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Recommend driver for SE KT88

144.253.15.46
Posted on September 12, 2023 at 12:34:36
Pabloaguas
Audiophile

Posts: 50
Location: East
Joined: August 3, 2006
 Hi Inmates, 



I'm wondering if any experts can recommend driver topology for my SE (Tetrode Mode) KT88 amp?



Background- this amp started as an Audio Note Kit 2 SE 6550 amp. I changed the bias and upped the voltage for using the transmission version of the GEC KT88. I have 555v of Anode to Cathode voltage and have 285 on G2, but might want to go into Triode mode someday... just not sure if 555 volts in triode mode would kill the tube... I guess that is another question.

This amp originally had a very poor design of a SRPP 12ax7. This SRPP got changed later to use 5751 and then recently changed droppers to use e80cc. I eventually deleted the input tube stage(the 6SN7) because the extra gain was not needed. 

So now, I'm wanting to just abandon the SRPP, and drive the KT88s with a SE topology driver- maybe just a good-old voltage gain stage with a single section of a dual-triode. I read here that a single section of a 5687 is an excellent driver for the SE KT88. 



Can anyone with direct experience comment on what types of circuits and tubes they have found to be best for driving the SE Kt88?

Thank you in advance for your suggestions.

 

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RE: Recommend driver for SE KT88, posted on September 14, 2023 at 08:28:32
Paul Joppa
Industry Professional

Posts: 7344
Location: Seattle, WA
Joined: April 23, 2001
 Depends on the KT88 operating point (voltages, currents, and load impedance) and on the feedback topology and amount.

 

You could run that tube in ultralinear mode, posted on September 14, 2023 at 09:56:29
Ralph
Manufacturer

Posts: 5100
Location: Minnesota
Joined: April 24, 2002
 If you had the right output transformer... any transformer manufacturer could make one for you.

I've done this in some smaller amps and its worked quite well. If the tap is placed properly you get triode linearity. I think there is merit to this since KT88s are easier to get, are easier to drive and you'll be able to run the amp with less likelihood of hum from the filament circuit.

Regarding driving the tube- a 6SN7 section will do that nicely if AC coupled and class A1. If you are planning class A2 or A3 then you'll need some driving current in the driver- a cathode follower direct coupled to the tube will work very well but you'll need a B- supply to make it work. A 6Sn7 will still work very well for that.

 

RE: You could run that tube in ultralinear mode, posted on September 14, 2023 at 13:33:14
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 "If the tap is placed properly you get triode linearity."

With a beam power tube or a power pentode, even wired in triode you don't get triode linearity. Wire UL they are less linear than if they were wired in triode mode but as I said, even wired in triode mode they are not as linear as a true triode and further away from the linearity of a direct heated triode like a 45, 2A3, 300b or 845.

Here's a 6550 in UL mode. You might be able to improve on this by changing the tap location but it will never be as good as when triode wired.



Here's a 6550 triode wired. Do you see how much more linear this is vs. UL?



Here's a 300b. Do you see that it is more linear than the triode wired 6550?




The bottom line? You can not get triode like linearity from a beam power tube or a power pentode. No matter how you hook it up.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Ultralinear patent, posted on September 14, 2023 at 14:00:22
Ralph
Manufacturer

Posts: 5100
Location: Minnesota
Joined: April 24, 2002
 The ultra linear patent, assigned to Arco, disagrees with you. Link below

Other manufacturers, like Dynaco and Marantz, recognized the benefit, but not wanting to pay royalties, moved the taps slightly so as to get around the patent. In fact the entire industry did this and that practice is still around to this day, mostly out of tradition and ignorance of that went down; which is why you are presenting the info you have.

 

RE: Ultralinear patent, posted on September 14, 2023 at 14:32:20
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 Are you saying that the plate curves for UL on the 6550 look bad because the UL tap is at 43% instead of 40%?

