KT88 PP amp

[uki]

Hey guys !!

Been searching for 2x KT88 amp schematic and came across this topic which is exactly what I'm looking for but no schematic.

I sort of winged it on this amp and don't have a schematic. I explain the circuit.

Input -> 1/2 6SL7 -> tone -> Volume -> 1/2 6SL7 -> Phase Inverter (6SN7) -> Pair of KT88's

The topology is a blackface normal channel with added mid sweep (variable slope resistor) and added my tweed control which bypasses the tone controls and provides a simple tweed tone control. It's tweaked everything to provide a clean input stage and shifted the tone control caps down to 47n and 500p. I used all the best parts I could dig up to provide the tone I was looking for. That's why I used Octals (for their warmth)

The power tube plates are at 500v, Screens at 495v. Biased around 60ma/tube. I think the OT is 6k. I had my friend at Mercury do all the calculations for the iron.  I specifically specified as small and light as possible.  The iron shipped to me was 18 lbs.

I hope all this helps.

I have found the preamp schematic from Sluckey's Rock-Ola.
So all it needs is to adapt a pair of KT88, by the way they are cheap just this month.

The goal is: a light weight big watt tube amp.

Around here is possible to order the transformers the way it have to be, custom made.

How to adapt the Rock-Ola to use a pair of KT88 ?  

Thanks in advance

[shooter]

here's what I did with Steve's creation, it's even got 1 88 

[sluckey]

How to adapt the Rock-Ola to use a pair of KT88 ?

Just plug'em in.

[shooter]

 

[uki]

How to adapt the Rock-Ola to use a pair of KT88 ?

Just plug'em in.

I did thought about it but, how about power? how many watts it will do just replacing the tubes? Since the transformers will be custom made for this project, the idea is to build something with around 60-80watts.

[sluckey]

Rocky is about a 25-30 watt cathode biased amp. You really wouldn't get any more power if you just plug in some KTs. About all you could use from Rocky is the preamp and phase inverter. You gonna need big iron and higher B+. And switch to fixed bias. Should be able to find a KT p/p power amp to copy.

Try to contact loosechange by pm. He was just logged on three days ago.

[uki]

Thanks Sluckey !!

Been cooking this amp idea for some time!

You gonna need big iron and higher B+.

I was looking at this calculator, and gave an idea about how much B+ it will need, Loosechange said in his post "power tubes plates are at 500v". His PT is giving some more than that yes? The calculator gives 527ish back with 385-0-385(with diode rectifier full wave). But how much watts of output does it generate? How to calculate power?
Going to use diode rectifier, which type should be used, full bridge ? That way can use a bit smaller iron and get more output yes?

About all you could use from Rocky is the preamp and phase inverter.

Roger !  With more powerful iron, more B+, the power supply for the preamp will need changes yes?

Should be able to find a KT p/p power amp to copy.

I think I've found something !!!

[VMS]

Going to use diode rectifier, which type should be used, full bridge ?

This depends if the HT has got CT or not, without CT full bridge and with CT full wave

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


If using Hammond iron here are some matching pairs from KOC:


https://www.londonpower.com/hammond/matching.htm

[shooter]

fwiw most of my SE builds lately have all been KT88, not sure how "values" hold in PP, but a loaded plate volts of 400-450vdc  seems to be the tubes happy place.  much lower than 400 gets you into compression before the 88 comes alive.  You also want a good drive signal, the 88 seems to want a bigger drive than most other bottles I've played with.

[Ritchie200]

https://el34world.com/Forum/index.php?topic=17548.msg234365#msg234365

Jim

[Ritchie200]

Sorry Uki, I could not help myself!  It will give you the power and certainly the tone (that I like!), but I'm afraid it may be a little heavy...

Jim

[shooter]

but I'm afraid it may be a little heavy...

ya think 
but to be fair, 2 KT88's really is a waste unless they are driving 4 speakers, and 4 speakers with box is by physics - massive 

[DummyLoad]

a pair of kt88 driving a pair of ev12l speakers would probably sound very, very nice. 


--pete

[shooter]

ok, i'll Agee with DL  , although I just use 1 to drive an EV AND a creamback
and it makes me smile 

[Ritchie200]

Bah!  If one is good and two is better - well you know what THAT means!

Jim 

[PRR]

> a pair of kt88 driving a pair of ev12l speakers would probably sound very, very nice.

Quads of 8417 (hot cheap 6550) driving single JBL E-130 (not clipping much) sounded very, very nice.

Agree that if you are going to beat snot out of a pair of big tubes, a single EV12L could be stressed.

[uki]

@Ritchie200  I did find the schematics on that topic !  Thanks !

