Stereo SE KT88 power amps for live rig

[SILVERGUN]

Haha!  I knew I could pull you to the dark side!

Jim

I saw the visual representation of the dark side last night.
 
I measured clean output and then proceeded to measure fully square output at around 10 watts.
 
But there was a point (around "11") where the tops of that square wave started to get a PITCHFORK shape to it, and that's where things got real nasty...
Not sure if I'll ever want to go back there again....kinda scary

[pompeiisneaks]

Not sure if I'll ever want to go back there again....kinda scary

LOL, nice :)

 

~Phil

[Willabe]

B+ @ 276mA is way more than you need. 

[shooter]

that's where things got real nasty...

Most all my builds are PSE and you're right, without NFB, (even with), you sometimes wind up in the
 death-metal, Satans on His way sound

[SILVERGUN]

B+ @ 276mA is way more than you need.

Ahh, right...especially since as my B+ goes up my current requirement comes down.
 
I got a little over zealous after watching that other PT get dragged down, but I do always err on the side of too much current capacity.. 
And I liked the EX cause it doesn't have a 5V winding

[Willabe]

And I liked the EX cause it doesn't have a 5V winding.

You could use a 5acv wind for relay's if needed. (If the rectified dcv is a little low/on the edge, they make 2.5v relays.) 

[jojokeo]

Now there's a double tranny schematic I've never seen before. Now I know why pete's got all those rectifiers?    It seems disconcerting to see the neg point on top bridge rect going to the lower winding like that but in series things like that happen? Now if this gets SG that higher voltage I've been wanting to see then a big    you go boys! SG - get it goin'! Too bad you just don't have a 400-0-400 PT though... wait just a second; would not this work perfectly? - http://www.ebay.com/itm/STANCOR-12558-1-POWER-TRANSFORMER-TUBE-AMP-12AX7-6L6-6V6-5Y3-12AU7-5E3-/321991180693?hash=item4af82b8195:g:UDwAAOSwEK9T9P2z

[SILVERGUN]

Now there's a double tranny schematic I've never seen before. Now I know why pete's got all those rectifiers?    It seems disconcerting to see the neg point on top bridge rect going to the lower winding like that but in series things like that happen? Now if this gets SG that higher voltage I've been wanting to see then a big    you go boys! SG - get it goin'! Too bad you just don't have a 400-0-400 PT though... wait just a second; would not this work perfectly? - http://www.ebay.com/itm/STANCOR-12558-1-POWER-TRANSFORMER-TUBE-AMP-12AX7-6L6-6V6-5Y3-12AU7-5E3-/321991180693?hash=item4af82b8195:g:UDwAAOSwEK9T9P2z

That looks good jojo..but also sold (?)
 
I don't really have a voltage shortage issue here as I can easily tap more off of my bench supply...only problem with that is it doesn't accurately show me how a real xfmr is going to perform.
 
I 'could' hookup per DL's schematic, but even that doesn't get me to a real world solution.
 
It might make sense for me to transfer the circuit to my bigger board and have unlimited control. This small board is somewhat limited...I realize now that I should have made another power pigtail so my bench supply could easily feed either board....considering that also, and should probably just do that since this will surely come up again.
 

[DummyLoad]

Alright DL, that'll take a little finagling
 
Going forward, what do you think about  https://www.hammfg.com/files/parts/pdf/290EX.pdf
 
I think I can hog out the stout chassis hole and fit this on there
 

Makes. sense right?..

I guess, my plate was about 100 more than yours, and fwiw, I used EL34, 6550 and KT88, the 88 sucked B+ down the most

I'll probably run some numbers with EL34 in there before I'm done experimentin'...and before I smoke anything

this one drops right in the v2 stout chassis. 112 bux from mr. hoffman. it's pretty stiff and the one i used to proof the mini plexi w/ dual EL34 SE. the hammond 290CX will work as well. i published a list of PT that would fit in the stout v2 chassis. click here for a list of PTs that should fit, or will fit with minor enlarging.

