2 x kt88 power amp

[AxelKay]

Hey guys! I want to build a 2x kt88 power amp to accommodate/accompany my ultra high gain preamp. I have come down to three different designs :

https://mhuss.com/MyJCM/JCM800_2204.pdf 

https://ax84.com/archive/ax84.com/static/corepoweramps/50W_PP/AX84_50W_PP_Poweramp_Schematic.pdf
  https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf

In your opinion, which one of the three power amp designs would give the most tight high end "unmuddied" and modern metal sound?
I want the most out of the kt88's (in a good way)
Let me know which one you would do and why (along with the necessary modifications of course)
P.S. my preamp can handle 250 - 300v or even a bit higher.
SERIOUSLY can't wait for your input!
Thanks

[sluckey]

They're all basically the same circuit. Just use big iron and big filter caps for what you want. You may also want to consider some Hiwatt designs.

[DummyLoad]

Major Minor. two schema attached: different power supply transformers.

--Pete

[AxelKay]

Major Minor. two schema attached: different power supply transformers.

--Pete

I already got those schems. Problem is that there's no layout around, especially for the iii, and also I'm not sure where the power amp stops and where I should hook my preamp up. I know the vr7 (MV) is included, but is the c7 to be ignored and go along with r22 -> r18 -> B directly?

[shooter]

P.S. my preamp can handle 250 - 300v or even a bit higher.

little confused, will the pre PS be powering the PA?

KT 88's like 400 - 450VDC at the plates as a starting place to get them all excited about showing off 

[SILVERGUN]

Major Minor. two schema attached: different power supply transformers.

--Pete

I already got those schems. Problem is that there's no layout around, especially for the iii, and also I'm not sure where the power amp stops and where I should hook my preamp up. I know the vr7 (MV) is included, but is the c7 to be ignored and go along with r22 -> r18 -> B directly?

Big dreams require big effort. You might have to make your own layout especially since this is a custom design. Look at other amp designs and layouts and try to see where the similarities are. You might get lucky and find one that matches up close enough that you can borrow. The more you look around the more you'll realize that the power amp sections of the layouts all start to look very similar and it should become very apparent where you need to connect your preamp signal to the power amp. That's it for the lecture.

VR7 is a volume control - do you have one in your design?
Ignore C7?-   why?
R18 is in the tone stack
R22 is a plate resistor that connects to the B+ rail (might look different in your preamp design)

I would highly suggest posting a screenshot of the changes/questions you have because I'm not sure if guys will have the patience to go back and forth from a pdf schematic to check your questions.

When I'm not being the fun police I am highly recommending that you look at the Bogner Ubershall schematic if you want "un-muddied modern metal".

[AxelKay]

P.S. my preamp can handle 250 - 300v or even a bit higher.

little confused, will the pre PS be powering the PA?

KT 88's like 400 - 450VDC at the plates as a starting place to get them all excited about showing off 

No no. Will be powered by the pa

[AxelKay]

Major Minor. two schema attached: different power supply transformers.

--Pete

I already got those schems. Problem is that there's no layout around, especially for the iii, and also I'm not sure where the power amp stops and where I should hook my preamp up. I know the vr7 (MV) is included, but is the c7 to be ignored and go along with r22 -> r18 -> B directly?

Big dreams require big effort. You might have to make your own layout especially since this is a custom design. Look at other amp designs and layouts and try to see where the similarities are. You might get lucky and find one that matches up close enough that you can borrow. The more you look around the more you'll realize that the power amp sections of the layouts all start to look very similar and it should become very apparent where you need to connect your preamp signal to the power amp. That's it for the lecture.

VR7 is a volume control - do you have one in your design?
Ignore C7?-   why?
R18 is in the tone stack
R22 is a plate resistor that connects to the B+ rail (might look different in your preamp design)

I would highly suggest posting a screenshot of the changes/questions you have because I'm not sure if guys will have the patience to go back and forth from a pdf schematic to check your questions.

When I'm not being the fun police I am highly recommending that you look at the Bogner Ubershall schematic if you want "un-muddied modern metal".

I've already built a pcb (painting by the numbers) uber with 4 x E34L's. Not the twin jet (180w)
I know c7 is part of the tone stack but I'd like to use my own, or if I may say; one input, four gain stages, post tone stack. That's what I have.
If I could alter the iii and turn it into a steroid ultra gain modern tight thing for detuned 7 or 8 strings, I'll be living the dream!
(I apologize but I only have a decade of experience in amp technology and that's from what I've read here and there. I have a huge experience in electronics but that's not enough and I'm fully aware of the fact)

[SILVERGUN]

I know c7 is part of the tone stack

Please know that I'm not trying to be a d!(%
Don't let me knock you off track. I'm just trying to make sense of your questions and help give you some direction.

