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In Reply to: RE: You could run that tube in ultralinear mode posted by Ralph on September 14, 2023 at 09:56:29
"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"
Follow Ups:
'
Edits: 08/23/24
go anywhere near 0v grid line.
this is the secret of UL.
it is in the ancient texts.
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"
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.
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"
Edits: 09/14/23
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-
~!
The Mind has No Firewall~ U.S. Army War College.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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.
"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"
Edits: 09/15/23
https://www.oestex.com/tubes/ul.html
I thought the link below of special interest as its a single-ended application.
and they are still not linear or triode like.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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.
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...
.
Have Fun and Enjoy the Music
"Still Working the Problem"
It doesn't look very linear to me.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/14/23
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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 !
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.
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.
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!
I should have added that the 6E6P was triode connected.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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"
Edits: 09/21/23
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