Firebottle Kt88 tube rolling

[Isaacc7]

Originally Posted by Huduguru

@Isaacc7 : I am beginning to roll tubes in a June 2021-production Firebottle amp w/volume control, and am curious about your experience with 807/5933 power tubes and associated rectifiers. How does the 807 compare to say a kt88? I like the availability of the 807 and the quirkiness of it (adapter and top cap), but am curious about sound. I love more solid low end of the bigger tubes, but want to check out the 807 and any compatible retifier/input tube combo with it.

Sorry about the late reply... The 807 is a great tube! I enjoy a Russian pair I have. They are labeled Firenti but are Russian. I also have a set of 5933 that I have not tried yet but will one of these days.

The 807 is basically a 6l6 with a top cap. Using an adapter essentially gives you access to all the different brands 6l6/6l6gb tubes but cheaper. Another tube you might want to think about is the 6bg6. It too started out as a 6l6 with a top cap though with a different base. What's cool about the 6bg6 is that over time manufacturers ended up putting their newer versions of 6l6 in the 6bg6. GE ended up with 6l6gc guts in their last 6bg6ga. The Philips 6bg6ga actually had the same guts as a Sylvania 7027a! I've bought them for $15 NOS, not going to find 7027a at those prices...

So my suggestion is if you want to play with adapters, get a 6bg6. You'll be able to get the same tubes as the 807 plus some better ones too.

[Isaacc7]

Let me tell you about my new favorite combo... The GE 6bg6ga with a Raytheon 6f8g is magic! It is a big, open sound but the mids to lows are out of this world. Rock solid bass. The openness gives a great sense of space. There is also a kind of slight, I dunno, coppery? sound to it. A warmish, slightly metallic sound that makes string instruments really come alive. It's a lively, dynamic sound that is super big and detailed. I will say that I almost gave up on this tube. I must have gone through 5 input tubes looking for something that sounded good. Most sounded dry. The Tungsol 6c8g gave a super big and airy sound. So much so it was almost dizzying. The Raytheon clicked immediately, it is a magic combo in my system.

I think both of these tubes have octal equivalents. The Raytheon 6f8g is probably the same as the Raytheon ladder plate 6sn7. The GE 6bg6ga I have looks like it's the same as the GE 6l6gb. There is another GE 6bg6ga that looks like it has the guts of the GE 6l6gc. but I haven't tried those yet. I haven't checked but it wouldn't surprise me if the octal equivalents cost more. I'm a cheapskate so I don't mind using adapters.

[Isaacc7]

The cv1102 is a wonderful tube by MOV. Later on it was released as the BL63 with the Marconi label. It is a rich, darker sounding tube. It has the same pin out as a 6f8g, lower amplification and higher heater current. I've been told that the sound resembles the Mullard ecc32. I'm never going to own one of those so I'll take their word for it. It has the sound signature that matches the descriptions I've heard of the ecc32 so I believe it.

Every time I use this tube. I'm blown away by the richness of the sound. It's a very chocolate cake kind of presentation. It lacks the air of some of the other tubes but makes up for it with its vivid timbres.

Lots of amps would choke on this tube because oif the heater requirements but the firebottle kt88 has no problems. The only thing to watch out for is the low gain. My regular preamp has developed a fault so I'm using a passive one right now. When I use this tube with a regular output tube like a 6l6 I just don't have enough gain. My preamp was right at 0db attenuation, full line signal and it wasn't quite enough. It sounds amazing when I pair it with a high gain tube like the el38.

If you're looking for the prototypical British sound but don't want to pay for an ecc32 or British 6sn7 the vt1102 or bl63 is highly recommended. They aren't the easiest things to find but with some patience you can find them at reasonable-ish prices.

[Isaacc7]

Finally got around to trying the Sylvania 5933. I can characterize these as soft and warm. I've tried 5 different input tubes and while all of them sounded different in terms of bass, soundstage, highs, etc. But in all cases there is a lack of detail and a pleasant warmth. I could also not get any better than fat bass. After the tight bass of the 6bg6 it's tough to live with flabby bass.

These particular 5933 tubes are from Vintage Tube Services so I know they are in great shape. They did come in the old green Sylvania boxes though. I do have some gold brand 5933 which are newer but I'm not sure when I'll get around to trying them.

[Isaacc7]

I have given up on the 6aq5. No matter which ones I try and the input tube used there is a distortion in the upper midrange that I can't get rid of. I was using them with a 5y3 rectifier but I think I was still feeding them too much voltage on the plates and the screen. Oh well. I have an amp coming later in the year that will be able to use them.

I saw an older post that mentioned using 6k6 tubes with a 5w4 rectifier. My friend gave me a pair of GE 6k6gt tubes. I popped them in with a Tungsol 5w4gt rectifier and a tall 7n7 in the input. Sounds quite nice. The initial impression might just be from getting rid oif that distortion lol. I'll listen for a while and try some other input tubes too.