stereo SE kt88 build ... abdellah diyaudioprojects design

[alecu7]

Alex

depending on voltage different transformer resistance values are required but also be carefull of not using a capacitor larger than specified. These values are required to prevent too large peak currents through the recitifier tube. Model the PSU in PSUD II (duncanamps.com), it takes only a short time to get familiar with the software and prevents a lot of troubles later on.

AM

I have been using PSU II and I got a nice CLCRC. 33uF is the max value for the first capacitor for this specific tube (5u4-gb). What I got is: 33uf - 10H - 220uF - 250R - 120uF. I don't have the transformer so I cannot check the resistive value. I'm all confused now because of that article. Any thoughts?

Thanks,
alex

 

[tim614]

Thanks Tim.

Awesome amplifier you built here.
Just waiting for an answer, I browse 5u4-gb datasheet. What they say here is that you MUST NOT connect the tube directly to the transformer but to use a resistor so that the transformer resistive value +the additional resistor should be in the range of 170Ohms. I didn't buy the transformer yet as I was looking at Edcor XPWR033 or XPWR002 , tube 5u4-GB 33uF - 10H (Hammond 193J) 220uF. Could you please share your working schematic?

Thanks,
alex

 

[tim614]

this is what i use for mine with 5u4 i get 426v B+ and the current rating is plenty the amp is dead quiet and the LED is a must try.

XPWR048 120Vrms, 60Hz. 700V(350-0-350)@300mA, 30V(15-0-15)@50mA, dual 6.3V(3.15-0-3.15)@4A & 5V@4A $90.33 USD

 

[alecu7]

Tim,

Thank you for the schematic. I want to try triode mode only with 120Ohm resistor. I may try the LEDs as per your schematic.

Thanks,
alex

 

[jmillerdoc]

PP vs SE

Hi all,

I am in process of building this amp but I got stuck in OPT.
Would it be sufficient to use GXPP15-8-5K from Edcor, considering it's a 15w transformer or I must go with CXSE25-8-5K ?

Thanks,
alex

Yes, you need to go with the SE. The PP will not work well because it is designed for a Push Pull application. The SE is for the Single Ended amp such as this one on this thread. The SE is air gapped to accommodate the current running through the SE design in such a way as not to saturate the core. The PP core will likely saturate at certain frequencies and give a poor bandwidth response. People have used over-sized and over-rated (watts/current) PP OPT's for single ended applications but I think the one you are attempting to use here wouldn't work well. If you are planning to buy new from Edcor but don't want to spend the $$$ for the top-line OPT consider using their second best line of SE OPT's. They have a rated response of 40-18K but you will likely get better than that. This would be much better than trying to use the PP OPT and should definitely outperform the PP OPT in bandwidth. I used the top-line Edcor SE OPT's in my build and they work super!

 

[alecu7]

Yes, you need to go with the SE. The PP will not work well because it is designed for a Push Pull application. The SE is for the Single Ended amp such as this one on this thread. The SE is air gapped to accommodate the current running through the SE design in such a way as not to saturate the core. The PP core will likely saturate at certain frequencies and give a poor bandwidth response. People have used over-sized and over-rated (watts/current) PP OPT's for single ended applications but I think the one you are attempting to use here wouldn't work well. If you are planning to buy new from Edcor but don't want to spend the $$$ for the top-line OPT consider using their second best line of SE OPT's. They have a rated response of 40-18K but you will likely get better than that. This would be much better than trying to use the PP OPT and should definitely outperform the PP OPT in bandwidth. I used the top-line Edcor SE OPT's in my build and they work super!

I'm an idiot. I meant GXSE15-8-5K I am worry about 15w. I can live with 40Hz (i think).

 

[jmillerdoc]

What I am doing is putting a variable resistor in the cathodes of the output tubes. There are some very nice and small 12.5 Watt, 500 Ohm rheostat from Ohmite on eBay at this moment at a very reasonable price....

( OHMITE 500 OHM 12.5 WATT RHEOSTAT - eBay (item 400092245495 end time May-15-10 18:07:40 PDT) )

This allows me to easily adjust the anode current for the "sweet spot".

AM

You may want to consider placing your pot in series with a minimum value resistor that way you could make it to where you never "dial in" to a dangerously low value. Just my 2c worth.
Jeff

 

[AmadeusMozart]

You may want to consider placing your pot in series with a minimum value resistor that way you could make it to where you never "dial in" to a dangerously low value. Just my 2c worth.
Jeff

Jeff

Should have mentioned that I am placing 120 Ohm resistors in series. Am still ponerding about a CCS in the plate/g2 lead to prevent the output transformer being destroyed should the tube short. Just set the CCS for a current about 10 mA larger than max operating levels at max mains voltage fluctuation.

