I have been playing with various cascoded input Driver stage for my 777 Amp and so far, the ultra linear cascode has performed the best in this Amp. I also tried self-biased cascoded SRPP and Conventional voltage divider cascode from B+ to ground and returned to this Ultralinear version.
This drives a triode strapped EL34 or 6550 operating at about 350 Volts B+ with cathode feedback for about 3-5 watts very well with plenty of dynamics and with the EL34’s a 3D Holographic midrange without a lot of euphoric 2ed harmonic to contend with.
Attached are the actual voltage measurements of various 9 pin Dual triodes of which the 12AT7 sounds the best. I would like your people view on optimization of this circuit for use with the 12AT7 AS WELL as the 12AU7.
This drives a triode strapped EL34 or 6550 operating at about 350 Volts B+ with cathode feedback for about 3-5 watts very well with plenty of dynamics and with the EL34’s a 3D Holographic midrange without a lot of euphoric 2ed harmonic to contend with.
Attached are the actual voltage measurements of various 9 pin Dual triodes of which the 12AT7 sounds the best. I would like your people view on optimization of this circuit for use with the 12AT7 AS WELL as the 12AU7.
Hi PPL,
Maybe you can post the circuit files here so that everybody can discuss together? I have been working on some 6sn7 Ultra linear cascode, result is not very impressing when compared to others 6sn7 topology. So, it is interesting to see the implementation on 12au7/12ax7 etc.
Maybe you can post the circuit files here so that everybody can discuss together? I have been working on some 6sn7 Ultra linear cascode, result is not very impressing when compared to others 6sn7 topology. So, it is interesting to see the implementation on 12au7/12ax7 etc.
thanks for your interest. Circuit Files?? the schematic and measured voltages are shown on the attachment on my previous post.
i surprised you get poor performance from a 6SN7 UL cascode that Tube has worked Great with a 47K plate load in My previously Posted Circuit, However could you elaborate more..
FWIW I did see the other day a tube - but cannot think about the name of it - that had half a 12AX7 and half a 12AU7 in one envelope (from memory).
That would be nice to use the 12AX7 for the amplfier and the 12AU7 for the cathode follower part.
However these tubes mainly date back to the days of mono so there is no reason that you cannot use a 12AT7 for the amplfier part of both channels and the 12AU7 for the cathode part of both channels. That would allow you to get the filament at cathode potential for the 12AU7 and as such you can go to voltages far higher than the Vkf breakdown. Would certainly open a new frontier to explore.
AM
That would be nice to use the 12AX7 for the amplfier and the 12AU7 for the cathode follower part.
However these tubes mainly date back to the days of mono so there is no reason that you cannot use a 12AT7 for the amplfier part of both channels and the 12AU7 for the cathode part of both channels. That would allow you to get the filament at cathode potential for the 12AU7 and as such you can go to voltages far higher than the Vkf breakdown. Would certainly open a new frontier to explore.
AM
If a particular tube's UL cascode works great, I would think of this circuit has either lower output impedance, high input impedance, higher mu, lower distortion or better PSRR etc when compared to grounded cathode, common cathode, grounded grid, white follower or any other topology one can think of. So, I would like to learn more about the advantage of UL cascode(50% of output signal feed back to the upper triode grid).
To be more direct, I wasn;t impressed with the result of UL cascode (50% feedback) in 6sn7. And I want to learn more in the case of 12at7/12ax7 etc.
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my previous circuit has evolved so my latest version in its complete Amp is attached. Works quite well however is on the verge of instability.
Except for the distorting driver, it will, as Atmasphere says, surely be instable. If going the cascode route why not go for high Gm-tubes?
The UL connection itself is not so bad even if it doesn´t do so extremely much for linearity but it helps.
Unfortunately you´ve got all wrong as you can´t have the capacitor from the upper grid to ground! No feedback at all that way. See sulcorebutia´s circuit above, it is correct.
The UL connection itself is not so bad even if it doesn´t do so extremely much for linearity but it helps.
Unfortunately you´ve got all wrong as you can´t have the capacitor from the upper grid to ground! No feedback at all that way. See sulcorebutia´s circuit above, it is correct.
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Older NE-2s may not conduct in total darkness.
Some activation energy is needed to initiate conduction and ambient light normally provides this.
