SEUL KT-88 Build, Help Appreciated

[Hotrodster]

This is a build I've wanted to do, but never got the motivation or ability to do it. I can paint by number pretty good, but don't have much vision. So this is going to be a learning experience.

Have been collecting parts for the day I was ready. Well the transformers came in and put them on top of an aluminum plate. With bad wrists Decided it is too much weight and needed to be split it. Ordered another power transformer and while I wait for it to arrive, decide what to do for chassis.

In a box I found a couple chassis was going to use for a different project. Set the transformers on them and while still heavy it is manageable.



My concern is maybe they are too small. Measure 9.75 W X 12 D. Would be 8.75 X 11 workable. 2.5" Deep
I Like the look and size, but with size of some of the other parts think may me too tight for my ability.

What would be a good size chassis to use for this build of mono blocks ?

 

[primosounds]

Actually, i think those chasses look about right. It depend on the size of your capacitors and how you situate them and the type you use. For instance if you are thinking of using the Russian teflon caps you would definitely check out its dimensions and set aside the real estate for that.

 

[dowto1000]

You are way too small in my opinion, for a KT88 Monoblock. There will be tight spacing, fields will interact, and you will have no room for circuit changes, because it will all be too tightly packed.

Do a Golden Ratio shape, suggest 18 by 11 1/8th, by 3.5 inches, out of 14 gauge steel, ( steel sounds beter to me, than aluminum, and steel shields nicely) , and have the corners welded. ( G.R. is 62%, 18 inches times 0.62 is 11.16 inches. )

Most DIYers I see, build in way too small a chassis, and they have NO future flexibility to use the chassis in a different version.

Suggest to use ALL film caps in a SE amp. I like WIMA 4 lead DC LINKS, and allow space for that. In SE, everything matters. Instead of one high DCR choke, commonly seen/used without thought, use two smaller ones, much lower in DCR, in a L1/C1/L2/C2 B+ filter ( to the Finals) configuration. L1/C1/L2/C2 rocks IMHO, vs all else I have tried. Ls all 20 Ohms or less. Cs are low in uF.

The Owner of Quicksilver, Roger, brought his KT-88 SE amp to RMAF one year, quite a while back, and it was truly GREAT sounding. He selected a 12AX7 driver, ( mu of 1000 ) DCed to the KT-88 grid as I recall.

A KT-88 is one ( of three ) tubes I would use in a SE amp, in 2018. But the KT88 build is MUCH harder to pull off, with low DCR, due to such high currents needing filtering. It would have to be monoblocks for my design goals to be met. Al chokes 20 Ohms or less, so its two L/Cs series.

Don't short change yourself on space.

Dowto1000

 

[Lavane]

Have you looked at Tube Labs SSE amp board? It's not point to point, but it does allow for a compact amp. I built a 300B using his SE board.

 

[Hotrodster]

Thanks for the suggestions and making me think a few things out.
Really have my mind set to do the Kegger version at this thread http://www.audiokarma.org/forums/index.php?threads/kt88-ect-single-ended-amp-seul.382162/
One shot deal, Have to do the best I can this round because not sure when able to afford to do another.
Have to think parts out to figure out size of build. Looks like type of capacitors is going to dictate size of chassis. Also space I'm going to use them in.
Small condo, so not a lot of space.

 

[dowto1000]

Thanks for the suggestions and making me think a few things out.
Really have my mind set to do the Kegger version at this thread http://www.audiokarma.org/forums/index.php?threads/kt88-ect-single-ended-amp-seul.382162/
One shot deal, Have to do the best I can this round because not sure when able to afford to do another.
Have to think parts out to figure out size of build. Looks like type of capacitors is going to dictate size of chassis. Also space I'm going to use them in.
Small condo, so not a lot of space.

Question from your photo,

Why position those input RCA jacks in front of power supply choke?

Why don't you relocate the RCA input jacks to be 1 1/4 inches in front of the Input / Driver tube, so the lead length can be short as possible, and use as high a quality wire as possible?

Under two inches in SE amps is best, for RCA jack's hot to grid of Input tube, and the RCA jack ground wire to ground.

Should all three of the transformers be rotated 90 degrees, so core faces tube??

Dowto1000

 

[jdsalinger]

Go for it.

