Stereo SE KT88 power amps for live rig

[pompeiisneaks]

oh gotcha, so the biasing of the tubes to optimize tone etc? 

Very interested in how this goes too.  Keep us posted!

~Phil

[SILVERGUN]

oh gotcha, so the biasing of the tubes to optimize tone etc?

Yeah...or just the use of a huge bottle in a different operating condition than what might be considered "normal"
 
The SE application is supposed to bring out the second harmonic, and I thought it might be cool to hear that through a KT88...we'll see
The guys pointed out pretty early that it will not be very noticeable if I'm hitting it too hard...so my main goal is basically to get the tonal advantage of the KT88 with approx. the same output as 2 6V6s...I have to try it to know if it's worth it.
 
The problem that comes with the low B+ is that you have to have a lot of current to get the tube going (for "tone")
Lots of current in an SE build means a big, heavy, expensive OT....and that was contrary to my initial concerns about cost and weight.
 
Not very sensible when you can have more clean power cheaper with push-pull...so I'll try one stereo side SE and the other side PP...we'll see

[SILVERGUN]

What do you guys think about the driving stage(s)?...and filling in some of the blanks now that we have some variables resolved?
How should I set up the screens and what value cathode resistor should be a good fit?
I guess I'll need more drive than a parallel AU7? ...how bout 2?
 

[Willabe]

Hi SG. 

Colas LeGrippa built a few SE KT88 amps. He LOVED them.  Do a search, he posted full schemos.

Last he posted he was working/living in Cuba.

[SILVERGUN]

Hi SG. 

Colas LeGrippa built a few SE KT88 amps. He LOVED them.  Do a search, he posted full schemos.

Last he posted he was working/living in Cuba.

Did an advanced search for posted by: Colas LeGrippa
Keyword : KT88
and only came up with this:
http://el34world.com/Forum/index.php?topic=16733.msg165853#msg165853

[PRR]

> one-electron UBT-2. it's 4.8K at 4/8/16 or 2.4K at 8/16/32

Am I snow-blind or is that a math error?

[PRR]

> The problem that comes with the low B+ is that you have to have a lot of current to get the tube going (for "tone")

Not "tone", for POWER.

You want to take a 6550/KT88 up near 40 W Pdiss to get 16W out. (Assume 40% efficiency at 5%-10% THD.)

100V 400mA
200V 200mA
300V 133mA
400V 100mA
500V  80mA
600V  66mA
1000V 40mA

You can get power at low current, just use insane voltage.

I don't think voltage (or current), per se, actually gives "tone". When we hear different tone at different voltages, we are often not changing the current or the load as we should. If the V/I/Load are off-optimum, of cource the overload "tone" will change.

The Optimum Load will be roughly V/I:

100V 400mA 250r
200V 200mA 1K
300V 133mA 2,225r
400V 100mA 4K
500V  80mA 6.2K
600V  66mA 9K
1000V 40mA 25k

> Lots of current in an SE build means a big, heavy, expensive OT

An optimized transformer "scales". For a given Pdiss, various volt/amps, use the same core (and gap), adjust the number of primary turns.

Your problem is that the Likely Suspects don't do this. Hammond gives you one primary winding, with a chart for several nominal impedances. Taken literally, 125ESE can only do 80mA. Looking above, that means 500V supply. But the 6.2K Optimum Load isn't on Hammond's chart. I worked 450V 78mA 35Wdiss and nominal 5K, came shy of 35W*40%= 14W out.

Impedances below ~~~2K, you won't find a single tube to pull well.

Impedances much over 5K, you won't get hi-fi bandwidth without costly fancy winding (even then is tough).

[SILVERGUN]

> one-electron UBT-2. it's 4.8K at 4/8/16 or 2.4K at 8/16/32

Am I snow-blind or is that a math error?