Sorry Uki, I could not help myself!  It will give you the power and certainly the tone (that I like!), but I'm afraid it may be a little heavy...
Jim

but I'm afraid it may be a little heavy...

4 speakers, and 4 speakers with box is by physics - massive 

4 speakers cab is a project for the future !

> a pair of kt88 driving a pair of ev12l speakers would probably sound very, very nice.

Quads of 8417 (hot cheap 6550) driving single JBL E-130 (not clipping much) sounded very, very nice.

Agree that if you are going to beat snot out of a pair of big tubes, a single EV12L could be stressed.

I'm aiming for something around 70watts, wouldn't be stressing the speaker yes?

Can't get those ev12l around here for a fair price, what is available is a national brand called Keybass

I got the tubes !! Already had the preamp tubes, 2x 6SN7 and 1 6SL7
199 bucks for the matched pair, its about 52 US dollars, the price before was 350 for the pair.

Next is to figure out transformers and circuit.

[PRR]

The Keybass K 15-169 ST series seem to be similar to EV15B/L speakers with respectable spec-sheets, and may serve well at 70 Watts in a proper enclosure. I do not know who really makes them or where, so I do not know how "good" they are.

I missed that we were talking about 12-inch speakers. I assume their Twelves are little Fifteens (possibly same magnet/motor), but it has been a long time since I abused Twelves with flat-out guitar.

[uki]

The Keybass K 15-169 ST series seem to be similar to EV15B/L speakers with respectable spec-sheets, and may serve well at 70 Watts in a proper enclosure. I do not know who really makes them or where, so I do not know how "good" they are.

I missed that we were talking about 12-inch speakers. I assume their Twelves are little Fifteens (possibly same magnet/motor), but it has been a long time since I abused Twelves with flat-out guitar.

The factory of Keybass isn't far from where I live, can't buy directly from them thou. I do have 2 of the K12-147 on my 6G8 cab they sound really good.

You can see/hear it here:

[PRR]

They look like very good speakers. Closely related to the classic E-V speakers of the 1970-1980s that I knew well. The E-Vs were the next generation after the JBLs of the 1960s such as what Dick Dale had Fender had JBL build to survive Dick's loud clear playing.

The 150W "RMS" rating seems reasonable for that coil and magnet. This would support 600 Watts of amplifier in lightly-clipped speech/music PA work. 150 Watts of heat in LOUD guitar amplifier overdrive (pure square waves) can happen with a "75W Sine" amplifier, so that is about the safe limit for crazy guitarists. Two such speakers on a 70W 2*KT88 amp seems quite safe.

For lurkers: In Brasil, imported electronics are HEAVILY taxed to encourage the local industries. US, UK, or Chinese speakers (or transformers etc) are not usually affordable. I'm glad to see this HAS fostered a strong local speaker company.

[Ritchie200]

Yes!  Back in another life I was a manufacturing liaison between engineering and manufacturing also working with import/export with international products.  Brasil was expensive to get our electronic "stuff" into - even if we had no domestic competition.  It didn't even help that I wore my Senna t-shirt when I visited...(he would have been 59 today )

Not sure if it is the depth of field of the pic but wow, that looks like a beast of a magnet!

Jim

[uki]

...Not sure if it is the depth of field of the pic but wow, that looks like a beast of a magnet!

Jim

Here a pic where you can see the whole speaker

[uki]

@Sluckey  What is the RAW control on Rock-Ola, is it same a middle ?

[sluckey]

From zero to five, the raw pots acts just like a mid control. But from 5 to 10 the raw pot begins to lift the tone controls from their ground reference, making them less effective. Crank it to 10 and the tone controls have very little effect, kinda a raw sound. You also get a gain boost.

This is a great mod to any AB763 amp that has a mid control. Simply replace the mid pot with a 250K-A pot. Walla. You have a mid control plus a raw control. Another good AB763 mod is to replace the 100K reverb pot with a 100K-A pot.

[uki]

Is it possible to have the mid control and the raw ? Does it will work well ?

Did some work on V1 schematic

[sluckey]

Is it possible to have the mid control and the raw ?

I've never considered that. Let us know if it is worthwhile. Some people just use a switch to completely lift the tone stack for a full raw sound.

[uki]

Is it possible to have the mid control and the raw ?

I've never considered that. Let us know if it is worthwhile. Some people just use a switch to completely lift the tone stack for a full raw sound.

Do you like the idea? I would need a breadboard to check that out... What about the mid pot value? I just copied from AB763.

How is the sound of Rock-Ola, clean or distorted ?

[sluckey]

I like the idea of a single pot as shown in my schematic. If I was gonna include a mid pot, I would use the value that Fender used. BTW, you don't need the 6.8K resistor when using a mid pot. Rocky is a clean amp. I've never really played it loud to see if it maintains the clean sound at high volume. Actually, I played with Rocky for a couple weeks after building it, then put it on a shelf. I just recently stole the 6L6s from it to go in a '67 Band Master.

