As I always do when someone alerts me that they have a Hilgen amp, I asked James to take some pictures. He responded immediately with some clear pictures, including some of the tube array. That was courageous of him, because James -- at that moment -- knew nothing about amp innards.
It was obvious that James's Challenger had a completely original set of tubes, including two Amperex Bugle Boys and a Mullard. Because my Challenger also appeared to have its original tubes, I asked James to try to read off the identification numbers on the tubes. I was assuming that our respective tube arrays would be identical. The power tubes and rectifier matched: two 7591A power tubes and a 5URGB rectifier. The pre-amp tubes did not:
Mine: v1: 6C4 v.2: 12AX7 v.3 12AX7 v.4 12AU7 v. 5 12AU7
James: v1: 6C4 v.2: 12AU7 v.3 12AX7 v.4 12AX7 v. 5 12AU7
"Only one can be correct," I thought. Therefore, I changed my tube sequence to match James's array. The reverb worked for the first time since I'd owned the amp. On the other hand, the tremolo did not work -- again for the first time ever. A lightbulb went off in my head: "V.2 must be the culprit in James's sequence. Let's try putting a 12AX7 back in there." I grabbed a brand new Groove Tubes 12AX7 and plugged it into V.2. Now both the reverb and the tremolo worked! However, the amp sounded like it had too much gain; lots of people would prefer that more conventional sound, but it no longer sounded like a Hilgen. Because I had never seen a Hilgen circuit that had more than two 12AX7's, I hypothesized that v.3 should be the proper location for the other 12AU7. The theory proved correct: the reverb and tremolo were both operating, and the amp sounded like a Hilgen again. (Back went the original 12AX7. No offense, Groove Tubes. This is electro-archaeology.)
However, because this tube array was different from either of the original arrays, I was still a little unsure about contradicting what seemed to be history. Blasphemy usually gets people into trouble.
Luckily, James was at his computer much of yesterday. I immediately sent him an e-mail in which I asked him if he was willing to try putting a 12AX7 into v.2 and to see if his tremolo came on. I told him that, because I could not exclude the possibility that his amp might blow up, he should feel free to decline. But I then self-servingly suggested that the odds of that happening were pretty low, given that the pin-outs for the two tubes were the same and the 12AX7 simply had more gain.
James took a leap of faith and switched his v.2 and v.3. Now both his reverb and tremolo worked!
Therefore, the proper tube sequence for the Model 2024 Challenger is: v.1: 6C4 v.2: 12AX7 v.3: 12AU7 v.4: 12AX7 v.5: 12AU7 v.6: 7591A v.7: 7591A v.8: 5U4GB
In sum, as a result of working together for several hours, James and I (a) fixed both our amps, (b) proved what was the proper tube array for the Challenger, and (c) discovered that even if an amp has its original tubes, that doesn't preclude the seemingly unlikely possibility that someone may have taken them out and put them back incorrectly. Neither of our amps' apparently original tube arrays had been correct. If James had not taken the time to fill out a response form and investigate this mystery as thoroughly as he did, none of this would have happened. Thank you, James!
Now James's $20 Challenger is an even better deal. But I'm totally delighted about that.