Lost in translation...and excitement
I'll leave the math police'ing up to you guys
OK....2.4K @ 2/4/8

[shooter]

Am I snow-blind

snow here also, but I think He mentioned a *mis-match* spkr to solve that.

might be cool to hear that through a KT88

My gigging son likes the SE KT, says it's chime-y, smooth, loves pedels
You're probably gonna want about 50Vac -p-p available for drive.  I was biased about 20-23v, my drive
was about 56Vac p-p, which just started to *bend* the KT when it was dimed.  Don't know your pre preference's but the CF TS after the pre sounded best in my world over the *fender* style, fwiw

[SILVERGUN]

The Optimum Load will be roughly V/I:

100V 400mA 250r
200V 200mA 1K
300V 133mA 2,225r
400V 100mA 4K
500V  80mA 6.2K
600V  66mA 9K
1000V 40mA 25k

Thanks for all of that PRR, but I especially wanted to point this out...
I knew the current to voltage relation,,,but never tied in the impedance as it relates to that equation.
Seeing it listed like that made a couple dormant brain cells perk up...ah-ha moment

[Ritchie200]

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

Jim

[Willabe]

Did an advanced search for posted by: Colas LeGrippa
Keyword : KT88

Maybe it was 6550? 

[SILVERGUN]

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

Jim

This guy got me with this one line..."if you were a real man......blah,blah.........KT88"
We'll I knew I'd never build a Major, so before these things collect too much dust, here we are.
You better be right Kemo-sabee

[DummyLoad]

> one-electron UBT-2. it's 4.8K at 4/8/16 or 2.4K at 8/16/32

Am I snow-blind or is that a math error?

yes. brain fart. 2/4/8


@$!#×@&!!

-pete

[SILVERGUN]

Don't know your pre preference's but the CF TS after the pre sounded best in my world over the *fender* style, fwiw

Thanks shooter...
Just a power amp driver circuit needed..no tonestack (see partial schemo above)
I never measured the preamp for it's output signal voltage, but I'm assuming I'm gonna want to set this up to keep a big signal pretty clean and just get it to the grid of that KT as simply as possible.

[SILVERGUN]

MAX idle current through UBT-2 is 120mA - use caution! 

How about a 1/8 amp FA fuse in line between plate and OT on the breadboard?

So, I should be right there on the edge @120mA @ idle(looking at PRR's chart)?...slightly lower than the 133mA he's got for 300VDC
With these PTs I am seeing approx. 315VDC plate to K at idle with just the 6V6 in there.
So cathode resistor value will be critical.

Now I see where/how the voltage/current/impedance conundrum comes into play, and I see why any logically thinking mammal would shy away from this application.

[SILVERGUN]

So cathode resistor value will be critical.

I'll give it a shot...based on ballpark #s

Lets say 120mA total plate plus screen current
Made up bias voltage of -30VDC (based off shooter's # of 56V p-p)

30 / .120 = 250ohm R-K

Any good?

[jojokeo]

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

Jim

This guy got me with this one line..."if you were a real man......blah,blah.........KT88"
We'll I knew I'd never build a Major, so before these things collect too much dust, here we are.
You better be right Kemo-sabee

Late to the party but better late than never there at all! My head is spinning at all the suggestions and had to stop reading before reaching the end of page 1...so where the heck are things currently?

[DummyLoad]

start with 220R-250R for Rk. Vk should be around 25V with 325V, assuming Vg2 is near plate V.


--pete

[shooter]

30 / .120 = 250ohm R-K  Any good?

You should be close enough, my B+ was tad higher, 360ish, Rk, I think was 270, down from 300, or 300 up from 270 .  No notes on final I, used a 3k OT,  I'm still pis-poor at final docs

[SILVERGUN]

Late to the party but better late than never there at all! My head is spinning at all the suggestions and had to stop reading before reaching the end of page 1...so where the heck are things currently?

