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Why use Cathodyne or Long tail pair PI?

 
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fusionbear



Joined: 01 Nov 2007
Posts: 265
Location: Southern California

PostPosted: Sat Nov 20, 2010 1:19 am    Post subject: Why use Cathodyne or Long tail pair PI? Reply with quote

Most amps nowadays seem to use a long tail pair PI, but I love the sound of the old Princeton with its Cathodyne arrangement and built a Weber 6S100 that also has the same arrangement. What are the plusses and minus' of each. Tone differences? I'm still a muggle, so whatever you guys discuss will be very much appreciated... Very Happy
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sliberty



Joined: 26 May 2006
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PostPosted: Sat Nov 20, 2010 1:51 am    Post subject: Reply with quote

The Cathodyne produces muchlower gain than a typical LTP. If you want to push your power tubes hard, the LTP may be a better choice. On the other hand, as I have recently been trold many times, some tubes prefer a lower gain PI. For example, I have been messing around with a 12AU7 power stage, which will now most likely become a 12BH7 power stage instead. These tubes are driven too hard by a LTP, and a Cathodyne is preferable I am told.

Of course there is also the obvious architectural difference. A Cathodyne uses one triode, and a LTP uses two.
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David Root



Joined: 04 Aug 2006
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Location: Lower Mainland BC

PostPosted: Sat Nov 20, 2010 2:44 am    Post subject: Reply with quote

In practical terms the cathodyne generally a la Fender (but not necessarily) uses a gainstage before it so you're still using a full tube.

Depends on the power tube you're driving too eg 6L6 vs 6550. I have never thought of 12AU7 or 12BH7 as a power tube, but if you hook up a cathodyne PI to a 6550 or KT88 you'd better have a good driver tube in between, which is where the 12AU7 or 12BH7 come in.
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ChrisM



Joined: 22 Oct 2008
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Location: Toronto, Canada.

PostPosted: Sat Nov 20, 2010 3:02 am    Post subject: Reply with quote

sliberty wrote:
The Cathodyne produces muchlower gain than a typical LTP. If you want to push your power tubes hard, the LTP may be a better choice. On the other hand, as I have recently been trold many times, some tubes prefer a lower gain PI. For example, I have been messing around with a 12AU7 power stage, which will now most likely become a 12BH7 power stage instead. These tubes are driven too hard by a LTP, and a Cathodyne is preferable I am told.

Of course there is also the obvious architectural difference. A Cathodyne uses one triode, and a LTP uses two.

The cathodyne is has a gain of 1 no?
Only uses one triode, so thats a plus. As David said though, there is usually a gain stage before though.

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sliberty



Joined: 26 May 2006
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PostPosted: Sat Nov 20, 2010 4:48 am    Post subject: Reply with quote

I believe the gain of a typical Cathodyne is just slightly below 1.
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fusionbear



Joined: 01 Nov 2007
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Location: Southern California

PostPosted: Sat Nov 20, 2010 5:01 am    Post subject: Reply with quote

In my Weber, there is a EF86 driving both triodes in parallel of a 12AU7 in the Cathodyne arrangement and then to a pair of KT88's. I am really happy with the tone and volume, but I like to tinker...
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Cliff Schecht



Joined: 30 Dec 2009
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PostPosted: Sat Nov 20, 2010 5:12 am    Post subject: Reply with quote

fusionbear wrote:
In my Weber, there is a EF86 driving both triodes in parallel of a 12AU7 in the Cathodyne arrangement and then to a pair of KT88's. I am really happy with the tone and volume, but I like to tinker...


Depending on your skill set tinkering can lead you to great places or lead you nowhere. Before I knew what I was doing with frequency response shaping and biasing with tubes, I would tweak endlessly as I read around the overwhelming amount of resources out there. Once you get the hang of it though you can take junker amps and make them awesome Smile.

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tubeswell



Joined: 27 Mar 2008
Posts: 1587
Location: Wellington. NZ

PostPosted: Sat Nov 20, 2010 8:21 pm    Post subject: Reply with quote

A cathodyne is also capable of giving a more closely-balanced set of output signals (assuming you don't bias it 'wrong' or overdrive it too much to end up with gain-spike or frequency doubling distortion). Ideally the plate load and the cathode load should be the same (and this is apparently why you find bypassed cathode bias resistors in hi-fi amps cathodynes - which I never knew until I read Merlin's 1st book).

Whereas AFAICT an LTP is more-or-less inherently unbalanced. You can get an LTP nearly balanced, by increasing the tail resistor and matching the plate resistors, but this comes at the expense of gain.
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TUBEDUDE



Joined: 04 Jan 2007
Posts: 215
Location: GA.

PostPosted: Sat Nov 20, 2010 8:44 pm    Post subject: Reply with quote

Or adding a negative supply to the PI tail.
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Andy Le Blanc



Joined: 22 Dec 2007
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Location: central Maine

PostPosted: Sun Nov 21, 2010 5:36 pm    Post subject: Reply with quote

cathodyne is an old standby, you can drive kt88/kt90/6550 into distortion with one.
A buffer is nice to handle current, more better drive, but you have to watch
the gain before the power side.

Each inverter has its own flavor, and plays with feed back in different ways.
The old cathodyne sound incredible up to a point and then falls apart into distortion.
Cathodyne was used very frequently by most old mfg at one time, more so than LTP.

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harleyboy2112



Joined: 17 Oct 2009
Posts: 12
Location: south dakota

PostPosted: Mon Nov 22, 2010 2:09 pm    Post subject: Reply with quote

Adding a grid stop resistor at the input of the cathodyne phase inverter gets rid of the nastiness when the amp is cranked. I built a 5e7 style amp and added the grid stopper and it sounds great when cranked. Tweed overdrive without the nasties.
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