--pete

[jojokeo]

Now there's a double tranny schematic I've never seen before. Now I know why pete's got all those rectifiers?    It seems disconcerting to see the neg point on top bridge rect going to the lower winding like that but in series things like that happen? Now if this gets SG that higher voltage I've been wanting to see then a big    you go boys! SG - get it goin'! Too bad you just don't have a 400-0-400 PT though... wait just a second; would not this work perfectly? - http://www.ebay.com/itm/STANCOR-12558-1-POWER-TRANSFORMER-TUBE-AMP-12AX7-6L6-6V6-5Y3-12AU7-5E3-/321991180693?hash=item4af82b8195:g:UDwAAOSwEK9T9P2z

That looks good jojo..but also sold (?)

Did you change your cell number? I've texted you several times...

[DummyLoad]

Now there's a double tranny schematic I've never seen before. Now I know why pete's got all those rectifiers?    It seems disconcerting to see the neg point on top bridge rect going to the lower winding like that but in series things like that happen? Now if this gets SG that higher voltage I've been wanting to see then a big    you go boys! SG - get it goin'! Too bad you just don't have a 400-0-400 PT though... wait just a second; would not this work perfectly? - http://www.ebay.com/itm/STANCOR-12558-1-POWER-TRANSFORMER-TUBE-AMP-12AX7-6L6-6V6-5Y3-12AU7-5E3-/321991180693?hash=item4af82b8195:g:UDwAAOSwEK9T9P2z

That looks good jojo..but also sold (?)

Did you change your cell number? I've texted you several times...

somebody bought it... hopefully one of you two characters.

--pete

[jojokeo]

somebody bought it... hopefully one of you two characters.

--pete

why what gives you that idea? 

[DummyLoad]

somebody bought it... hopefully one of you two characters.

--pete

why what gives you that idea? 

imma clairvoyant texican!    no, not really, just WAG...


--pete

[jojokeo]

I like the first one 
So I figure around 525 B+ loaded? That should get there for Vp but looking at datasheets they show Vg2 about 300v - why? Why not just below Vp say 500-ish then drop down for preamp voltages? Especially if I use VVR only on the 88?

Re-thinking again  - that's likely because of hi-fi BS specs in wanting a pristine pure sign wave...

[Ritchie200]

Haha!  I knew I could pull you to the dark side!

Jim

I saw the visual representation of the dark side last night.
 
I measured clean output and then proceeded to measure fully square output at around 10 watts.
 
But there was a point (around "11") where the tops of that square wave started to get a PITCHFORK shape to it, and that's where things got real nasty...
Not sure if I'll ever want to go back there again....kinda scary

If it looked like this, I'd say you were on the right track!

Jim

[shooter]

If it looked like this

When I see that, I hear gnashing of teeth

[DummyLoad]

revised stacked FWB. this rev., 1a, has less ripple at C1. connect top bridge (-) to lower bridge (+). see attached for clarification.

--pete

[jojokeo]

I like the looks of this one better but wouldn't balancing resistors be wanted?

[sluckey]

The bridges put out different B+ voltages so you don't want to try and force them to be the same with balancing resistors.

[jojokeo]

Good point on that. Okay, the big unanswered question I have is about the KT88's screens...I've seen a number of designs that allow the plates to go well into the 500 volts area but why do the screens need to be kept so low at around 300v??? Most other tubes use a 1k resistor which allows them to be not far under the plate voltages. What is the big deal about KT88s screens?

[SILVERGUN]

I went back and tried to find anything that might have been skewing those #s and found a couple loose screws in power rail, so I ran numbers again with everything all tight. Included screen current measurements for jojo. EL34 does not have the same pitchfork effect on the wave before going square (make your own conclusion)
 
KT88
280VDC a-k
23V across 250 ohm
87mA @ 24W pdiss
approx. 8W clean out - 10W square
Screen current: (measured across 1K screen resistor)
5mA clean
17mA at fully square max output
 
EL34 under same conditions:
303VDC a-k
17V across 250ohm
65mA @ 20W pdiss
approx. 6W clean output up to 8W fully square
Screen current:
10mA clean
16mA at fully square max output

[SILVERGUN]

Tapped into bench supply and got plate voltage up for more testing...also did some play/ear testing and seemed to prefer the 88 (at either voltage)
Not very scientific but, something.
For anyone keeping score, I had also left the primary load at 2.4K (accidently) for this set of testing. I'm assuming that I should have gotten rid of the mismatch and returned the pri to it's spec'd 4.8K for 'best' results.
 
