Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on October 30, 2014, 08:59:18 am
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I have done a search on this forum and several others to try to get an idea about the KT66 tube. This is one I am considering trying in the D'Mars just to hear what it's like? I have Tung-Sol 5881's already that I'm happy with. Having said that, I would be open to another tube IF it adds even more smoothness and sustain.
I read quite a few posts and threads about the tubes ..........
The more I read, the more I got conflicting reviews such as: the KT66 is muddy sounding vs. it has more clear headroom, the KT66 in push/pull each need their own cathode resistor vs. they can simply be substituted for 6L6/5881 in a cathode biased amp, they have more bass and less midscooped vs. mushy bass .............. etc.......
So my questions are:
1) how would you compare the tone of KT66 to a 5881/6L6?
2) using a shared cathode resistor of 270R/10w, would I have to change that with individual cathode resistors &
of what value if I needed individual cathode resistors?
3) does the KT66 tend to have a smoother tone and more sustain at the same volume as 5881's?
4) Is there a brand that you'd recommend considering?
5) Anyone have a schematic of a cathode biased KT66 amp they can share?
Data sheet here: http://www.drtube.com/datasheets/kt66-mov1977.pdf (http://www.drtube.com/datasheets/kt66-mov1977.pdf)
IF I can just plug in and play without rebiasing ............ and it has more smoothness and sustain, I"d probably be interested in trying this.
With respect, Tubenit
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Ciao Tubenit
I give a search for KT66 cathode biased amp and I find only one guitar amp, a Weber kit, other are HiFi Amp some of which are UL
The Quad II has a very unusual configuration with windings dedicated to the catodes
The two schematics that are in yellow remember in some way the Staldel 25L15 that remember the Williamson configuration
(https://taweber.powweb.com/store/5f2h_schem.jpg)
M.Huss draw this, but it is fixed bias
(http://mhuss.com/SmallBox/1987.gif)
attached some schematic
Franco
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2) using a shared cathode resistor of 270R/10w, would I have to change that with individual cathode resistors & of what value if I needed individual cathode resistors?
I don't believe you 'need' individual resistors. But anytime you replace a shared cathode resistor with two individual resistors, you need to double the value. Wattage could be halved. This is true for preamp tubes also.
Using separate resistors allows you to easily/accurately check bias of each tube.
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KT66 for the D-Mars is intruiging.
Watch out for the filament current at 1,3mA. 5881 is only 0,9mA.
I have a stash of NOS GEC KT66 but have yet to try them.
I had an old PA that used a single cathode bias resistor of 1,8k in parallel with the filaments of V1 and V2.
In the end I rebuilt it for 5881 and individual 470R cathode resistors / 360 V Plates
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no experience with the 66's but did 4 builds/proto-types with the 88's. I like 2 resistors so I can balance them easier. 3 were SE and they all gave a clean, hard to distort sound, one of the SE was a PSE n I believe you'd want 2 R's both G2 and cathode. the 1 PP cathode biased seemed to come up a little short, somewhat thin sound, after lots of tweaks I have it close to a buttery, mellow bluezy tone. anyway, fwiw.
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I am thinking (per data sheet), that at around 385 volts on the KT66 plates that either of these approaches would be reasonable.
BTW, I noticed the Tung-Sol KT66 has similar heater current draw as a 6L6.
Thoughts?
With respect, Tubenit
I am still interested in a tone review also, please.
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I am still interested in a tone review also, please.
Hopefully Ed will chime in on this. He has experimented with KT66's in different amps.
(He mentioned them in your Marshall Major mini amp build post, IIRC?)
Brad :icon_biggrin:
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On this link there is a project about building a 50W (HiFi) KT66 amp
there are also some interesting considerations/details about the use of the KT66 tube that might be worth reading
http://www.r-type.org/articles/art-085.htm (http://www.r-type.org/articles/art-085.htm)
Franco
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I did some research on this recently and found the preferred current production KT66 is the Gold Lion by what appeared to be a large margin. Kt66s can handle 500 volts on their plates which the 5881 of course can not.
Thanks
Mike
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BTW, I noticed the Tung-Sol KT66 has similar heater current draw as a 6L6.
For any new production tube claiming the "KT66" or "5881" or "6L6" name, I'd personally treat them all equally. You may find you have a brand preference, or not.
Like Tony, I have a few GEC KT66's hanging out, but haven't really tried them. I did once play my Super copy with Groove Tube KT66HP's for a while. And with the old Tung Sol 5881's. Bottom line in that amp was clean they all sounded like a 6L6; I never had much opportunity to play that amp loud enough to characterize the distorted sound of each type.