C7 is not in the tone stack. It is a filter cap.
Unless you are looking at Doug's Plexi 50 schematic and I am looking at Dummyloads Major Minor iii

[AxelKay]

Yes yes! You are correct, I'm on my phone and switching back and forth between schematics can be tricky!

[jojokeo]

Hey there SG (and others) - it's been a while thinking of kt88 uses aside from Jimbo's constant banter but - serious question here - do the '88 screens need regulation and/or be kept at a lower voltage than the plates? Or can they simply be used like the other power tubes in the same way as EL34s, 6L6s, etc...? And I'm assuming that the screens don't need or even maybe want an OT with UL taps? Did you SE OT use a UL tap?

[SILVERGUN]

Did you SE OT use a UL tap?

No, just a standard big SE OT running @ 420V plate, 395 screen, 33V across 300ohms. These are new numbers since I recently changed the tube, but it sounds good to me so numbers don't matter. I'm cool with a 1 year duty cycle. I used a 4.7K dropper between the plate and screen supply and a 1K screen resistor intentionally just to try to show some mercy.

[AxelKay]

@silvergun if you were to choose between the mhuss jcm800 6550 and the minor iii in terms of quality saturation and articulation, which one would it be?

[SILVERGUN]

@silvergun if you were to choose between the mhuss jcm800 6550 and the minor iii in terms of quality saturation and articulation, which one would it be?

Of those choices, for modern metal, the 6550.( You do mean the Peavey 6505 right?)
I have a close friend who plays in a touring metal act and he has found his happy place in a 5150iii...I agree with him that it's his best tone by far. Fat, tight, hard hitting.

[SILVERGUN]

This is a snip of the NFB circuit in a 5150ii
Look at how different it is than 99% of all classic amps.
If you look at the Ubershall it also has a similar approach of "tuning" the NFB.
This is where some of the metal magic happens.

[AxelKay]

@silvergun if you were to choose between the mhuss jcm800 6550 and the minor iii in terms of quality saturation and articulation, which one would it be?

Of those choices, for modern metal, the 6550.( You do mean the Peavey 6505 right?)
I have a close friend who plays in a touring metal act and he has found his happy place in a 5150iii...I agree with him that it's his best tone by far. Fat, tight, hard hitting.

Nope, M. Huss' take on the jcm800 50w with 6550 valves (direct drop-in replacement to the kt88).
Wish I had a 6505+. The evh iii yes, I will agree with you, perhaps THE most perfected amp for the job so far! Got the schems for both but I have too many el34 amps, plus it's half the price buying them brand new!... Need some 88 into my life right now 

[SILVERGUN]

So just to be clear, what preamp are you using?

Of those choices, JCM800, ho-hum, Major Minor, maybe cool if I was doing a Deep Purple cover act.

I keep going back to your original post... "modern metal" power amp.
I would do any of the well known modern metal power amps adapted for KT88 and refer back to sluckeys original post:
BIG caps, BIG iron.

[AxelKay]

About the big caps... In terms of capacitance or V ?
He also mentioned something about hiwatts. They aren't famous for their presence in the general metal scene, so what should I be looking for in a hiwatt?

[SILVERGUN]

About the big caps... In terms of capacitance or V ?
He also mentioned something about hiwatts. They aren't famous for their presence in the general metal scene, so what should I be looking for in a hiwatt?

V rating will matter once you figure out what your B+ is gonna be.
In this case we are referring to capacitance.
The bigger the uF the stiffer the PS will be and the better the bass response. Very important for metal and critical if you'll be using low tuned 7 and 8 strings.

The reason I was asking you about the preamp is because there are basically 2 schools of thought.
Build a big clean crunch machine like a Hiwatt and drive it into Metalville with pedals OR build a gain monster preamp into a very efficient power amp (like a Soldano SLO100).
A Hiwatt would make a perfectly good platform to drive hard with pedals.

True "modern metal" amps are highly tweaked machines....as I'm sure you know

[sluckey]

I should not have mentioned hiwatts. Sorry.

[AxelKay]

About the big caps... In terms of capacitance or V ?
He also mentioned something about hiwatts. They aren't famous for their presence in the general metal scene, so what should I be looking for in a hiwatt?

V rating will matter once you figure out what your B+ is gonna be.
In this case we are referring to capacitance.
The bigger the uF the stiffer the PS will be and the better the bass response. Very important for metal and critical if you'll be using low tuned 7 and 8 strings.