KT88
362VDC a-k
31V across 250ohm
118mA @ 43W pdiss
approx. 12W clean up to 16W full
Screen current:
12mA clean
26mA full
 
EL34
392 a-k
22V across 250ohm
84mA @ 33W pdiss
approx. 9W clean up to 11W full
Screen current:
12mA clean
22mA full
 

[SILVERGUN]

Last but not least, change load back to 4.8K into 8 ohms...raised input voltage again
 
KT88
411V a-k
37V across 330ohms
107mA @ 44Wpdiss
approx. 19W fully square
ran out of room on scope to judge/guesstimate clean
Screen current:
30mA at full clip

[Ritchie200]

Okay, the big unanswered question I have is about the KT88's screens...I've seen a number of designs that allow the plates to go well into the 500 volts area but why do the screens need to be kept so low at around 300v??? Most other tubes use a 1k resistor which allows them to be not far under the plate voltages. What is the big deal about KT88s screens?

I would tell you (again), but I would have to kill you.  There is no violence allowed on the forum.  Sorry!

Jim

[Ritchie200]

Please correct me if I'm wrong...  We want the screens low to limit dissipation when the plate swings lower than the screen volts on max signal flogging - which is very important in Class A?  Since we are close to max safe dissipation all the time anyway?  Gotta keep those screens warm, but not glowing! 

Jim

[PRR]

Take plate voltage up. Current must go down to stay inside Pdiss.

The peak current for audio will be about twice the average (idle) current).

Screen voltage determines maximum plate current.

So as plate voltage goes up, screen voltage "can" go down.

Lower screen voltage allows smaller G1 bias and signal voltage, easier drive.

Lower screen voltage indirectly affects the knee voltage. The effect is not large, but you want the lowest knee voltage for best plate voltage swing, so why over-volt the screen more than needed for the current you need.

OTOH.... a low screen voltage is awkward. Screen current is uncertain and changes a lot, so it needs to be a fairly solid supply, not a dropping resistor. For *this* reason, audio designers favor picking tubes and supply voltages so that screen can eat nearly plate voltage. And for historical reasons, most "audio" pentodes can work very happy at 250V to 350V supply for P and G2.

[shooter]

play/ear testing and seemed to prefer the 88

same conclusion I came to, fwiw

[jjasilli]

Good point on that. Okay, the big unanswered question I have is about the KT88's screens...I've seen a number of designs that allow the plates to go well into the 500 volts area but why do the screens need to be kept so low at around 300v??? Most other tubes use a 1k resistor which allows them to be not far under the plate voltages. What is the big deal about KT88s screens?

KT-88's were developed for hi-fi to be used in ultra-linear for hi power w/ lo distortion, or in triode mode.  Checkout this article:  http://education.lenardaudio.com/en/14_valve_amps_4.html  But they don't have to be used that way.

[jojokeo]

KT-88's were developed for hi-fi to be used in ultra-linear for hi power w/ lo distortion...  But they don't have to be used that way.

Exactly! (...what's happening???)

[PRR]

The KT88 was marketed for audio (few hi-fi ran to 50W-100W sizes).

It specifically says "similar to 6550", which is well documented as an Ampex motor servo driver.

It isn't really designed-for ultra-linear. But in the late 1950s, UL was all the rage, every new tube-sheet had to show UL conditions.

The EL34 really lucked-out because it came on the cusp of the UL craze, and its heavy G2 current (normally bad) was turned by UL into a minor advantage. The '34 is a re-pack of a loooong line of low-price larger power pentodes (to pre-WWII EL3).

