when listening to orchestral works.Well, I set the bias at 0.75 mA as recommended by Cary, but what I heard, among other things, excessive sibilance and lean bass in a Nat King Cole CD, and the "magic" of EL-34's disappeared, though the soundstage became wider.
Follow Ups:
Before switching to SET amps, I owned 2 Cary amps (SLI 80 and SLAM 100s) and I used NOS GEC KT88s. For push pull, those tubes are killer. Fat, big, and very musical. Not as musical or detailed as SET amps, but as good as you can get for push pull designs. A close 2nd are NOS Tung Sol 6550s. Some even like those better. Your SLI 80 should sound _wonderful_ with genuine NOS GECs. Are you sure the tubes were not used? Used GECs can test good after thousands of hours, and the magic is spent. What are you using for octals?BTW if your GECs have the smaller Lion logo, they are not original NOS. They are later versions made in China and sound nothing like the real deal. Not trying to pour cold water----just trying to help trouble-shoot. That amp can sound very very good.
I purchased them as NOS from a HK dealer (or at least they were offered as such) three years ago. They are three-getter type and have blue GEC labels on glass, so they should be 60s or early 70s vintage. I have approx. 50 hours on them. I have yet to decide whether I like that overblown "fat" sound or not. Yesterday I felt that they tend to smear the focus (or it was by contrast with EL-34 tubes).I currently use Sylvania VT-231 as driver tubes, Amperex 6922 orange globe gold pins (which replaced Mullard CV 4110) and Sylvania 5U4G rectifiers.
You don't mention whether you've given the GEC's time to burn in or not... the excessive sibilance and lean bass are classic signs of unburned-in KT-88's, so if you haven't burned them in yet then do so before you go playing around with biases.
They were well burned in before my today's attempt. After lowering the bias setting, I am going to continue my exercise by changing the 6922 tubes.
Follow-up #2: To explore this further, I switched into another amp (Unsion Research Sinfonia) which is also equipped with GEC KT-88 tubes (70's vintage). Though this tendency persisted, it was not so irritating and was even barely noticeable.My conclusion is: either the former GEC tubes are incompatible with Cary (whether tonally or technically) or the Sinfonia amp can handle complex or problem passages in a more civilized manner.
...will not be likely to get you more dynamics.The EL34 is a 25 watt anode dissapation tube and a pentode.
The KT88 is a 40 watt anode dissapation tube and a beam tetrode.
Very different animals, the only thing in common, and for this you are lucky - is that they have similar (but not identical) pinouts.
To get the extra power capabilities of the KT88, the amp would have to be redesigned to supply much more B+ (high)voltage to the tubes, to have a bigger powersupply to provide the 70% increase in power the KT88 could allow, and perhaps even a bigger output transformer to take the extra power.
If you need more in-room loudness, then the only two solutions are either a much bigger amp, or much more sensitive speakers.
Please note - somewhere there is a confusion regarding the bias current. It's far more likely the amp's tubes are running at 75mA of bias. At only 0.75mA, it would sound horribly distorted!
Maybe you mean 0.75 volts at a test point, not actual mA of current?
Sorry, I meant to say 75mA.This amp was designed for 6550 or KT88 tubes, but also accepts KT-66, KT-90 and EL34 tubes.
Follow-up: setting the bias at 0.60mA gave a more relaxed picture but did not remove excessive "SSSS". Either it due to the poor quality of recording or I should play with other tubes and cables...
With my SLI-80 I recently moved from the Mullard EL34 to a PentaLabs Shugang KT88 Smooth Plate tube. You will need to put at least 100+ hours on the KT88 tube for them to settle down. At first these tubes were bright and harsh but did provide for a tighter bottom end. A quicker paced tube over the EL34....more punch and drive so to speak.I have since moved the bias up to 90mA and noiticed the sound is more fuller, not as bright on top and the bottom end is even more impactful. Still not an EL34 which has that midrange magic, something that I miss.
Yes the amp is designed for KT88/6550s and some feel EL34s just dont sound right in this amp. They could be right.
I have used Sylvania VT231s but they provide way to much detail for my liking, hence the GTBs listed below.
My current tubes: Siemens 6922 Gold Pins, Sylvania GTB 6SN7s and Valvo 5U4G Rectifiers.
Darrin...
Forgot to mention, here is a tip on Bias levels for 6550s I came across at TubeWorld. This is where I got the idea to crank up the KT88 to see what would happen, similar experience for me:COMMENTS:
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"I am using a Cary sli80 integrated tube unit. In case you get another customer with one I thought I would pass on the results of my bias tweaking with the smooth plate Tung Sol 6550's. Cary says anywhere from 75ma-100ma per pair. 75 is cool, detailed but little warmth. 85 is warm, clean and has nice high end extension. 90 is too warm and loses the high end. So 85ma it is. Yes I tried 80 but 85 is still better."
Yes, I remember this quote from Tubeworld. Meanwhile, I cured this problem by changing the interconnect. I have several cables, incl. VH Audio Pulsar, Kimber Select 1021, but every time I hear some artefacts I get back to the Straight Wire Virtuoso R which I think is underrated but has a "conservative" analog-like sound and fits very well with my setup.With the GEC KT-88 tubes, I initially set the bias at 75mA but did not hear the "magic" expected from these tubes. The sound was sort of overblown, though a bit more transparent, while a 60mA setting gives a more relaxed sound, with tight bass and magical midrange.
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