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Old 12th November 2006, 04:15 AM   #1
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Default Pentode vs Ultralinear

Hey guys

for the past three months (at least) I have been debating buying a huge upgrade in my OPTS for my EL34 SE amplifier. I currently opperate the '34s in Triode Mode with a Hammond 125CSE

The issue comes when I shop. The One Electron UBT-2 seems to be the Gold Standard for this purpose, opperates in UL, Pentode, or Triode with no problem at all. It comes with a $92 dollar price tag, which makes a Stereo Pair roughly $200 (with all Tarrifs, Taxes, and UPS Fees).

On the other Hand, there is a steady stream of Transcendar Transformers on ebay that are 135 for a stereo pair.ltra These opperate at a primary of 3.3k instead of the UPT 4800, but thats not my issue. These have no CT for UltraLinear. People have said that these two OPTs are very close to each other in preformance, and in power capability

so this is where I face a huge question, Is a pentode operating mode worse than UL? I hear a resounding "YES!" most of the time, but Its just soooo much cheaper. Is there anything I can do to make it sound the same? or at least $100 the same?

any help would be appreciated beyond belief

-Moose
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Old 12th November 2006, 06:49 AM   #2
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I'll get in before Johan Potgieter, this is his specialty!

Pentode mode well implemented can sound as good as UL, IMHO, as well as providing significant extra power. Some people are actually prepared to admit that they prefer it!

UL gives you a head start in the area of low OP impedance and low distortion, because of the local NFB built into the design. (We shouldn't forget the need for a very well designed and built OP transformer for UL. A good tranny is expensive; a mediocre tranny will work against the apparent advantages of UL.) However, a local NFB arrangement can be made with pentodes so that, in the end, there isn't a great deal of difference in that respect.

Pentode mode is usually considered more expensive (or at least, more trouble) to implement than UL. This is because it is necessary to do something to stabilise the g2 supply in pentode mode. UL just needs a tapping for g2 on the OP transformer primary (two tappings if you're talking of PP) and no stabilisation is necessary. That is a sweeping statement, I realise; a UL design in Class AB1 still requires a 'tight' PS but it's not as critical, we are told, as the g2 supply for a pentode design.

The purist pentode practitioner will have you believe that you must provide a totally separate PS for g2, with its own power tranny, rectifiers and choke-input filter. This costs quite a bit, takes up a significant amount of extra space and adds a lot of weight.

Other pentode fanciers (self included) prefer an easier and simpler approach, to get, hopefully, just as good a result. As long as your B+ supply for the plates is 50v or more above the required voltage for the screens, you can derive a stable screen supply from that same B+ using a MOSFET regulator. Such a regulator costs only a few dollars and doesn't take up much extra space nor add any great weight. You do need a good heat sink, but there are no heavy iron components or additional capacitors of any size.

A small additional requirement is that, if you regulate the screen supply in absolute terms (i.e. using a reference voltage source like a gas discharge tube or a zener), so that it doesn't vary even if the line voltage changes, then you need to regulate the fixed bias supply too (assuming that's the type of bias you use). If you don't, the current through the OP tubes will vary as the line voltage changes because the bias will also change but the g2 voltage will not.

Johan, any comments?
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Old 12th November 2006, 08:04 AM   #3
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Quote:
A small additional requirement is that, if you regulate the screen supply in absolute terms (i.e. using a reference voltage source like a gas discharge tube or a zener), so that it doesn't vary even if the line voltage changes, then you need to regulate the fixed bias supply too (assuming that's the type of bias you use). If you don't, the current through the OP tubes will vary as the line voltage changes because the bias will also change but the g2 voltage will not.
Just a fast question. When both bias and g2 voltages are regulated, doesn't the B+ voltage need to be regulated too? Because when using triodes, one should not regulate one without regulating the other, but with pentodes I don't know...

Erik
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Old 12th November 2006, 08:50 AM   #4
Tweeker is offline Tweeker  United States
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Its ok to regulate just screen and bias in a pentode or beam tube, operation is much more sensitive to screen than plate.
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Old 12th November 2006, 08:58 AM   #5
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Hi Tweeker

Quote:
Originally posted by Tweeker
Its ok to regulate just screen and bias in a pentode or beam tube, operation is much more sensitive to screen than plate.
Thanks for the answer!

