KT88 SE Abdellah (modified by Alex Gendrano) + AK..

[Kegger]

No problem, done it myself from time to time..

Defiantly something to remember in a cathode biased amp verse a fixed bias amp.. :thmbsp:

 

[AmadeusMozart]

Update

<snip>Have completed the rebuild of the amplifier. <snip> AM

Have been very happy with the amplfier, it is running very well.

A couple of minor changes have been made and I am attaching the updated schematics. First change I did was to add an extra RC to the driver's power supply part which got rid of the slightest hum. It was not a problem with 89 - 91dB efficient loudspeakers but when starting to use 100dB efficient loudspeakers it becomes audible. Result of this had a pleasant side effect: depth and stage width was improved.

Second change was instigated by getting a Behringer UCA 202 interface which puts out maximum 2dBv (about 4 dBu or 1.23 V rms or 3.47V pp) and the amp was 2.2V pp.

Any decrease in feedback leads to the inevitable increase in distortion. Two schematics are presented: one for an input of 4dBu (about 1.78 dBv) and one for 0 dBv (1 Vrms).

Best to all - arthritis is getting so bad that I can hardly move my fingers and won't be posting more. Thanks to all for their support, in particular Kegger. (I won't be building anything more)

 

[AmadeusMozart]

errata

there is an error in the text regarding the 12AX7 in one of the schematics: at the supply voltage of 300V the 12AX7 is limited to about 30V rms output before distortion gets too high.

edit: R15 was added for checking grid current to the driver tube and is not implemented in real life.

 

[Kegger]

Good work, and I hope you can get your arthritis under control, I don't have that
as of yet but I do get hand cramps pretty bad sometimes where my fingers lock up.

 

[cgraf]

Hi Guys

First of all thanks a lot for all this nice information about the KT88 design.
I decided to start a project with the "LED" version of Kegger and have one question:
Would you build this amp by designing a nice PCB ? Or would you build i using classical wiring without PCB ?
Both methods are ok for me (I design pcb's since a long time), but I would like to know from the Pro's if there are differences concerning the sound.

Best regards

Chris

 

[PakProtector]

Kegger I am having a hard time wrapping my head around how to choose an LED from data sheet info. The voltage is obvious and I presume that we want a V-I curve that is as vertical as possible so that bias is stable WRT to current but what about dynamic resistance? Is there a way to determine that from the sheets?



mike

hey Mike,
The 'dynamic impedance' you speak of is the slope of the i-V line where you're running it. The curve is near flat until you get to the glow voltage and then it heads toward vertical. It is not quite vertical, though the slope is hard to measure from the usually provided graph. You're looking for the steepest, straightest one.
cheers,
Douglas

 

[PakProtector]

Unfortunately my transformer only supplies something in the vicinity of either 300V or 350V which meant that I would not get full advantage out of runnhing UL. The 6L6GC is rather happy around 300 - 350 V with a dropped voltage on the g2 which then keeps h2 > h3. The higher the voltage the more h3 starts rearing its head and the less h2 becomes.

Regards
AM

This H3 is most likely due to driving the load line through the knee where the curves bunch up as in cut-off conditions. If the g2 voltage is low enough the load line hits along the horizontal portion when g1=0V and you get even or asymetric distortion products.

In my 6CB5A amp I have in progress I am building an adjustable regulator so I can set g2 votlage for best results...likely a bit power limiting, but with a minimum of odd distortion.
cheers,
Douglas

 

[Kegger]

Hi Guys

First of all thanks a lot for all this nice information about the KT88 design.
I decided to start a project with the "LED" version of Kegger and have one question:
Would you build this amp by designing a nice PCB ? Or would you build i using classical wiring without PCB ?
Both methods are ok for me (I design pcb's since a long time), but I would like to know from the Pro's if there are differences concerning the sound.

Best regards

Chris

On a build like this with such low parts count a PCB probably doesn't help much in building simpler,
other then that I have no problem with an well designed PCB as far as sound/noise is concerned..

 

[mashaffer]

As I have mentioned before I am using a variant of this for my mini-console project. I was in the process of slinging some solder tonight when I realized that I had mis-ordered (or used elsewhere) the 1.2K 2W MO resistors for the Screen resistors. I have on hand either 1K or 1.6K. I am inclined to think that either of these (esp. 1.6K) should be and OK substitute. Would you agree?

mike

 

[Kegger]

Myself, I would use the 1k, should be just fine.

 

[mashaffer]

Thanks Keg.

mike

 

[AmadeusMozart]

Update

Have been very happy with the amplfier, it is running very well.

Update: Have bought an Apple Mac Mini 2010 and am using that as a source (running Windows 7 and Foobar / FLAC). The output level of the Mac mini is 1.4 V RMS and I have made a small adjustment to input sensitivity, picture is attached.

At present running them with Rogers LS3/5A in my office but recently my daughter gave me back my Kef Constructor CS1 (B110 - SP1057 / T27 SP-1032) which I built in 1994/1995. Am now restoring those (at the time could not get exact filter parts, ordered the exact filter now from the UK thanks to the wonders of the internet) since I prefer those above the LS3/5A.

 

[Kegger]

I would strongly suggest if you want more sensitivity go back to a 12at7 for driver
verse a 12ax7, and either bypass cap on the drivers cathode or LED for the bias V.
(12ax7 is to high of plate resistance an low current to "drive" the output properly)

 

[AmadeusMozart]

I would strongly suggest if you want more sensitivity go back to a 12at7 for driver
verse a 12ax7, and either bypass cap on the drivers cathode or LED for the bias V.
(12ax7 is to high of plate resistance an low current to "drive" the output properly)

Hello Kegger, good to see you're posting. Actually at this level and with the 6L6GC still running in class A (and not drawing grid current) one can get away with the 12AX7.

Would not be happy going any lower in driving voltage though.

Working on the speakers is agonising slow going - have to rely on another daughter to do the work since I almost completely have lost the use of the fingers on my right hand, frustrating. But as they say in China: don't complain there is always someone worse off who will gladly trade places.

Cheers for now.

 

[Scuzzer]

Would not be happy going any lower in driving voltage though.

Wouldn't a 12AT7 actually offer more drive voltage here though?

 

[Kegger]

Wouldn't a 12AT7 actually offer more drive voltage here though?

Yes it would, with the low current the 12ax7 would net you.

And it would have better frequency response, as the load on
the 12ax7 is to low, but if you raise it then even less current.

The 12ax7 is just not a good choice for driver duty, some may
like the rolled off and "tubey" sound, but it's just not good hifi.

Anyway just wanted to give the info that an 12at7 is a better
choice for driver duty and still give you plenty of gain. Cheers..

 

[RevMen]

Just finished my build of this amp last night. I am very pleased with its sound, though I do think it needs more gain. I'm going to look into substitutes for the 6N1P in the future. For now, though, I'm gonna listen to a lot of tunes through my Planet 10/MarkAudio EL70s.

 

[Kegger]

If you give a complete amplifier schematic, I might have some suggested mods.

Regardless though that is a very pretty amp, nice job there..

 

[MX117.MC7150]

Some very nice work here. Can see it will be a longer-term project to build a set of monobloc amps based on this circuit design.

 

[Shadowdog]

RevMen! Great looking Amp!



I think the original schematic on that SE amp used a 6NIP with a 12AT7 as a sub.

Randy