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Old 14th July 2010, 06:22 PM   #1
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Default Cathode Follower driven Phase Inverters

Hello!

I'm new here. Let me start by saying I'm more of an enthusiast and home tinkerer with a background in broadcast more than anything else.

I have a few guitar amplifiers, I have built 1 myself and modded others, but I've been wanting to try a few things out and I've been looking for tips and examples on this site and others...

two things in particular, would be using a cathode follower to drive a phase inverter. It was something suggested to me a long time ago that I never got chance to try out. I've heard the Komet Concorde has this setup but that its debatable whether it actually has a benefit in its configuration.

Ive also thought about using a switch for the bootstrapped cathode follower in this application.


now... I'm also trying to read up on phase splitters. I'm used (probably like most people) to the LTP. Thats what i'm familiar with both in design and tone. However, I'm considering using the Floating Paraphrase. I don't think I've heard it... i MAY have played an amp that uses that, but it might have used a standard paraphrase... (it wasn't a production amp).

The amp im aiming to build will be of moderate gain, marshall JCM800 maybe... not red channel dual rectifier. I'm using only using 3 12AX7's so I won't have options for lots of cascading. I want to play around with a few less common techniques.

any pointers would be much appreciated.
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Old 14th July 2010, 06:51 PM   #2
TheGimp is offline TheGimp  United States
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Use shielded cable from your input jacks to the input grid of the tubes. 12AX7/ECC83 will oscillate. Look at the JCM800 2203 schematic, the high gain input (V1a) is most susceptable to this.
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Old 14th July 2010, 07:00 PM   #3
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Originally Posted by TheGimp View Post
Use shielded cable from your input jacks to the input grid of the tubes. 12AX7/ECC83 will oscillate. Look at the JCM800 2203 schematic, the high gain input (V1a) is most susceptable to this.
Im not building a JCM800 clone... Cheer for the advice, I might be ahead of you. i was going to use shielded cable for the input to the first grid, probably with a 33K grid stopper. The chassis i have is a spare prototype chassis i got cheap from a guy helping me out. It has first preamp socket RIGHT by the input (single input) which is really convenient as far as keeping that cable run short goes.

I'm playing with some ideas like parallel triode input and a few other bits... I know what vibe I want, but I don't want to get there by copying a marshall... if that makes sense... which is why i was asking about the Phase Splitters and possibly driving them with a Cathode Follower. (I might not use a parallel triode input either, and If i did i might use values closer to a linked 1959 SLP or something)

cheers for the help!
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Old 14th July 2010, 09:41 PM   #4
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Are you sure you're seen a commercial design that uses a CF to drive the PI? I haven't come across one, although of course there are many that go CF > tone stack > PI; these of course are primarily using the CF to drive the tone stack. The common PI's are all high-ish input impedance, I don't see why you need a PI, especially for a commercial design where you are trying to keep costs down.
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Old 14th July 2010, 09:59 PM   #5
SY is offline SY  United States
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Originally Posted by leadbelly View Post
Are you sure you're seen a commercial design that uses a CF to drive the PI?
C-J did it between voltage amp and an LTP phase splitter. If the phase splitter is a cathodyne, there's no possible reason to do so.
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Old 14th July 2010, 10:53 PM   #6
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When it was suggested to me, it was a few years ago, and I was playing around for the first time with an amp. something very JCM800'ish, and I had a spare 12AX7. It was suggested to run it as a CF into an LTP.

It was also suggested to try using both spare triodes as cathode followers following the PI, like the 180watt Fender Super Twin... but that seemed a bit over the top to drive 2 EL34's. (the Super Twin having 6 6L6's)

I've been digging around and I found a discussion about the Komet Concorde, which is meant to be similar but not the same as a Komet 60. I read a comment from one person saying he had one on a bench, and the spare triode usually found in those amps had been employed at the last gain stage to drive the PI (LTP) but others (randall aiken) suggested it did nothing in that configuration because there was a 220K resistor between the CF output and the PI grid.

the Komet being input gain stage > Tone stack > more gains stages > PI > Output.

I've also seen schematics with a similar topology to the above, but with the input stage as a cathode follower driving an early tone stack. (I say early, as its not the usually bunch of cascaded stages before a CF driven tonestack, which seems more common).

Leadbelly... I would like a PI in my design. I'm using 2x EL34's in push-pull. at least ive decided that! and i want a PI to drive them.
If you mean I don't need a CF... that is also true... but then I could just copy any old schematic, but I don't want to. I want to try something new (to me at least).

So... I might try and find some more out about this if i can.
at the moment im toying between the idea of Floating Paraphrase and LTP PI's. Cathodyne isn't on the list really.

Last edited by BobHimself; 14th July 2010 at 11:14 PM.
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Old 14th July 2010, 11:57 PM   #7
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BobHimself,
Just a note to pass along my experience of PI for Guitar Amps. I built quite a few amps using the "USUAL" Schmitt Splitter as is used in 90+% of commercial tube guitar amps (basically a diff amp splitter).
As an experiment I then built an amp using a Cathodyne (Concertina) Splitter.
I have never gone back to the Schmitt. The Cathodyne with a high mu tube (12AX7, 12AT7 or 6SL7) is to my ears just far superior with earlier, smoother overdrive. The overload is asymmetrical which means even order harmonic distortion products. Why a high mu tube? If you look at the maths then it clearly indicates that for best linearity (that is HiFi use) a low mu tube should be used. The high mu tube emphasizes the cathodyne "warts" and those "warts" just happen to suit (soundwise) a guitar amp power section.
For a pair of EL34 I would recommend a 6N9 or 6SL7 cathodyne splitter.
Cheers,
Ian
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Old 15th July 2010, 12:06 AM   #8
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that sounds like a strong recommendation!
Out of interest... did you try it with a 12AX7?
I have no problem with using an AT7 but I just thought i'd pop the question... not THE question mind!
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