[shooter]

the version I did was also clean where I played it, but the guitar guys dimed it out, pedaled it up, and had it through a 2 X12 and she broke up nice with good distortion.
I think one comment was "It's not like a Fender, or a Marshall", I counted that a good thing 
  I also re-purposed iron & tubes

[uki]

This is just a draft , I dunno if it works or if is correct but is an idea.    I haven't decided yet about the power section, thou.
Since this is based on the 6G8, it would give 100w power yes? Not really what I want but it is an exercise.
Im still looking for more schematics, ideal power would be 70w.

[pdf64]

...Since this is based on the 6G8, it would give 100w power yes? Not really what I want but it is an exercise.
Im still looking for more schematics, ideal power would be 70w.

See the manufacturer's info, the operating conditions of a 6G8 (430V idle HT, assume 2k p-p OT) would be nowhere near what a pair of KT88 need to deliver 100W http://frank.yueksel.org/sheets/086/k/KT88.pdf
There's a good chance that the plates would overdissipate heavily.
If the OT is mismatched to an 8ohm load, then you may get ~70W though.
Note that KT88 need a greater magnitude of bias voltage than eg 6L6, so a consequently bigger signal is needed to drive them fully (ie to Vg-k=0); guess about 100V p-p here.

[VMS]

There is also conditions for 70W UL power section on above datasheet.
I think UL amp without nfb-loop sounds pretty good.

[Ritchie200]

There is also conditions for 70W UL power section on above datasheet.
I think UL amp without nfb-loop sounds pretty good.

Oh yes it would!

Jim

[uki]

There is also conditions for 70W UL power section on above datasheet.
I think UL amp without nfb-loop sounds pretty good.

Oh yes it would!

Jim

Would this have a marshal like distortion? Looking to build clean sound amp.

...Since this is based on the 6G8, it would give 100w power yes? Not really what I want but it is an exercise.
Im still looking for more schematics, ideal power would be 70w.

See the manufacturer's info, the operating conditions of a 6G8 (430V idle HT, assume 2k p-p OT) would be nowhere near what a pair of KT88 need to deliver 100W http://frank.yueksel.org/sheets/086/k/KT88.pdf
There's a good chance that the plates would overdissipate heavily.
If the OT is mismatched to an 8ohm load, then you may get ~70W though.
Note that KT88 need a greater magnitude of bias voltage than eg 6L6, so a consequently bigger signal is needed to drive them fully (ie to Vg-k=0); guess about 100V p-p here.

I will have the transformers custom made. I need to learn how they have to be.
A modification on the bias circuit is necessary then to be able to use this schematic yes?

[Ritchie200]

https://el34world.com/Forum/index.php?action=dlattach;topic=17548.0;attach=48413

Jim

[Ritchie200]

I'm not sure how the circuit you have will work.  One thing that is constantly preached around here is pick a WORKING circuit and tweak to taste from there.  The link I posted is to a circuit Dummyload posted on the Major thread.  It also references transformers you can look up for specs to custom wind.  It is based on the original Major circuit.  The Major has a big bold sound and will kick you with clean.  It really takes a lot to overdrive it but is very musical when it does. See early SRV or Ritchie Blackmore.  I'd say listen to my recordings I've posted but I've run so much crap into the front end it really doesn't capture it (it does take pedals very well!).  I lay the 4x12 face down on the floor with a mic under it and pack pillows around it. I still cant turn it up past 4 without being very uncomfortable - with headphones on.  You asked if it sounds like a Marshall?  No.  When you hit a big drop D chord on a cranked EL34 Marshall it squashes into that woody tone we all love.  With the Major, the UL KT88's hold up and will kick you in the gut - in a good way.

Here is a great example of this amp turned up with a little front end push.

Jim

Jim

[VMS]

Here is one option of a KT88 amp. Drz surgical steel. And with good eye you can probably reverse engineer the circuit from the gut shot. Only tricky part is the fx-loop with 12dw7 tube:


http://www.proguitar.de/ProGuitar.de/Surgical_Steel.html

[uki]

 

... About all you could use from Rocky is the preamp and phase inverter. You gonna need big iron and higher B+. And switch to fixed bias. Should be able to find a KT p/p power amp to copy.

How about this, use the major power supply as well ?

[sluckey]

Looks clean to me. Something worth mentioning... I changed over to 6SL7s soon after converting Rocky. I think those KTs will like the extra gain also.

[uki]

Looks clean to me. Something worth mentioning... I changed over to 6SL7s soon after converting Rocky. I think those KTs will like the extra gain also.

