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Isaacc7 08-14-2021 07:37 PM

Firebottle Kt88 tube rolling
Because the KT88 Firebottle is a tube roller's dream I think it deserves its own thread. Too much info is buried in the monster thread, I hope this will be a place to share experiences with this particular amp.

My amp came with a Sovtek 5ar4, PSVane Kt88 Treasure, and a GE 6sn7. Out of the box my system (Topping D90, Schitt Freya+ passive mode, and Phase Technology PC 3.5 speakers) sounded very clear and open if just a bit hi-fi-ish. I have gone through a ton of tubes since. Here we go...

Yikes! This got really long. I'm going to split up the input and output tubes across a couple of subsequent posts.

Isaacc7 08-14-2021 07:39 PM

Input tubes
These impressions seem to mostly hold up regardless of which output tubes I use.

GE 6sl7
Sylvania 7f7 "chrome dome"

I found both of these to shift the emphasis up in frequency. While the 7f7 had remarkable bass control and extension ultimately I didn't enjoy the sound signature. It just gated on me and I found it fatiguing.

Sylvania 7af7. This is an odd tube not officially supported but I have heard of it substituting for 6sn7 tubes in other amps. It's gain is between the 6sn7 and the 6bx7. It's a round plate Sylvania tube so what the hell, I gave it a shot. I don't know if I just have a bad tube or if the 7af7 just isn't compatible but it did not sound good. Closed in, collapsed sound stage, grainy. Just bad.

My favorite input tubes have been various 7n7 tubes. As far as I can tell they are all Sylvania regardless of the label. Old Sylvania 6sn7 tubes are all well regarded and so I don't think you can go wrong. My favorite one so far is a tall bottle with a short silver getter on the top. From the 40s, it is open, balanced, and with impressive bass.

I have a slew of other conventional Sylvania 6sn7 and 6sl7 of various vintages that I plan on trying too. I'm actually hoping my 7n7 stash ends up being just as good. Then I can sell the regular tubes lol.

Isaacc7 08-14-2021 08:28 PM

Output tubes
As I mentioned earlier, the original KT88 tubes were impressive but I wanted a little more warmth and better bottom end. I have always had a soft spot for the 6l6 family of tubes. I told a friend of mine about wanting to try some tubes. He had a few sets of tubes in the 6l6 family that didn't suit his taste in his Elekit amp so he sent me his rejects lol. I ordered an EH 5u4g to get ready for them.

First up was a pair of 807 tubes with adapters. I had always wanted to hear 807 tubes so I was excited. They were unlabeled. My friend thought they were supposed to be Brimar but speculated that they were actually Russian tubes made in the 80s. They were dreadful. Thin sounding, screechy, and generally unpleasant. I told my friend and he said yeah, they were pretty much the same in his amp. I threw them away they were so bad.

The other set were much better. New Russian Golden Lion KT66. These had more of the warmth and "tone" that I was looking for. Now we were talking! Everything had more weight and sounded more natural to me. My opinion was further solidified once I replaced the GE with a 7n7 (see fist post).

I was happy with the sound but couldn't resist the siren call of trying other tubes in case they sounded better....

New Russian TungSol 7581a tubes slightly improved on the things that were important to me. a little better bass, little better guitar tone. Plus I felt comfortable using the 5ar4 with them for a bit more power.

I also ordered some new TungSol 6v6 tubes. Had heard good things about them. I wasn't too impressed with them. Pretty meh to me so I gave them to a guitarist friend of mine.

My friend felt bad about offloading his reject tubes on me and so ordered some 807 tubes from TubeDepot for me. Turns out they screwed up the order. He ordered the tubes he liked so much, Sylvania 5933. What they sent looked like proper 807 tubes (tall and curvy instead of short, straight, and squat like 5933). I had no idea they were the wrong tubes. They didn't have a manufacturer but were labeled CV 124. Ooh British! Were they Mullards? The 5u4g went back in and I fired them up. They certainly sounded like what I had assumed Mullards would sound like, big, warm, lush... I was in love!