Thanks for the reminder, it is an important detail that I forgot to mention.

AM.

 

[jmillerdoc]

Jeff

Should have mentioned that I am placing 120 Ohm resistors in series. Am still ponerding about a CCS in the plate/g2 lead to prevent the output transformer being destroyed should the tube short. Just set the CCS for a current about 10 mA larger than max operating levels at max mains voltage fluctuation.

Thanks for the reminder, it is an important detail that I forgot to mention.

AM.

What were you planning to use as a CCS?
Jeff

 

[Nikon1975]

Last Sunday I met with the people from diyaudio forum here in Tokyo, and we did some listening also of mine SE KT88.

I have a switch to choose between CCS with DN2450 and 47K resistor as a load for the driver, and everybody preferred the CCS. This is also something to try, very easy to implement. My CCS is set to 6.5 mA, with the 47K 4.5 mA are drawn.
Led for the cathode 3.4 V.


Davide

 

[seel34]

tungsol kt120

i could buy a pair of tungsol kt120 ,any body can tel if they are suitable for our amps?
i found this treath when i was googling on it

New Tung-Sol KT120 output tube - AudioKarma.org Home Audio Stereo Discussion Forums

would i be correct to asume that in the se kt88 in triode it will give much more power?
and are there some more advantages to use these over the sovtek kt 88 i am using now?
i pay here in the netherlands euro 54 +sending for a matched pair wich is about the same as a pair of kt88
would they be a plug and play repacements in our amps ?
looking forward to your coments and tips
kind regards🙂
ko

 

[Bas Horneman]

would i be correct to asume that in the se kt88 in triode it will give much more power?

Unless you change the operating point of your amp you won't have more power.
If you do increase current and maybe voltage. Can your output transformer handle it? A SE transformer often has a maximum current rating. And while we are at it. You psu transformer also needs to be able to supply the "juice"


Other things you need to know:
You have to know if your filament current is sufficient. Because the KT120 draws more current.

 

[Nikon1975]

I have two monoblock of this amp, with CCS and led for the driver.

So I have one triode that is not used. I have the temptation to connect the two triodes in parallel and double the current of the CCS. Any reason why not to do it ?



D.

 

[seel34]

srpp line stage

after playing my amp for almost a year ,i was still left with the feeling that the gain of the 6n1p ev wasnot enough
i tryed the schematics of this link ,but then the gain was much to much
6DJ8 / ECC88 / Symmetrical SRPP Tube Preamplifier Schematic
so i made the following modifications
i left out the 1 resistor of 1 meg ,the 2 resistors of 150 ohm i changed for 680
ohm resistors [25wat ones i found in a electronica dumpstore ]and i left out the elco over the first resistor , the last resistor of 1 meg ohm i left out to , there is still the 470k one from the kt 88 schematics
the gain is now perfect , the soundstage is great with more detail
the only other modefication i had to do was making the power suply
a bit better , to get ride of the small hum i had with the srpp
i am enjoing the amp like this a lot more 🙂
tubes used 2 times 6n1p ev voskhod 5ar4 sovtek , sovtek kt88



kind regards seel34

 

[alexg]

I don't think the gain of the 6n1p is inadequate to drive the KT88 to full power.

The SRPP that you linked will not have bigger gain than the 6n1p driver of Mikael's KT88 amp.

 

[seel34]

volume/gain

the differents in gain with a single 6n1p with 2 red led biasing and the srpp
with the 680 ohm biasing is clearly there in my amp
if i turn the volume at 12 o clock i have a nice room filling full sound ,like i am used from my other amps [this with the srpp]
with a single 6n1p i need to put the volume at 3 to 4 oclock to get the same volume
the gain was even more with the capicitator over the first resistor of the srpp
but i find it sounding better with out this capicitator
kind regards se el 34 🙂

 

[6L6PPP]

Hi, I am reviving this thread. I just bought the amp below from jmillerdoc and this amp is ABSOLUTELY INCREDIBLE. Any one who has the skills to build one of these should do so immediately. With a few tube changes the clarity, soundstage and dynamics of this KT88SE top any amp I have ever heard. The bass is in a league of its very own. The amp is alive! 😱

He has the amp set up to run either triode, UL or pentode. He graciously added a feedback loop for pentode ( I like pentode). I am listening to some Allman Brothers in pentode right now and I cant turn it off. The sound is simply perfect.

