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Single ended pentode: UL vs triode connection

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Posted on March 26, 2014 at 20:46:57
greg7
Audiophile

Posts: 943
Location: AZ
Joined: January 12, 2003
 Power output differences aside, is there any inherent advantage to connecting in triode versus ultralinear for, say, an EL34 or KT88? Or are the sonics are dependent on implementation, quality of OPT, etc.?

 

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RE: Single ended pentode: UL vs triode connection, posted on March 26, 2014 at 21:59:29
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 This question is a little too open ended to be answerable.

The sonics will be influenced by everything you mentioned. A lot of people just take pentode/UL amps, then triode wire the outputs and draw conclusions.

Generally, one would be able to remove the global feedback network, reduce the driver stage gain, etc, which makes an incredible difference. On the other hand, if you have some marginal output transformers, they may have been addressed in the design of the feedback network.

 

RE: Single ended pentode: UL vs triode connection, posted on March 26, 2014 at 23:11:38
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 Damping factor. In UL it's a bit less than 1.0 usually, without added feedback(*). In triode mode you can get 2 to 4 without feedback. Pentode mode absolutely needs feedback to get a damping factor better than 0.10 or so.

The above does not consider the few exotic, usually horn, speakers that are actually designed for current-source drive.

(*) Note that UL is actually pentode mode with feedback to the screen grid. I would expect it to have the same sonic effect as a pentode amp with enough feedback to match the damping factors.

 

RE: Single ended pentode: UL vs triode connection, posted on March 27, 2014 at 06:32:36
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015






Here's the amp I built and listen to every day. It is cathode biased, and can use EL34, 6550, or KT88. It has a Triode/UL switch. To me, it sounds much better in UL mode.

 

RE: Single ended pentode: UL vs triode connection, posted on March 27, 2014 at 07:50:03
SteveBrown
Audiophile

Posts: 2454
Location: Portland, OR
Joined: November 14, 2002
 If you're just thinking about EL34's another neat trick is that you can connect both the screen and supressor to the plate for a bit different triode connection which, IMHO sounds excellent. I have used this configuration in SEP amps several times and it really is very nice. Good thing is, none of what you're considering is very hard to undo if you don't like it but as Paul mentioned, UL will need some feedback (which I don't care for).

 

RE: Single ended pentode: UL vs triode connection, posted on March 27, 2014 at 20:27:19
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002
 "...is there any inherent advantage to connecting in triode versus ultralinear for, say, an EL34 or KT88?"

Yes.

Have a look at the plate curves, UL vs. triode.

The harmonic distortion produced by the tube is directly related to the whether the grid lines are evenly spaced along the load line.

The more even the spacing, the lower the distortion. (not necessarily the distortion of the amp as a whole but the distortion produced by the output tube)

BTW, CB makes some very good points below.

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

 

My take..., posted on April 2, 2014 at 08:59:49
Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000
 Guys,

First why every use something that not what it is. In college we were taught to investigate parts by what it was designed for and that would tell us how best to use it.

Using a pentode as a triode is just silly. That's why they made triodes so you didn't have to muck with pentodes.

UL is not a good idea for SET's, its way to noisey and mess up the direct current flowing through the output transformer. What you want 1/2 the transformer to see X amount of current and the other 1/2 to see 120% of X??? Not a good idea...

SET Pentode should be just that make the damn screen regulated at some set place and you will have better output.

Thanks,
Gordon
J. Gordon Rankin

 

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