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stereo SE kt88 build ... abdellah diyaudioprojects design

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#321
88man said:
The externals of the amp are finally assembled, the red oak stained with rosewood finish, all terminals, switches, and tubes mounted, and the chassis work is completed, transformers have rubber grommet washers between the chassis plate and their bolts to eliminate any potential electromechanical coupling. For the RC filter (one for each channel) before the 6N1P plate, I bought parts for 10, 15, 47uF caps and 100R, 240R (2W). I am going with be using 10uF + 240R/2w.

As you can see, the tag board is ready to go, but I am holding off on soldering the final tag board - I am deciding on which design to use.

Tag board 1:
-with 330k feedback resistor between KT88 and 6N1P plates
-with 1k series KT88 grid input resistor
-with 1M 6N1P input resistor to ground

-OR-

Tag board 2:
-NO 330k feedback resistor between KT88 and 6N1P plates
-NO 1k series KT88 grid input resistor
-NO 1M 6N1P input resistor to ground

Questions:
1. Which tag board/schematic will yield better sonics?
2. Will it matter what combination of RC I'll use for the filter?

Thanks guys, I could really use your advice since this is my first DIY project.
Click to expand...

I like that you take great atention to detaile and research
the tagboard looks great the chassis works loooks really nice and well thought out i can see the result will be excelent.
BTW; im builting an Se kt88 also but in RH mode and im using the same tagboard.
 
#325
Excellent info on grid stoppers. I see that they behave like an RC filter to help in oscillation, RFI, distortion. I'm using it!
The Aiken article suggests 1.5 to 470k ohm values for the grid resistor, however, I haven't seen such high values commonly implemented in schematics on this site...

I've also wondered about the output screen grid on the KT88. Some like to use:
1. 100R for all modes
2. 1.2kR for UL and 100R for Triode
3. 100R for Triode only and no R on the other modes.

Poll question: what do you prefer?...
 
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#326
cjctan said:
Hi Glenn,
The recommended value for the grid stopper range from 1.5K to 470K.
Does the current value of 1K effective in our amp design or should we increase to a higher value?
Click to expand...
I only tried the 1k because I've seen it used in a lot of existing Hi-Fi amplifier designs. Typically I see larger values in guitar amps (how I learned to build amps in the first place).
Like this EL34 SE at Angela's site:
Stereo Single-Ended EL34 Hi-Fi Amplifier
 
#327
88man said:
Excellent info on grid stoppers. I see that they behave like an RC filter to help in oscillation, RFI, distortion. I'm using it!
The Aiken article suggests 1.5 to 470k ohm values for the grid resistor, however, I haven't seen such high values commonly implemented in schematics on this site...

I've also wondered about the output screen grid on the KT88. Some like to use:
1. 100R for all modes
2. 1.2kR for UL and 100R for Triode
3. 100R for Triode only and no R on the other modes.

Poll question: what do you prefer?...
Click to expand...

I prefer #1 (it's on my schematic) because George Tubelab Home recommended it, and he's a lot smarter at amp design than I am 😉

Glenn
 
#329
Electro close to cathode resistor

88man,
I love the looks of you amp, great job. One thing you might consider is moving your electrolytic bypass caps a little further away from the cathode resistors from the power tube. The electro's look like they almost touch and sit right above them. Heat is a killer to electro's and this could be a set up for early failure. If anything, place the 3K resistor as the one that sits right under the electro as it dissipates the least amount and is the only one that will at times not have current running through it. I used a very similar tag-board like yours but just put a bit more space between the power cathode resistors and the bypass caps. Of course it will work the way you have it, question is how long? These resistors can get pretty hot.

I am still lovin' mine, listen to it almost every day. It just seems to get better and better with time. It is going to be a tough amp to beat as it offers so much others do not (flexibility out the wazoo).
Cheers!
Jeff
 
#330
Thanks, Jeff for the praise and great advice. I haven't soldered the tag board yet. I do remember your advice about the heat generated by the cathode resistors in an earlier thread. I leaned the 220uF cap next to the resistor only to balance it upright for the picture. Once soldered I'll be sure to flare them outward.

As far as looks go, I can't take credit, I used Glenn's chassis layout. The only thing I did was to mount the terminals, and few switches to the back and side.

I can't wait to hear this amp! I not going to rush into it as I'll be checking everything tenfold.

Thanks to Alex, Glenn, Jeff, Kegger, Scott for all your advice, otherwise this project would have never been built.
 
#331
88man said:
.......I can't wait to hear this amp!
Click to expand...

I can't wait for you to hear it either!

I am working on a couple of small mods for mine now. First is another rotary selector, 2 pole 6 position, to switch out different coupling caps. Also, I am going to add the feedback resistor between the anodes on a STDP switch in hopes of reducing some of the distortion in Pentode mode I get at higher volumes. Lastly is the paralleling by switch of another 1K cathode resistor on the 6N1P to be able to swap out the 6N1P with the 6922/6DJ8 tube.

I am having so much fun with this amp! I am constantly switching between modes and tubes.....soon it will be modes, tubes, caps, and feedback...

