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Difference bewteen ultralinear & triode mode?
Difference bewteen ultralinear & triode mode?
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Old 8th January 2013, 05:32 AM   #81
DualTriode is offline DualTriode  United States
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Another view of distortion and negative feedback.
Same bottom line as DF96 says.
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Old 20th August 2015, 11:32 AM   #82
James Freeman is offline James Freeman
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Originally Posted by DF96
Actually, the opposite is true! In triode mode there is little increase in gain if the speaker increases its impedance. UL gives some increase in gain.
Pentode mode gives a huge increase in gain. It is the lack of an increase in gain which is the advantage of the triode mode.
However, the global NFB often used with pentode or UL mode can negate this advantage so the net effect may simply be that triode mode gives smaller peak output power.
Triode mode is acceptable if you want to avoid global NFB and don't mind a poor damping factor (so presumably either have suitable speakers or prefer one-note bass).

Originally Posted by DF96
When I had almost finished building my UL EL34 amp I listened to it with feedback removed, just to check that the feedback was going to improve a good amp rather than paper over a bad one.
It sounded OK to me, although I didn't leave it in this state for long.
To hear a pentode with no feedback listen to most 1950's radio receivers. Distortion may be low at reasonable volumes, but the speaker resonances are not well controlled because there is almost no damping.
Originally Posted by DF96
Yes, if you don't do feedback loop stability then triode-mode is a quick and easy way to reduce distortion and output impedance - but it also reduces power.
So DF96, do you think that UL with NFB is an ideal combination for Hi-Fi (ie, more power, less distortion, better linearity)?
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Old 20th August 2015, 03:45 PM   #83
DF96 is offline DF96  England
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UL with NFB is a good combination for hi-fi. It is the combination I use.

Triode mode gives lower gain and less output power. Without NFB it will probably give higher output impedance. Useful for those who wish to avoid NFB.

Pentode mode gives higher gain and slightly higher output power, and higher distortion. It can't do hi-fi without significant NFB.

Sorry, that is just repeating what I said earlier.
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Old 20th August 2015, 05:32 PM   #84
smoking-amp is offline smoking-amp  United States
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The never ending discussion of neg. feedback effects on distortion harmonics has another shark hiding in the dark, which mostly invalidates even carefully derived conclusions. Take a 2.0 power device generating just 2nd harmonic, then a 1.9 power device generating a tapering tail of higher harmonics above the 2nd H. No discernible difference in sound though. One has to take the phase of the harmonics into consideration as well. The 1.9 power device generates a tail of alternating phase harmonics, it's just the mathematics modeling the less curved transfer function. Sonically, the sum of the higher harmonics cancel.

Similarly, neg. feedback straightens out the transfer function of distorting devices. A tapering tail of harmonics may well indicate improved linearity. But the typical FFT magnitude displays give you no clue whether it is good or bad. FFTs need a post analysis program to calculate the transfer curvature. Neg. feedback should always improve the "real" linearity (assuming it is covering the bandwidth), even at low amount. (it may increase some higher harmonics slightly, in a beneficial way, to do that) Whether it sounds better is another matter, since 2nd harmonic is the easiest to remove. People pay good money for 2nd H. dist. No 2nd H. gets called "clinical", "sterile", "flat"......

Last edited by smoking-amp; 20th August 2015 at 05:38 PM.
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Old 20th August 2015, 06:01 PM   #85
payloadde is offline payloadde  Europe
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Judge: This court appreciates that you invented physics, Mr.Newton, but unfortunately you can't have a patent on it.

Last edited by payloadde; 20th August 2015 at 06:03 PM.
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Old 21st August 2015, 02:47 AM   #86
celestar32 is offline celestar32  Singapore
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Just curious to know is there any side effect or damage to the tube amplifier if we switch triode to pentode or vice versa when listening to music.
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Old 21st August 2015, 09:28 AM   #87
DF96 is offline DF96  England
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Never switch while the amp is on. Your ears, speakers or amp may be damaged.
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Old 21st August 2015, 10:05 AM   #88
M Gregg is offline M Gregg  England
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Originally Posted by DF96 View Post
Never switch while the amp is on. Your ears, speakers or amp may be damaged.

I wanted to switch between quickly many years ago so I fitted a b+ switch that I could turn off the B+ to the power tubes change over and power up again.

However you have to make sure you don't kill the Bias supply when you do this and that the reduced load on the PSU creating voltage rise that may exceed the PSU cap working voltage.

Nearly every amp I have built has a change over switch even single ended I can't live with just one output configuration and after disconnecting and changing over so many times about 40 years ago I fitted switches. I find it to limiting..I'm not a fan of U/L many are YMMV. I prefer triode..or ES U/L.

I like to be able to use different tubes as well so I make the bias so I can change tubes..that way you don't end up with a pile of tubes you can't use.
A bit like a diesel/petrol car in case of shortage Max Max tube amp can use anything with octal base..

NB watch the bias setting when you switch..the current can be different at idle

Its interesting to be able to run most octal tubes..and switch between triode etc..I have switches that have Pentode/U/L/triode/ES U/L. I even did it with an amp running PPP..and on a friends amp Vellman and it worked well. Its not until you have switched back and forth about 100 times while listening that you will find a preference but even then its nice to have a change occasionally.

I guess if you build a PP and a SE with switchable configuration for different tubes / feedback/ input tube/ bias setting etc you only need two amps with possibility of specialist Triode and OTL..but I don't see the point in building different amps for octal tubes etc..(unless you like building amps )

Universal PP and Universal SE..

M. Gregg

Last edited by M Gregg; 21st August 2015 at 10:28 AM.
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Old 21st August 2015, 11:16 AM   #89
James Freeman is offline James Freeman
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Interesting post Gregg.

I love the ability to have it all, with as many switches and knobs as possible.
But I also appreciate a single trick pony that does absolutely the best for what it's meant to.
It is nice to have endless options on a guitar amp but with Hi-Fi?

Here is a quick photo of the back of my modded guitar amp.
It has so much unused holes front and back, children with pokey fingers should be kept in sight...
On top is my DIY load box which mimics exactly a guitar speaker impedance (200W), for recording, measuring, re-amping with SS amp for cranked tone in bedroom levels, etc..
An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.
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Old 21st August 2015, 01:22 PM   #90
gyro is offline gyro  United Kingdom
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Triode mode is sometimes useful for making use of decent high Pa tubes like the NOS Soviet era GU50 (40W Pa), which is limited to 250V on g2 in Pentode or UL but will happily run at 400V in Triode mode.
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