Because even if you are in A1 class you have to override the miller capacitance of the next tube. Try for instance to drive an 211 using a 12ax7 and you will hear the difference. have a look over here, Miller Capacitance
The Miller capacitance of 211 is ridiculously small, smaller than Cm of 12AX7.
Mu of the 211 = 12, but when the stage is loaded, the gain is some 7,5...8.
According to the RCA 211 data sheet, Cgp = 14 pF and Cgf = 5,4 pF.
This gives the total input capacitance: Cin = 14 pF x 8 + 5,4 pF = 117,4 pF.
As a reference: Cin of 12AX7 is some 150...170 pF.
So it is obvious that the Miller capacitance of 211 or 845 does not require EL34 to be (over) driven.
So can somebody give other reasons ?
Hi!
If you want to go for the DHT driver, I would rather suggest the 801A or VT62 because you can run it at higher plate voltage and thus it has more headroom.
I have built a 211 amp with the 801A. Some pics can be seen here:
VinylSavor: Making of a 211 Amplifier, Part 1: Planning
The article is about another 211 amp but it contains pics of the 801A/211 monos.
DHT tubes are very linear and they can provide very clean drive voltage swings which such tubes require. Since the 801A runs at about half the plate voltage of the 211 you can use a power transformer with a center tap. use a full wave bridge for the output stage B+. Put another filter chain at the center tap and it will give half that voltage.
The filament supply is even more critical at the driver stage than the output stage.
However keep in mind that DHT driver will only have a mu of 8. if you want to have reasonable input sensitivity an input transformer withs tep up is needed, say 1.4
Best regards
Thomas
If you want to go for the DHT driver, I would rather suggest the 801A or VT62 because you can run it at higher plate voltage and thus it has more headroom.
I have built a 211 amp with the 801A. Some pics can be seen here:
VinylSavor: Making of a 211 Amplifier, Part 1: Planning
The article is about another 211 amp but it contains pics of the 801A/211 monos.
DHT tubes are very linear and they can provide very clean drive voltage swings which such tubes require. Since the 801A runs at about half the plate voltage of the 211 you can use a power transformer with a center tap. use a full wave bridge for the output stage B+. Put another filter chain at the center tap and it will give half that voltage.
The filament supply is even more critical at the driver stage than the output stage.
However keep in mind that DHT driver will only have a mu of 8. if you want to have reasonable input sensitivity an input transformer withs tep up is needed, say 1.4
Best regards
Thomas
Hi!
Grid current effects start to kick in way before the grid of the output tube gets positive. Even if the amp is designed to operate in Class A1, it will still have to fight with that at volume peaks. In case it is over driven it can recover more quickly with a power driver.
Another reason is headroom. Few small signal tubes can provide the voltage swings needed to drive a 211 or even a 845 cleanly. If you want headroom this becomes only viable with power tubes. I want 6dB headroom minimum.
Best regards
Thomas
Grid current effects start to kick in way before the grid of the output tube gets positive. Even if the amp is designed to operate in Class A1, it will still have to fight with that at volume peaks. In case it is over driven it can recover more quickly with a power driver.
Another reason is headroom. Few small signal tubes can provide the voltage swings needed to drive a 211 or even a 845 cleanly. If you want headroom this becomes only viable with power tubes. I want 6dB headroom minimum.
Best regards
Thomas
I have just done some tests with "small", but hi-gm tubes.
Two tested were Soviet pentodes (but triode connected) 6j11p and 6j9p, two triodes 6s3p and 6s45p-e and one triode-pentode 6f12p.
All of these were used together with active load done with NFET.
In general, all of these can produce over 200 Vpp with THD below 0,1 % to 0,5 % depending on the type. Used +Ub was 400 V.
I would like to see what sort of THD results others have got with power tubes as drivers.
I remember seeing a test that showed the 45 having the lowest distortion as a driver tube at 0.02% with 30 other tubes tested. Can't find the link though.
I made some LT-Spice simulations with triode 45 as a high voltage driver stage.
The lowes THD-value I got was 0,78 %.
The test conditions were as follows:
Choke loaded (10 H) circuit with 3k3 cathode resistor bypassed with 100 uF.
+Ub = 400 V, Ik = 22,8 mA
Uout = 300 Vpp to 100 k load resistor.
Uin = 90 Vpp
So it is obvious that 45 and similar low mu triodes are far from optimum when
a linear high voltage driver stage should be built.
The lowes THD-value I got was 0,78 %.
The test conditions were as follows:
Choke loaded (10 H) circuit with 3k3 cathode resistor bypassed with 100 uF.
+Ub = 400 V, Ik = 22,8 mA
Uout = 300 Vpp to 100 k load resistor.
Uin = 90 Vpp
So it is obvious that 45 and similar low mu triodes are far from optimum when
a linear high voltage driver stage should be built.
Try a CCS. It will lower the THD. But yes, 400V isn't in the 45's most linear region. 300B would be better.
With 300 V +Ub the linearity was worse. THD was 1.18 % (at Ik = 34 mA).
The distortion spectrum is such that 2nd harmonic is some 15 dB above the 3rd, so in practical circuit a lot of distortion cancelling will take place and improve the final performance.
With a CCS the performance do not change, which is obvious.
Only the frequency response improves at low end.
The distortion spectrum is such that 2nd harmonic is some 15 dB above the 3rd, so in practical circuit a lot of distortion cancelling will take place and improve the final performance.
With a CCS the performance do not change, which is obvious.
Only the frequency response improves at low end.
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Hi!
I would not use a 45 to drive a 211. It as a very low mu and does not have a lot of headroom in voltage swing. That's why I suggested the 801A itcan provide a much larger swing and has a higher mu. The 801A also maintans the thoriated tungsten filament theme throughout the amp.
Best regards
Thomas
I would not use a 45 to drive a 211. It as a very low mu and does not have a lot of headroom in voltage swing. That's why I suggested the 801A itcan provide a much larger swing and has a higher mu. The 801A also maintans the thoriated tungsten filament theme throughout the amp.
Best regards
Thomas
Just listen with your ears. If you keep doing simulations, you will end up with something that sounds like an SS amp you can otherwise build for much cheaper. I would personally use the 300B. Its low output resistance can pretty much drive anything, multiple 211's in parallel if you needed to. The gain can come from a 5842 stage before it.
45 driver at 400V has been used with great results in real amps. CCS has been measured to significantly reduce a triode's IMD.
45 driver at 400V has been used with great results in real amps. CCS has been measured to significantly reduce a triode's IMD.
801A is quite similar low mu power triode, thoug having higher mu.
I would use instead some high gm triode or high gm triode-connected pentode with CCS. These works with excellent linearity and with a fraction of power dissipation.
For me it is useless to listen only. At the moment I am listening a pentode connected PL519 SE without any NFB and it sounds perfect to me even though THD is above 5 %.
Simulation is a great tool for advanced designers. With a couple of hours with LT Spice correspond a week with soldering iron.
Still i have not found technical reasons to use low mu power triodes as drivers. Hi gm (small) triodes are a lot better.
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