Good to see you jojo....here we are in a nutshell
- was thinking about a stereo SE EL34 power amp rackmount style build for SE tone (harmonic content) at a reasonable volume
- originally was thinking about weight issues and cost issues and wanted to rule out specific toroidal xfmrs and OTs for current concerns
- got great advice and opinions from people I respect....listened intently
- realized that I should just try 2 small power amps in conjunction with 2- 1x12" speaker cabs for a live wet/dry rig
- went back and forth about stage volume and headroom vs. drummer in small/med club settings
- thought of trying one SE power amp for one side (wet or dry) and one PP for the other
- Jimbo pushed me towards the KT88 through PM by attacking my manhood
- realized that I should just try a SE KT88 to cross it off the bucket list and hope it fits the bill for either the wet or dry side of the rig (because I have a pair here)
- was (and still am) being taught how to properly 'design' it
- DL hooked me up with a very nice OT
- will put the circuit on the breadboard as soon as the OT arrives and then try to work backwards through the drive stage(s)
- may try to manipulate final output with speaker selection...i.e. SE dry side through high efficiency speaker to compete with PP wet side through low efficiency speaker (or opposite)...may use VVR on PP side to balance volume
- will use 2 of Dougs stout chassis because they mate up well with my 'thiele' type cab dimensions (and because I already have one here)
- will worry about PP side later....thinking either EL34 or 6L6 into Hammond 1760L (because it's here)
- we had to discuss concerns about 340V B+ and the current / impedance demands on the OT for KT88 for any kind of sensible operation. (because I have PTs in stock with 240VAC secondaries)

....besides me flip flopping a couple times, that's about it

[jojokeo]

Thanks for the info., with so many ideas and suggestions it's hard to know which direction you're going. I agree with PRR's tone assessments above (big shocker here) and without reading it prior to my own gut instincts, was immediately thinking of a higher B+ with less current KT88 situation. You can use the OT you & DL listed but in reverse mis-match to get up to near the optimum load but isn't necessary to get it perfect. Which would be the range of ~9K @ 8/16/32 option for the windings. I tend to like pushing towards the smaller side of the plate loading listed match in general. Smaller current = smaller size, weight, & core, etc. with power rating of 20w should be fine. (didn't read ratings for OT you are thinking of using).

I was thinking of using only the 240vac outside windings (no CT use) 240-240 > FWB > ~600-620Vdc loaded on plate & screen while using VVR would be fun. Big tubes like big voltage (just ask Jimbo) but with VVR you can tailor whenever wanted and the nice thing especially SE when you dial it down a tad you will notice the floor noise hiss drop as well. It's a small issue in a small gig/bar scene but more valued if ever recording or simply practicing at home. I think you should VVR only the KT88 if you decide to incorporate it. I changed the PT for a slightly higher voltage & current rated one after this schematic was made but it didn't make any tonal difference other than running cooler (one of my original goals on that amp was also seeing how close I could design that amp to the edge of various parameters successfully).

http://el34world.com/Forum/index.php?topic=10769.msg99089#msg99089

I ran my preamp V1 tube to bias two stages simultaneously in three ways and helps with a nice variety of tones & gain levels to suit your mood or a minimum of two at least on a DPDT On/On switch. I then also made a tone stack bypass mod for another given a big signal boost and tweed feel. I suggest doing something similar to provide various flavorful options to the mix and you'll have a nice fun versatile amp there.

http://el34world.com/Forum/index.php?topic=9954.msg125523#msg125523

"....besides me flip flopping a couple times, that's about it"
That's fine unless you're a certain female running for president?

[kagliostro]

Ciao SILVERGUN

Here is my provocation

Mesa Boogie 20/20™ Stereo Power Amplifier

http://mesaboogie.com/support/out-of-production/2020.html

http://mesaboogie.com/media/User%20Manuals/20-20%20Manual.pdf








Based on the original idea to fit all in a head space and in a power of 20 + 20 W (however not very much nor very low)

Franco

[jjasilli]

The problem that comes with the low B+ is that you have to have a lot of current to get the tube going (for "tone")Lots of current in an SE build means a big, heavy, expensive OT....and that was contrary to my initial concerns about cost and weight. Not very sensible when you can have more clean power cheaper with push-pull...so I'll try one stereo side SE and the other side PP...we'll see


Yes, that well summarizes the issue of big iron SE, especially if there's 2 of them. 


