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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 24 Jun 2010, 01:25 
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Just got my Tung_Sol KT120s today and ran a few tests on them. Also since the tests I have managed to get a few hours listening in. Tubes this big should take 100hours plus to burn in. After the power test (below) I have listening notes. My current amp is the Be Bamp which usually has matched JJ Blue Glass KT88s running at 93mA idle current.

The amp is SE UL with 0NFB.

Image

I ran a power test with the KT88s to see if simply by plugging in the 120s I'd get more power. And I did!

The way I tried to test initially failed so a resistive 9.4ohm load with the digital CRO and the function generator was used.

All tests @ 1khz into 9.4 ohms.

The KT88s produced 6.6W RMS. By simply replacing the 88s with the KT120s the max power jumped to 8.0W.

My speakers are 6ohms and with the 88 I have measure 7.8W in the past. I had to use two 4.7ohm resistors and just read across one to keep the input voltage peak across the digital CRO down.

You can see a 17% jump in power simply by swapping tubes out. I upped the plate idle current to 111mA but this gave no more power so I left the idle current at 93mA. This way I can roll between 88s and 120s.

Worth the money for sure. I bought a quad matched set. The tubes are big but not a patch on my 211s. Think of over-sized KT88s and your in the ball park.

Sound appears a little more open with the 120s. Good authority and transients, balanced. Lighter sound than the 88s. The 88s are a little darker, these feel brighter and crisper through the mid range. The bass starts to roll off after 31Hz but still plenty of energy at 20Hz. The bass tests are from a CD so the whole system is still delivering down at 20Hz!! There is a room resonance at 50Hz (windows rattle, floor (wooden) shakes.

UPDATE - see this blog post for my Tung-Sol KT120 Tubes - SE Listening Impressions and Notes (29 June 2010)

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PostPosted: 24 Jun 2010, 19:59 
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After upping the idle current on the KT88 and enjoy big strong deep bass I was worried I would lose it with the KT120s. I played a track which I knew had an almost sub-sonic bass notes. After playing the track through twice I could not hear the very low and runbly bass notes I was use to.

Then I realised the KT120s were producing the same very low note at the same volume level only now they had complete control over the bass driver. With the KT88 the bass note got lose, flabby and bloated. The KT120 had the note by the bass clef.

Because I use 0NFB there is the chance of amp instibility. The only time I have seen this on any of my KT88 based amps is in the bass. I think with the KT88s in place the amp was becoming unstable at very low frequencies. With the KT120 this was not happening.

It appears my comments about a crisper more open mid-range have been agreed with by another KT120 user. And now I can also agree with his comments about very good bass extension. When developing these amps I used, for a short time, 5881s. Until when testing the low frequency response I found another 10hz just by using KT88s. So tubes will not olnly predict the over tone of your amp but things like bass weight, authority and bass extension.

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PostPosted: 24 Jun 2010, 23:55 
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Hi,
So in laymensterms you are saying that instead of the bass note 'rolling' off and the speaker waffling back and forth, the KT120's actually had control of this note?

:D

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PostPosted: 25 Jun 2010, 02:24 
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Yes. More control than the KT88. It wasn't until I compared the two that I could hear the difference. With the KT120s in from a test CD, I can hear my system starting to roll-off at 31hz. But even 20hz had lots of energy but maybe at 20hz even the KT120s were starting to lose it.

Bass is good though and I feel better control than the 88s. Mids a little clearer and crisper.

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PostPosted: 25 Jun 2010, 05:36 
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Hi again, Now i hate to be the man of a thousand questions but do you think it can handle the bass because it isnt having a large amount of power drawn from it, I.E. rather than using it to build a 60W class A amp is it better to use it in a low wattage KT88 one and take advantage of its extended range

Hope im not too annoying : :wizard:

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PostPosted: 25 Jun 2010, 06:37 
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Mate ask away. I'm very happy to talk to and help others where I can. I'm going to ask Bruce to chime in here, he is the tube guru and the one who put me on to these tubes a few weeks back. Once again I can only say every tubes has a certain signature and Bruce, on another thread, out-lined a number of tubes and made a comment about how he found their individual sonics.

I can only guess a bigger tube like this may have better bass grip. But that is a simplistic way of looking at. Bias and loading comes into it. I noticed and measured more power with these tubes changing nothing else. More power could relate to more control. I'll let Bruce fill in the gaps.

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PostPosted: 25 Jun 2010, 08:14 
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I experienced a very similar phenomenon when switching between a KT88 and 6CA7. The 6CA7 had more control and at first sounded like less bass, until my wife and I realized that the KT88 sounded "flabby" by comparison. This was in a SE Triode amp.

