Joshua,
You can view a cascode as a "synthetic" pentode. Like a pentode it offers greatly reduced Miller capacitance and large single stage gain. It too has a very high O/P impedance. Unlike a pentode, cascoded triodes don't introduce partition noise. The PSRR of cascodes is WRETCHED, which makes regulated B+ a must.
Cascodes that function well employ triode's with high gm, at the bottom of the stack. Gain in cascodes is roughly = (gmLower Device) (net ZL).
BTW, the example you provided is a SRPP, not a cascode. Both arrangements are examples of "Totem Poles".
You can view a cascode as a "synthetic" pentode. Like a pentode it offers greatly reduced Miller capacitance and large single stage gain. It too has a very high O/P impedance. Unlike a pentode, cascoded triodes don't introduce partition noise. The PSRR of cascodes is WRETCHED, which makes regulated B+ a must.
Cascodes that function well employ triode's with high gm, at the bottom of the stack. Gain in cascodes is roughly = (gmLower Device) (net ZL).
BTW, the example you provided is a SRPP, not a cascode. Both arrangements are examples of "Totem Poles".
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That is not a cascode... it is an SRPP variation.
http://www.freewebs.com/valvewizard/index.html
Cascode gives high gain, and wide bandwidth as the arrangement negates the Miller effect. Downside is very low PSRR.
dave
An externally hosted image should be here but it was not working when we last tested it.
http://www.freewebs.com/valvewizard/index.html
Cascode gives high gain, and wide bandwidth as the arrangement negates the Miller effect. Downside is very low PSRR.
dave
Note that the circuit shown is an SRPP which is not a cascode amplifier at all.
If you take the output from the lower triode's plate it is analogous to a common cathode triode amplifier stage with a CCS plate load. Take the output from the upper tube cathode as shown and depending on load impedance it is arguably a PP output stage.
SRPP offer their best distortion performance into a specific load resistance based on tube choice and operating current. Types realized with high mu triodes generally have rather high source impedances and do not realize either the gain or low output impedance of a common cathode triode amplifier stage driving a CF. Medium mu tubes like the 6SN7 seem to work well and provide good performance in SRPP configuration. (In some of my older designs I used them to drive 300B output tubes and on 400V rails they can swing more than 200Vpp with good linearity.)
Cascode stages take their output from the upper plate and generally have a fixed (and well decoupled) bias voltage applied to the grid of the upper tube. Transfer function is pentode like, and gains are comparable. The cascode unlike a true pentode does not have partition noise and may have a significant advantage in noise performance. Note that the effective rp of the upper triode is quite high and large plate resistor values are required to get good gain. The noise advantage lead to their widespread application in the front ends of tube FM tuners. (Fisher Golden Cascode and the Scott front ends are fine examples of this.) See Planet10 post for cascode connection.
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go to the link that planet10 gave. Good quick explanations of all the standard topologies are there
The SRPP gives a good clean signal and less distortion than a standard stage.
If your looking for valve type distortion such as for a guitar amp then I would steer clear of the SRPP.
If your looking for valve type distortion such as for a guitar amp then I would steer clear of the SRPP.
Thanks.
I'm looking for the most neutral and natural sound possible. It looks like SRPP is a good choice to experiment.
I'm looking for the most neutral and natural sound possible. It looks like SRPP is a good choice to experiment.
Actually my measurements indicate this is not the case except where the upper tube is used only as a CCS and the lower tube is not called upon to deliver any significant load current. The SRPP used into the proper load with the proper tube does exhibit good linearity, but not necessarily as good as a properly implemented common cathode stage with resistive loading driving a good CF, and in a single stage with a proper CCS instead the SRPP is clobbered as long as significant output current is not required. (Generally the case where voltage amplifier stages are used.)
Something else to note with the SRPP when implemented with tubes like the 12AX7A the source (output) impedance of the stage will be typically an order of magnitude higher than a simple common cathode resistively coupled to a CF. (Figure 10K vs 1K and you will be in the ball park for an Ip of 800uA - 1mA) This discovery came at my expense with an inexpensive, but nice sounding phono pre I designed that could not drive any boutique audio cables because of the excessive capacitance and high source Z - it was not picked up by a prospective distributor for this reason, and that may have contributed in a very small way to the eventual failure of my business.