Digging around I figured out they were Ferranti tubes. Also a British supplier, but it looks like these were actually Russian tubes. I think these are a screaming deal. You can get them for $15-$25 a piece. Well worth it if you like that kind of sound. Plus, they look amazing:)

Time goes on and my friend asks me if I'd be interested in some more of his "rejects" lol. His amp can take various tubes and I was benefitting from his experiments. This time it was the 6ar6. I had always been intrigued by these so was happy to try them out. Another winner! They resemble the 807s to quite a degree which shouldn't be a surprise. While they don't look nearly as good as the 807s they have better bass so they because my new standard. Keep in mind that even those these are octal tubes they are not plug and play! You need adapters in order to use them in the amp.

By this time I had been scouring ebay and tube sellers for more of those 807s, new input tubes, and now more 6ar6 tubes. I saw a pair of black glass Ken Rad 6v6gt tubes and ordered them on a whim. These sounded quite a bit better than the TungSol versions to me. Better bass, better depth, batter timbre. Still not as compelling as the 807 or 6ar6 though.

Once again my friend offered me another tube to try. This time they really were Mullards. I had never heard of the el38 before but wow, what a tube! First of all they are just sexy looking tubes. Still curvy like the 807 but taller. And they have the most prominent blue glow of any tube I have ever seen! But the real story is the sound!

In my amp, with the tall 7n7 input tube, and my speakers, the el38 gives me the best, well rounded sound of anything aI have heard so far. It has the clarity of the kt88 but better defined, possibly deeper bass than the 807/6ar6. The better details make the soundstage deeper. It's an impressive tube. It impressed me enough that I promptly bought out the rest off the Mullard branded supply of a UK tube supplier. I'm trying not to go into full on hoarder mode with this tube. I have never been this obsessed with a tube before, and that's saying something...

I prefer the 6ar6 when I want to rock out and the el38 for everything else. Man, I love this amp so much! So much fun!

Isaacc7 08-14-2021 08:36 PM

Tubes I'll be trying soon
I was happy to hear the Ken Rad 6v6 tubes sounded so good. I have a bunch of 7c5 Sylvania tubes I've been sitting on for over a decade. Back then I had DIY aspirations so I bought a ton of useful audio tubes with different bases and voltages. The 7c5 is a loctal 6v6 so it'll be fun to try them out. Just got the adapters in so stay tuned in...

I also had another tube put in my radar. The 2e22 is a directly heated pentode that appears to have the same pin out as the 807. Looking at the specs it certainly seems like it should be able to be run in this amp. I don't know if it being a directly heated cathode would screw things up though, I can't seem to find much info about popping it into a 6l6 circuit. Anyone out there have any ideas on the potential danger if any of doing that?

I also found some el39 tubes in my stash from years ago. I'm trying to figure out if they are suitable for this amp and if any of my current adapters could be used with them. Anyone ever hear of using these for audio?

Isaacc7 08-15-2021 11:10 PM

Sylvania 6sl7wgt
My previous experiment with a 6sl7 was promising but ultimately didn't satisfy. I had used a 7f7, a Sylvania 6sl7 with a loctal base. This particular one would be considered a "chrome dome" because the getter flash extended down most of the length of the glass. As a whole they have a great reputation but I found it fatiguing despite the fun of the bass extension and control.

While organizing my tube stash I came across a Sylvania 6sl7wgt with a brown base. The box said it tested at 70%. Popped it in and have been listening for most of the day. I am still using the 6ar6 outputs and the RCA 5u4g. My initial impression is one of smoothness. I don't hear the fatiguing sound of the previous Sylvania 6sl7 (7f7) but I got the bass back! Woo! This is my favorite tube so far, just edging out the tall 7n7 in this position.

Earlier in the day I had found a 50s vintage Sylvania 6sn7. It is a two hole cousin of the so-called three hole "bad boy" 6sn7. At least one of the online tube gurus (Brent Jesse? Andy at Vintage Tube Services?) claims they are sonically the same. In any case I wasn't impressed. Everything seemed subdued, I actually found myself wondering if I need a more powerful amp. There was also a dullness to the system that I wasn't used to. The 6sl7wgt replied this. Wonder if my positive views of that tube are because of my negative views of this one lol.

