Crack as plate amp

[Dr. Toobz]

I am wanting to try out the Crack as a plate amp (given that I no longer own any high-Z headphones, since I have yet to find one that I could live with long-term), and wanted to run a few ideas past the experts to see if my thinking is sound. So far, the two headphones that have lasted a while in my collection are Grado SR-325 (since 2006) and AKG K701's. (I also seem to like the Sennheiser HD598 better than the 600 or 650, which sound like a blanket is covering them). All of these cans would like to see a low source impedance, so my thinking is that if I wire the Crack as a standard plate amp with a transformer output, it will look the same on my rack but work better "under the hood" where these headphones are concerned.

The first option would be to ditch the Speedball on the 6AS7 in favor of the standard cathode resistors, bypass the existing 3k cathode resistors with at least a 100uF capacitor on each half of the tube, and use a pair of 1.7k to 8 ohm Edcors in the traditional, serial-feed arrangement. This should keep the tube running at similar dissipation parameters as the stock OTL arrangement, correct? The cathode bias isn't changing, and the transformers have a very low DCR, so there shouldn't be a ton of voltage drop at the plate compared to the current arrangement of taking the B+ right there without a plate load. My only concern with this arrangement (aside from the typical serial vs. parafeed preferences) would be regarding the filtering of the power supply, since a serial feed arrangement basically puts the supply in series with the musical signal (correct?). A second idea I had would be to use a plate load resistor and go parafeed, but I don't know the exact formula for calculating a suitable load. The third option would be to re-fashion the Speedball into a plate load by swapping out the two transistors, but again, this exceeds my feeble electrical expertise. In all three cases, the transformers can simply mount on the standoffs I currently have propping up a pair of Triad chokes. Any ideas here, or am I better off selling the Crack and building another SEX amp (or saving for a Mainline?).

[Paul Joppa]

I'd expect the first option to work reasonably well. The PSU is well filtered so hum should not be a problem. You can probably use the existing output capacitor as the cathode bypass ...

The gain will be low, however - you might want to get at least a 16 ohm output winding - I don't now what Edcor offers.

[Dr. Toobz]

I had figured gain might be a problem, given the presence of a 12AU7 followed by a low-gain tube like the 6AS7 and an OPT to add further insult. What about switching the driver tube to a 6922 (circuit left as-is) or 12AT7 (with diode bias to get around .7V at the cathode)?

The lowest primary impedance Edcor makes (without a special order, anyway) appears to be 1.7k, and no secondary above 8 ohms is offered for that primary impedance in the GSE series. There is a 1.7k to 16 ohm model in the GSXE series, but the narrower, 4-hole mounting scheme won't allow for the simple attachment idea I had in mind. There's nothing with as small (or smaller) of a step-down ratio otherwise, which could prove to be a problem, given that the plate impedance would be around 400 ohms, IIRC. I may look into a 600 ohm to 8 ohm transformer instead (which would reflect 2400 ohms to the tube with the Grados, which is still kind of high). No wonder why the 6080/6AS7 ins't a terribly popular SET tube....

At least this may work! If I do try it, I'll report back with any findings.

EDIT: This may work: Hammond 119DA, 600 ohm to 8 ohm, rated 12W (http://www.hammondmfg.com/collins.htm)

[Paul Joppa]

The Hammond 119DA is not airgapped.

Edcor does do custom work for a modest charge. I imagine it would be very easy for them to change the secondary impedance, if you ask for a specific design with the standard primary and airgap.

You can use a 5998 or 7236 to get more gain, as well as a 6922 driver. I have doubts about the viability of very low bias, as would be needed for anything with a mu higher than about 35.

[Paul Birkeland]

I would sell the Crack and go to the SEX amp.  IMO, the 6080 really shines as a cathode follower, but not so much as an actual voltage amplifier. 

When I think about how I'd pull the project together, I kind of end up thinking that I would just put two octal sockets into it and make something resembling the SEX amp, lol. 

If you dumped the 6080 and went with a 6BX7, you might have better luck.  Driving the 6BX7 with a 6922 or 12AT7, then using the SEX amp upgrade iron, you could throw something together. But, with all the time and energy invested, I still wouldn't performing well up against the SEX amp...

-PB

[Dr. Toobz]

Kind of what I am thinking! I actually have my original 2009 SEX amp still, which I can't bear to part with, and recently pulled out to use with the Grados (which works perfectly, of course).

Originally, I was going to re-build the SEX as a gift for my dad (in order to justify buying a Stereomour), but that hasn't happened yet. I may refurbish the Crack and sell that, keep the SEX amp for my headphones and/or speakers, and either sell or give away the Stereomour. (As nice as the latter amp is, I'm not sure it was much of a jump from my nickel-transformer SEX amp through the Klipsch Heresies). Funny how I keep coming back to the SEX amp.....must be one hell of a product. :-)

[Dr. Toobz]

More experimenting leads me to believe that I should maybe look into class-A, solid-state for the Grados, such as a Schiit Asgard, until I can spring for a Mainline! I'm going to stick with the original plan to gift that SEX amp, as it's also becoming clear that the Stereomour is indeed much better; it's the speakers that are the limiting factor. (A test with headphones, despite the presence of a little hiss/hum, showed much better delicacy and extension than either my Crack or SEX, suggesting that I need to get speakers worthy of showing that difference). 

In any case, thanks for answering my questions about re-fashioning the Crack into something more Grado-friendly. I'll leave this idea at "why bother?"

[Paul Birkeland]

You could always thrown an 8 Ohm stereo L-pad on the output of the Steremour for use with the Grados