Following 2 Power Tube Amp Design

[Andy]

Thats really one of the beautiful things about this amp. You can swap the power tubes without re-biasing. You do have to be mindful of the impedance, but the switch in the design makes that very easy too.

I like a big bottle in mine, a KT66 or 6550. But I also like them with a vintage 6V6, its pretty nice to be able to swap them with a flip of a switch.

The book describes the various wattages, but I think a single 6V6 is around 4W, a 6L6 is 8-10W. Pretty easy to get the blend of tone and watts you are looking for.

[Rifflicks]

So the 6v6 can still be used? I must have misunderstood because I thought the bigger tubes were the only options….Is the Impedence controlled by switches??? If it is I’m convinced….so this single tube version is a 5F2A with a choice of 3 different steroids???

[Andy]

Yup, you got it. I’ve run 6V6, 6L6, 6550, KT66, EL34, KT77 and KT88’s in it. Its super versatile that way. The chassis is a 5F2A that allows the tube swap, plus the cathode bypass options for different voicing. Its pretty brilliant.

There is a small chart in the book that describes how to wire the impedance switch. I have a small printout that can be used to attach to the cabinet to remind you what direction works for what tubes here:

https://www.tubeampnetwork.com/images/impedance_chart.png

[Rifflicks]

O K great….one more question…..it appears from the forum threads I have read on the newer 2 Stroke builds that everyone is using a 12 inch speaker…is the 10 and 8 inch configuration not advisable? surely the larger tubes should be able to drive them better.

[Andy]

Its purely preference. I use the 10 x 8 combination in all my builds. I like the richness that the 2 speakers imparts. Rich and a bit more complex, but not so much that it overshadows the amp. LOVE that part of the design.

There are a lot of reasons one might just use a 12. You can get a more standard cabinet from Mojo or something that way. But I think you will find many on the forum here extolling the virtues of the 10 x 8.

[Robin]

I tested several speaker combinations and agree with Andy and DH, the 8 & 10 combination is outstanding. A 12 cuts through in a band situation but so does the 8/10 combination. I also tested ceramic and Alnico speakers. My favorite is the ceramics (the Jensens listed in the book) with the Two Stroke and a Tele. Recently, I’ve started using the new Jensen Jet Falcon 10, it’s great. I received the 12″ version today and will test it soon.

The cathode bias design does a lot more than just allow for self biasing.
The single-ended, cathode biased circuit (although not as efficient as a
push/pull design like your 5F2a amp) has a singing, compressed tone feel that is fantastic.

[Rifflicks]

The 8&10 speaker configuration was one of the features of the original design that reeled me in enough to want to build it…I have an old Gibson GA-30 with the 8&10 and it sounds great. I play mostly blues so I’m planning on a set of Webers..an 8A125(lightly doped)and a 10A125-0 …I’m theorizing that the “tightness” of the 8 will rope in, and fatten a little of the early breakup of the 10.

[Andy]

That should be a great combo. I just showed one at NAMM with a Weber 10F150 and a Jensen C8R. The A125’s are what I use in our commercial amps and I love them. I like the ceramic with the Two Stroke because the 8 adds some of the highs that is missing. Y

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