Note, "When Hafler & Keroes talked about a 20% tap, they were talking in terms of impedance. This correspondes to a 40% tap in terms of turns ratio. They are actually the same thing but expressed in different units."

"When the screen grid of a tetrode is connected directly to the anode of the tetrode the resulting tube is efiectively a triode"

That is simply not true. Or at the very least, a tetrode connected as a triode does not automatically become a "good" triode.

I think you need to take Hafler/Keroes' words with a grain of salt.

BTW Hafler/Keroes didn't have just one number in mind. It was a range depending on what tube was being used.

"6. The combination in accordance with claim 1 wherein the tube types employed are selected from commercial types 6L6, KT-66, 5881, and 807, and wherein said screen grid loading falls in the range 18 per cent to 26 per cent."


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

From a Acro pamphlet on UL operation showing different tap values, posted on September 14, 2023 at 15:22:58
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002



.
Have Fun and Enjoy the Music
"Still Working the Problem"

 

tung sol 6550 UL plate curves at the recommended 40% tap, posted on September 14, 2023 at 15:58:19
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002






It doesn't look very linear to me.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Ultralinear patent, posted on September 15, 2023 at 08:00:47
Ralph
Manufacturer

Posts: 5100
Location: Minnesota
Joined: April 24, 2002
 To my understanding, yes, a number of manufacturers used slightly different taps with as little as only 2% difference in order to get around the patent. This results in a bowed curve.

Of course the tap values must be different depending on the tube; tubes being different from one another and all.

Your Tung Sol reference does not specify what UL was used, only that it was UL. The curves I've seen when the taps are set up properly look very much like a triode. Apparently though the family of curves you get varies quite a lot depending on the tube.

Since Arco controlled the patent and Hafler later went to Dynaco, I find it amusing that he knew how to get around his own patent and proceeded to do so.

 

40% Just what Acro claims is optimum, posted on September 15, 2023 at 13:32:19
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Ultralinear patent, posted on September 15, 2023 at 14:39:39
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 "The curves I've seen when the taps are set up properly look very much like a triode. "

Can you provide a link please?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Ultralinear patent, posted on September 15, 2023 at 17:32:51
tube wrangler
Manufacturer

Posts: 2493
Location: USA
Joined: January 29, 2007
 Pentode types: There will always be a time-delay
between plate and G2-- unless this
anomaly is corrected for-- a pentode
will never become a "good" triode no
matter how triode-connected or UL connected it is.

However-- there is a correction that
can be applied by designing the output
trans for the brand tube you are going to use--
(UL connection), OR by
designing a separate G2 supply that
applies the correction there-- or BOTH as I
am currently doing..

I am pursuing this KT88 thing
lately and over the last 2 years
I have made a few changes, as needed,
decided by direct comparison against
my own and other pure DHT triode amps.

Incidentally this pentode thing started
originally by using the KT-150.

I soon found SOME KT88 types that are
better built (don't rattle like a tin can from
crappy construction), and sound really good,
so the KT-150 quickly became an also-ran.

When a KT88 is run correctly, and
at the right plate current, and corrected
as stated (above), it can and does
sound just like the DHT triode amp with
some delightful additions:

(1) It has more bottom-end "crunch" and
layering of music, it has a deeper soundstage,
it has less hum (if both tube types have balanced A.C.
operated filament).

(2) The top-end extends further. In daily use, on
all kinds of music and sources such as Blu-Ray
movies, etc., this isn't needed, as the pure
triode DHT has enough of that anyhow, when used
with excellent parts.

(3) Once perfected, the two topologies sound
very much alike-- in practical usage terms--
but they won't measure alike....

All in all-- I love them both, and so do the
guys I know who are building both. All are SE
amps.

