stereo SE kt88 build ... abdellah diyaudioprojects design

[scitizen17]

Originally Posted by 88man

Jeff, don't keep us in suspense any longer - plug that baby in! Seriously, I am curious on your take on the sound. I see that your layout is different than Glen's or Alex's - but esthetically it looks balanced. I really like it!

A safety question: While handling and testing ASC caps in PSU, how do I bleed the charge safely and completely to prevent an accidental shock?... Do I use a 5kR across the terminals??...

Thanks!

5K will work but I would recommend around a 5 Watt resistor

[alexg]

Originally Posted by 88man

Jeff, don't keep us in suspense any longer - plug that baby in! Seriously, I am curious on your take on the sound. I see that your layout is different than Glen's or Alex's - but esthetically it looks balanced. I really like it!

A safety question: While handling and testing ASC caps in PSU, how do I bleed the charge safely and completely to prevent an accidental shock?... Do I use a 5kR across the terminals??...

Thanks!

I use 330K to 470K as bleeder resistor (2 watt). It takes a while for the charge to be removed, but it works.

[88man]

Thanks scitizen17 and Alex... I thought I felt a little tingle when I touched the terminals from a NIB ASC cap while I was doing the layout for this amp. I just wanted to be sure - No question is too dumb to ask when it comes to HV. (BTW resistance of skin is 15k ohm - not a good bleeder resistor).

[jmillerdoc]

Yeah, A higher K is what I use, I used a 680K 1 watt across the ASC cap terminals. The higher the K the less wattage you have to use as I understand it. I could be wrong but I think I read that somewhere in an old book written in the 50's. Apparently the more resistance the less current that actually will flow through it while the power is on. When the power is off the same amount of power will ultimately flow out through the resistor regardless of the R value, it just takes longer, so the amount of power dissipated per unit time is less thus, the wattage of the resistor can be less. Somebody pipe in if I am explaining this wrong. That is what made sense to me when I read about bleeder resistors.

I wonder if using a lower R value resistor make much difference in the power supply while the unit is operating...does it effect the power available? Does it create much of a voltage divider to lower the final voltage on the B+? I always wondered about this b/c if it did it would seem to me it would be best to use a high value then wouldn't it?

Well I came to have an early Thanksgiving with the family down in San Antonio this week (I work through Thanksgiving week) so I am here in South Texas. I actually brought the amp and toolkit with me to try and complete it while I am here. I used the excuse that my dad was interested in my latest project to be able to bring it with me....explaining why I needed the toolkit was a harder story. As you can imagine it would be pretty unpopular with my wife if I spent much time actually working on it while I am here. She and my mom are going shopping tomorrow so you know what I will be doing while they are out....

I hope to get it done tomorrow. I don't have much of a system to test it on here, I think dad has a pair of old Sanyo speakers in the garage and a DVD player in the living room. I am not sure that is what I want to hear its first utterances from....LOL.

The circuit is not too much different than the one posted at the beginning of the thread. Just some minor changes in the power supply that seemed to look better on PSUD2. I sans'd the 5H choke for the driver tubes for now as it takes up too much real estate under the chassis. I have it if needed and can squash it under there. I also have a 30H 595R choke I was thinking of trying that is a bit smaller. The voltage drop is about 30V more though and I am not sure how that will affect the loadlines and dynamics of the driver tube. I will probably try all of them before it is all said and done.
Right now my ripple at the main B+ tap is about 0.01 p-p, if I recall correctly. After the second RC filter (for the driver tube) it is essentially gone.
Also, my OIL cap is not actually an ASC, it is a Eudya. Never head of it before but I had it in a box of stuff that was given to me. If it doesn't work well I will go ahead and buy 2 50uF ASC caps and get rid of the 82uF Solen and try it more similar to the original design.

Jeff

[jmillerdoc]

Originally Posted by 88man

(BTW resistance of skin is 15k ohm - not a good bleeder resistor).

And thankfully for your sake it is that high!
Jeff

[jmillerdoc]

I did mention a while back in this thread that I decided to use x2 6N1P tubes, 1/2 of each per side. I set it up this way for 2 reasons amin reasons and a couple of less important ones too. 1st is the ability to use an SRPP as the driver stage. Just a few wiring changes and there it is. I already have a couple of the resistors mounted on the green peg board for this. Second is the symmetry of my layout. Another reason theoretically is it would be easier to match the 6N1P tubes this way. I bought a batch of 12 tubes for about $18 on ebay, not too bad.

I am also toying with the idea of using the standard configuration with a lower gain tube running with both triodes in parallel. I have also modeled a driver stage with a 5687 SRPP that I may try down the road.

Nice thing about this amp is it is very flexible, you can make hundreds of different combinations with the output tubes and driver stages with some very minor alterations in the wiring.

I only own a set of KT88s and 6L6GCs at the moment.....I was wondering what different output tubes others have tried in this circuit with good results.