The reason I was asking you about the preamp is because there are basically 2 schools of thought.
Build a big clean crunch machine like a Hiwatt and drive it into Metalville with pedals OR build a gain monster preamp into a very efficient power amp (like a Soldano SLO100).
A Hiwatt would make a perfectly good platform to drive hard with pedals.

True "modern metal" amps are highly tweaked machines....as I'm sure you know

I'd swear that I read on several occasions, that the higher the cap value, the muddier (boomier ?) the sound, and keeping lower cap values in general brightens the amp up

[tubenit]

I already got those schems. Problem is that there's no layout around, especially for the iii

Look at this thread and Reply #1 which has Pete's Major Minor schematic attached.  You will see on that same reply that there is a SCH editable layout available. I think it could be edited in under 20 min to match Pete's schematic.

http://el34world.com/Forum/index.php?topic=17556.0

With respect, Tubenit

[sluckey]

I'd swear that I read on several occasions, that the higher the cap value, the muddier (boomier ?) the sound, and keeping lower cap values in general brightens the amp up

I specifically said big filter caps.

[SILVERGUN]

http://www.valvewizard.co.uk/smoothing.html

[shooter]

I read on several occasions, that the higher the cap value, the muddier (boomier ?) the sound

In the pre i'll agree, in the PS and PA, you want BIG.  I typically use 100uF and up for the PA taps, I ALWAYS mostly build self biased, so I use 47 - 100uf as PA tube bypass caps (ya that's not metal :)

[DummyLoad]

cleaned up the schematic and adjusted power rail resistors to comp for the lower B+. oversight on the cathode resistors for the 2 x KT88 - this output stage is published in the GEC KT-88 datasheet. for the PT the hammond 370FX will work well, the 370HX will loaf and run cooler. a JCM 800 blank chassis would be a good fit for this circuit. i may breadboard this ckt. later in the week.

https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf

schema attached. check for errors - feedback welcomed and appreciated.

--pete

[AxelKay]

cleaned up the schematic and adjusted power rail resistors to comp for the lower B+. oversight on the cathode resistors for the 2 x KT88 - this output stage is published in the GEC KT-88 datasheet. for the PT the hammond 370FX will work well, the 370HX will loaf and run cooler. a JCM 800 blank chassis would be a good fit for this circuit. i may breadboard this ckt. later in the week.

https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf

schema attached. check for errors - feedback welcomed and appreciated.

--pete

Right on!

[66Strat]

cleaned up the schematic and adjusted power rail resistors to comp for the lower B+. oversight on the cathode resistors for the 2 x KT88 - this output stage is published in the GEC KT-88 datasheet. for the PT the hammond 370FX will work well, the 370HX will loaf and run cooler. a JCM 800 blank chassis would be a good fit for this circuit. i may breadboard this ckt. later in the week.

https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf

schema attached. check for errors - feedback welcomed and appreciated.

--pete

Nice!

The only thing that I see is that VR6 differs in value from the Marshall Major schematic, 1M vs 250K.

https://el34world.com/charts/Schematics/files/Marshall/Marshall_major_200w.pdf

[jjasilli]

Hey there SG (and others) - it's been a while thinking of kt88 uses aside from Jimbo's constant banter but - serious question here - do the '88 screens need regulation and/or be kept at a lower voltage than the plates? Or can they simply be used like the other power tubes in the same way as EL34s, 6L6s, etc...? And I'm assuming that the screens don't need or even maybe want an OT with UL taps? Did you SE OT use a UL tap?

Per bogie specs, KT-88 screens have max V of 400; and should be 1/2 -to- UL% of plate voltage.  Plate V max = 600.  E.g., Stromberg-Carlson 1100 100W PA amps, 2X KT-88's, use separate PT secondaries for 600 plate volts, 300 screen V.  But for guitar amps, seems no one bothers with screen specs.

[66Strat]

The data sheet that I have shows g2 V max of 600 volts.

Unless I'm mistaken, the Stromberg Carlson 1100 is tetrode connected rather than UL.

https://el34world.com/charts/Schematics/files/Stromberg_carlson/Stromberg_carlson_aph1100.pdf

[DummyLoad]

The data sheet that I have shows g2 V max of 600 volts.

Unless I'm mistaken, the Stromberg Carlson 1100 is tetrode connected rather than UL.

https://el34world.com/charts/Schematics/files/Stromberg_carlson/Stromberg_carlson_aph1100.pdf

JJ was just pointing that out and using SC 1100 as a reference of typical tetrode schema. the SC 1100 has two delon doubler HVPS - 625V for anodes, 325V for G2.

