Tremolead (my take on a 5E9-A Tremolux) - 6K6GT question

[johnnyreece]

Hello, all!  I started dreaming this up a while back.  I had previously built a 5E3, and, while it was great for what it did, it was a bit of a one trick pony.  So, I started thinking about maybe a more versatile 5E3.  Enter the Tremolux - very 5E3 front end with tremolo.  I modded the preamp with the lead/cascade mods from Rob Robinette's site.  I also incorporated some Tweedle Dee stuff.  Some Valve Wizard/Sluckey stuff in the trem.  A few other things (Tilt control instead of Tone, screen resistors, power tube grid resistors, adjustable bias, dual PT secondaries).  My other change is using a 6SL7 for the tremolo, because I'm building this in a Weber 6MX chassis, which has two noval holes and four octal holes.  I got a preliminary version built and discovered I needed to rearrange my bias a little (6K6 tubes ran too hot; big bottles ran too cold).  I'm also waiting for some parts to come in (used some things that I had on hand in order to get it running, but the tremolo section isn't quite right).  Anyhow, in brief testing, there are definitely three distinct flavors.  The least-modified channel still sounds very "tweed".  The lead channel feels a lot more Marshall, and the cascade switch allows some pretty gnarly higher-gain sounds.  I wish I still had my Telecaster, as that's what my old 5E3 sounded best with, and I'd like to compare, but my Les Paul will have to do for now.  The schematic/layout I've attached are what I hope to have together soon when my new parts arrive.  If I tweak anything, I'll update these files.  Thanks for looking!

Edit 5-16-24:  Updated to as-built schematic/layout.  For anyone interested, my favorite tube combo was KT66 with GZ34 rectifier.  Couldn't find a sound I didn't like with those tubes.  YMMV.  If anyone would like, I can get some voltage measurements. 

[johnnyreece]

I think I've settled on the *final* version of this, but I wanted to get a little advice before getting faceplates made.  Here's my dilemma:  I have multiple cathode resistor and B+ settings.  Depending on the rectifier and B+ settings, I get plate-cathode voltages generally between 350-400 (I could probably get higher with straight diodes, but I've not tried that yet).  My OT is for a JTM45, and its ratings are 3400 primary with 2,4,8,16 secondaries.  I know that some people have dropped 6K6GTs in a 5E3 with no modifications, but I'd like to be a little gentler with them.  As such, I was thinking about the following settings for the OT:

EL34 and 6L6 - 3400 primary with 2,4,8,16 secondaries
6V6 and KT66 - 6800 primary with 4,8,16,32 secondaries
6K6 - 13600 primary with 8,16,32,64 secondaries

I know some of the numbers (32 and 64 ohm, especially) aren't really practical, but I included them for completeness.  My biggest concern is for running 6K6 tubes.  I've increased the cathode resistor to 600, but I can't really get below 350 on the plates without neutering the bigger bottles.  My dissipation was at 101% when I measured it.  Again, though, I've read anecdotal evidence of people subbing them in a 5E3 with no modifications with no ill effects.  Am I overthinking this, or is it a valid concern? 

[sluckey]

As said in another place and time, I would be nervous running 6K6s at 400V.But, give it a try. If it doesn't work out you still have other tube options.

[johnnyreece]

My apologies for not being clear!  I would *only* run 6K6 on the low B+ option with a 5Y3 (or 5R4, which ends up pretty much the same) which brings it to roughly 350 on the plates.  I suppose I've got a dozen or so 6K6, so if I don't hear anything else, I can try it 'til I fry it. 

And, thank you for your assistance in another place and time.  Your input (not only on my thread, but other posts you've made over the years) has been instrumental in getting this together!

[sluckey]

I can try it 'til I fry it.

I like that spirit.   

[sciurian]

I just did an octal 6SJ7+6SL7 18W-ish with 1750PA OPT (8k4 18W iron), SS rectifier. Got some redplating past 370-380V in cathode bias from most of my 6V6GT and 6K6GT tubes until I added a 5Y3 worth of sag resistor. Swapped to fixed bias, reduced the screen-grid supply sag, and added tremolo, and it's dandy. It's made to run either 6V6GT or 6K6GT. It idles at about 350-360VDC with 20-30ma cathode current per tube. Those 6K6 screens grids are thirsty!

I ran a bunch of VTADIY loadlines for 6K6GT @ 8k and 300/325/340/350/370V and my impression was that you can get away with a steeper load than 12k a-a. I switched to fixed bias to optimize for 5G9 tremolo, but also because it seems that cathode-bias conditions on datasheets usually call for a higher-impedance OPT.

(edit to add: all of this is to say that you may be able to get away with a 7k a-a load and 6K6GT)

Good luck!