I am driving a pair of Proac Response 2.5 clones built with Morel MDT33 tweeters in place of the ScanSpeak 2905 series tweeter (the Morel MDT33 is tough to beat, excellent tweeter that is very hard to find these days). The sensitivity is about 91dB maybe a little higher. I am in a 20x16' room. I have driven these speakers with as little as 1.5 watts with plenty of volume (even bass). I will try the tubes I have of course but was wondering what others would suggest as my next purchase of tubes to try out with my set up.
Jeff

[alexg]

Originally Posted by jmillerdoc

I only own a set of KT88s and 6L6GCs at the moment.....I was wondering what different output tubes others have tried in this circuit with good results.

I have tried KT88, 6550, EL34 and 6L6. The 6L6 did not cut it on my personal sound preference, but the rest is gorgeous. I used a pair of Siemens El34, Tungsol 6550 (reissue) and NOS GE 6550, as well as EH KT88, NOS GEC KT88 and reissue GEC KT88.

My favorite for vocals is the Siemens EL34 and the KT88's and the 6550's for the rest of my music.

[porkchop61]

I'd agree with Alexg, 6L6's (and KT66's) sound kind of "mushy" to me.
EL34's & 6CA7's sound really nice and tight. My daily tubes are just EH EL34's in Pentode mode (believe it or not).
I'm running these with a pair of customized Altec A7's, and I really don't seem to notice more distortion in Pentode mode. This is mostly for Rock music, the KT88's are really nice for symphonic pieces.

Glenn

[88man]

Hi Guys, how does the layout look?... I just got the T-6061 0.125in aluminum sheet 12.5x16in. I took everything out of its boxes. Here is an approximate dry fit of top panel components for a stereo set up. However, I was disappointed to see very thin leads on the Hammond iron - looks like 20ga stranded wire. I might try Edcor, or better next time...

Set Up:

PTX: Hammond 274BX 375-0-375 @175ma
OPT: James 6123HS
PSU: ASC Motor Run Caps, 440VAC
Tubes: Genelex Gold Lion MP KT88, Sovtek 6N1P, Sovtek 5U4-G
Coupling Cap: Auricap 0.22uF 450V
Bypass Cap: Black Gate 220uF
Driver Cathode bias: (2) 2V LEDs

Triode: Grid to plate 100R
Ultra-linear: Grid to OPT tap 1200R
Pentode: Grid to B+ OPT

Will Experiment With:
- 10uF, 20uF, or 40uF first cap in CLC to get output of 400-420V B+
- (2) 2V LEDs for driver cathode bias
- Siemens-Halske CCA 6922/6DJ8 with 560R + 100uF BG cathode bias
- 330k plate-to-plate resistor.
- 100R + 47uF RC filter to driver plate
- 1kR before KT88 grid
- 5U4-G, 5V4, 5AR4
- Coupling cap: Mundorf Silver Oil 0.22uF
- various speakers: Paradigm S20, S40; Klipsch La Scalas; DIY 3-way Scanspeak 218555-01/18W8545/9800 (eVeII)

[jmillerdoc]

Any Idea which way the laminations run in those James OPTs? Just try and make sure they are not parallel to the power trafo. I might also move the driver tube a little further forward, the farther from anything the better IMHO. The rectifier tube is awfully close to your ASC's. Some would recommend 2" if possible. I am not sure if the oil caps are as sensitive to heat as regular electrolytics but I wouldn't chance anything that might cause early failure. You might try swapping the rectifier tube with your 40uF cap, that would give you a little more distance from the caps (heat) and tubes (noise). You could even save a little surface real estate by tucking the smaller 40uF cap under the top plate (I did this with all mine). I like the split chassis design with all of the power on one side and signal on the other. Have you seen the ventilated socket plates at vt4c.com? I am placing a very large order with them this week and if you would like it wouldn't cost any extra in shipping for me to have them add 3 more to my order. I imagine it would cost a couple of dollars to ship them to you in an envelope to Boston. This offer goes for anybody watching this post at the moment, I expect to place my order in about 2-3 days. I have 10.1kg left in my order at the current shipping price I am at. I am already having to eat the cost on several items (shipping $100) so I might as well pass the savings on to any USA watchers who might be interested. This store in HK is awesome for the price/quality ratio but shipping sucks. I am buying the James OPTs for $200 here, I have priced them elsewhere for about 260-270 with an additional 20 or so bucks of shipping. so I figure it is worth it. Plus I got an additional 6% discount for my order quantity. Not too bad. So if anybody needs something from this store I am willing to front the cash to get it here to USA and you guys can pay me to ship it to ya when it gets here. My delivery time from HK is 3-6 days, I should have all this stuff in in about 2 weeks top. Let me know by private message if you need something in the next 3 days.
Jeff

Here they are: Audio Catalog

Very clean looking addition for $4 each.