Another thought.  Do you really want power tube overdrive?  This is not an easy question for your application.  KOC says power tube overdrive is unnecessary & wasteful, because you can get any tone you want in a preamp.  The power amp need only supply a clean boost to drive loudspeakers.  The clean power amp can boost overdrive tone created earlier in the preamp.  With your Triaxis preamp you probably don't need power tube overdrive to get your desired tone.  But if you want the option, you may find the amp too loud for the venue.  Some alternatives:  overdrive only 1 SE amp & keep the other clean; install a triode SW or use Tubenit's switchable cathode resistor-cap circuits to cut power for less headroom & earlier overdrive.

[jojokeo]

or use Tubenit's switchable cathode resistor-cap circuits to cut power for less headroom & earlier overdrive.

VVR does a nice job for this by lowering PT's voltage while it's grid gets hit normally since it gets much more sensitive (lowers power tube headroom considerably) running at lower voltages. It also preserves normal high end and harmonic content from being diminished which resistance normally affects.

[Ritchie200]

Another thought.  Do you really want power tube overdrive? 

YES!!!!!  I'm not sure about SE, but there is nothing that sounds as sweet as whipping KT88's in PP.  Preamp shmeamp...

Jim

[jjasilli]

Yeah, I'm with you; but for Arena Rock! 


SG wants this rig for local bars

[Ritchie200]

No, I'm sayin that if the KT88 sounds as good flogged SE as it does PP, I think he will be very happy!    Sorry, didnt esplain meself very well...


Jim


ps Preamp shmeamp

[shooter]

if the KT88 sounds as good flogged SE as it does PP

It's lots harder to flog the 88 in SE mode, but if you do, and have no NFB, expect some mean n ugly replies

[SILVERGUN]

OT got here on Monday, but I've been tied up all week with BS...sorry if I didn't address all above posts, but I do appreciate all of the input.
Big THANKS again to DL for his kind gesture, incredibly fast shipping, design help, and all around good dude-ness...I even got a special surprise in the box!
 
I know how much you guys like big words and pretty pictures, sooo....
There it is! put the 9V in there for scale

[Ritchie200]

Ooooooooooo!!!!   Ahhhhhhhhhh!!!!  Cant wait to hear the results!

Jim

[Ritchie200]

Also wanted to give my .02 on cabinet(s) for this build.  You mentioned 2, 1x12 cabs.  I'm sure that will work fine and may be the best for portability.  However, I did want to throw this out to you.  I have a 4x12 that can be split for stereo.  I tried it with my Scholz/Rockman setup which is basically the modules running into a 150 watt/channel SS PA.  The sound was great, just a glorious swirl in a small package. (The response was horrid because to get the true Boston vib, you need full range speaks)  I'm not suggesting that you build a 4x12, but a 2x12 with a mono and stereo jack might be more useful onstage and with other builds?  Separation (as in two cabinets spaced out) is great, but if it's miced, it will not matter.  Remember the angled 2x12 cabinet with ports that I posted a couple of years ago on that speaker thread?  That design might be just the ticket.

Jimbob

[sluckey]

Big THANKS again to DL for his kind gesture, incredibly fast shipping, design help, and all around good dude-ness...I even got a special surprise in the box!

Yeah, he's like that. A lot of people from Texas are. Always a surprise in the box. He sent me some rocker switches recently. Used about 50 FWB rectifiers as packing  material! 

[jojokeo]

SG - Aren't you glad you have that glorious test rig set up paying off for you once again? Watch out for that Ritchie guy's advice next thing you know he'll have you label the input jack "No Tele's" 

[DummyLoad]

SG you're welcome.use it for pp amp of other. it drove 4 x el84 in pp with 8 ohm load in a baldwin organ.
sluckey i was goung to send more but ran out of room... :p

no tele input jack - i like it!! ;-)

thanks for the kind words... y'all !

--pete

[Willabe]

Wait a minute, are you working with outer space beings with their technology?     

(Dang it, mine won't enlarge, look at reply #79 and you'll see the critter.)   

[jojokeo]

I see it too! There's some sort of plate-being or getter-guard in there? Definitely appears to be lacking sunshine either way?