Isaacc7 08-16-2021 10:32 PM

There's a twist with the 807 tubes. As I went through my tube stash trying to get it in order I found a pair of 5933 tubes. The only ones I would have had were the ones that my friend sent me. That also means that the 807 tubes I like so much are the ones I initially hated so much I decided to throw them out!

What changed? The only thing I can think of is that I got rid of the GE 6sn7 tube. The 807 tubes sound completely different than that initial session. It pays to experiment with both the input and output tubes. If you change one it's a good idea to see what changing the other can do for you.

Isaacc7 08-17-2021 11:12 PM

Turns out the 2e22 has a max voltage of 250 volts on its screen grid. That is a ways below the 290 from the Firebottle. Oh well. It would have been fun to run a directly heated pentode. Though I'm not sure if a heater circuit not designed for directly heated tubes would cause audible problems.

Isaacc7 08-23-2021 11:58 PM

I've been loving the Sylvania brown based 6sl7wgt with the 6ar6 tubes. An amazing combo. Thinking about what that 6sl7 had brought to the other tubes I figured it wouldn't work with the el38 tubes.

I was right. That combo was very clear, almost ethereal sounding but it lacked low end and felt flighty.

Finally got the 7c5 adapters and put in some NOS Sylvania clear 7c5 tubes with the Sylvania 6sl7wgt and wow. No, it didn't exhibit the hifi pyrotechnics of the 7n7/el38 combo or the weighty and warm presentation of the 7n7/6ar6 but I think the 6v6 tubes brought the most balanced sound I've heard from this amp. The entire spectrum high to low felt well represented. It is just ever so slightly warm but I think has that magic tube-type of neutrality.

If you can live with the 4 watts or so they give you should definitely try some old 6v6 tubes. I haven't done any direct comparisons but these Sylvania sound very much like the black glass Ken Rad 6v6s I used before. Even the really sought after 6v6 tubes aren't all that expensive, 7c5 even less. Check them out!

Isaacc7 08-27-2021 02:18 PM

I know I've said it before but I really do think I'm done rolling tubes for a while. I've gone back to the el38 tubes, tall 7n7 input but now I'm using a Japanese 5ar4 instead of the 5u4g I had been using before. Oh, and I'm using this with my active preamp now.

I think this combo would be one of my go-to configurations if I was using a passive preamp. I also love the brown base Sylvania 6sl7wgt with 807, 6ar6 (with adapters), or sylvania 6v6, all with a 5u4g rectifier. The 807 and 6ar6 tend to be too warm and bassy when I use the amp with my 6sn7 based preamp.

Anyway, I do hope others will add their experiences with tube rolling!

Bombadil 08-28-2021 01:49 AM

The uber long thread of >6500 posts contains hundreds of posts from people telling of their tube rolling experiences.

On my KT88 HO extra hyped up Inspire, which produces 15-17 wpc instead of the standard 12, I like the big tubes best. KT88, KT120 and KT150. With some of the smaller, lower output tubes, they sounded strained and even harsh. 6F6, 6V6, and 6P3S-e all sounded harsh. To the point of where it was a pain to listen to them.

The KT88, KT120 & KT150 have the most neutral presentation, with a natural smooth sound but not overly warm. Good treble without being harsh. Good extended bass. All three sounded very similar. After a lot of comparisons, I decided I liked the KT120 best. I think.

Likewise for using a 6SL7. Extremely detailed and forward. Also tried a 6BX7 but didn't like it.

I have a number of 6SN7 which I like very much. The best of them create a wide, open soundstage with pleasant midranges and fairly strong, defined bass.

For a warmer, more bluesy sound, kinda a warm nightclub-like presentation, I like 1960s Sylvania fat bottle 6CA7s. Smaller soundstage, but warm & pleasant. Tried the new production EH 6CA7, they sounded overly warm and fuzzy. Less high frequency detail. Sounded like I set tone controls to bass +2 and Treble to -2.

For a punchier, edgier sound, which was still not harsh, I liked the new Gold Lion KT66. Electric guitars sounded a bit rawer. Horns had a little more bite, but just a shade. These can sound great on rock music.

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