Bottlehead Forum

Bottlehead Kits => Crack => Topic started by: Doc B. on March 10, 2010, 10:26:35 AM

Title: loaded on Crack
Post by: Doc B. on March 10, 2010, 10:26:35 AM
Shawn and I have C4S'd the production prototype - woo hoo! We put C4S active loads on the 12AU7 drivers last night - and easy upgrade - and got a really nice improvement in dynamic punch. Today we took on the more challenging job of putting together a test C4S for the 6080 cathode. That required our biggest Avid heat sinks on the MJE 340s and a little adjusting of the 6080 cathode current down to allow for the additional draw of the bias LEDs on the active loads. After making sure it all worked exactly as planned and test firing it on my HD600s we put PBs HD800s on. Wow. I'm going to have to go back and compare it with my big reference headphone amp again. So far our loads are holding up and the power trans is running at the right level of warmth. We've got the 600s back on right now and we're just letting the amp run. Keeping our fingers crossed that we have achieved enough cooling for it to run happily for several hours.
Title: Re: loaded on Crack
Post by: VoltSecond on March 10, 2010, 05:04:57 PM
Doc, a resistor from collector to emitter on the MJE will cut the power loss considerably. At max output signal if the MJE still conducts 10% to 20% of the idle current, it will still work great. A power resistor is sometimes less expensive than a big (or bigger) heatsink.

If you are concerned about tube arcs, put a diode across base-emitter of the MJE to keep it from avalanching.  If you don't care, leave it off and save the nickel.

Note: The LM317 based CCS I've heard sounded very brittle in the highs. I've had slew rate problems with the LM317 at work so I'm blaming the brittleness on the LM317.
Title: Re: loaded on Crack
Post by: Doc B. on March 10, 2010, 05:59:40 PM
Thanks bud! We'll give your suggestion a try. So far the full tilt version is holding together quite nicely. But if we don't experiment we don't learn anything new, eh? At startup the voltages come up very smooth and slow, so at this point I'm not too concerned about arcing. One nice advantage of IDHTs.