8417 and 7591 (and a few others) were specifically re-designed (from 6550 and 6L6) at the request of the hi-fi makers (8417 is all Fisher's fault). 7189 is re-rated EL84, an old car-radio tube.

[Willabe]

 

Colas, have you seen this thread?

(He was on line, but I guess I bumped this too late?)

[SILVERGUN]

OK, I'm onto something here..
Play testing tonight with the Triaxis into the '88 Tone Blaster' power amp and really just couldn't be more delighted with the tones.
 
I was previously left with the impression that the Triaxis wasn't quite right and it was somehow contributing to my questionable tone choices of the past...it appears now that I was dead wrong and it was an OPERATOR ERROR. I think I just always pictured this big scary issue that was going on between the pre and power amps and it was just a big tone suck hole and there was nothing I could do about it (this was 15 -20 years ago when I still believed in Santa).
I don't know what was wrong before, but now that I have this power amp and the ability to set the volume of the preamp and drive the 88 as hard as I want, I have found a huge vat of the special sauce that I came here for initially.
Big, round, honking, clear, fat, tight, elastic, punchy, singing, sweetness!
 
I figured out just how much drive I wanted by trial and error and going back and forth with driving stage changes. I settled for an AU7 input tube into a paralleled AU7 driver, and it don't matter what the numbers say because it just plain ole sounds frackin' perfect.
The preamp has an output control, the power amp has both an input volume and output volume control. And it's in the interaction of those level controls where I was able to find my center, and now I can go forward......
 
Just thought that was worthy of an update, because at one point I thought to myself 'that's the sound/control I've been looking for right there!'
I'll be working on a postable schematic, but there's really nothing to it....this has more to do with the interaction of the preamp volume and being able to drive the output at will...brought out such a greater presence and body in the sound. The preamp has a lot to do with it, but my glory has been found in the interaction of the two.
 
THANKS AGAIN GUYS!!
Without all of you're input, experiences, opinions and shipping prowess,,I never could have made it here this quick or accurately.
.. and I'm very happy to report that one KT88 can be and is enough for 'this' specific application. (considering that I will have a whole other power amp running the wet side)
 Oh yeah, that's right,,,I still have to build 'em.
For the record @idle:
373VDC a-k
350 g2-k
33V across 330ohm
95mA @ 35 pdiss
 

[Willabe]

Hey, that is great news! 

Thanks for posting your findings SG! 

[shooter]

I settled for an AU7 input tube

I to have been using AU in the V1 spot for the last 3 builds!

when you get a chance, run it through a good 4X cab just for fun 
when you really go off the deep end, look into the iron required for a PSE KT88

[SILVERGUN]

At first I tried it with just the parallel AU7 barely pushing the 88. It sounded good but kinda sterile, lifeless, and just polite.
The breakthrough came when I added the single AU7 input stage and it wound up driving the driving stage and that just did wonders. Fully cranked it's slightly over the edge, but rolled back just a hair and the response is just incredible. I feel like I got lucky because I got it to the edge and then worked it back from there without losing the magic.

[jojokeo]

Nice to see you've found your happy place SG. I've done a bit of research on the 88's g2 and from what I glean there is a supposed sweet spot at appx 250v. And this is for guitar not hifi use. But It's not good enough to simply feed just any voltage this way using a dropping resistor, rather it is supposed to be regulated at this voltage and/or possibly up to 300v or so? This was found using VVR in the same way as PRR suggested to another member wanting to control his el84 screens in another thread. I've heard also that g2 ran too low caused the 88 to turn darker... Just some more food for thought for you in case you feel like experimenting any further?


Questions, And what did you get if you tried an ax or ay there?



But, now you realize you have sold your soul to the (kt88) devil right? Jimbo will not let you forget or live this down - unless you've accomplished this all with strictly only a tele? 

[SILVERGUN]

Questions, And what did you get if you tried an ax or ay there?

