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Old 16th June 2016, 02:38 AM   #1
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Question Ultra-Linear Midget Amp ?

For the users of horns or other high efficiency speakers who build amplifiers with 700 or 900 miliWatts maximum power; would a small signal pentode running Ultra-Linear make a musical amplifier?

Anyone have a few thoughts on this?

Brian
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Old 16th June 2016, 03:37 AM   #2
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Brian Albin View Post
For the users of horns or other high efficiency speakers who build amplifiers with 700 or 900 miliWatts maximum power; would a small signal pentode running Ultra-Linear make a musical amplifier?

Anyone have a few thoughts on this?

Brian
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Old 16th June 2016, 04:56 AM   #3
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You don't have to use UL mode "finals", if you regulate g2 B+. Full pentode or UL, some form of loop NFB is (IMO) indicated. Even horn speakers need some damping factor help.

FWIW, I'd avoid the small signal tubes and go with the 6BM8. About 2 WPC is what you get. Better to have power that's not used than have a speaker wrecking amp that's clipping most of the time. I've taken the liberty of uploading a 6BM8 design that's based on something Mullard did.

A "weird" option would be 3X 3V4s running full pentode PSE. If you regulate g2 B+ at about 70 V., cranking the anode B+ voltage up to 110 V. should be safe enough, provided you watch the anode dissipation carefully. Each 3V4 requires its own "floating" filament supply of 2X alkaline "D" cells, which should last a reasonable amount of time. Also, "fixed" bias, with individual trim pots., is needed or "current hogging" will be a problem.
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Old 16th June 2016, 10:44 AM   #4
316a is offline 316a  England
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Quote:
Originally Posted by Brian Albin View Post
For the users of horns or other high efficiency speakers who build amplifiers with 700 or 900 miliWatts maximum power; would a small signal pentode running Ultra-Linear make a musical amplifier?
Why not just use a triode connected 6E5P ? That would hit your power requirements . Use a 3k-5k output transformer , 330 ohm grid stopper , 220k grid leak and 3 x schottky diodes in the cathode circuit for the entire power stage . I found the 6E5P to make a better sounding spud amp than the 6S45P , trouble is a slight lack of gain with my 97dB co-axes but should be fine with horns

316a
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Old 16th June 2016, 03:29 PM   #5
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Hi Eli! It is good to hear from you again.
 
I see several of those caps are electrolytic. Are any of them in a position where a change to film & foil, or metalized plastic would improve the sound?
 
I understand little of how Ultra-Linear operation improves the sound. When you say:
"You don't have to use UL mode "finals", if you regulate g2 B+. Full pentode or UL, some form of loop NFB is (IMO) indicated. Even horn speakers need some damping factor help."
Is the idea that U-L increases damping factor over triode operation?
Does voltage regulation on grid2 B+ increase this damping factor?

Brian
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Old 16th June 2016, 04:25 PM   #6
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EF86 is a very low current device. If I recall correctly WE417 is the tube that the russian 6S45P is copied from; it is very current-capable.

Why not have the WE417 as the output tube? It's perfectly capable.

I'd go for the 6E5P as mentioned in a message above. As a curiosity have a look at my small power SE amp: Small power all russian tubes single ended amp
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Old 16th June 2016, 04:44 PM   #7
rongon is offline rongon  United States
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Quote:
Originally Posted by Brian Albin View Post
Is the idea that U-L increases damping factor over triode operation?
No, UL operation increases damping factor over pentode operation, but is still not high enough to be used without negative feedback in most cases.

Of the triode, UL or pentode choices, triode offers the highest damping factor before negative feedback ('open loop'), but lowest power output.

A triode output tube may have high enough damping factor to allow its use without negative feedback, and it can sound nice that way (although adding negative feedback will lower distortion and increase damping factor).


Quote:
Originally Posted by Brian Albin View Post
Does voltage regulation on grid2 B+ increase this damping factor?
Not that I'm aware of. Regulating the screen grid supply improves distortion and overall performance of a pentode, but doesn't reduce its high plate resistance.
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Last edited by rongon; 16th June 2016 at 04:47 PM.
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Old 16th June 2016, 08:18 PM   #8
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Thank you, MrCurwen and Rongon for the replies. That cleared up some of my confusions.
I do not understand what advantage UL operation can provide. It sounds like I am being told triode is better. Does UL in any circumstance produce the better sound?

Brian
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Old 17th June 2016, 12:09 AM   #9
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"Better sound" is a subjective term. This forum is the only place on earth where I've encountered folks with sufficient maturity and restraint to avoid discussions of subjective phenomena.

Objectively, UL operation yields a sort of middle ground between pentode and triode operation of the same tube. It was developed at a time when 'powerdrive' wasn't practical in consumer audio equipment. The primary advantage of UL is that it yields nearly as much power output as pentode operation with conventional drive circuits, but with reduced distortion figures. My opinion is that triode operation with powerdrive often makes more sense now that it's easy to accomplish.

My advice is to go ahead and build something. Don't wait. Listen, study, and experiment. Don't waste money on boutique components -- just buy stuff that's adequate and put in the effort to make mistakes and learn from them. High-end components are useless without good circuit design, and sometimes even more useless WITH good circuit design. I'll get down off my soapbox now.
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Old 17th June 2016, 03:22 AM   #10
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Quote:
Originally Posted by BinaryMike View Post
"This forum is the only place on earth where I've encountered folks with sufficient maturity and restraint to avoid discussions of subjective phenomena."
Well that's disappointing! Subjective sonic descriptions are what I find useful.

Brian
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