Mainline vs. others

[mete]

If one wire is moved in the Crack, the 6CG7 can be used in place of the 12AU7 (the 6CG7 is a 9 pin 6SN7).

Is it the heater wiring ? No change necessary for biasing ?

[mete]

The HA-2 used a 6SN7 as the input tube.  Personally, I much prefer the sound of the 6SN7 in this position to the 12AU7 that the Crack uses.

Pete Millett made fewer than 40 HA-2s before pulling the plug on Wheatfield Audio back in 2000 or so.  He's admitted on other forums that he was “the world's worst business guy.”  I believe he sold the rights to the HA-2 to HeadRoom Audio back when it was Tyll Hertsen's company, but AFAIK no more were made.

I bought one of those original HA-2s from Headroom.  I was shopping for a headphone amp and was looking online at the Headroom amps (made in house) when I noticed they also had the HA-2 for sale.  I don't think anyone else ever sold Wheatfield amps.  Given Headroom's generous return policy, I ordered both a HeadRoom Max and an HA-2, thinking I'd return the one I liked less.  I returned the Headroom Max, even though it had way more resolution and power than the HA-2.  The HA-2 just had something magical going on, the kind of component that sucks you in without drawing attention to itself.

As I recall, the HA-2 sold for $799, $899 for one with upgraded tubes.  I got one with upgraded tubes (Mullard CV378 rectifier, GE 6SN7, TungSol 5998--boy, those were the days).  It was kind of ugly, but very well built.  Almost all of the resistors were Rikens.

I gave away my HA-2 to a friend who now lives across the country, so I've never heard a stock HA-2 next to a Crack, but I have made 4 or 5 of my own versions of the HA-2 circuit over the years.  The latest one uses solid state rectification but keeps the choke.  As much as I love glowing tubes, I have to admit the solid state rectifier sounds better (I tried it both ways).  And I use blue LEDs to bias the input tube (instead of the resistor plus bypassed cap array Pete used on the HA-2: he bypassed the electrolytic bypass caps with 1uF Solen caps).

It sounds really, really good.  Not better than my S3X, but different.  A friend brought his modified Crack over to compare it my own modified HA-2 circuit, and he thought it was a draw.  I wasn't so sure.

I think the 6SN7 plays a big part in why I like the HA-2 circuit so much.  Plus, the 6AS7/6080 tube is pushed harder: 2K cathode resistors instead of the 3K used in Crack.  As Paul points out, that makes for one toasty chassis, so I have a hefty heatsink in mine.  I also decided to use a toroidal PT and choke because I just could not get the Hammond iron I'd been using to be quiet.

Great info, thanks Larry.

[mete]

The HA-2 also runs quite a bit more B+ voltage to help increase the value of the cathode bias resistor in the output stage, but the larger Speedball board eliminates the need to do that and keeps the power dissipated inside the chassis reasonable while maximizing available signal current in the output stage for the intended load (rather than burning it off in that resistor).  Looking at the HA-2, each 2K cathode resistor dissipates 18W and that's a huge challenge to deal with inside a chassis. 

Was an active load/a current source also used in the old tube amplifiers (pre-70s or so) or is it a new/more feasible concept because of the solid-state components ?

[Paul Birkeland]

Is it the heater wiring ? No change necessary for biasing ?

Yes, I would expect the plate voltage to be close to the same with the 6CG7 compared to the 12AU7.

Was an active load/a current source also used in the old tube amplifiers (pre-70s or so) or is it a new/more feasible concept because of the solid-state components ?

It's relatively rare to see tube current sources in vintage gear.  It's a lot of complication and heater/cathode insulation limitations can really make this difficult to pull off.  (Atmasphere does use them though)  Back in the day, if you needed a higher impedance plate load, you'd just crank up the voltage.

[mete]

Yes, I would expect the plate voltage to be close to the same with the 6CG7 compared to the 12AU7.

Is it common to wire a Crack like this ? It sounds like a good and easy alternative. If Speedball also moves the operating points up, closer to HA-2, then Crack with Speedball wired for 6CG7, becomes like a HA-2 with solid rectifiers with a simpler and/or better bias scheme.

It's relatively rare to see tube current sources in vintage gear.  It's a lot of complication and heater/cathode insulation limitations can really make this difficult to pull off.  (Atmasphere does use them though)  Back in the day, if you needed a higher impedance plate load, you'd just crank up the voltage.

Sounds like an exemplary story to me, how the good old is combined with the new.

[Paul Birkeland]

Is it common to wire a Crack like this ? It sounds like a good and easy alternative.

It's a pretty uncommon modification as far as I can tell.

If Speedball also moves the operating points up, closer to HA-2, then Crack with Speedball wired for 6CG7, becomes like a HA-2 with solid rectifiers with a simpler and/or better bias scheme.

The HA-2 runs a lot of extra voltage to help increase the value of the cathode bias resistor on the 6080 to get it well above the impedance of the headphones one would be expected to use while still running lots of quiescent current.  This resistor is in parallel with the headphones themselves and signal current in the output stage is shared between these parts, so the higher the value of this resistor, the more current is available for the headphone load.  The larger half of the Speedball presents a very high AC impedance, so nearly all of the available signal current will be delivered from the headphone, and the drastically higher operating voltage isn't required.