 

[srb02]

Here is that "Kegger" amp, built on a 12" X 13" plate.

http://audiokarma.org/forums/index.php?threads/another-kt88-se-ul-build.595868/

It's an everyday player, love everything about this amp, Bass, mid-range, beautiful top end. It's all great.

My two cents Stu

 

[vfr800fiman]

You're going to love it.
I built this one:http://diyaudioprojects.com/Tubes/KT88/

I did make some changes due to Kegger's suggestions. The best one was changing the bias method on the
driver tube to LED's, totally different sound.

I also have mine wired with a switch so I can run UL/Triode/Pentode modes. I also have a switch for changing
the bias resistor value on the power tubes. This lets me run KT88/EL34/KT66's, even 1625's with an adapter socket!

Plenty of knowledgeable people here for help.
Good luck!

Just found a picture:
http://webpages.charter.net/porkchop/tubeamp/kt88_finished.jpg

 

[Hotrodster]

Nice

 

[Hotrodster]

Probably picked a bad time to start this. Between Vacation, long weekends, and the rain it's been hard to get to this.



I've managed to get the holes drilled and started mounting parts.


Next to start wiring it.

 

[primosounds]

For me, deciding how to layout the transformers and then the tubes is one of the most time consuming. Once that is determined, the rest is much simpler since you are determined by the layout of the trans and sockets. The next thing to be concerned about is the layout of the heater wires. Laying the heater circuit is the next thing i always do in a diy amp. There is a lot written about that so if you have not looked at that before it is a good thing to check it out.

 

[Hotrodster]

Real life got in the way for a bit. Seems like I may have some time, back to this.
Sold things to make other things happen. Couple transformers changed.

Have to figure out the power supply with items I have. Trying to figure out PSU Designer.
Could someone tell me if this will work? Looks good to me, but not sure.



Thanks,
Don

 

[prelius]

Hotrodster,
In order to figure out whether your simulation in PSUD is good you need to post the results/screenshots of the simulator. There have been quite a few discussions within SE building threads on how to properly set up PSUD, what measurements to do on your transformers to make sure the simulation is close to real life result. Just search AK for PSUDII.

 

[macaltec]

Just watching to see what I can learn.

 

[Andyman]

I have a Stancor P-6165 here and was wondering if it would work in a Kegger SEUL KT88. I'm pretty green on tranny's, but see this one is rated 400-0-400 and read somewhere the KT88 SEUL needs 350-0-350; what ever that means. I'm also wondering if it could be an upgrage for the Dynaco ST70 so it could use KT88s/120s, etc.

TIA,
Andy

 

[jdsalinger]

I have a Stancor P-6165 here and was wondering if it would work in a Kegger SEUL KT88. I'm pretty green on tranny's, but see this one is rated 400-0-400 and read somewhere the KT88 SEUL needs 350-0-350; what ever that means. I'm also wondering if it could be an upgrage for the Dynaco ST70 so it could use KT88s/120s, etc.

TIA,
Andy

View attachment 2992020

I can't comment on the ST-70. Personally, I would leave that stock and use it for your SE amp. This PT is rated 200mA and that's also important, but very sufficient for this amp. It also has appropriate current rating for the 5.0V and 6.3V windings.

Bluglow's power supply schematic actually shows a 380-0-380 (360-0-360 min for el34 tubes), so yours is not much higher than that.
(there are two ways to indicate the voltage on the secondary. Yours says "800 Volts C.T." which is equivalent to 400-0-400.
There are 3 wires connected to the secondary of your power transformer. The two on the ends of the secondary are +/-400V. The "0" is the 0 volt center tap (C.T.).

You could use an appropriate dropping power resistor to get the B+ down to within the ratings of your filter caps (and supply to your tubes) if necessary.

Congratulations on your project. In my 15 year audio journey I've built/refurbished over a dozen amps and this is the best (PP or SE) I've built/heard. It was my first DIY build, then I sold it to fund other exploratory projects, but came back around and built a second one because nothing else sounded so good and was as simple as this amp.

 

[gadget73]

The ST-70 idles at 200ma stock, and swings a fair bit higher when its actually passing signal. This would not be an upgrade for the stock power transformer.

 

[Andyman]

@jdsalinger Thanks, the application for the SEUL was my primary question. The question about the ST70 was just an curiosity I had. I may try to complete the SEUL project I have here with it.