Express w/cathodyne PI & 6v6s??

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jph118
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Express w/cathodyne PI & 6v6s??

Post by jph118 »

Hey guys, just thinking out loud about this... using a 5E3 deluxe chassis that i have on hand, i was inspired by some of the "Blues Express" schematics featuring a cathodyne split-load PI into 2xEL84, no NFB. I do a lot of builds with cathodynes and have my personal tweaks to get the most out of them, especially with 6V6s... i'm just curious how the cold clipper stage feeding directly into the PI will sound, or if that might be better moved to before the tonestack with a common cathode driving the PI like a 5E3/5F11/6G2, and also providing a place to tie in NFB if desired. I'll prob try it first with the original gain structure, and use snubber caps on the plate resistors in lieu of NFB to smooth things out. Just curious if anyone has tried this yet.

I know, i'm not really building an Express at this point, but it's fun to experiment!
jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

Something like this for the preamp & PI since i'm limited to two 9-pin sockets:
Preamp.jpg
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dvnator
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Re: Express w/cathodyne PI & 6v6s??

Post by dvnator »

I've tried it and didn't think it really worked. With the 10k stage right before the cathodyne I didn't think the power tubes were driven enough. It came out more like a blackface amp.

I ended up going with this:
Image
And it works pretty well. I look at it as a sort of 18 watt/Trainwreck mash up.
jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

Nice! I think if mine isn’t driving the power tubes enough I’ll bypass that cold clipper cathode for a more conventional stage (maybe on a switch). Getting further from the Express, but still fun.
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johnny riff
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Re: Express w/cathodyne PI & 6v6s??

Post by johnny riff »

I had built something very similar years ago and still play it. 6v6 output,
I wanted to save a 12ax7 in the pre so i used a split load PI.
Not a wreck but sounded good to me,
jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

johnny riff wrote: Tue Feb 20, 2018 10:44 pm I had built something very similar years ago and still play it. 6v6 output,
I wanted to save a 12ax7 in the pre so i used a split load PI.
Not a wreck but sounded good to me,
I'm kind of hoping for something more on the bluesier side, so i think this may work out well... if anything, it'll be fun to hear what a pair of 6v6 can do thru the Classictone TW OT compared to the much smaller 20w iron I usually use.
jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

Happy to say this is coming out pretty nicely... couldn’t crank it tonight, but with the level up to about 3-4 the tone was very good, bold & full with an edge. Had a slight lead dress issue coming off the Vol pot, but that was an easy fix once I found it on the scope.
ChopSauce
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Re: Express w/cathodyne PI & 6v6s??

Post by ChopSauce »

You mean you built it right after the schematic you posted earlier? That's really interesting. I'm looking forward to reading more details about this build.

Congrats, anyway!
jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

Yep. I’ll post an updated schematic soon (ignore that treble/vol pot confusion up there, drew it quickly), but after cranking this guy to 10 this morning, I’m thrilled with it. Early ZZ in a box, basically. Upped all the plate resistors to 220k on the first three stages to push the output a little more, that was the only big change.
jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

Updated schematic, i'll draw a full one soon for anyone interested... may add treble snubbers after trying it with a few more guitars...

IMG_9898.JPG
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jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

added a 500pf snubber to the 220k cold clipper plate load to cut down on some light HF oscillation at high levels, also went back to 100k on the first two stages to soften things up... the added 5dB or so on the clipper was enough extra gain, and the earlier stages were a bit too stiff. Equally satisifed with single coils as well as buckers, here's a quick thing I did for instagram...

https://drive.google.com/file/d/1QuYbqh ... sp=sharing
ChopSauce
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Re: Express w/cathodyne PI & 6v6s??

Post by ChopSauce »

That thing rocks. So, 100k slope and 6k6 output transformer primary?

Thanks for sharing, anyway 8)
jph118
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Re: Express w/cathodyne PI & 6v6s??

Post by jph118 »

ChopSauce wrote: Thu Mar 01, 2018 9:54 pm So, 100k slope and 6k6 output transformer primary?
yep! Used the massive TW iron from Classictone, which seems like it can take a lot more than 30w. Def trying this circuit with EL34s soon.

WGS ET65 12" in that open-back combo, too. I think it sounds just a little bit better thru a closed-back 2x12" with a celestion H30/V30 mixed pair. Maybe I'll try to do a recording of that, but the nuance may not come thru.
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johnny riff
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Re: Express w/cathodyne PI & 6v6s??

Post by johnny riff »

I have not built an git amp in awhile,but getting the itch. I didn't do a search but has anyone tried a pot/vari resistor
for the grid leak before the cold clipper stage. I believe the stock is 150k, so maybe 56k to 156k with 100k lin. pot or with a 250k pot 56k- 306k.?
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martin manning
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Re: Express w/cathodyne PI & 6v6s??

Post by martin manning »

Yes I've done that and it worked quite well. I used a 100k trimmer and 47k resistor to ground, with the wiper going to to V2's grid. That provides up to a 10 dB cut. Others have done the same and reported liking it.
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