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Old 9th November 2013, 03:35 AM   #11
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Correction: Using the 2 stage design ala AA764, but retaining the single pot tone stack on the 5E3.
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Old 10th November 2013, 01:34 AM   #12
jjman is offline jjman  United States
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Must to add a grid (leak) resistor and perhaps a (grid) stopper resistor to the 3rd stage and PI.

Since you don’t have signal loss in the tone section, the 3rd stage may clip before the PI, unless some attenuation is placed before the 3rd stage. Although non “Fender” cathode resistor values might avoid that.

If the 3rd stage clips before the PI clips the output tubes will never reach clipping nor full output potential. All of this (preamp distortion) could be an intentional outcome if so desired. Using a pot instead of the grid leak in front of stage 3 could be a way to "dial in" the attenuation value needed for whatever result you want on this. Then you could replace the pot with resistors. Marshall uses that attenuation approach on some amps.

A “master volume” could be used on the input to the PI to keep speaker volume down while allowing the 3rd stage to clip via increasing the “normal” volume control.

Slamming that type of PI hard is also known to be an issue and is mitigated with a very high value grid stopper on it’s input.
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Old 11th November 2013, 11:37 AM   #13
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Quote:
Originally Posted by LATUBEGEEK View Post
Thanks Dave! I wanna get started on something, so decided on the 5E3 Cathodyne. I am going to drop the 2nd pair of inputs, use both 1/2s of the preamp 12AX7 on one input (and the cathodyne PI gives me a 3rd stage of gain!), but use a single tone pot (ala AA764) to keep it simple. Attached is the schematic and the eyelet board on my breadboard. So far so good. Now to build the PI eyelet board and put them both together!
Don't forget your anode and cathode bias resistors on the cathodyne (not shown on your schematic). Also, you'll want 22uF decoupling at each tube. I would also recommend at least 1K between each B+ (so with the 22uF and 1K's you'll have Pi filters along your high voltage power rail) to isolate each stage and keep it nice and clean. Read this for more details on power supply design. Getting the power supply right the first time will save you a lot of annoyance later. It's not as critical with a low gain amp such as this, but it will still help the amp be noise free.
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Old 11th November 2013, 11:41 AM   #14
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Quote:
Originally Posted by jjman View Post
Must to add a grid (leak) resistor and perhaps a (grid) stopper resistor to the 3rd stage and PI.

Since you don’t have signal loss in the tone section, the 3rd stage may clip before the PI, unless some attenuation is placed before the 3rd stage. Although non “Fender” cathode resistor values might avoid that.
+1

Definitely 1M to ground on the grid of each stage, plus at least 100K grid stoppers between each stage. You can experiment with grid stoppers for subtle variance in tone once you're amp is built.
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Old 11th November 2013, 02:58 PM   #15
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Default 5E3 BUILD UPDATE

Doozer - My hastily-drawn schematic was more for showing the preamp mods, and I plan to use the 5E3 for the PI and PP sections.... but I will probably turn the 1M grid leak resistor to the PI grid into a pot, to control the 3 stages of gain to it. Will give me a leak and a stopper in one pot? And keep the 0.1mfd de-coupling caps (per 5E3). Why 22ufd? And increase 1500R stoppers if needed on the 6V6s (suggestions from other posters THANKS ALL!). Am slowly assembling it on breadboard (see pic), with separate eyelet bd for PI, and more submodules for PI and power tubes. Tested preamp section already, works fine.
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Old 11th November 2013, 03:24 PM   #16
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Quote:
Originally Posted by LATUBEGEEK View Post
Doozer - My hastily-drawn schematic was more for showing the preamp mods, and I plan to use the 5E3 for the PI and PP sections.... but I will probably turn the 1M grid leak resistor to the PI grid into a pot, to control the 3 stages of gain to it. Will give me a leak and a stopper in one pot? And keep the 0.1mfd de-coupling caps (per 5E3). Why 22ufd? And increase 1500R stoppers if needed on the 6V6s (suggestions from other posters THANKS ALL!). Am slowly assembling it on breadboard (see pic), with separate eyelet bd for PI, and more submodules for PI and power tubes. Tested preamp section already, works fine.
When the pot is at full gain you will have no grid stopper. I have the same setup on my current build (master volume control) and haven't had any problems, but I think I'll add a fixed grid stopper anyway.

I meant power supply decoupling, not inter-stage signal decoupling.
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Old 11th November 2013, 04:23 PM   #17
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OK, you caught me trying to short cut using one pot for both grid leak and stopper..... Might go with a fixed 500K leak and 500K pot for vol/stopper? Will keep input impedance up, but still have full volume control (and who knows what will happen when stopper goes to zero?). And when you say "power supply decoupling", you mean the filter caps? I am using 22ufd instead of the 5E3 values (16ufd), and expect to play with the power rail dropping resistors, to adjust the circuit plate (and screen) loads.
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Old 11th November 2013, 04:53 PM   #18
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You can use a 1M grid leak without concerns, unless you want to attenuate the signal before the stage.

Yes, sorry, I meant power supply filter caps. I sitting at my desk at work doing digital design and mixed up the terms
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