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Old 14th October 2004, 05:01 PM   #1
percy is offline percy  United States
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Default PP-Triode , PP-UL , PP-Pentode-strapped-as-Triode

What are the pros and cons, and differences (technical as well as sonic ) between these three output topologies ?

PP Triode
PP UL
PP Pentode/Tetrode strapped as Triode

Thanks!
-Percy.
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Old 14th October 2004, 05:20 PM   #2
SY is offline SY  United States
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I'd grab a copy of Crowhurst's "Understanding HiFi Circuits" and look at the second chapter. He discusses this in great detail, with examples, then sums up with a comparative chart. It's a much less clear tradeoff than many would have you believe.
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Old 14th October 2004, 08:37 PM   #3
percy is offline percy  United States
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Thanks SY. That seems to be a good direction to go into. By the way do you have the book ? How is it ?

Quote:
It's a much less clear tradeoff than many would have you believe.
So assuming all else is same(tubes, design, etc..) there really isnt much difference between the way one sounds than the other ?
I really wanted to get some impressions from people who have had a chance to listen to these types of output stages.
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Old 14th October 2004, 09:04 PM   #4
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Quote:
So assuming all else is same(tubes, design, etc..) there really isnt much difference between the way one sounds than the other ?
You've got to be joking.
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Old 14th October 2004, 09:39 PM   #5
percy is offline percy  United States
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Quote:
You've got to be joking.
It's a question!
Although you did answer it partly suggesting that there is a difference.
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Old 14th October 2004, 10:22 PM   #6
SY is offline SY  United States
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Quote:
Thanks SY. That seems to be a good direction to go into. By the way do you have the book ? How is it ?
It's a GREAT book, one that any tube aficionado should own.

As for sonic differences, there are good and bad amps out there with all of those topologies (and a few more you haven't mentioned). The universal best choice is... well, it's more a matter of faith than of actual data. Maybe that's why there's so many different kinds of amps.
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Old 15th October 2004, 12:00 AM   #7
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Default UL vs Triode PP

I can give my own personal views ONLY based upon -
1) a EL84 Push Pull Amp I built and have run in Ultralinear and "strapped as Triodes". Hi quality 1970's EI Output transformers.
2) A prototype monoblock using 4 x KT88 driving Plitron PAT4006 Toroidal Output Transformers.

You are trying to balance a number of issues:
For low distortion the impedance of the Output Transformers Primary Inductance needs to be much higher than the output impedance of the output tubes. At low frequencies this is often not the case and distortion (2nd Harmonic in particular) rises. When this is excessive it gives the amp a "covered" sound. To get over this you need to do one of 2 things.
a) Get an output transformer with higher primary inductance
OR
b) Lower the output impedance of the output tubes.

Most people opt for the second solution as it has an additional benefit that the reduced output impedance of the tubes reflect to the secondary of the output transformer as lower amplifier output impedance giving you a better damping factor.

The tube output impedance is about 2.5 times less in strapped as triodes mode than in ultalinear mode.

With the little EL84 Amp (10W per channel) I run in Ultralinear Mode and put up with a slightly covered (but glorious) sound. I tried it in strapped as triodes mode but it didn't have enough power.

With the big 4 x KT88 Amp power wasn't an issue and the output transformer has a massive 590H primary inductance so I wasn't chasing impedance issues and could compare the sounds more easily.
The Ultralinear is lightening fast and has very a "live" feel.
The strapped as triodes mode is subjectively "slow" which is a complaint that some make about Single Ended Triode Amps too.

I prefer the Ultralinear Mode BUT the damping factor is NOT as good as I would wish and some speaker colouration is evident at higher frequencies (gets stronger as the amp delivers more power imto the higher impedance load at higher frequencie)

All of the above should have one other caveat. I run all my amps with zero global feedback. This means that there is no extra reduction in output impedance of the amp and so damping factor is usually an issue. I have found that any global feedback destroys time coherency and therefore sound stage (stereo imaging) - actually up to about 5 dB is sometimes an acceptable compromise.

I am currently running the 4 x KT88 in strapped as triodes mode (even though I like the pace less than ultralinear) as a compromise to get better damping. In Ultralinear Mode it just a little too raw. In the mean time I have ordered a pair of VDV2100-CFB/H Output Transformers to try inplace of the PAT4006. The VDV2100-CFB/H have 30% Ultralinear Taps and 10% Cathode Feedback whereas the PAT4006 I have now have only 40% ultralinear.

My target - ultralinear plus cathode feedback

A bit of a rave - BUT I hope there is something useful in there.
Cheers
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Old 15th October 2004, 02:04 AM   #8
percy is offline percy  United States
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http://www.decware.com/paper16.htm
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Old 15th October 2004, 02:42 AM   #9
SY is offline SY  United States
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Quote:
Originally posted by percy
http://www.decware.com/paper16.htm
There's some pretty questionable stuff in there along with some things that are correct. Take it all with a big grain of salt. Buy and read Crowhurst.
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Old 15th October 2004, 05:02 AM   #10
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@percy

www.audioxpress.com/resource/audioclass/

It's not the book, but it's a good read by Norman Crowhurst.
Nine PDF files, Audio Classroom: Desiging Your Own Amplifier

Wayne
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