I got only one 6SL7, use for PI ?

Does the power supply need chages? I see nodes B and C on Rocky for the preamp. But Major have only A and B nodes... 

Here is how the PT ca be ordered. With or w/o CT.

[sluckey]

I got only one 6SL7, use for PI ?

No. Put it in the preamp. If you get another later on just change out the PI.

And make a node C for the preamp.

[shooter]

I see nodes B and C

use the last tap on the major PS, "feed" it to the turret board, and "build" each tap near the node

[uki]

And make a node C for the preamp.

A bit lost here, dunno if this works and no clue about the values.
Can it be copied from Rocky using same values?

[sluckey]

Use 47µF for the cap. Start with 22K for the resistor but be prepared to increase the resistor value. The preamp is only gonna draw about 2mA, so a 22K resistor will only drop about 44V. Half watt is sufficient for testing but use 2 or 3 watt metal oxide once you have found the value you like.

[Ed_Chambley]

Well, let me add to the stew.  I have built a couple of variations of this amp.  It is not the same as a Route 66, I passed a lot more lows and mids because hey I am using big tubes.   Dummyload gave me a couple inside hints and using teh 12At7 is better, much better than !2Ax7 for the PI.


It will play about 77 watts or so clean and then the slide into sustain, not grit, just singing tone.  It is very loud and I run mine as a small Bass Rig sometimes, but it shines over a Bassman Center Board 2, 12 with Altec 417 and poor old greenbacks scream in a 4 12.


Remember the ad for JBL or Memorex where dude sitting in his chair with a speaker blowing his hair straight back.  It is KWEL to play, but remember it will play nice too.  The deal is the EF86 has little headroom so if you push it the pentode tone says CRANK!


The reason I thought of it as I have used a 6Sn7 PI, but the 6SN7's I have will not slam the crap out of the KT88 or KT120, well I could have, but the 6SN7 are selling for stupid money.  I like 6sl7 because I have a few really nice ones.  I have a friend who just changed KT120 in one of these. Tubes ran 3 years of 5 to 6 days a week hard play a week.  I retubed it just because he felt he should.  They were getting a little noisy, but my gowd man.


I have been running KT120 for over 3 years and before Gold Lion KT88 Russian.  I tried some JJ's, but just not the same in this amp.  The old GE 6550a will actually run hotter without screen problems than the KT120.  I am not teling anyone to install KT120's.  I purchase new tubes from Upscale Audio and they cost more and he has burned in and tested up to 550 volts.  I am good with that.


Anyway, another to think about.

[DummyLoad]

ed, you should be making about 100W with that OT & PT combo even in UL mode. suspect with those values that your LTPI is not driving hard enough to do that. i guess that's what you want?

likely you're not capable of driving output stage into clipping with current LTPI.

bump B+ to 400V for the LTPI.

suggestion:
Ra1 = 82K (inv)
Ra2 = 91K (non-inv)
Rtail = 15K
Rbias = 1.2K @ 2.9mA
B+ = 400V
Vout ~90Vpk

Rdrop of B+A to B+B is = 75K now, lower to 33K-36K. so, change two resistors and you should get near to 100W and allow power amp to clip. the OT can take it since it's rated at 120W @ 30Hz

--pete

[uki]

fwiw most of my SE builds lately have all been KT88, not sure how "values" hold in PP, but a loaded plate volts of 400-450vdc  seems to be the tubes happy place.  much lower than 400 gets you into compression before the 88 comes alive.  You also want a good drive signal, the 88 seems to want a bigger drive than most other bottles I've played with.

What is the PT high voltage on your schematic?

So thinking on the above
I did look at several schematics old ones, and the PTs go around 400-0-400 but that for old tubes, for this new Sovtek KT88, the data sheet says 440 max screen voltage, using that much B+ after the diodes which will be about 560, will the screen have as much? 350 from PT will do about 500v after diodes, what would be the ideal high voltage on PT for this amp to achieve around 70w ?

[shooter]

look at what Ed used in post #44, he's got north of 500vdc plate and UL taps for screen, which probably measures north of 440vdc, but G2 current is what'll kill the tube faster than volts, and he likes it and that's good enough for me 

[uki]

I think I found what I'm looking for with this info and this

1- My concern is, with 400-0-400 OT, the rectified voltage will be about 560v or more, this much voltage will drop with tubes in place and with bias adjust yes?

The picture shows what I'm looking for

2 - So the plate should be about 460v to get 70w, so between 460-500 plate voltage is the ideal, is it right?

[shooter]

 
I missed the 120 
You're correct Uki, 400-0-400 will get you over 500 loaded based on Ed's #'s
360-0-360 I believe will be closer to your design #'s, wait for more chime in's before you order.