Why use Cathodyne or Long tail pair PI?

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fusionbear
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Why use Cathodyne or Long tail pair PI?

Post by fusionbear » Sat Nov 20, 2010 1:19 am

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... :D
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sliberty
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Re: Why use Cathodyne or Long tail pair PI?

Post by sliberty » Sat Nov 20, 2010 1:51 am

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
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Re: Why use Cathodyne or Long tail pair PI?

Post by David Root » Sat Nov 20, 2010 2:44 am

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
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Re: Why use Cathodyne or Long tail pair PI?

Post by ChrisM » Sat Nov 20, 2010 3:02 am

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
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Re: Why use Cathodyne or Long tail pair PI?

Post by sliberty » Sat Nov 20, 2010 4:48 am

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

fusionbear
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Re: Why use Cathodyne or Long tail pair PI?

Post by fusionbear » Sat Nov 20, 2010 5:01 am

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
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Re: Why use Cathodyne or Long tail pair PI?

Post by Cliff Schecht » Sat Nov 20, 2010 5:12 am

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 :).
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tubeswell
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Re: Why use Cathodyne or Long tail pair PI?

Post by tubeswell » Sat Nov 20, 2010 8:21 pm

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
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Re: Why use Cathodyne or Long tail pair PI?

Post by TUBEDUDE » Sat Nov 20, 2010 8:44 pm

Or adding a negative supply to the PI tail.

Andy Le Blanc
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Re: Why use Cathodyne or Long tail pair PI?

Post by Andy Le Blanc » Sun Nov 21, 2010 5:36 pm

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
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Re: Why use Cathodyne or Long tail pair PI?

Post by harleyboy2112 » Mon Nov 22, 2010 2:09 pm

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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