-Dennis-

 

Dennis, this guy is looking for you, posted on September 16, 2023 at 19:56:18
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Circling back to the OP, posted on September 18, 2023 at 12:19:33
Pabloaguas
Audiophile

Posts: 50
Location: East
Joined: August 3, 2006
 It's interesting to hear about the UL debate and see some curves showing UL effects in push-pull amps. However, as the original poster, I'm looking for advice for a driver for my SE KT88, as well as looking at the debate between fixed-power-supply G2 mode vs "triode" mode. This is partly Because my existing transformers have no UL tap, and I have no time/money/ or interest in experimenting with UL.

So, trying to just redirect this thread to the two original questions asked, which were:

1. Favorite drivers/circuits/tubes for driving SE (including fixed-tetrode or triode) KT88

And

2. Debate on fixed-voltage g2 vs "triode" mode G2 using fixed value resistor from Anode to G2

Thank you in advance for your wisdom !

 

RE: Circling back to the OP, posted on September 18, 2023 at 13:50:14
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 The KT88 in pentode mode is very easy to drive. Depending on how much gain you need, I would just use your favorite signal triode. If you have a preamp and if your speakers are reasonably efficient one triode stage (per output tube) should be all you need.

Hooked up as triodes KT88 are harder to drive but they require less feedback.

Edit, I think I am wrong here. Tetrodes have high plate resistance and a lot of distortion and they need a lot of feedback for both of those things. I think you would need a lot of gain so you can apply a lot of feedback.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

Try the Russian 6E6P or perhaps the 6E5P, posted on September 18, 2023 at 14:41:58
coronadope
Audiophile

Posts: 102
Joined: October 31, 2007
 Several years ago, under the guidance of an OEM, I ran the 6E6P to a direct coupled KT88/KT 150.

EBB of about 390VDC, Plate resistor 18K, cathode resistor 500R. About 205 VDC 'cross the tube at 9.55MA. (Some may say that this is tube abuse and will claim max P K to be 150VDC.) My friend has been running the 6E6P at these values and has even goosed things up a bit and reports zero problems.)

Here's my comments from May 14 2020 when I breadboarded this design:. Explosive! Dynamic! Clean! Clear! Holographic! Detailed! Drive! Build it!

 

RE: Try the Russian 6E6P or perhaps the 6E5P, posted on September 18, 2023 at 14:50:36
coronadope
Audiophile

Posts: 102
Joined: October 31, 2007
 I should have added that the 6E6P was triode connected.

 

RE: Try the Russian 6E6P or perhaps the 6E5P, posted on September 18, 2023 at 17:18:43
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

you would short stroke the driver in an UL amp and never, posted on September 20, 2023 at 20:14:09
elblanco
Audiophile

Posts: 3682
Joined: August 20, 2004
 go anywhere near 0v grid line.


this is the secret of UL.

it is in the ancient texts.

 

RE: Circling back to the OP, posted on September 20, 2023 at 22:06:29
Paul Joppa
Industry Professional

Posts: 7344
Location: Seattle, WA
Joined: April 23, 2001
 tl;dr -- the choice of a driver depends on the kind of feedback used - the requirements are quite different.

My view is that feedback is feedback, and UL is a form of feedback. Therefor other forms of feedback can be substituted for the UL connection. There is nothing magic about UL.

My current interest is in plate-to-grid feedback, a.k.a. Schade or anode follower or transimpedance. This is mostly theoretical; I've only designed two such amps that have been built and listened to, and the jury is still out - there are too many variables for a couple of samples to judge by. But the theoretical advantage is that the feedback is confined to the output tube and does not involve the driver or the output transformer, thus the loop can be quite stable.

If the feedback is large, the driver tube sees a low impedance load - the output tube becomes a current-controlled voltage source, i.e. a transimpedance. To get a large amount of feedback, you need to have a high source (plate) impedance in the driver or a low-resistance feedback resistor (which would waste a lot of power as heat). This is the opposite of the usual rule for a driver, which calls for a high load impedance and a low plate impedance (such as a 5687).

