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jrobson 26th August 2011 12:34 PM

Tubelab SE - XLS tubes

Sorry I'm another newb.... so here goes... :)

I ordered a board yesterday, I would like to build it around the XLS type tubes:
KR Audio : KR 300BXLS :

For a power transformer I can't find something suitable, I've only looked at Edcor's site though.

I was thinking along the lines:800V/660V (400-330-0-330-400) 300mA CT(is this too much?), 6.3V at 5A and 5V @ 2A CT.

Would this be suitable? The 660 is just there to test the new board with a set of cheap tubes.

The caps on C4-C7 are 550-600V.

Evenharmonics 26th August 2011 12:52 PM

If you are after quality sound, spend the most on output transformers.

jrobson 26th August 2011 01:23 PM


Thank you, I have spoken to Electra-print to make a good set of outputs for this amp.

Edcor does have a 720V 220ma power unit that should also work.
EDCOR Electronics Corporation. XPWR002-240

And also 760V 200ma:
EDCOR Electronics Corporation. XPWR144-120/240

It is not entirely clear to me though how to get the best out of the tube, should one stick to the 450V and 100mA average figures or bump something up or down a bit?

I've searched the net extensively and haven't found information on power transformers specifically tailored to xls tubes, only reviews which say they are really good when replacing a "standard" 300B and they would be even better when adjusted properly.

jrobson 26th August 2011 02:01 PM

The second link appears unsuitable, too little A on the 6.3v.

Evenharmonics 27th August 2011 12:29 AM

I would work on speaker quality and room acoustic treatments for more gain in sound quality. I use cheap-ish 300B tube set at 370V and 70mA bias. It sounds good because my speakers are really good (high efficiency horn) and I treated my room.

Jpeg 28th August 2011 04:55 PM

Since My Tubelab SE just went up in smoke, I may be the wrong person to reply, but I did a lot of research before I built mine. I can think of several things that you may want to consider.

The first of which would be is that you have a fair amount of control over the voltage by what resister you put in the R4 slot and what size cap you put in the C4 slot. This is all explained in detail on the Tubelab web site. If you lower the reister value to 75 ohms, and bump up the capacitor to 65-70uf, you can probably get somthing like 425-440 volts out of a plain old 660v X-former that so many of us have used. If you are conviced that voltage makes a big differance to how this tube sounds, I would buy serveral values of each and try different ones to see what sounds best.

The 2nd consideration would be how much extra sound you would get for your extra watts. If you have a pair of 97db sensitive speakers and feed them 10 watts, you will get about 107db, or about twice as loud. If you want to get twice as loud as that (117db) You will need 100 watts. Bumping up an amplifier 9 watt amplifer to 12 watts might get you somthing like 1db, which you might not even notice. My personal experiance is that 2 watts on an 87 db speaker is loud enough to keep anyone in the house from sleeping.

The 3rd consideration is tube life. The harder you run a tube, the shorter it's life. If I were to pay $525 for a pair of output tubes and I didn't get 5 years of life out of them, I think I would cry. If that 300b really is good for 450 volts (there are many that say they are but are not, though for that price they had better be) it should still last much longer at 400 volts.

The forth thing I would consider is tube avalibility. Lets say you do get 5 years of life out of them, but by then this company is out of bussiness. Getting stuck with an amp that takes special tubes can be pretty painful.

I also agree with Evenharmonics, output transformer, the right speakers and your room all have a large impact on sound, choice of output tube and voltage generally has a much smaller impact.

If you do go this route, I don't forget you need a 5 volt tap for your rectifier. I used the XPWR131-120, which is good for 200ma at 660v, so my eduated guess is you could get 420-440 volts and safely bias to at least 80ma with a little bit of fiddling.

Anyway, that is my 2 cents, take it or leave it. I am off to go fix "smokey" Oh, and take a great deal of care wiring those transformer wires, the center tap does NOT go in the center...

jrobson 28th August 2011 05:40 PM

Thank you Jpeg, the "application page" referred to on the tubelab page that is supposed to have a chart doesn't contain this chart Tubes and Transformers, likewise the "upgrades" page htgjl doesn't exist. I also can't find a page to show how to connect the power transformer, output transformer or voltage pot. Hopefully this comes with the board?

I believe the higher current and voltage is supposed to improve the low end, I'll see if I can find that chart on here to see what the situation is regarding the relationship with R4 and C4 and the voltage, I've already bought the suggested choke for R4, but I'll have a look what can be done with C4 since I haven't bought it yet.

Thank you for the help both of you.

Ps. speaking about C4, why would one "need" to use electrolytic caps at all, why not just use motor run (well for up to 150uf) and PIO caps all round? Obviously not for C1-C3, C8, C10.

I've ordered everything already except for the expensive tubes and some of the caps, I've ordered the suggested 100uf 450v motor run for C5 bypass but I can't find an explanation as to why I can't use 3 100uf motor runs in here and no electrolytic cap.

The "quality" electrolytic caps are quite expensive.

Jpeg 28th August 2011 08:43 PM

I believe you could use all motor-run caps, but they are huge.

I borrowed this pic from the "Pictures of your tublab" thread.
I think most people consider a medium-priced cap bypassed with a motor-run to be a good compromise.

The board really doesn't come with instructions other than what are on the website, and George been too busy with family emergancies and 80-hour workweeks to fix it. But we can't blame him, who else would design a world-class amp like this for $35? All the info you seek is here in this forum.

Here is the thread for wiring the x-formers:

There is one for the volume pot too. In fact when you have a few extra hours I would scroll through all the past tubelab threads, there is some really good info there, you can beneit from every one else's mistakes...

Jpeg 28th August 2011 08:45 PM

I guess the pick isn't going to copy. Click here, it's the 4th one down...

tomchr 28th August 2011 09:03 PM

How much power you get from your 300B's depend on which operating point you choose. If you're going for power, you want to run the tube at close to the maximum plate dissipation. This means 400 V, 100 mA or so. I've run some JJ 300B's at that operating point with Edcor CXSE25-8-5k OPTs and gotten about 13 W at 3 % THD. I think the THD was mainly limited by the driver I was using so I'm redesigning that. But high power output with a 300B SET is certainly possible. Note, however, that the difference between 13 W and 26 W is only 3 dB. That's audible, but really only slightly higher volume. I wouldn't get too hung up on squeezing every available mW out of the tube.

Some people have run into trouble running 300B above 400 V. Trouble includes some sort of thermal run-away where the tube won't shut off completely even at very low grid voltage. This causes a rather spectacular self-destruction to happen.

So my recommendation is to not go beyond 400 V measured from anode to cathode on the tube. As I recall the TubelabSE schematic, this means a B+ no higher than 400~410 V.


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