JJ, ALL 3 KT88 datasheets that i have, give 100W UL conditions with 560V to the plates. that condition will have more than your noted 400V maximum applied to the screens.
max applied VG2 depends on the mode of operation. because of VG2 limit of 600V in tetrode mode, one would expect that max Va in triode mode would be less than 800V and most datasheets indicate that is so - Va,g2 = 600V max. for UL/triode modes, where Va max in tetrode mode is 800V.


--pete

[AxelKay]

The data sheet that I have shows g2 V max of 600 volts.

Unless I'm mistaken, the Stromberg Carlson 1100 is tetrode connected rather than UL.

https://el34world.com/charts/Schematics/files/Stromberg_carlson/Stromberg_carlson_aph1100.pdf

JJ was just pointing that out and using SC 1100 as a reference of typical tetrode schema. the SC 1100 has two delon doubler HVPS - 625V for anodes, 325V for G2.

JJ, ALL 3 KT88 datasheets that i have, give 100W UL conditions with 560V to the plates. that condition will have more than your noted 400V maximum applied to the screens.
max applied VG2 depends on the mode of operation. because of VG2 limit of 600V in tetrode mode, one would expect that max Va in triode mode would be less than 800V and most datasheets indicate that is so - Va,g2 = 600V max. for UL/triode modes, where Va max in tetrode mode is 800V.


--pete

I can also verify this, wherever I've looked kt88 / 6550 max voltage dissipation is 600v

[Ritchie200]

Original Major runs about 600V on screens - really beats them up.  New manufacture tubes have problems with that.  I've only found a few that will play.  Change to 1k on screens to give them a break.

Jim

[jjasilli]

Yes, bogie specs are 800 Plate, 600 screen for KT 88, but then don't use 6550!  Virtually nobody uses full KT88 voltage,except Ritchie200, and it set his hair on fire! Anyway, 600 or less plate, 450 screens seems pretty standard actual use for guitar amps.

And yes the SC 1100 have 2 HT secondaries as stated for plate / screen; hence are not UL.  I think KT88's were designed with UL in mind (but I think PRR dis-agrees).  But for guitar amps, UL is seldom used. Just ask, Ritchie! 

[AxelKay]

Yes but with age comes wisdom and forgetfulness... forgetfulness... back to wisdom, the kt88's are starting to get favored by the modern player and not necessarily in UL. As far as I have researched, the max v dissipation is 600v, 1k res, and running them at somewhere around 470v works fine. Only issues are the xfrms. 400something - 0 - 400something. Ot is a tricky business because of the impedance! And fortunately or not, I have no other choice but to go with Hammonds (lowest price : quality ratio for the EU market)

[Ritchie200]

Axel,

If you read that entire Marshall Major/Minor thread you saw that the idea was to recreate the Major at lower volume.  The Major is a very unique design not found in any other musical instrument amp.  That design contributes greatly to its sound when driven hard.  That was the secret to Ritchie's iconic sound.  If your aim is to sound like Dimebag (RIP 15 yrs ago yesterday), that design may not be what you are looking for.

If you run the KT88's in the 500v range you will have a load of headroom.  As you saw, the Major is also UL which provides even more headroom.  If you are just interested in reproducing a high gain preamp design, a non-UL KT88 design will be fine and definitely still give you a kick in the gut.  I would have to disagree with SG in that the KT88 is MUCH more musical to my ears.  The 6550 is sterile and brittle in comparison.  HOWEVER, if you are trying to reproduce an existing proven 6550 design that is the "benchmark" for metal players, have at it.  Hammonds are fine.

Whatever you build a soundclip is a REQUIREMENT!

Jim

[66Strat]

The data sheet that I have shows g2 V max of 600 volts.

Unless I'm mistaken, the Stromberg Carlson 1100 is tetrode connected rather than UL.

https://el34world.com/charts/Schematics/files/Stromberg_carlson/Stromberg_carlson_aph1100.pdf

JJ was just pointing that out and using SC 1100 as a reference of typical tetrode schema. the SC 1100 has two delon doubler HVPS - 625V for anodes, 325V for G2.

JJ, ALL 3 KT88 datasheets that i have, give 100W UL conditions with 560V to the plates. that condition will have more than your noted 400V maximum applied to the screens.
max applied VG2 depends on the mode of operation. because of VG2 limit of 600V in tetrode mode, one would expect that max Va in triode mode would be less than 800V and most datasheets indicate that is so - Va,g2 = 600V max. for UL/triode modes, where Va max in tetrode mode is 800V.


--pete

Apologies, I misinterpreted JJ's post.