If I were you, I'd get some to be a replacement pair of "6L6's". Data sheet says the tubes have a little more Gm than 6L6's (7mA/v vs. 6L6 @ 6mA/v) but that can vary tube-to-tube and with operating condition. What that means is they'll bias up similarly to 6L6's, and shouldn't be different-enough to warrant a cathode resistor change.
My Guess is that if there is a tonal difference, you'll only hear it under output-stage distortion; and given that, an amp with no feedback loop around the output stage is more likely to allow the differences to be heard (remember, the loop tries to make the output stage "behave"). My overall experience with all tubes is that you can hear distortion differences between true pentodes (6K6, EL84, EL34) and beam power tubes (6V6, 6L6, 5881, KT66), but that swapping for different output tubes makes much less sonic difference than swapping between different input stage preamp tubes.
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HBP & others who responded,
Thanks for the info and perspective! At this point, I am thinking that I will just stay with 5881's & if somewhere down the line I can borrow a pair of KT66's then I might give it ago. Not sure I want to invest in a pair and I already have an extra set of 6L6's.
Thanks, Tubenit
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i built a Plexi style amp recently with KT66s in it and i really like the tone. lots of bottom end and nice clear warm tone overall. i don't think they distort as quickly as 5881s and they do have higher filament current requirements. i think i am using the Gold Lions in this build. I have also used them in an AB763 style amp with excellent results. i have not used them with cathode bias but i was thinking of trying that. hope this is helpful.
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I use KT66's instead of 6L6's in all my amps of that vein , Cesar Diaz loaded SRV's Fenders with them, I'm pretty sure he has KT66's in his Vibroverbs in the El Macombo Video , it was around that era from what I've managed to find out
Love em
:icon_biggrin:
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I really like the Gold Lion KT66's. To my old ears they drop in between a 6L6 and an EL34. Not as glassy as the
6L6, but not quite as much grind as an EL34. In my amps they seem to have a nice big wide sound stage (in Hi-Fi terms) and very sweet overtones. Clapton Beano tones in a JTM 45 with a Les Paul. Beautiful sounding tubes at 400V
and nice sounding cathode biased as well. Yum!
The reissue Tungsols are killer as well. I just built a simple pushpull amp using 5881's at 350V with a 63bassman
normal channel for the front end. Rockabilly heaven!
I have also used the 5881'reissue's in a dual channel tweed / BF single ended cathode biased amp.
Great sounding grind for rock, sweet overtones for blues.
Not as recognizable in the midrange as EL34's and to me they don't sound as "big" as 6L6's or KT66's but still one of my favorite tubes.
Both of the tubes above are reproductions of the originals. Close but not the real deal.
I have had good luck with both but I run them conservatively (within the modern specs from the current manufacture).
I have a set of Blue New Mexico Iron that I have been saving for a build for myself. I was trying to decide which
type of output tube I was going to use. The cool thing is spec wise, I can use KT66 or 5881's with the iron that I bought. A great excuse to buy an extra set of output tubes.
enjoy.
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I really like the Gold Lion KT66's. To my old ears they drop in between a 6L6 and an EL34. Not as glassy as the
6L6, but not quite as much grind as an EL34. In my amps they seem to have a nice big wide sound stage (in Hi-Fi terms) and very sweet overtones. Clapton Beano tones in a JTM 45 with a Les Paul. Beautiful sounding tubes at 400V
and nice sounding cathode biased as well. Yum!
The reissue Tungsols are killer as well. I just built a simple pushpull amp using 5881's at 350V with a 63bassman
normal channel for the front end. Rockabilly heaven!
I have also used the 5881'reissue's in a dual channel tweed / BF single ended cathode biased amp.
Great sounding grind for rock, sweet overtones for blues.
Not as recognizable in the midrange as EL34's and to me they don't sound as "big" as 6L6's or KT66's but still one of my favorite tubes.
Both of the tubes above are reproductions of the originals. Close but not the real deal.
I have had good luck with both but I run them conservatively (within the modern specs from the current manufacture).
I have a set of Blue New Mexico Iron that I have been saving for a build for myself. I was trying to decide which
type of output tube I was going to use. The cool thing is spec wise, I can use KT66 or 5881's with the iron that I bought. A great excuse to buy an extra set of output tubes.
enjoy.
When you talk about the Tungsol tube you are only referring to the 5881 correct? I agree that the tube sounds great. I have quite a few NOS Tungsol 5881s and cant tell the difference between the NOS tubes and the reissues. Thinking about buying a pile of them to stash away. I really liked the Winged C 6L6GCs and kept saying I was going to buy a good supply of them but didn't. Their current prices exceed their value in my opinion since they are no longer being made.
Thanks
Mike