Again, can't wait to hear how you like it when it is done.
Jeff
 
#335
I am considering building this amp as my first hifi amp build (built guitar amps/preamps in the past and plan on building a phono preamp first). The part that draws me to this one is the ability to use different tubes and Pentode/Triode/UL on the outputs. I think this can really get me started to really hearing the difference the different operating modes and tubes can make.

My question though is that a lot of schematics have been posted, each with their own modifications. Which schematic is going to be the fully-flexible amp? I know someone mentioned they only wanted to use KT88's, etc. Are the 2 posted by 88man (post #318) capable of swapping different tube types? (I believe the paralleled 3k Cathode resistor on the KT88s was done to compensate for the Plate dissipation when using an EL34).
 
#336
You should be aware that while you can swap output tubes in this design, you will never be operating at the optimum biasing unless you have a bias resistor that can be switched into place for each tube type. I think this get's you into the more linear operating point of the tube.

Now I've seen this done by one person, and it's not too big a deal. I think he just had a rotary switch that selected a different resistor depending on the tube being used.

Don't get me wrong, I think the amp sounds good with just a two position bias selection, I run mine this way. I just wanted to pass on that the design won't be perfect if your that kind of person 🙂

Glenn
 
#337
Forgive my noob-ness... but supposing I built this KT88 output amp and also had the 8-tube Aikido phono pre... would I need a 2nd preamp between the phono pre and the output amp? a "line stage" pre so to speak? Or if I did not wish to have equalization controls could I connect the Aikido directly to this circuit?

Thank you.
 
#338
joelap said:
I am considering building this amp as my first hifi amp build (built guitar amps/preamps in the past and plan on building a phono preamp first). The part that draws me to this one is the ability to use different tubes and Pentode/Triode/UL on the outputs. I think this can really get me started to really hearing the difference the different operating modes and tubes can make.

My question though is that a lot of schematics have been posted, each with their own modifications. Which schematic is going to be the fully-flexible amp? I know someone mentioned they only wanted to use KT88's, etc. Are the 2 posted by 88man (post #318) capable of swapping different tube types? (I believe the paralleled 3k Cathode resistor on the KT88s was done to compensate for the Plate dissipation when using an EL34).
Click to expand...

Porkchop's schematic at the beginning of this thread is the one to start from IMHO. Also, take a look at the filter on the driver stage here: KT88 Single Ended (SE) Tube Amplifier. I used a single 30H/15uF filter and then split to each channel although I plan to replace this with a 9H/15uF on each channel to get better separation (It works great as it is but I am curious if I will be able to hear the difference.

There are quire alot of variations on this amp including a feedback resistor between the anodes of the power and driver tubes (I did not try this).

Just be sure to read this entire thread it is load with great info.

Lastly, I think the importance of using ASC type oil/film motor run caps for the power filter cannot be emphasized enough. You can buy them dirt cheap here (GE variant): https://www.surpluscenter.com/sort.asp?UID=2228011921541531&catname=electric&keyword=EMRC
Or buy the ASC from Allied Electronics. I used both and can't tell the difference between the ASC and GE but some would say the ASC are the best. 370VAC is enough for the circuit but I went with the 440VAC ones (DC rated at 810V per ASC engineering dept, I inquired a while back).

This is a great amp, you won't be disappointed if you build it well.

Also, you can consider using the 6922/6DJ8 tube in place of the 6N1P if you drop the cathode resistor to 470-510R.

Again, this thread has great info and a lot of variations on this general theme, all of which from what I read are solid designs.

Have fun!
Jeff
 
#339
joelap said:
Forgive my noob-ness... but supposing I built this KT88 output amp and also had the 8-tube Aikido phono pre... would I need a 2nd preamp between the phono pre and the output amp? a "line stage" pre so to speak? Or if I did not wish to have equalization controls could I connect the Aikido directly to this circuit?

Thank you.
Click to expand...

This circuit has enough gain to be run without any preamp at all. I don't see why you couldn't hook your Akido right up to it.
Jeff
 
#340
porkchop61 said:
You should be aware that while you can swap output tubes in this design, you will never be operating at the optimum biasing unless you have a bias resistor that can be switched into place for each tube type. I think this get's you into the more linear operating point of the tube.

Now I've seen this done by one person, and it's not too big a deal. I think he just had a rotary switch that selected a different resistor depending on the tube being used.

Don't get me wrong, I think the amp sounds good with just a two position bias selection, I run mine this way. I just wanted to pass on that the design won't be perfect if your that kind of person 🙂

Glenn
Click to expand...

Use two 5 position switches for each channel: switch 1 has five 100 Ohm resistors and switch two has five 20 ohm resistors. This gives 0 - 600 Ohm in 20 Ohm steps. Or use a variant starting halfway, endless possibilities. I am using cheap guitar switches on oBoy from HK (hongkongsuperseller) which are ceramic and can handle high voltage. (Recently bought 10 of him.)

I have an 100 mA meter in each anode lead and use the resistors to set the current.

AM
 
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