Long tail PI vs cathodyne

[titser_marco]

Hi guys! I'm in the middle of planning my next (re)build and I've been toying with the idea of using a single triode PI.

What are the advantages (aside from just using one side of a 12A_7) and disadvantages compared to a long tail PI?

Thanks!

[JMFahey]

You'll *always* need 2 triodes.
The LTP provides 2 out of phase signals and has gain, so, say, 1V input provides, say, 40V RMS to drive power tube grids.

The cathodyne/single triode can also provide the same 40V RMS BUT it has a gain of 1, so you must feed *it* those 40V RMS ... guess what you need to turn 1V input in 40V suitable to drive the PI ?
You said it: an extra triode

[David Root]

If you heavily overdrive a cathodyne splitter triode you'll get nasty unmusical distortion. This can be alleviated by putting a gridstopper of 470k to 1M on pin 7. Other than that the cathodyne sounds good.

[titser_marco]

If you heavily overdrive a cathodyne splitter triode you'll get nasty unmusical distortion. This can be alleviated by putting a gridstopper of 470k to 1M on pin 7. Other than that the cathodyne sounds good.

Yeah I was reading the material on the Valve Wizard website and it does indeed recommend something like that.

You'll *always* need 2 triodes.
The LTP provides 2 out of phase signals and has gain, so, say, 1V input provides, say, 40V RMS to drive power tube grids.

The cathodyne/single triode can also provide the same 40V RMS BUT it has a gain of 1, so you must feed *it* those 40V RMS ... guess what you need to turn 1V input in 40V suitable to drive the PI ? Rolling Eyes
You said it: an extra triode Smile

I see. So if I go with that approach (driver into cathodyne), what are the advantages / disadvantages compared to an LTP?

[Littlewyan]

Depends what power valves you're trying to drive. I read the Cathodyne will distort itself before driving the output valves hard enough for them to achieve full power.

[pdf64]

I read the Cathodyne will distort itself before driving the output valves hard enough for them to achieve full power.

You may have been reading about a specific example, which was likely due to design choice / error.
The key problem with driving say a KT88p-p, with up to -80V fixed bias on g1, from a cathodyne is that the HT required to get the required max signal swing may push the constraints of regular tubes; dc elevated heaters could help.
But for the vast majority of our phase splitter applications, a cathodyne could work as well as an LTP.

[tubeswell]

The main limitation of a cathodyne is the PP output signal swing is limited to around 1/4 to 1/3 of the HT supply node (for the cathodyne). AS JM Fahey said, you need to drive the cathodyne with the same sized input signal that you want to get from each output. So you need another gain stage in front of the cathodyne.

Whereas a LTP can potentially deliver a bigger signal swing for the amount of input signal (for a given HT voltage). But overall, a LTP isn't capable of providing as much gain as an inverting stage that uses the same type of triode paired with a cathodyne - tube for tube, with the same HT, and the same input signal, and the same AC load/impedance bridging. This is because the plate-to-cathode voltage in a LTP will always be less than a conventional inverting triode stage, for a given HT supply voltage (because of the tail voltage in a LTP).

[Phil_S]

Selection of the PI, in many respects, is related to what you want for your power section. For example, if you use a 6SN7 as a power tube (you can and should try it if you haven't), it doesn't really like a big voltage swing. That makes a cathodyne a very good choice.

[The Ballzz]

I find it interesting that many of the most revered Tweed Fenders (5E3, 5F4, 5F8, etc.) use cathodyne PI's with cathode biased power tubes. Of course, it appears that only the 5E3 uses no negative feedback? I'm curious as to whether or not that is a big factor in the "magic" of these amps?

Just Sayin'
Gene

[Colossal]

I find it interesting that many of the most revered Tweed Fenders (5E3, 5F4, 5F8, etc.) use cathodyne PI's with cathode biased power tubes. Of course, it appears that only the 5E3 uses no negative feedback? I'm curious as to whether or not that is a big factor in the "magic" of these amps?

Just Sayin'
Gene

Gene,

I built a modded 5E3 circuit with a feedback switch. Feedback is nice to add a polite setting for lower volume work and is useful. But the native mode (sans feedback) is still great and has a bit more gain.

[The Ballzz]

While adding NFB may be an nice option down the road, I may have confused folks by the way I presented! When I spoke of the "magic" of these amps, I was referring to the possibility of it being the cathodyne PI.
Thanx 4 Your Thoughts,
gene

[gingertube]

David covered overdrive of the cathodyne splitter itself.
The other thing to note is that when you start to overdrive the output tubes (when grid swings positive with respect to cathode) the impedance looking into that output tube grid drops dramatically, the anode and cathode loads on the cathodyne become unbalanced and the cathodyne splitter then behaves badly.
Usually this will occur before the cathodyne itself is overdriven (well if they got the design right anyway).

This should be mitigated by the use of much larger than normal grid stop resistors on the output tube. I'm suggesting 39K or 47K here.

I wrote this about a goldentone amp I recently restored. It has a cathodyne splitter with 100K anode and cathode resistors.

As the loads on Anode and cathode become unbalanced (as when overdriving the output tubes) then

Zout anode increases from 1/gm toward RL (from 650 Ohms toward 100 KOhms in the Goldentone)
Zout cathode increase from 1/gm by maximum factor of RL/u (from 650 Ohms toward 650 + 100K/100 = 1650 Ohms in the Goldentone)

You can see that the output impedance at the anode and cathode which are both approx. 1/gm with balanced loads, become massively unbalanced when either the load on the anode or on the cathode drops dramatically.
The trick is to minimise the dramatic load change by using large gridstops on the output tubes.

If you are designing the amp to be a metal shreader I recommend the differential splitter instead.
If you are looking for a vintage folk blues 70s rock amp then the cathodyne can sound lovely (think of all the Fender 5E3 Clones which have been built)

Cheers,
Ian

Skip if maths gets on your wick - here is my derivation of the above.

If the Anode load drops significantly then:

Zout cathode = RL+ra/(u+2) x ra/RL The ra/RL term insignigicant so

Zout cathode approx = RL+ra/u+2 At usual values of u

Zout cathode approx = RL/u + ra/u = RL/u + 1/gm

That is it increases by RL/u


If the cathode load drops significantly then:

Zout anode = RLxRL(u+1)+RL.ra / RL(u+2)+ra

RL squared (u+1) is much larger than RL.ra and RL(u+2) is much larger than ra so

Zout anode approx = RLxRL(u+1)/RL(u+2)

and at reasonable values of u

Zout anode approx = RL

[martin manning]

Thanks, Ian!

[ic-racer]

Good sounding cathodyne amp I have been working with lately. A little different than the usual Fender circuits. May give you some ideas.

https://tubeamparchive.com/viewtopic.ph ... sc&start=0