Erik
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Old 12th November 2006, 09:14 AM   #6
Yvesm is offline Yvesm  France
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Quote:
Originally posted by ray_moth
I'll get in before Johan Potgieter, this is his specialty!

Pentode mode well implemented can sound as good as UL, IMHO, as well as providing significant extra power. Some people are actually prepared to admit that they prefer it!

UL gives you a head start in the area of low OP impedance and low distortion, because of the local NFB built into the design. (We shouldn't forget the need for a very well designed and built OP transformer for UL. A good tranny is expensive; a mediocre tranny will work against the apparent advantages of UL.) However, a local NFB arrangement can be made with pentodes so that, in the end, there isn't a great deal of difference in that respect.

Pentode mode is usually considered more expensive (or at least, more trouble) to implement than UL. This is because it is necessary to do something to stabilise the g2 supply in pentode mode. UL just needs a tapping for g2 on the OP transformer primary (two tappings if you're talking of PP) and no stabilisation is necessary. That is a sweeping statement, I realise; a UL design in Class AB1 still requires a 'tight' PS but it's not as critical, we are told, as the g2 supply for a pentode design.

The purist pentode practitioner will have you believe that you must provide a totally separate PS for g2, with its own power tranny, rectifiers and choke-input filter. This costs quite a bit, takes up a significant amount of extra space and adds a lot of weight.

Other pentode fanciers (self included) prefer an easier and simpler approach, to get, hopefully, just as good a result. As long as your B+ supply for the plates is 50v or more above the required voltage for the screens, you can derive a stable screen supply from that same B+ using a MOSFET regulator. Such a regulator costs only a few dollars and doesn't take up much extra space nor add any great weight. You do need a good heat sink, but there are no heavy iron components or additional capacitors of any size.

A small additional requirement is that, if you regulate the screen supply in absolute terms (i.e. using a reference voltage source like a gas discharge tube or a zener), so that it doesn't vary even if the line voltage changes, then you need to regulate the fixed bias supply too (assuming that's the type of bias you use). If you don't, the current through the OP tubes will vary as the line voltage changes because the bias will also change but the g2 voltage will not.

Johan, any comments?
Read and approved

Must I add:
No miller effect and slightly higher sensitivity makes the driver happier.

Yves.
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Old 12th November 2006, 12:29 PM   #7
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Ray touched important points. Regulating g2 B+ at 50-70% of plate B+ goes a LONG way towards improving multi-grid power tube linearity. NFB of some kind is needed to deal with the HIGH O/P impedance of multi-grid tubes. Without NFB, the damping factor of the amp will be unacceptable.

BTW, to obtain absolute maximum performance from ultra-linear mode, a separate screen grid winding on the O/P trafo and regulated g2 B+ are needed.
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Old 12th November 2006, 12:45 PM   #8
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Quote:
The One Electron UBT-2 seems to be the Gold Standard for this purpose, opperates in UL, Pentode, or Triode
The UBT-3's that I have here do NOT have the UL tap. I see no mention of a UL tap on the specs for the UBT-2 on One Electron's web site, but I can't get the entire spec to load.

My experience with Triode - UL - Pentode experiments in an amp similar to yours were not in favor of Pentode operation. I could not get decent sounding bass out of my big speakers and their eficiency is high enough that triode is plenty loud. Cathode feedback helps, but I still wasn't happy with the sound.

The big Edcor transformers have the UL tap, and are priced in between the two transformers you mention. I am not crazy about the "Pimp my Ride" blue paint job, but I am puting them on an equally rude polished aluminum Diamond Plate chassis for effect. They sound pretty good and have big bass. They are twice the size of the other two transformers.
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Old 13th November 2006, 05:50 PM   #9
Jaap is offline Jaap  Netherlands
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Default mosfet regulator

Can anybody post a schematic or link for a mosfet screen regulator that can be used in a KT88 PP amp ?

thnx,
Jaap
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Old 13th November 2006, 08:48 PM   #10
SY is offline SY  United States
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I'd use the Maida circuit. Piece of cake.
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