Most plate-to-grid feedback amps designs I've seen on the web use a (relatively) low-rp driver so the amount of output-tube feedback is usually small. The RH-88 is a widely recognized example. The feedback is sometimes augmented with an ultralinear connection, which partially brings the output transformer into the loop.

 

RE: you would short stroke the driver in an UL amp and never, posted on September 21, 2023 at 08:09:08
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002
 I would like to understand what you just typed but I don't. Can you explain it to me?

Thank you.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Ultralinear patent, posted on September 27, 2023 at 08:23:01
Ralph
Manufacturer

Posts: 5100
Location: Minnesota
Joined: April 24, 2002
 https://www.oestex.com/tubes/ul.html

I thought the link below of special interest as its a single-ended application.

 

RE: Ultralinear patent, posted on September 27, 2023 at 20:13:30
grindstone
Audiophile

Posts: 197
Location: GMT -6
Joined: August 10, 2008
 Thanks for that--I had forgotten all about Mr. Grimwood for years and years and it was good to see he apparently has been updating over time.

(Thought I?) recalled Patrick Turner doing a pile of work on those things and lost hours diving usenet posts and archives of his website only to achieve full-bleery.

Seems as though the OP has proceeded in a parallel thread of the same subject line at AK...

 

these UL curves, from your link, are the same as the ones I posted., posted on September 28, 2023 at 09:35:55
Tre'
Industry Professional

Posts: 17968
Location: So. Cal.
Joined: February 9, 2002



and they are still not linear or triode like.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: these UL curves, from your link, are the same as the ones I posted., posted on September 28, 2023 at 09:57:51
Ralph
Manufacturer

Posts: 5100
Location: Minnesota
Joined: April 24, 2002
 Yes (I'd have replied sooner but have had my hands full recently). Once you get the plate voltage high enough, the curves look pretty decent. It seems as if you were careful and didn't get greedy, you could avoid that squiggly bit on the left easily enough.

Something else that might make this a fun little project: Norman Crowhurst pointed out a problem with how feedback is applied in tube amps, yet over the last 65 years or so since I've not seen anyone (except us, apparently) do anything about it. The problem is non-linearity at the feedback node (the cathode of an input tube) causing the feedback signal be distorted before it can mix with the incoming signal. This has led to feedback having a bad rap in the high end audio world. The solution is fairly easy though- mix the feedback with the input signal before it hits the tube. IOW on the grid, using a divider network, like the way opamps do it.

Then you could put a bit of feedback on the circuit with a number of benefits and take advantage of the fact that the tube is easy to drive by comparison to a DHT, allowing the circuit to be simple. Two tubes would be enough to do it.

 

There is merit and validity in what Dennis says here, Sigh, nothing beats empirical knowledge!~nT, posted on October 29, 2023 at 10:30:35
Cleantimestream
Audiophile

Posts: 7613
Location: Kentucky
Joined: June 30, 2005
 ~!
The Mind has No Firewall~ U.S. Army War College.

 

EL84... :), posted on November 16, 2023 at 05:43:47
PakProtector
Audiophile

Posts: 12412
Joined: May 14, 2002
 If the gain structure is acceptable, 6V6 will have lower gain in comparable circuits. But call the plate load 10-15k Ohm, and tune the OP with their g2 voltage.

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Circling back to the OP, posted on November 20, 2023 at 09:01:18
PakProtector
Audiophile

Posts: 12412
Joined: May 14, 2002
 mmmmmhhhh...plate to grid. I recall one of Gary Pimm's pictures of doing that sort of thing to a MOSFET. Resulted in vertical, evenly spaced gate lines of drain current. IMO, it is slightly easier to use pentodes as getting the heat out is slightly easier... :)

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Recommend driver for SE KT88, posted on August 21, 2024 at 00:29:04
tube wrangler
Manufacturer

Posts: 2493
Location: USA
Joined: January 29, 2007
 I might add a few bits here-- this is
not to be taken as advice to any builder, but rather these
are impirical observations only, decided upon by listening to
the best system wiring, source components, and speakers.