While I am no longer a major fan of the mu-follower, and actually prefer the SRPP - in terms of linearity the mu follower will usually outperform the SRPP by at least 6 - 10dB depending on tube chosen into an appropriate load and offers a gain that ~ = mu whereas the SRPP does not. This is important in places like the front end of a phono pre-amplifier where the higher gain often results in an improved noise floor for the entire pre-amp as less gain is required in the next stage and other downstream noise sources become less significant due to the higher signal levels..
I've used the SRPP a lot and prefer it to the common cathode driving a CF for reasons that I seriously doubt would bear close technical scrutiny. Something about simplified signal path, fewer parts, eh..😉😉 I.e. I admit to having an irrational prejudice in favoring SRPP over some other topologies..😛😛
Edit: Actually in the light of the above comments I will amend it to say I favor SRPP with low to moderate mu tubes with low rps and higher transconductances. IT drive is where I am headed at higher currents.
Nigel is right though that there is no application in a guitar amplifier that would particularly suit an SRPP.
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Kevin,
You wrote:
>>Note that the effective rp of the upper triode is quite high and large plate resistor values are required to get good gain.<<
This is only half correct. The effective rp is high, yes, but large anode loads are not required, and in many cases would be impossible to use.
The gain of a cascode is approximately the gm of the lower tube x the anode load of the upper tube. Rp's don't come into it. As cascodes benfit from hi gm tubes, which are generally run at high current (10mA and more), anode loads of 25k will give stage gains of circa 250 for a single casccode stage using 6DJ8's at 10mA, and more with even high gm tubes like the WE417.
10mA across the mentioned 25k load = 250v, so with typically 150V across the two tubes means a B+ supply of 400V. High anode load R's would mean prohibitive B+ voltages, but also extreme gain!
Regards, Allen
You wrote:
>>Note that the effective rp of the upper triode is quite high and large plate resistor values are required to get good gain.<<
This is only half correct. The effective rp is high, yes, but large anode loads are not required, and in many cases would be impossible to use.
The gain of a cascode is approximately the gm of the lower tube x the anode load of the upper tube. Rp's don't come into it. As cascodes benfit from hi gm tubes, which are generally run at high current (10mA and more), anode loads of 25k will give stage gains of circa 250 for a single casccode stage using 6DJ8's at 10mA, and more with even high gm tubes like the WE417.
10mA across the mentioned 25k load = 250v, so with typically 150V across the two tubes means a B+ supply of 400V. High anode load R's would mean prohibitive B+ voltages, but also extreme gain!
Regards, Allen
The SRPP is a power amplifier, and is specifically suited to relatively heavy loads (not more than a few kilo-ohms usually). A cathode follower usually has a very high input impedance, in which case the SRPP is a silly choice to drive it.
A cascode followed by a cathode follower would be logical. Or, if you prefer, a 'half-mu' stage would be a good choice too. This is the basis of the Aikido line stage http://www.tubecad.com/2004/AikidoAmp5.gif, which uses a half-mu stage feeding a cathode follower.
HINT: If you want the tubes in a SRPP to last more than a few months, you must provide two seperate heater supplies. One for the lower triode, and one for the upper, with the upper heater line raised above gnd some 10-30V HIGHER than the cathode of this second tube will be at. This is to advoid heater cathode breakdown.
Regards, Allen
Regards, Allen
Thats sounds like a real pig as many tubes have a common lead to teh heaters for 6/12 volt operation.
I cant say I have had any problems with my SRPP amps and I have put the heaters in series to 12 volts.
None of the sources of info I have say there is a problem with the heaters.
SRPP was very popular in TV sets.
These are not the best use of that circuit. Compared to a well-designed common cathode stage, you sacrifice linearity and signal-to-noise in order to... uhhh... uhhhh...
I can't think of a single reason you'd use an SRPP in an RIAA stage or even a line stage.
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