5e3 power tube grid stoppers

[John Clifford]

Like many 5e3 builders, I’ve been looking for the best way to reduce that harsh, blatty, broken-speaker-like distortion that tends to happen midway between clean and full overdrive. I tried adding a 500k pot to the PI grid which did nothing I could hear, then tried a 1M pot which made a small difference, but also seemed to muffle the overdriven sound. I tried lower-value coupling caps after V1 which helped some but also ruined (for me) the beautiful, bassy clean sound of the stock amp, so I restored the originals. Didn’t try smaller bypass caps because I figured that would have a similar effect. Then I read Merlin Blencowe’s discussion of the cathodyne phase inverter, where he recommends larger grid stopper resistors on the output tubes. I hooked up both sides of my PI output (post caps) to the oscilloscope, and lo and behold, saw exactly the pattern of anode signal distortion shown in Merlin’s book (see photo). So I replaced the two 1.5k grid stoppers on the 6V6s with 56k (about middle of the range Merlin suggests). That really did the trick for me. The harshness (blocking distortion?) is now minimal at higher gain, but the original amp’s clean sound is still there in all its glory. The only trade-off, to my ears, is a little less clean headroom (there wasn’t much to begin with) and a little more compression. On the scope, the PI output is much more symmetrical. So I recommend trying this mod if you’re having the same issue. Sorry to bore the experts on here who’ve known this for years - this is aimed at other beginners like me.

{EDIT: Valve Wizard Link - http://www.valvewizard.co.uk/cathodyne.html --HBP}

[jordan86]

Very cool. I assume this could be done on the Princeton Recerb, AA1164? I read Merlin’s page on the cathodyne PI and nipple distortion. I did a 1meg into the grid of the PI but like you, didn’t think it was that much of an improvement. Will have to try this soon.

Thanks!

[John Clifford]

I assume this could be done on the Princeton Recerb, AA1164?

Don’t know about the Princeton, sorry. All I know is it worked on my 5e3 clone. I left the 1M pot in place on the PI grid. I can leave it turned up (0 ohm) for stock, and might still use it the other way occasionally. Good luck!