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Old 31st July 2007, 01:22 AM   #1
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Default Phase inverter

Cathodyne, LTP, or schmidt?



It seems a lot of amps use cathodyne, I have seen some guitar amps that use Schmidt, which is very similar to the LTP.

Cathodyne seems simplest, will it be sonicly rewarding? I don't need much gain, I am only using line level sources with my amp and not phono level.
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Old 31st July 2007, 01:28 AM   #2
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Opinions are split. Andy Evans likes cathodyne (concertina, split load), and I like the singly-driven diff-amp (long tailed pair). With the LTP you can sometimes lose a gain stage in your circuit, which is bound to be a plus. The 'dyne, of course, has less than unity gain.

That didn't help much, did it?

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Old 31st July 2007, 01:44 AM   #3
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Cathodyne has limited voltage swing - less than half supply for sure. So it's best used with output tubes that don't require a lot of grid swing like 6BQ5, 7591, 6V6... never mind what Dynaco did with it!
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Old 31st July 2007, 01:56 AM   #4
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Id be using it for 6v6/el84 type tubes. Also noted that my 7868 amp seems to use a type of cathodyne PI. Only weird difference is that the cathode resistor is in series with one of the grid resistors on the lower op tube. I can elaborate with a schematic if needed...
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Old 31st July 2007, 03:14 AM   #5
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In a case like this, it's almost a case of six-of-one, a half-dozen of the other. The cathodyne has excellent AC and harmonic balance, so long as it isn't going to be looking into grids driven positive (in that case, the unequal impedances at the plate and cathode will throw it out of balance).

I tend to prefer the LTP since I use fixed bias and cathode follower grid drivers, and the LTP eliminates one extra stage.
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Old 31st July 2007, 04:49 PM   #6
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I looked at it again and it does appear to be a LTP. However, the resistor that normally goes from v1B to ground goes to the ground where the cathodes of the output tubes tie in, and one of the 470k grid resistors is tied to the top of the 12k resistor.
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Old 6th August 2007, 02:43 AM   #7
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I have to use a LTP or schmidt if I want to do fixed bias on a pp amp , right? it seems most amps use these in this case.

Also.. I would like to use 12au7 if possible, as I have some RCA cleartops with matched GM readings ( two tubes, four identicly matched triodes)

Any way to sub An AU7 for an AX7 with 220k plate resistors instead of 100k?
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Old 6th August 2007, 05:33 AM   #8
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Quote:
Originally posted by ThSpeakerDude88
I have to use a LTP or schmidt if I want to do fixed bias on a pp amp , right? it seems most amps use these in this case.
You don't "have to". For fixed bias, it is best to use a DC coupled driver of some sort. I prefer to use cathode followers for this purpose. However, it is also possible to DC couple to a driver that uses a triode with a good amount of g(m), like 6SN7s, or those 12AU7s. You would need an extra source of HV so that you could elevate the cathodes of the drivers above ground to establish your Q-Point bias.

The main disadvantage to the cathodyne is that it doesn't produce any gain, and so you have to separate the gain function from the phase splitting function. An LTP can do both at once, and you frewquently find that you don't require an extra gain stage.

Quote:

Also.. I would like to use 12au7 if possible, as I have some RCA cleartops with matched GM readings ( two tubes, four identicly matched triodes)

Any way to sub An AU7 for an AX7 with 220k plate resistors instead of 100k?
Draw a loadline and see what you get. The 12AU7 has a much lower u-Factor (~20 v. 100) and a much higher g(m) (3.1mA/V v. 1.6mA/V) and a higher Pd (2.75W v. 1200mW). It will have less gain, but higher current sourcing capability.
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Old 6th August 2007, 12:34 PM   #9
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Though I haven't built the amp yet I now have a pair of phase splitter autoformers from Jack Elliano at Electra-Print. He has a schematic on his site for a PP EL84 using them.

My plan is to use a 6N1P for input into the PSA-2N for phase splitting and straight to the EL84s for output. A bit different from Jack's schematic but I had already designed it with the exception of the phase splitter.

I tested several other trafos for phase splitting and found some that worked really well and at low cost ($20 for the Edcors). Just something else to consider.
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Old 8th August 2007, 01:37 AM   #10
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Making a phase inverter using an interstage certainly cuts down the cost , parts count, and makes it simple. However , sound quality is tremendoulsy dependant on the transformer, so it makes it a little harder than one would think.


With PP 6BQ5's, what are the advantages/disadvantages of adding fixed bias?
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