Good schematic for SE EL34/KT88 amp

Actually no, in addition to the UL, (a little more is "needed" than just the UL)

On this schematic and others posted around here, and other places as well it is the resistor from the plate of the output tube to the plate of the driver tube.
If you follow it logically what you see is some of the output tube plate is fed back to it's grid, just connected to the other side of the coupling cap to block dc.
(It's an simple but effective way of getting a bit more feedback to clean things up without adding the output transformer in the loop, I prefer this method)

On my later builds of the same basic circuit it has been tweaked for "better" performance, I like triode, but for me this UL setup is a bit more refined output.
(having said praise for this topology, one should definitely experiment for ones self and see what they prefer to there ear and there gear/listening space)
 
Yeah I hear you. Thanks for the banter. Much appreciated. I didn't build the Abdellah version I actually built the JE Labs version, but used bigger iron. I did do a lot of reading on the various versions and also didn't just throw that huge 880 ohm 25 watt resistor in the cathode read; it was not a misprint, that is exactly what he designed it to use. I thought a resistor value closer to the Abdelleh version would be better. I think I started with a 560, but after I got it running I did the math and biased it properly, to the tubes I had. Using about 70% of max diss. I think I will play around with the UL tap this week though, just to see what it sounds like.
Enjoy!
Also considering picking up a pair of EH KT-88's. Any good word on them? Or bad words even. Lol
 
The EH's are fine,

Just more food for thought, when going from Triode to UL.

To really get the full affect of the circuitry with going UL I strongly suggest the feedback resistor as well.
(that would be R3 on the schem I had posted, try values from 150-270k, I've been more around 180k)
 
Oh great. Now I have something else to try. Much abliged señor Kegger. I will most definitely give them both a try. The way I do things, may be silly but, I might do one at a time. See if I can hear a difference for both. That work for ya? Or a should I just jump in, get it all done at once, see if I like it. I'm gonna go back, take a gander at your schematic. Maybe I can dig in tomorrow. Thanks brother!
 
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Ok my friend, that NFB circuit has me a bit baffled. You got a laments explanation?
Ok, edit, you said R3. I can't delete the question so lemme go look again. Sorry.
Ok, output plate to pre plate. Got it. I was looking at it wrong. :eek:
 
I truly love the way mine sounds in triode.

I preferred triode too, but it was REALLY hard to tell a difference other than volume.

I tried NFB (280K ohm, I think). It may reduce distortion on a scope, but I couldn't hear a difference.

I ended up removing it just cause I wanted to maximize power.

Oh, and I used EH KT88s and 6CA7. Both were nice. Also, the Sovtek preamp tube was nice and clear. Tried a NOS RCA and it seemed dull and veiled.
 
Both. I have the toggle switch It was a couple of years ago, so take that for what it's worth.
280K plate to plate. Maybe that wasn't enough, but it was just a value that was recommended on either the AK or DIYAudio thread.

All I can say is that I couldn't hear any improvement.
 
Gotcha,

Yah 280k won't provide a lot of feedback, so I'd agree may be hard to tell. (my newer schems do run a much lower value)
In Triode your not really needing/wanting that feedback resistor there, it's UL that resistor can make a pretty big difference.

When the driver tube and that feedback resistor are working well with each other what you'll get is a cleaner and more extended frequency extremes.
(but it's really only important if that's what someone wants, and there speakers could benefit from it as well, lower bottom end and higher top end)
 
As for trying UL and the Feedback, sure try them separate, then try um together to get a better feel for what works and what your hearing.
 
I preferred triode too, but it was REALLY hard to tell a difference other than volume.

I tried NFB (280K ohm, I think). It may reduce distortion on a scope, but I couldn't hear a difference.

I ended up removing it just cause I wanted to maximize power.

Oh, and I used EH KT88s and 6CA7. Both were nice. Also, the Sovtek preamp tube was nice and clear. Tried a NOS RCA and it seemed dull and veiled.

Maximize power and minimize parts count!
Glad you both like the EH, I'd love to try something affordable that sounds good. I've been running, most of the time, what were NOS RCA 6CA7 fatties, they're fantastic. I did try a few different 6SL7's too. Started with GE I think, tried sovtek and didn't love them but they had more gain. Much more. I ran them for a while to see if they'd warm up, but then someone gave me a pair of Philips JAN's. Blows everything else away. I hope they never die. Aside from trying some 6L6's, some crappy EL34's and a pair of KT88's, I've pretty much had the same tubes in there for almost ten years. And believe me, they get PLAYED. Sometimes ten to twelve hours a day.
Gonna give Kegger's idea a go, changing coupling caps made such a difference, maybe this will too. Who knows. We all like to tinker anyway right? Can't hurt.
 
Alright, since my cabinet with the switch drawer is buried under a mountain of other bins, I desoldered the triode strap, soldered on the UL tap, and soldered that 280 in. Gonna give her a run for a few days, see if it's worth adding the switch. Definitely louder. The bass didn't seem to change all that much, not nearly as much as adding better coupling caps did, and it seems to have lost a LITTLE bit of magic. We'll see. Ordered a set of EH KT88's, so I guess I'll leave it as is until they come. Play some different genres. Thanks for the help. Enjoy the long weekend.
 
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My suggestion, try lowering that feedback resistor. ;)

Like say 180K, no higher than 200K to get the full affect..
 
An update, if you care. Didn't like the feedback, tried several values, just didn't like it. Which is ok why? Because I really wasn't diggin the UL either. So, no switches. I also got my EH KT88's today. They've been playing since noon-ish. They do sound pretty dang good, hoping they'll open up a bit as they burn in. And although the power tranny is running hotter, form the extra heater current mostly I suppose, the tubes themselves seem not hot enough. That's probably great for tube life expectancy, but would biasing hotter sound better? I'll have to do pull the unit again and take some readings, do the math, cuz I doubt they're anywhere near 70% pd. I've got plenty of transformer, so I'd appreciate if someone would say, yeah bias hotter it'd sound better. I'm all over it. Thanks.
 
Thanks bro. I might've built yours if I'd seen it first. Ten years is a long time! Sort of.... Anyway, yes. I'm thrilled. Probably build another one at some point. But for now what I'd like your opinion on is the bias. Leave it? Change it to hotter? What's your take on that sir?
 
First I would need to know ALL the Details to make any real comment.

But the 70% rule is pretty good, and if your lower I would raise it though,
Will it sound better? Well that's up to you again, normally for me it would.
 
I'll dig in and get measurements tomorrow if I can. As I recall b+ to the opt is around 420 (was with the 6ca7's anyway) and cathode resistors are 680. I agree, it's all about what sounds good to the listener. That's why I avoid all the arguments about caps all found the same and all that. If it sounds good to me, I don't care if it's a placebo, it still sounds good.
 
I'm hoping some burn in time will bring the magic to these KT88's that the 6CA7's had. Wouldn't be a problem for me to re-bias it too though. Also could swap out the 5U4g for a 5AR4 and bump up the voltage if its lower than I remember. I was just reading through that thread with your schematic. Very interesting preamp setup! Looks neat. Where are the pics of your builds?
 
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