High Plate Voltage 6L6’s In Lichtlaerm Prometheus?

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braintheory

Well-known member
So I got last week a Lichtlaerm Prometheus that I’m really enjoying and wanted to explore all the various powertubes in it. I tried everything so far besides 6L6GC’s and KT66’s, so I put in some recent Russian made Tung-Sol STR 6L6GC’s with the bias pots all the way down and the amp immediately blew its 2A fuse. I’m guessing maybe the 520 plate voltage was too high for those tubes. Can anyone here recommend me some 6L6GC’s and KT66’s that can handle this plate voltage? Has anyone here tried these tubes in their Prometheus? Thanks!

I also emailed and messaged the maker on fb messenger as well about all this, but haven’t heard back from him
 
You’re going to have to step up to the big boy club of 6CA7 for beam powered tetrodes which are triode strapped to act more like a 6L6 or step up to KT77. 5881s which are more robust 6L6s aren’t really rated for the plate voltage either. Most 6L6 are going to struggle with 520 on the plates. That’s definitely 6CA7/E34L/KT88/6550 big boy club.

Next step for Abaddon is 525V plates loaded. I will only recommend the new Shuguang 6CA7 or E34L models for stock OTs. I have had good luck with Russian Svetlana EL34s in the 480V range though but you’re asking for 6L6 variants.
 
520vdc unloaded is common. That’s probably 480vdc or so with the tubes in.

I’m guessing it’s probably a bad tube and not the plate voltage. Be good to know where the screen voltage is as well. That’s what’s really going to kill things.
 
glpg80 said:
You’re going to have to step up to the big boy club of 6CA7 for beam powered tetrodes which are triode strapped to act more like a 6L6 or step up to KT77. 5881s which are more robust 6L6s aren’t really rated for the plate voltage either. Most 6L6 are going to struggle with 520 on the plates. That’s definitely 6CA7/E34L/KT88/6550 big boy club.

Next step for Abaddon is 525V plates loaded. I will only recommend the new Shuguang 6CA7 or E34L models for stock OTs. I have had good luck with Russian Svetlana EL34s in the 480V range though but you’re asking for 6L6 variants.
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I’ve already tried those tubes in it and while I like them, they still don’t have IME the same sound as 6L6GC’s, especially in the highs, but closer to that direction. It says on his site it can take 6L6GC’s though, so I’d love to try some that can handle it
 
Do you guys think 7581A’s will have a better chance of working in it? I do have some of those including a brand new quad of Russian made Tung-Sol 7581A’s
 
Try the 7581’s but have some way of monitoring the bias when you fire it up. It might be going super hot. I’m still guessing it was a bad tube though.
 
RedPlated said:
Try the 7581’s but have some way of monitoring the bias when you fire it up. It might be going super hot. I’m still guessing it was a bad tube though.
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Will do. Good news is the amp is externally biased with my multi-meter, so very easy to do. What about trying KT66’s in it? How do you feel about that in safely using them in it?
 
Jeremy is right - screen grids are indeed the crux of many tubes - 6550s are very susceptible to damage with high screen grid voltages relative to the plates. They prefer to be ran in ultra linear mode or pentode with a very low screen grid voltage.

I personally don’t like the sound of KT66 but as always YMMV. Let me know what you think if you try them - I’m interested in your opinion.

Where did you source the suspected bad tube?
 
glpg80 said:
Jeremy is right - screen grids are indeed the crux of many tubes - 6550s are very susceptible to damage with high screen grid voltages relative to the plates. They prefer to be ran in ultra linear mode or pentode with a very low screen grid voltage.

I personally don’t like the sound of KT66 but as always YMMV. Let me know what you think if you try them - I’m interested in your opinion.

Where did you source the suspected bad tube?
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I've tried in it so far 6550's, KT88's, 6CA7's, and EL34's without any issues. They all sounded good in their own ways and I can mix the tubes too, which I tried also

I don't generally like the sound of KT66's as much either, but will still try it as you guys know I like to be very thorough and leave no stone unturned lol. I have also legit vintage GEC KT66's and '90's Sovtek KT66's.

Do you mean who did I buy the tubes from that we think went bad? I don't wanna hurt the shop's reputation as they otherwise have been very solid with me
 
braintheory said:
I've tried in it so far 6550's, KT88's, 6CA7's, and EL34's without any issues. They all sounded good in their own ways and I can mix the tubes too, which I tried also

I don't generally like the sound of KT66's as much either, but will still try it as you guys know I like to be very thorough and leave no stone unturned lol. I have also legit vintage GEC KT66's and '90's Sovtek KT66's.

Do you mean who did I buy the tubes from that we think went bad? I don't wanna hurt the shop's reputation as they otherwise have been very solid with me
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Safer to go to PM
 
peterc52 said:
Not cardboxy?
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Not sure if that was meant as a joke or not, but to answer for real it doesn’t sound cardboard-y to me like a UU or plastic-y at all like the EVH’s or too filtered as long as I keep the cut control all the way down. It does get nasal and weird sounding when I turn that up. Most of my other keeper amps I have to say sound more organic still, but it’s not bad and I love the voicing itself of the amp and the feel is also nice. It has a killer grind and growl to it, pretty tight, well balanced voicing and sounds very big. My only bigger sounding amps overall I think are the Hell Razor KT150, Megalith Beta, Blueface and Rev 1 Uber

One of the better cleans too of high gain amps IME. The medium gain crunch sounds are very lack luster though. For high gain it has a lot of range and can do almost anything well imo: leads, tight/fast riffs, big wall of sound type riffs, etc. I’m liking it a lot overall. I like the bias set though a bit lower than what’s recommended for added clarity and focus. It’s throaty growl can almost get like the recto orange ch and a grind sort of like some of the Engl models I liked, but to me better and without sounding as artificial or fizzy like them
 
Braintheory, brotha’, you got no worries being you in my book man.
You’re amp experience & opinion are the standard I bounce off to save herds of cash my man!!!

Any of you RigTalk guys know the NOS tubes
collection this man has??
It’s a library of tubes in alphabetical order.by country.
Seriously, he’s got’em down to the countries.
Them real SED’s🤘🏻, the really real ones, LOL!
Dude is HOLDIN’!

I know that bogus feeling when an amp sounds/feels cardboxy.
The EVH one is spot on.
I almost bought a full stack EVH , the good one for next to nothing.. it was HUGE , the head & whole stack.
Beyond biblical amounts of gain., like Voodoo amps etc.
I remember feeling Stonehenge Deja-vu flavor of the vibes just from looking at that monstrous stack.
 
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