I swapped out the 6N1Ps for NOS RCA 6GU7 and ditched the Chinese rectifier for a NOS Raytheon blackplate 5U4GB. I lost a little gain but not to much. This amp has the shunt pot mod so I am not sure how the gain compares to my other amps normal volume pot levels. But, there is definitely enough gain even with the lower mu NOS tubes.

NOS tubes elevate the sound well above the stock level. The amp is very good with the new production 6N1P and a Chinese rectifier. But, with NOS tubes this amp will take on anything. I am going to hunt some NOS power tubes down next. But, the transformation you get upgrading just the input tubes is amazing. The Shuggy KT88s get the job done extremely well. But, I cant help but wonder what a good NOS power tube would do.


In fact, I bet Jeff would never have let this go if he tried NOS tubes in this amp. IMO, I would guess his stated preferance for triode is possibly the fact that triode mode reduces the glassiness, grain and grit of the 6N1P. I have done this switch with 6N1Ps before and compatible NOS tubes best the 6NIP every time in my experience. Pentode with feedback is truly excellent (big, powerful and explosive) when done right using refined input tubes. I expected to prefer UL.

So guys, try some 6GU7 or 6CG7s to replace the 6N1Ps and invest in a good 5U4G/GB and be prepared to listen .... because you wont be able to stop!! 6GU7s are great unknown tube that are only $5 each at RES.


Jeff you are the man . I cant thank you enough. I have dibs on whatever you build next. 😀




http://i79.photobucket.com/albums/j124/6bx7gt/Dads20camera20232.jpg

http://i79.photobucket.com/albums/j124/6bx7gt/Dads20camera20231.jpg

http://i79.photobucket.com/albums/j124/6bx7gt/Dads20camera20229.jpg

Here are a few fairly low res pics taken of my nearly completed KT88SE amp. A few things done differently is the power supply which I have modeled on PSUD2 to be right at the border of maximal operation of the 5U4-G tube. It uses a combination of Solen and ASC oil caps. The arrangement is a 25uf Solen -> 20R -> 47uF Solen -> 10H 77R DCR Wesinghouse choke -> 80uF ASC coupled parallel to an 82Uf Solen -> B+ tap -> 20R Wire wound -> 15uF Solen -> driver tap. The tube complement is a set of Mc Intosh KT88's and 6N1P Russians. The Coupling caps are 0.33 Jupiter Beeswax and tinfoil, All resistors are PRP or Mills (a couple of Xicon WW in the PSU), and Blackgate bypass caps. All wire used is silver stranded with some home built shielded cables to the RCA inputs and from a blue velvet pot to the driver tubes (white cotton covered cables). The power and OPT's are of course Edcor.
The Chassis is all hand-built from exotic woods including Rosewood, Madeira, Canary, red Cedar, and some oak. Binders and RCA's are topline Vampire's. Most of the passives were sourced from Soniccraft.com. The remainder of the parts were gathered from here and there like tubedepot.com and tubesandmore.com.

I need to finish the filament wiring which I decided to rectify and filter. I have 2 small modules with the rectifiers and filter caps not shown that will go on the opposite side of the large Westinghouse choke from the Solens seen on the right of the choke. power input is a simple filtered IEC jack bought at Jameco if I remember correctly.

I haven't powered it up yet and will likely have to make some adjustments to the power supply filter when I do. I find PSUD2 to be off every time I have used it but not by much. Once I am done I will try to supply some scope traces on it for ya.
Jeff Miller
Lawton, OK

 

[jmillerdoc]

Next is you know what....a super KT88PSE!
Everything is already coming together for the build. Soon I will start a new thread outlining the details of a parallel SE version of this amp. If anyone knows where to source a good 4 pole 3 pos rotary switch I would love to know...I will need this to operate the switching between UL, Triode, and Pentode modes. I am also going to implement the same switchable anode feedback circuit I added to the one above.
Jeff

 

[porkchop61]

He graciously added a feedback loop for pentode ( I like pentode).
Can you elaborate on this? Schematic? I'd like to give this a try on my amp that I built.

I'm currently running NOS GE GL-807's in mine, so far I like the sound. Be careful as 807's do not have the same pinouts as KT-88's, I made adapter sockets.

Thanks
Glenn

Edit:
I also want to try using a 5AR4 rectifier to get my B+ up a little, any changes required to do this?

 

[jmillerdoc]

Very simple...just added a 330k resistor (switchable) between the anodes of the power tube and the driver tube. Adjust amount by varying the resistor value...larger=less feedback, smaller=more feedback. It would be conceivable to use a stereo pot paralleled to a resistor (to keep from going below a minimum value, would not use a pot alone) to have a fully adjustable feedback loop...I plan to do this on my next build...