[Willabe]

I see it too! There's some sort of plate-being or getter-guard in there? Definitely appears to be lacking sunshine either way?

       Well I guess stranger things have happened in Philly? I mean just look at their baseball mascot.   

[SILVERGUN]

SG - Aren't you glad you have that glorious test rig set up paying off for you once again?

Yes indeed...it took all of about 30 minutes to get it cooking..priceless
I wound up putting a 12AT7 at the input just to push the AU7 driver enough to get a big signal in...my signal gen. only puts out about 1.5V max
Here's some #'s
No load B+ = 357VDC
Idle:
plate - K = 265VDC
25VDC across 250R K
95mA
25W pdiss
 
Loaded clean output @ approx.7 watts before going square pretty quickly...
But that's a VERY loud 7 watts, and square doesn't sound so bad either
 
I hooked up that little Deluxe Champ Reverb through a load - line level box and ran it into the input and it sounded pretty big for a little SE 7 watter
 
All, preliminary fun, but I figure I'd better report something after teasing you  guys with that pic
 
 

Wait a minute, are you working with outer space beings with their technology?     

Are you talking about MY reflection?...yeah I need to get out more
 
 

[Willabe]

Are you talking about MY reflection?...yeah I need to get out more.

Conspiracy cover up! 

Tryin to keep all the good outer space secrets to yourself. 

[shooter]

before going square pretty quickly

just the output of the 88?, or were you driving it squared?

[SILVERGUN]

before going square pretty quickly

just the output of the 88?, or were you driving it squared?

The KT plate signal went square well before the signal at the grid.
Makes. sense right?..with that stupid low B+

[jojokeo]

The KT plate signal went square well before the signal at the grid.
Makes. sense right?..with that stupid low B+

That's why I was trying to coerce you to drop the CT and wire up both outer ends of that PT to a FWB config and get those volts up??? Any chance you might give it a go? Of course you'll have to drop the preamp back down after the screen node.

[DummyLoad]

PT seems marginal for application. 62V drop w/ cap loaded FWB.


--pete

[SILVERGUN]

The KT plate signal went square well before the signal at the grid.
Makes. sense right?..with that stupid low B+

That's why I was trying to coerce you to drop the CT and wire up both outer ends of that PT to a FWB config and get those volts up??? Any chance you might give it a go? Of course you'll have to drop the preamp back down after the screen node.

No CT on this xfmr...it's basically just a 120v to 240v isolation transformer with what I thought was a very impressive 570mA rating

PT seems marginal for application. 62V drop w/ cap loaded FWB.

Yeah, I don't get it...I'll post the spec sheet tomorrow and you can correct my thinking...
Good thing I ain't married to it. Didn't think it was gonna dump it's guts like that.

[DummyLoad]

seem like a 135VA transformer should have better regulation than that with 170-180mA AC load. sorry that's .62 * .1A


http://www.hammondmfg.com/pdf/5c007.pdf



--pete

[SILVERGUN]

Here it is:
https://www.hammfg.com/files/parts/pdf/185F230.pdf

I got a deal on some of these and used them on a few small amps...still have a couple left, which is why I was trying to use it

[DummyLoad]

so, ya say ya have two of those PT? 


try this (see attached), it's a stacked FWB. wire it up carefully as depicted. no tolerances for error. bad wire job = bad juju mon: it leaks magic smoke. lots of it. bring it up connected to a lamp limiter.
makes 485V or thereabouts. screen supply should be around 325V. preamp stages are set to something less than 480 if tap the plate power rail or less than 325V if you tap the screen power rail. i chose the screen power rail in this scheme. throwing darts i'd say around -30V to -32V at Vg1 for 40W Pdiss at 450Va-k you need around 90mA Ik. so 30V/90mA 350 ohm Rk. start at 430R and work down. Rk should be 5-6W: 10W would be better.

works on SIM so should work for you. CAUTION! unloaded B+ at the plate power rail can potentially be over 500V, so me mindful of cap voltage ratings. for this PS, 350V radial cans are low-cost and should work well.


--pete

[shooter]

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

[SILVERGUN]

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