There are several factors that will infulence bass performance. Here's a list in no particular order of preference. (Not counting any local or global feedback)

1 The Power Supply type and configuration.
2 The coupling cap size and manufacturer
3 The topology of the output stage to include the output impedance
4 The primary inductance of the output transformer
5. The driver tube and it's loading
6. The RP of the output tube

And to a lesser extent even the power cable used for the amp can have an audible effect on bass performance.

This also doesn't take into consideration the reactance, capicatance and inductance of the loudspeaker and it's effect on the frequency response of the amplifier in question, also this doesn't take into consideration room treatments, room size and speaker design.


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PostPosted: 25 Jun 2010, 12:10 
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Hi All, A can of worms for sure. I have been using the KT120s for a number of days now. The amps are my reference set of OddBlocks. I can change the CCS current and other factors pretty much instantly. My initial thoughts were similar to Mark's that the bass seemed less. Additional listening convinced me that was no the case. As much as I like the JJ Blue Glass KT88s, they probably don't control the vey bottom end as well as the KT120s. I did not measure them yest, but I suspect they will also go well below 20HZ in the amps. Everything else does (most tubes go to 12-15 HZ flat in these amps). I did notice that the 120s like to feed a lower Z than many other tubes. I suspect that about 6-6.5K is the correct match in place of the 8K I use. When I increased the idle current to 112 per tube (pp-U/L) they worked better into 4K than the 8K. The KT88s work about the same into either Z. As soon as I get a few more hours on the tubes I'll do some bench testing and let you all know what I find.

A note of caution for folks with fixed bias amps set for KT88s.... KT120s bias at a different point. They need about 25% higher (more negative) bias than the KT88s to maintain the same idle current. I suggest if you have adjustable bias to measure the actual current to keep from smoking something.

Good listening
Bruce

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PostPosted: 25 Jun 2010, 18:05 
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rock4016 wrote:
I experienced a very similar phenomenon when switching between a KT88 and 6CA7. The 6CA7 had more control and at first sounded like less bass, until my wife and I realized that the KT88 sounded "flabby" by comparison. This was in a SE Triode amp.

There are several factors that will infulence bass performance. Here's a list in no particular order of preference. (Not counting any local or global feedback)

1 The Power Supply type and configuration.
2 The coupling cap size and manufacturer
3 The topology of the output stage to include the output impedance
4 The primary inductance of the output transformer
5. The driver tube and it's loading
6. The RP of the output tube

And to a lesser extent even the power cable used for the amp can have an audible effect on bass performance.

This also doesn't take into consideration the reactance, capicatance and inductance of the loudspeaker and it's effect on the frequency response of the amplifier in question, also this doesn't take into consideration room treatments, room size and speaker design.

I have 6ohmm speakers on an 8ohm tap.

Also I am about to run some more tests on mine: idle current, power and frequency response. Post the results soon.

By the way my OPTs are 5K with only a 8ohm tap. At this point 93mA at -34V bias. To be confirmed.

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PostPosted: 25 Jun 2010, 19:39 
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I have run the tests on the Be Bamp with KT120s. Few supprises and confirmations:


HT+ =400V, idle current =112mA, cathode R =300, bias =34V.

All test done into 6.8ohm 5W resistor (this was a mistake-haha). The reason for 6.8ohms and not 8ohms is my speakers are 6ohms. I figured tests I did with this resistance would come closer to what I could expect to hear in listening.

Lower 3db point =26hz upper 3db point =36khz. My OPTs are Edor open 15W 5K units. Max power was with 23Vpp across the 6.8ohm R. This translates to 9.7W RMS. The reason for the "haha" above is the load R was rated 5W. I touched the resistor to see if it felt like it was disipating close to 10W and burnt my finger - haha! Yes there was definitely 9.7W there - ouch!!

One observation was the 400V HT+ did not sag at max power. It sayed dead on 400V. But my power tranni is rated 175mA on the HT. I'm now pulling excessive current. The power tranni does get very hot. I can still hold my hand on it for a short time so I think it is OK.

The suprise was 112mA idle current. I thought it had not changed from the KT88s at 93mA. But as Bruce has said the current should have been higher for me to see a few extra watts. He was right. With the KT88s I get 7.5W. Just dropping in the KT120s I'm getting a full 2W more in class A SE UL mode. For me that's a good improvement and confirmation of original rough test. I think with the extra power I'm getting better bass control. Definitely but possibly not related, is a better mid-range. With a vocal track I am playing now and have played to death in the past, vocals are much clearer and crisper, snappier!

Image to follow.

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