Back in the 80s this problem was identified, and Krypton gas (low level radioactivity) was added to the neon gas to insure proper operation.
If you have a stash of older neon bulbs they may have this characteristic (poor ignition in dark).
Some activation energy is needed to initiate conduction and ambient light normally provides this.
Back in the 80s this problem was identified, and Krypton gas (low level radioactivity) was added to the neon gas to insure proper operation.
If you have a stash of older neon bulbs they may have this characteristic (poor ignition in dark).
Hey guys,
I guess the NE-2 has very low current capability. If I am wrong I´ll try them myself🙂.
The idea is good but probably it´s better to use a 100V and a 150V regulator tube(7-pin noval) on top of each other. They work in the same way as neonbulbs but is usually capable of ca 30mA.
I guess the NE-2 has very low current capability. If I am wrong I´ll try them myself🙂.
The idea is good but probably it´s better to use a 100V and a 150V regulator tube(7-pin noval) on top of each other. They work in the same way as neonbulbs but is usually capable of ca 30mA.
thank you all for your thoughts however some of them i don't understand
(1) Distortion in the Driver stage. I don't understand what type of distortion you are referring to since the standard THD and IMD measure exceptionally well.
(2) I amuse you are referring to the Driver stage tube and the cap bypassing the lower half of the Voltage divider? is this cap not installed on almost every other cascode circuit not using the grid leak method? is it not present so as to provide the required low AC impedance to ground?
(3) yes there are alto of feedback loops however having the feedback distributed locally around each stage not only linearized it but also requires less overall loop feedback for the same Distortion performance.
(4) yes I'm thinking the neon Lamps may have to go as they have had previous fail to ignite issues and you can just imagine what that did to output tube current.
(5) the stability issues in this amp are limited to the 8 ohm tap when they appear. the Loudspeaker connected presents a complex load as it is 24 dB per octave filters crossed over at 2.5K.
Thanks for your input on these issues
(1) Distortion in the Driver stage. I don't understand what type of distortion you are referring to since the standard THD and IMD measure exceptionally well.
(2) I amuse you are referring to the Driver stage tube and the cap bypassing the lower half of the Voltage divider? is this cap not installed on almost every other cascode circuit not using the grid leak method? is it not present so as to provide the required low AC impedance to ground?
(3) yes there are alto of feedback loops however having the feedback distributed locally around each stage not only linearized it but also requires less overall loop feedback for the same Distortion performance.
(4) yes I'm thinking the neon Lamps may have to go as they have had previous fail to ignite issues and you can just imagine what that did to output tube current.
(5) the stability issues in this amp are limited to the 8 ohm tap when they appear. the Loudspeaker connected presents a complex load as it is 24 dB per octave filters crossed over at 2.5K.
Thanks for your input on these issues
That would be the 12DW7. A good many gee-tah amps used this one, with the high-u section as a voltage preamp, and the medium-u section as a cathodyne phase splitter.
As for cascodes, I used cascoded 6BQ7s to implement an LTP with active tail loading. Even thought the spec sheet for the 6BQ7 doesn't mention any audio applications at all (it's intended for use as a VHF cascode ahead of the first detector in TV front ends up to 300MHz) and has a squirrely plate characteristic (undocumented variable-u feature?) that makes finding a good audio loadline somewhat more difficult, 6BQ7s gave excellent performance as an LTP -- decent gain margin for gNFB, and very low distortion as these sound very good indeed. (Besides the larger than usual cathodes add extra glowy bottle coolness.)
I don't see any reason why cascodes can't sound good with types like the 12AU7 or 12AT7 which aren't exactly premier audio VTs. There isn't a whole lot of information concerning the sonics of hollow state cascodes, however, the topology is used quite frequently in solid state designs to enhance linearity, so I don't see why this wouldn't be the case with hollow state cascodes as well (at least when implementing LTPs with them).
Thanks Miles. I will give the 12DW7 a try as soon as i receive a set from ARS. I tried the 6BQ7 hear as well as the ECC189
I did not have any issues with the varr MU, however i believe that don't come into play until about 3mA. This evening i will remove the cap from the lower voltage divider network and note the results empirically.
I did not have any issues with the varr MU, however i believe that don't come into play until about 3mA. This evening i will remove the cap from the lower voltage divider network and note the results empirically.
