Cascoding tubes

[Joshua_G]

What are the benefits and/or drawbacks of cascoding tubes, versus conventional common cathode topology, like in the attached schematics?

[Eli Duttman]

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 = (gm_{Lower Device}) (net Z_L).

BTW, the example you provided is a SRPP, not a cascode. Both arrangements are examples of "Totem Poles".

[planet10]

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

[kevinkr]

Originally Posted by Joshua_G

What are the benefits and/or drawbacks of cascoding tubes, versus conventional common cathode topology, like in the attached schematics?

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.

[Joshua_G]

Thanks all.
So, what are the benefits and/or drawbacks of SRPP?

[aardvarkash10]

go to the link that planet10 gave. Good quick explanations of all the standard topologies are there

[Joshua_G]

Thanks.

[nigelwright7557]

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.

[Joshua_G]

Thanks.
I'm looking for the most neutral and natural sound possible. It looks like SRPP is a good choice to experiment.

[kevinkr]

Originally Posted by nigelwright7557

The SRPP gives a good clean signal and less distortion than a standard stage.
<snip>

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.