So please consider these ideas as not mathematically provable,
but relevant to good musical enjoyment:

The SE- KT-88 has continued to be my go-to amp for
personal musical pleasures, because I just LIKE IT better
than anything I ever heard.. or have played with since.

Remember DeHaviland Electric? They built SE amps using
the 845 triode, and later a Russian designed higher
voltage DHT triode with more amp-factor than the 845. This
DHT tube was more popular with the buyers at the time.

The Lady in charge, who I won't mention here, got
curious and had her tech guy modify a Fisher 50-watt,
PUSH-PULL ULTRALINEAR amp for her. I believe they
succeeded in eliminating all feedback, but my memory
might not be correct here.

In any case, she loved it, and it became her go-to
amplifier.

There is something about a good pentode that simply speaks
music when operated in ultralinear mode. No, I can't
tell you what this is, but it is THE BEST sound I have
ever heard and for many years I did SE DHT triodes to
maximum sonic results.

My current KT-88 fave is a "pseudo-triode" 6E6P directly
connected to a KT-88 running ultralinear with a 2500 ohm
output transformer primary driving a (Altec design) 604
series driver in an approximately 11+ cubic foot cabinet,
that is a transmission-line design utilizing dual ports
that are symmetrical to each other, delivering very linear
bass response.

The speaker system is linear over the entire audio
range, and this feature makes it both spectacular and
musically pleasing with less listener fatigue and more
musical detail and dynamics than anything I've ever heard
or used. This makes it very telling of any and all amplifier
characteristics as far as altering music signals goes.

The 6E6P is run at about 15 ma. Plate and cathode voltages
were adjusted to get the KT-88 to operate at about 68ma,
with approximately 490 VDC on the KT-88 plate, with near
245 VDC on the cathode. This is because I always try to
get the cathode VDC at 50% of the voltage applied to the
plate. I find this more dynamic with lower distortions.

The KT-88 may very well be the most musically accurate
vacuum tube ever designed. I can't tell which ones of
the current crop are THE best, but I like the Russian
made "Gold Lion" copies, and the TAD (Tube Amp Doctor)-
Germany.

There is "something" about the KT-88 design that
SPEAKS MUSIC, linearity-be-damned.

Best advice is to find out WHY.

Thanks for listening to me! -Dennis-


 

RE: You could run that tube in ultralinear mode, posted on August 22, 2024 at 11:59:34
Best audiotransformers
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Joined: December 4, 2023
 '

 

RE: Recommend driver for SE KT88, posted on January 28, 2025 at 16:46:55
hifipaul
Audiophile

Posts: 739
Location: NY
Joined: December 22, 2008
 555v is too high for SE KT88. typical is about 350v across the tube (P-K) at 80 - 90 ma....550v would be more suitable for a PP circuit. You may want to reconsider the original PS design.

How much gain you need (ie, tube type) is related to how much feedback you use. An octal socket will let you experiment with several tube types, 6sn7,6sl7 6n7,6j5,6f8....I'd suggest to not use 5687.

In triode you can get away with 0 feedback or low feedback, which is what I prefer.

I used a choke loaded 6n7 in my last SE kt88 amp. Good sound.

 

RE: Recommend driver for SE KT88, posted on April 30, 2025 at 22:41:51
Minusss3db1
Audiophile

Posts: 2
Location: Wind Blown Prarie
Joined: April 30, 2025
 I think it is out there using the 6N1p choke loaded, direct coupled to the KT88.
This has a very good reputation, you have to float the cathode of the KT88 with a power resistor, but the heat is not too great.... I started with a 12AT7 actually loaded with the back to back Hammond chores for 300H...It sounded excellent, but was even better when I redid the circuit with 5670....
And of course sorry, circuit lost in storage move, and amp given away long ago..... Good Luck
Minusss3db

 

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