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Old 17th April 2011, 06:10 PM   #11
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A few thoughts. Your cathodyne phase splitter has no bias, you need a grid leak resistor to the bottom of R10. Also I doubt the need for a bypass capacitor on that bias resistor.

Unless you are looking for clean only I don't think that the 6DJ8s are going to give you nearly enough gain. A couple of 12AX7 or 12AY7 stages would be more normative for a guitar. The high mu triodes have rather high rp though so a cathode follower is often used to drive the tone stack. Also cathodyne PI will have different sound at overdrive than the LTP such as used in Marshalls.
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Old 17th April 2011, 09:44 PM   #12
rman is offline rman  Canada
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Hi all.

Mike: Good catch on that missing grid resistor. I was going to say his circuit should work now. Interesting comment about the cathodyne as compared to the LTP. I had no idea as this was my first guitar amp, built or bought.

Tauri : The tubes in series as you say in my circuit are called a cascode. It is a high gain stage like you guessed. I did it that way because as Mike says 6dj8's don't have that much gain. That is also why I added a bypass switch around the tone stack which looses most of the gain from the first stage.

Cheers.
Rolf.
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Old 17th April 2011, 10:00 PM   #13
Tauri is offline Tauri  Estonia
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Thank you very much for responses, gained a few good facts today. Took care of the phase-inverter resistor too.

@rman: I don't really want that much gain, got a pedal to switch between cleaner and distortion before the preamp. Though my amp should distort when playing harder on guitar.
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Old 18th April 2011, 12:15 AM   #14
rman is offline rman  Canada
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Right on sounds like you are good to go.
Good luck.
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Old 18th April 2011, 01:00 AM   #15
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Rman, on yours you might consider changing the cascode to a mu follower and putting the tone stack after the mu follower which has better drive and lower zout than the common cathode gain stage. Another thing to consider is that at least with the mu follower and I think also with cascode the over load characteristics are a lot more abrupt than with common cathode gain stages so you may find the distortion a bit nasty (my be what you want though).
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Old 18th April 2011, 01:12 AM   #16
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Quote:
Originally Posted by Tauri View Post
what am I missing now?
K1 L1 L2 L3 1
Don't use leading dot at the beginning, I don't know why...

You are also gonna have to guess some inductor values
and provide a load resistor.

Last edited by kenpeter; 18th April 2011 at 01:15 AM.
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Old 18th April 2011, 01:35 AM   #17
rman is offline rman  Canada
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Mike:
Interesting thoughts. There is a fine line on the gain control between "crunch" and a big mess. I originally tried the cascode first where it wouldn't get overloaded but it couldn't drive the tone stack. I don't really feel like rebuilding it right now, but I will keep it in mind.

Thanks.
Rolf.
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Old 18th April 2011, 02:12 AM   #18
TheGimp is offline TheGimp  United States
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Are you using 6N1P and referencing 6DJ8 in your schematic since you are actually using 6P1Pand using 6V6 symbols?

You have one spare triode not shown on the schematic. Convert your Cathodyne to a LTP and you will gain some gain as a side benefit.

If you are using Svetlana 6P1P-EVs don't worry about the 250V limit. I run them at 325V and have no problems. Just keep the screen voltage down at 250 or less and watch your anode dissipation. Keep your anode dissipation below the 12W limit and you will be fine.

In experiments I have cranked 6P1P-EVs over 375V on the anodes for short periods (IIRC) with no apparent long term problems. One of these days I'm going to build an amp to do long term testing at 350V AB1 to see how they do.

You don't need (or want) polarized caps as coupling caps.

Last edited by TheGimp; 18th April 2011 at 02:15 AM.
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Old 18th April 2011, 07:56 AM   #19
Tauri is offline Tauri  Estonia
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@TheGimp: No, I do use 6DJ8's for this preamp. I think i'm going to leave it as cathodyne, because I don't want that much gain, though with 6DJ8's there is very little compared to 12AX7.
If they can run on so much higher anode voltage, why isn't it marked on the datasheet? Anyhow, I can't get higher than +270V with my transformer, so have to settle.

Last edited by Tauri; 18th April 2011 at 08:08 AM.
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