Something new is coming

[Grainger49]

Alder is a hardwood, unlike pine, fir, spruce, etc.

PJ, I just meant that there was a lot of Alder in the Pacific NW.  Like pines in the South.  Alder is the best wood for smoking salmon.

 

[Todd R]

What would you recommend as the minimum impedance for a set of headphones to use with the kit?

The output impedance of the 6080 cathode follower is around 100 ohms. So in theory the most ideal load will be 300 and 600 ohm headphones. You could push this down to 100 ohms and just expect somewhat increased distortion - in fact we tried it with 32 ohm phones and it sounded pretty darn good, but that is not the intended usage. The current wisdom seems to be that a S.E.X. amp with the 120 ohm resistors removed from the jack works best with low ( like 32 ohm)  impedance headphones. The curves of some 6080 relatives might give a little lower output impedance in the given circuit, but I have not pursued that yet. Remember, this kit is only about 10 days old.

Doc,
I have a Wheatfield HA-2 which also uses a 6080 tube. They suggest using a 6528A for low impedance phones, would that work with this kit?

BTW, how does it compare to the SEX amp when driving high impedance phones like Sennheisers?

 

[Doc B]

Hi Todd,

I mistakenly said that the amp might work with a 6528A, but the 6528A heater draws twice as much current as the 6080, so it is not a possibility. So far I like Crack better with high impedance cans, and S.E.X. better with low impedance cans.

 

[Dr. Toobz]

Wouldn't a 5998 tube work here? I think the current draw is a little less than the 6AS7/6080 and the plate impedance is supposedly lower, too. I've seen it recommended in 6AS7 OTL circuits where a little lower Z-out is needed for 32-ohm phones.

 

[Todd R]

Hi Todd,

I mistakenly said that the amp might work with a 6528A, but the 6528A heater draws twice as much current as the 6080, so it is not a possibility. So far I like Crack better with high impedance cans, and S.E.X. better with low impedance cans.

OK, good to know.
Guess I'll go buy some, I mean a Crack

 

[Todd R]

Hey Doc,
Just paid for a Crack.
Couldn't see where I could order the official Bottlehead badge, do you still offer them?

 

[Doc B]

Yes, we do have badges. My bad, I will get an order button up tomorrow.

 

[corndog71]

Hey Doc,
Have you thought of producing a nice heavy foil type sticker as a cheaper alternative to the badges?

 

[vanofmonks]

Is there enough overhead in the heater circuit to use a 7n7 instead of the 12au7?  Curious because they are cheap and I have some nice teflon sockets.

 

[Paully]

I would be curious to know that too, I have collected a bit of a stash of those.

 

[Doc B]

I had my transformer current capability figures confused and you should be able to use a few more different tubes than I originally said. I'm not going to figure every tube out that you guys ask for, but here's the general scoop:

The power trans can deliver 3.8A AC to the 6.3V heaters. Right now the 6080 draws 2.5A and the 12AU7 draws 0.3A. So right now the total draw is 2.8A. One could change to a 6SN7 (0.6A heater) by punching out the socket mounting hole and wiring in an octal socket. Or just do it the easy way and use a 6CG7 in the existing socket with the heater pins rewired from 9 and 4/5 tied to 4 and 5. 12BH7 should be pretty easy to do as well.

The 12AU7 is running at 75V and about 3.8mA - the FPIII operating point. You could also rewire the heater to 4 and 5 and use a 6922 without any other changes - essentially the Seduction operating point. Gain will be higher, which I don't necessarily consider an improvement.

One could also bump up the current through the 6080. However we are designing a C4S load for the 6080 that will work great at the present operating point, but will be running very hot if we increase it. So we will stick with the current point we have chosen (150V plate, 100V cathode, 30mA) and let builders play with hotter points if they want to.

 

[jrebman]

Is the input stage directly coupled to the output, and if not, what's the value of the coupling caps?

Thanks,

Jim

 

[Paul Birkeland]

The two stages are directly coupled, there are just 100uf caps at the output.

 

[Todd R]

Yes, we do have badges. My bad, I will get an order button up tomorrow.

Don't see the link yet. Is it under parts?

 

[jrebman]

PB,

Thanks.  No interstage coupling caps -- have to love that too.  This is sounding like a really sweet design... congratulations.

-- Jim

 

[VoltSecond]

Use the C4S that uses the 2N2907. Use a POT for the current set resistor. Put the OTL cap to the wiper.

At one end you'll get a standard CCS fed cathode follower.
In the middle you'll get a hybrid push-pull amp with a reasonably low output Z.
At the far end, the solid state handles all the drive current and the cathode follower pretty much sees a high impedance.

The pot can be tweaked for tone.