Too much gain, and clipping...for what i could tell
I had it set up with an input 12AT7 into parallel 12AU7 and it was way too much drive

But, now you realize you have sold your soul to the (kt88) devil right? Jimbo will not let you forget or live this down - unless you've accomplished this all with strictly only a tele? 

Oh no...now that megalomaniac isn't gonna be able to fit his head through the imaginary gates of the forum...
I can just hear it now... "Hey SG, how do you like that amp I "designed" for you?"

It does have me thinking more about the effect of that 12AU7 driver CF in the Major, and how that might have something to do with his delusional obsession.

Cant wait to finish assembling the Tele so I'll finally have a grown up's guitar to plug in and play some Beatles.

Don't feel like I need to mess with the screens...don't want to limit max output too much.
I have a 4.7K dropper between the plate and screen nodes, and a dedicated 1K screen resistor. I do understand what you're suggesting for a separate screen supply, and might try it before i wrap it up.

[Ritchie200]

https://www.youtube.com/watch?v=4WrTZweHXTI

 

[DummyLoad]

send PM if you'd like to buy another UBT-2 to round out as a stereo setup.    


--pete

[SILVERGUN]

send PM if you'd like to buy another UBT-2 to round out as a stereo setup.    


--pete

PM coming as soon as I get done rocking out to Sabbath...our secretary came in my office and immediately just turned around and walked right back out. When she got back to her office she came over the intercom and said "what is that?"...I guess anything that deviates from Happy101FM is not work appropriate.
It's amazing what a little tube breakthrough (that no one around you understands) can do for your mood.
 
I have officially changed the name to "The Smokin' 88" power amp....you watch, he's gonna try to hit me up for royalties.
(or the  Silvergun Smokin' 88 as it will appear in my full line catalog next year)
 
I'm gonna send Jimbo a hat..."Make SG Great Again!"  (he's been a huuuuge supporter)

[jjasilli]

 

[SILVERGUN]

You know JJ,,this harkens me back to the Herzog experiment (to which I had a similar reaction of how pushed everything felt), and how enamored I was with that concept...except in this case it makes more sense, and is way less noisy and much more controllable.
The biggest difference here is that the cleans stay clean but just feel more alive and sound fuller....and if you want some slight breakup, you can hit the 88 a little harder, and it works. The OD is just KILLER, and you can really dig in without it mushing out. (I'm pretty sure that 6V6's would be calling for their mommy at this point)
 
For me, there really is something to having the preamp juiced up pretty good and then knocking some of that down(with the Master preamp out) to be able to "re-amp" it and blast the grids of the output tubes....and have some sort of ultimate control over it, rather than an amp with fixed values in the PI and so-forth..
And without resorting to a PPIMV.....when I reached that sweets spot last night  it left a huge grin.
 
Sometimes I'll get to a place in a build/experiment and I'll think "we'll that sounds acceptable", and I'll just leave it because it conforms to the topology(s) of the past....in this case I'm very glad that I decided to push a little further and drive a driving stage into those KT's....it paid off.
 
There are a couple fine points to be worked out in the schematic, and I'm working on getting that together with a couple more questions.
I would like to think that I hit an impedance matching sweetspot where the output of the Triaxis is a good match for the input of the Smokin' 88.
The output impedance of the Tri. is 25K
 

[jojokeo]

For me, there really is something to having the preamp juiced up pretty good and then knocking some of that down(with the Master preamp out) to be able to "re-amp" it and blast the grids of the output tubes....and have some sort of ultimate control over it, rather than an amp with fixed values in the PI and so-forth..
And without resorting to a PPIMV.....when I reached that sweets spot last night  it left a huge grin.

...the input of the Smokin' 88.

SG, keep the name simple...Smokin' 88 is perfect (despite a certain person's coercing).
 
I'm a little confused on your comment regarding that first paragraph in that you're saying you like the Vol control right before the pi (to control the signal amount right before) but then "with the fixed values in the pi"...and "without resorting to PPIMV" -> is confusing without a schematic to understand picture what you're actually doing?