Hey ppl,
You had some thoughts, hope this will help:
1 The cascode, with the mediocre 12AU7, of yours will have in the ballpark of over 0,5% THD at clipping point for the 6L6. This without any NFB from your SE-amp. A more suitable E88CC UL-cascode will lower THD to maybe 1/10. Another thing the one-triode stage will have a Zout of maybe 4k while the cascode will have the same Zout as Rl=91k.
Take a singletriode GC-stage with 1/2 E88CC and THD will be less than 1/3 of the above at the same gain. Using a CCS on top will lower THD even more to maybe between 1/4-1/5 the cascode. So is it worth the effort using the elaborate cascode as also the overtone structure of the it is worse?
2 What you have is an almost conventional cascode. Remove the cap and you have an UL as 50% of the signal is fed back to the upper grid. Check Broskie, think he has something about it. A patent is held by Bill F. at Conrad-Johnson.
By the way ECC189 is close to E88CC but needs another working-point than 12AU7, go for 10mA with 180V on the upper anode to get really lowTHD at 1/10th,at 100V you get 1/4 of the 12AU7 . 12DW7 will be even worse than 12AU7.
You had some thoughts, hope this will help:
1 The cascode, with the mediocre 12AU7, of yours will have in the ballpark of over 0,5% THD at clipping point for the 6L6. This without any NFB from your SE-amp. A more suitable E88CC UL-cascode will lower THD to maybe 1/10. Another thing the one-triode stage will have a Zout of maybe 4k while the cascode will have the same Zout as Rl=91k.
Take a singletriode GC-stage with 1/2 E88CC and THD will be less than 1/3 of the above at the same gain. Using a CCS on top will lower THD even more to maybe between 1/4-1/5 the cascode. So is it worth the effort using the elaborate cascode as also the overtone structure of the it is worse?
2 What you have is an almost conventional cascode. Remove the cap and you have an UL as 50% of the signal is fed back to the upper grid. Check Broskie, think he has something about it. A patent is held by Bill F. at Conrad-Johnson.
By the way ECC189 is close to E88CC but needs another working-point than 12AU7, go for 10mA with 180V on the upper anode to get really lowTHD at 1/10th,at 100V you get 1/4 of the 12AU7 . 12DW7 will be even worse than 12AU7.
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Hey Merlinb,
What should be the problem😉?
Check ppl´s circuit again. First of all, cascode gain is Rl*Gm giving us not more than 36dB gain due to the lousy Gm of the 12AU7(at this low Ua and Ia Gm is really low).
Then there is local current feedback through the unbypassed cathode resistor, -7dB. Then feedback by UL, -5dB. This leaves us with only 24dB=16x gain left. Even though feedback is 12dB THD is still quite high.
This is even less than an E88CC GC, with unbypassed cathode and a 22k anoderesistor, that should have 25-26dB gain.
One thing struck my mind: As a cascode acts close to a pentode one could build a really good Schade-amp around the input cascode and the 6L6. This gives triode-character to the amp together with pentode-efficiency so 6-7W or more can be excpected. Nothing but local feedback needed and no feedback from the OPT secondary either. But still something like E88CC is called for.
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I thought you meant ECC88 cascode v. ECC88 GC-stage.
(Even at those voltages, the 12AU7 cascode will give you at least 42dB gain.)
But then, if THD and gm are what you are really obsessed about, then why on earth would you use valves at all?!
What is a schade amp?
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What obsession🙄? Designing the better gainstage have never been that to my knowledge. Maybe it´s you who are obsessed by gain issues😉?
Can agree the tube needs more voltage to perform better. About the 42dB it will probably be below 40dB(with the same load as before). even without the two feedback paths, still with more than double distortion of the 88. Lets face it, 12AU7 will not make the best cascode for hifi at any voltage. The tube is OK in guitaramps and we use it at some places in ours.
An E88CC cascode can be tailored to give the same gain as the GC it´s just a matter of choosen Ra//load.
About Schade, why not do a search here at diyaudio there is plenty. Schade was the "inventor" of 6L6 and presented his solution in 1938. Can mail a schematic of an E88CC/6L6 if you want to.
That implies a gm of only 1mA/V! You clearly have very low expectations of that poor 12AU7!
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