• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Noob question on triode

I saw a tube amp kit where it can switch between triodes tetrode and pentode mode. So basically to switch to tetrode it's just means cutting the power one of the grid. Switching to triode is cutting power to both grids.

A true triode system would sound better than above system because the grids are in the way of the cathode and the plate even if it's not enable?

Yep I am a noob.

A pentode system should have the least distortion but less fedelity?
 

artosalo

Member
2010-02-16 9:00 am
Pentode and tetrode mode is the same thing. Then the screen grid (g2) has fixed dc voltage.
In triode mode the screen grid and anode are connected together.

Triode mode is more linear (lower THD) and has lower output impedance, which gives better loudspeaker damping, but less power than pentode mode.
 
There are many beam tetrodes and pentodes that are supremely linear when triode strapped. Look at 4P1L for example.

Simply switching between triode and pentode mode without changing the circuit more profoundly strikes me as a very bad idea. In guitar amps it's getting common, but that's another thing entirely.
 
It is not significant whether a tube is a true triode, or a triode strapped pentode / beam tetrode / tetrode. What matters most are

1) the curves
2) plate resistance at desired op point
3) can the tube provide desired amount of current (meaning, deliver enough output power)

What kind of circuit are you looking to build?

4 ohm speakers eat up more current than 8 ohm speakers. Damping factor basically is your amp's ability to deliver current to the speaker load. You'll have better damping factor (all other things being equal) with 8 ohm speakers.
 

billshurv

Member
Paid Member
2014-03-01 11:53 pm
If you want a triode amp build one. But just because some guy on the internet says triodes are better doesn't make it true. Take for example this rather nice 15W design from one of the forum members The Red Light District: A 15W Push-Pull Amplifier .Easyish to build, nothing expensive or unobtainium and great performance.

Unless you want something with buckets of tube 'warmth'. AKA effects boxes....
 
It is not significant whether a tube is a true triode, or a triode strapped pentode / beam tetrode / tetrode. What matters most are

1) the curves
2) plate resistance at desired op point
3) can the tube provide desired amount of current (meaning, deliver enough output power)

What kind of circuit are you looking to build?

4 ohm speakers eat up more current than 8 ohm speakers. Damping factor basically is your amp's ability to deliver current to the speaker load. You'll have better damping factor (all other things being equal) with 8 ohm speakers.


From my limited knowledge. Right now I am leaning toward a SET. So what you saying is that I should stick with well known tubes like the EL34 KT88 300B etc?
 
So we are talking about negative feedback.

I did some research and someone says
negative feedback also increases the bandwidth of the power section.

It also helps to increase the clean headroom range of the power amp since the nfb will force the power amp to stay cleaner for longer until the nfb loop falls apart.

It really doesn't matter how the preamp is configured. 1 stage plus a phase inverter will be enough to drive most power amps into overdrive.

without negative feedback the amp will start distorting as soon as it runs out of clean headroom.

and you got it right about the cut control.

You have less high frequencies going into the pa and less distortion.

nfb isn't something that's used to cover up anything or a way to fix mistakes. There's reasons for using it and reasons for not using it.

If you had the amp completely opened up nfb with the guitar volume on full it wouldn't matter because the loop would have fallen apart at that point.

However as you lower your volume the loop will kick back in and linearize the power amp stage.

It sounds like that return tube on the top of a car's coolant reservoir prevent spilling over and feed the excess back into the reservoir.
 
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Hearinspace

Member
Paid Member
2008-06-03 5:18 am
There's one issue that requires thought and a decision. Your speakers are said to sound very good. One reviewer considered them better than comparable bookshelf speakers which says something . . . . but he drove them with a 100 watt tube amp. If you like their size and want to stay with them then you might be best to keep in mind their sensitivity is spec'd at 89dB and you might be happier with 20-25watts at least. Though a 300B might satisfy in a small room for some kinds of music you might end up wanting a more powerful amp after a few months of listening. Of course all of this is vague and how true it is depends on your listening tastes and environment but in a general sort of way I think it applies.

If you're interested in taking up building and exploring the the world of amplification then it doesn't matter where you start - wherever you like is good, but if you are thinking of a one time endeavour then I'd suggest you go for more power.

This takes you into decisions about where to get that power. If you're already firmly fixed on tubes then you have to decide between a single triode that can produce it or start looking at parallel single ended or differential / push pull.
 
There's one issue that requires thought and a decision. Your speakers are said to sound very good. One reviewer considered them better than comparable bookshelf speakers which says something . . . . but he drove them with a 100 watt tube amp. If you like their size and want to stay with them then you might be best to keep in mind their sensitivity is spec'd at 89dB and you might be happier with 20-25watts at least. Though a 300B might satisfy in a small room for some kinds of music you might end up wanting a more powerful amp after a few months of listening. Of course all of this is vague and how true it is depends on your listening tastes and environment but in a general sort of way I think it applies.

If you're interested in taking up building and exploring the the world of amplification then it doesn't matter where you start - wherever you like is good, but if you are thinking of a one time endeavour then I'd suggest you go for more power.

This takes you into decisions about where to get that power. If you're already firmly fixed on tubes then you have to decide between a single triode that can produce it or start looking at parallel single ended or differential / push pull.

I would be honest that I wanted to keep using my RTI4 but I am planning to build my own full range Fostex or kangband speakers for the few more notches of sensitivity down the road.

I assume out of all the schematics, a SET is the simplest? I am extremely novice when it comes to tube amp so the simpler the better. I listen only to jazz and pop. No head banger music for me.

Shoog said a 10w should be plenty for me. My living room is 15 x 15. But I can move it to the bedroom if needed which is 12 x 10.

To me, learning just as important as listening. So I don't mind building something that will not fully satisfy me in the long run. I will probably get another amp in the next 10 years.

Let me put it this way, my friend tells me to get a 500cc if not a litre bike. But I am okay with a 350cc and later move onto a litre bike. Learning how to ride is just as important as going to speed of light.:p
 
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NFB in tube amps is basically trading voltage gain for linearity.

Most SET builders go the route of no NFB and using the most linear tubes, such as DHTs. EL34 in SET would be quite bad without any NFB.

There are a great number of cheap and easily available tubes that can be triode strapped (set up as triodes) and provide good results.
 
Your profile says you are an EE. Which does beg the question why you have started the three threads that you have so far. If there is anything you would like to confess now is a good time...

Yes, I have to confess I love learning new things! Multiply things at once. We take 9 courses in one semester while other faculties only take 4.

Now confess who you work for, the Feds or the CIA or Donald Trump's British Branch? :p