[SILVERGUN]

I'm a little confused on your comment regarding that first paragraph in that you're saying you like the Vol control right before the pi (to control the signal amount right before) but then "with the fixed values in the pi"...and "without resorting to PPIMV" -> is confusing without a schematic to understand picture what you're actually doing?

Sorry jojo, I was mixing my SE results with PP ideals...I should have never said PI other than to mean a "driving stage" before the power tubes.
 
What I'm suggesting to take away from this is the idea that we could have more control after the preamp (in a standard SE amp topology)
i.e...So you build a Champ and want to push a 6L6 but those two little gain stages aren't quite whipping the grid of the 6L6 (it sounds OK, but still ho-hum),,,try inserting a volume pot, feeding another gain stage, but preferably an AU7 so we don't add TOO much gain,,,but the AU7 swings the stock Champ preamp into the 6L6 grid and you get a screaming little banshee (or super crisp, rubbery cleans)
 
I'd like to run with this ball a little after the amps are built and I can just leave the KT on the b-board with the current driving stage in place...my thought would be to try the Deluxe Champ Reverb I built, and instead of running the last stage into the 6V6 (as it currently is), I will "re-amp" it by running it into the driving stage and then to the 88. I have a feeling it's going to be pretty badass.
 
 

[jjasilli]

Glad you like the KT-88's.  I almost didn't make the suggestion, as you originally had other tubes and trannies in mind.

The output impedance of the Tri. is 25K
Wow, that's high; but if it works maybe OK.  For further tinkering. . .


Some thoughts:  Buffer pedal; or various fx pedals might be used just for their buffering function between the Triaxis and the power amp; or an acoustic guitar piezo preamp, which expects to see a hi impedance source.  This might be a cheap & simple way to see if better impedance matching helps.  Or, input matching trannie on the power amp.  Or, for those of us who like tubes      in his Preamp book, Merlin has a cathode follower input stage designed to accept hi impedance sources. (That seems to make at least 2 of our members who are devout "cathode followers".    )

[shooter]

I will "re-amp" it by running it into the driving stage

If you're into tinkering, I did an octal *driver* using a 6SN7 driving a quad SE of EL84s  power amp.
the 6sn7 added some much needed *butter* to the sound, while still being able to send the quad,

calling for their mommy

[SILVERGUN]

To expand (running the risk of babbling):
 
Most guys will say I want a little more gain out of my amp, and the first thing they do is try to add some gain to the existing triodes by increasing resistor values (...whatever)...or re-bias output tubes, etc.
I'm guilty of this as well, and have in the past, wound up with a muddy mess of too much front end 'gain'
 
For a (future) PP example:
What if we interrupted the hardwired connection of PI to power amp, and added in a pot controlled AU7 stage on each side of the wave after the PI, to "re-amp" the signal before it gets to the power tubes. So it's a PPIMV with an AU7 'recovery' stage after the pot. Not so much for the purpose of providing a Master volume, but for changing where the gain is applied, and giving us an adjustable 'grid drive' control.
 
Imagine a simple 5E3 with each side of the inverter feeding an AU7 to drive 2 6L6s...anyone tried it?
That kind of thinking....
 

[jjasilli]

Wait, I think I over-reacted to the 25K output impedance of the Triaxis.  For voltage matching (bridging, actually) the input impedance should be 10X the source impedance.  So if the 1st stage grid leak resistor is at least 250K this should be fine.

[SILVERGUN]

Wait, I think I over-reacted to the 25K output impedance of the Triaxis.  For voltage matching (bridging, actually) the input impedance should be 10X the source impedance.  So if the 1st stage grid leak resistor is at least 250K this should be fine.

That 25K rating for the Tri. output is the printed spec for a MAX output setting...does that number increase/decrease as I lower the output from MAX to about 50% ?
 
Here's the output part of the Triaxis circuit...I'll be using the ones circled in red
 

[SILVERGUN]

Here it is as it sits,, and will sit for the weekend, cause I wont be around: