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I'll keep it short. I'd like to try KT66 in a 6L6GC amp. Info 'out there' seems slightly sketchy as to whether this is possible. If so, the big hangup seems to be the extra heater current on KT66 stressing a transformer (OT? PT?). I have yet to see any explanation as to HOW you would determine if the transformer could handle the extra current draw. This is a Gibson amp - GA42RVT - and Gibson has been no help whatsoever in this particular line of questioning. Anyone? Thanks!
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My data sheets show the heater current for the 6L6 is 0.9 amps. For the KT66, heater current is 1.3 amps. Otherwise, they seem to be pin-for-pin compatible. Now, I'm not knowing the spec's on your power transformer, if it can handle the additional 400 mills of heater current. You would have to plug the tube in, and see how much it pulls the heater voltage down with it installed. I think anything below 6.0 volts is going to make the power transformer run pretty warm. Anything below 5.7 volts, and that will make the rest of the amplifier unusable. That low of a filament voltage, and it will just kill the tone coming out of the pre-amp.
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Thank you for the response. Is there any way to determine the specs of the power transformer if Gibson is not forthcoming? Is the label on it in any way informative? Assuming not, how would one measure the heater voltage - is this too much for a mb description?Comment
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"Is the label on it in any way informative?" Does the label say anything like "6.3VAC 4A"? If so, you really need 5A or more if you're going to keep all the 12AX7s in there (6A+ would be nice).
If your B+ voltage drops by more than 20vdc then I'd assume that the load on the PT was excessive. A true KT66 may also draw significantly more plate current.
Pulling any 'surplus to requirements' preamp tubes (can you live without a channel/ reverb/trem?) will free up 300mA each of heater current.
There is a Russian, so called, KT66 that's really a 6L6 in a KT88 bottle (any decent tube vendor will have details), this has the same heater current draw as a 6L6 and might be the safer option. Whether it sounds like a true KT66 is another matter.
I'd be even more cautious than Mooreamps and exclude anything that pulled your heaters under 6.2VAC and/or pulled down B+ over 20v. You would measure you heater voltage from pin 2 to pin 7 of a 6L6, heeding the usual advice regarding safe practices for working around tube circuitry - read up on these (e.g. power down & unplug the amp from the wall to make/unmake any connections, use voltmeter leads with clips, double check integrity of connections so you don't short against an adjacent tube socket pin, don't power up without a speaker load connected)...a lot of current in that heater winding.Comment
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They are a pin-for-pin swap aside from the increased heater current. Bias is even similar.
Plug them in, and monitor the transformer heat with your hand (check it with the old 6L6's first). If the transformer becomes substantially warmer and uncomfortable to hold for an extended period, then you will need a separate heater transformer. You CAN try them to see how they sound, and if you even like them, without worrying about transformer damage.Comment
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Run them in the amp for awhile and see if the heater voltage drops much below 6.3v. If it does, then the power transformer needs some help and you should add a seperate filament transformer.
GregComment
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This is very helpful! Thank you all. Power transformer reads "IMAG 7002 / 680VAC / Mexico." I assume this is 6.8 VAC. I can yank #3 and #4 preamp tubes - at least temporarily - as I absolutely never use tremolo and I can live without reverb if in in the right room. Too bad one tube wasn't a dedicated tremolo tube as I could live without that permanently. I think I'll give it a go to try out the sound and see where things stand.
So, is it a safe assumption that if you can hold your hand on the transformer w/o pain, there is no need to worry (as sort of a blanket generalization in this specific situation)?
Can't thank you guys enough!Comment
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Yes there is.....Originally posted by EFK View Post
First, you pull all the tubes, and then measure and record the output voltage ; unloaded. Then, you install all the tubes, and then re-measure and record the output voltage. From these two values, plus knowing the value of the load, one can approximate what the rated max load current would be for that device.
-gComment
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"Power transformer reads "IMAG 7002 / 680VAC / Mexico." I assume this is 6.8 VAC - No, 680VAC most likely refers to B+ secondary winding voltage.
Pulling 2 preamp tubes will pretty well redress the balance.
"So, is it a safe assumption that if you can hold your hand on the transformer w/o pain, there is no need to worry (as sort of a blanket generalization in this specific situation)?" No, it is NOT a safe assumption. As Greg says, measuring voltage drop at the secondaries (more than 20v B+ drop, heaters running at less than 6.2VAC) is a more tangible way to establish safety margins. Some PTs run too hot to touch for years with no ill effects, others run cooler and one day they just crap out. In either case after a set's worth of playing I'd expect your PT to be uncomfortably hot to the touch, even with the stock tubes.
If you have doubts there are still the Russian KT66 (rebottled 6L6s), same current draw as 6L6s.Comment
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"If you have doubts there are still the Russian KT66 (rebottled 6L6s), same current draw as 6L6s."
True, but there are much better 6L6s available! Seems silly to pay for a differently-shaped bottle only!
This has been very sobering and helpful. I think I'll pick up a pair of 66s and try them out, and if I like the sound I suspect I had better buy some books and get to reading. I have a very healthy respect for electricity. Thank you all very much! I hope this thread pops up in the search engines - there is a lot of talk about swapping 6L6s for KT66s, all with the caveat, '...make sure your amp can handle them...' but with no specific details on exactly what to do to ensure no long-term damage. This ought to redress that.Comment
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For the record, there are a lot of underrated transformers in guitar amps, particularly for the way we used them nowadays. Transformers that are consistently run too hot (i.e. too hot to touch after use), will eventually break down and short. It is only a matter of time until the Formvar insulation on the magnet wire breaks down totally. Newer magnet wire that uses Polysol and other similar poly-based lacquers, have a better chance at survival.Comment
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That would be the Sovtek KT-66. It is the same guts as their 6L6WXT in an ST-shaped envelope.Originally posted by MWJB View Post
Compare:
http://thetubestore.com/sovtekkt66.html
http://thetubestore.com/sovtek6l6wxt.html
The KT-66 version is almost twice as much as the 6L6WXT. Too much to pay for a fancier envelope, IMHO.
chuckComment
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Back from the dead! Well I finally got a pair of Groove Tubes KT66-C to try out in this amp. I pulled V3 and V4 (reverb and tremolo drivers) to ensure ample heater current and WOW. I could never go back to 6L6. The sound is spectacular. OR I should say, was.....
I put about 10-15 hours through it with no problems at all. Not so much as a pop, click, ring, nothing. Did I mention how beautiful the sound was? And then, here I am playing along happily, volume at @ 1 o-clock (which is where I generally keep it - I have no neighbors) when suddenly it sounds like the volume gets rolled back to half; thin and weak. ???? I hit a few notes and fiddle with the volume knobs to make sure I didn't bump anything, and then put it on standby. Tubes look fine - no red plate, heater filaments are glowing nicely, faint blue glow in tubes. Alright, turn full power back on and the sound is back. So I play about 5 more minutes give or take, and it happens again. Sounds like half volume. This time it doesn't go away.
Yanked the tubes, they *look* fine although there are a few areas of very, very faint light brown tinge on the otherwise gray plates. Hard to see unless I shine a drop light on them. I popped in a few different sets of my old 6L6GC and all sound just fine (well, they're no KT66 but they sound as they ought). No problems at all.
What is going on here? I have to add - and I'm not making this up - in 25 years of playing tube amps, I have never blown a power tube. IS this what a dying power tube does? Sorry to sound stupid. What would kill these tubes at only 10-15 hours? I keep a pretty close eye on things and never saw any red plating or sign of overheating. Transformer never got too hot to put my hand on it and as I said the amp with the old 6L6GC works just fine.
This is a cathode bias amp (Gibson GA42RVT) and I would imagine 6L6GC and KT66 are supposedly close enough in bias that it ought to self adjust. At least, as I've been led to believe, the bias ought to be 'close enough' that while it may not be optimal, it shouldn't kill a tube/tubes in 10 hours!
Sorry to be so wordy. I'm in an extremely rural area and there aren't any local techs.
Schematic of the amp is here:
http://music-electronics-forum.com/s...ead.php?t=8911
Is there anything visible in this schematic which would be contrary to KT66 usage?
This guy has the 1X12 version of the same amp, offers some electrical info on his which I can't offer:
http://music-electronics-forum.com/s...ead.php?t=8801
Hope someone can offer some information - thanks!Comment
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"This is a cathode bias amp (Gibson GA42RVT) and I would imagine 6L6GC and KT66 are supposedly close enough in bias that it ought to self adjust. At least, as I've been led to believe, the bias ought to be 'close enough' that while it may not be optimal, it shouldn't kill a tube/tubes in 10 hours!" - You've based this purely on assumptions, which can easily come back and bite you on the ass. In the same circuit a true KT66 wil usually draw more plate curent than a 6L6. Did you check what current you actually had?Comment
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I installed a pair of those Sovtek "KT66's" in a Boogie MKII for someone (against my warnings) and they lasted about 5 hours.Although I rebiased the amp (installed a bias pot for just that reason) those tubes cant take the voltage,forget the exact voltage,but it was in the 450 zone at 30ma's,I set the bias so as to be safe for a plain 6L6 as opposed to the higher rated 6L6GC and they still fried in about 5 hours.I suspect the GT's are just re-labeled Russian or Chinese crap.As a comparison,I built an amp for my son around a NOS pair of GEC KT66's and that amp has been run 3-5 nights a week for almost 3 years with the tubes running 62ma's at about 375v's,which is HOT.I recently gave the amp a once over,and the tubes are still fine,sound great and test the same on my testers as the day I installed them.I dont know what they are putting in these current production KT's,but it aint a real KT66.Comment
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