RUmad
Super Member
So, I don't won't to be too long winded, but I feel this deserves a little backstory. Honestly, I can't remember exactly where I even picked this thing up. Had to've been a swap meet or a tag sale, and it had to've been cheap, because I've had it for many years and hadn't even looked at it. I dug it out of storage about five years ago while looking for something else. It is a Bogen Challenger CH-18. Got it on the bench, inspected it for obvious signs of....well....things that might blow up. Looked pretty dang good, except that some knucklehead had spray painted the entire unit silver, including speaker terminals, inputs, the pilot light jewel etc. So, bringing it to life with the variac, immediately the first cap can committed suicide. Right in front of me, I was scarred for life. Put it back on n storage. Fast forward to about a month ago. I decided I wanted to put together a mono rig/guitar amp. Nothing crazy, not necessarily 'hifi', just something to do really. Replaced the can with two flying lead 20uf caps (incorrect) and the first resistor between the sections because it looked pretty fried as well. Attached to the variac and an Ampeg speaker cab, and she came right to life. No hum, just music. Ok, I'm happy, I figure I'll put it together correctly now. I had to order the right size psu caps, somehow I didn't have the right sizes on hand, two 8's and 4 5's @ 500/450. While I waited for them I replaced all the signal caps and any out of spec resistors. Tidied up, tested tubes, and researched. Now, after the 'resto' I'm playing an iPod into the amp (through a blending circuit I borrowed from je labs) to an 8" Tannoy studio monitor and it sounds fantastic. Here's where the "challenge" comes in. I can't stand that the signal is forced through a four way selector switched tone stack. No way to play it clean. On 'deep bass' it sounds great at low volume, bass is tight and hits hard, but, obviously, all the sparkle is filtered out. On 'crisp' it sounds fantastic, especially at higher volume, but a lot of the bass is filtered out. The other two are complete rubbish. (Mellow and brilliant)
I've been banging my head on this for over a week. Do I figure out a way to bypass? Or a way to send the signal through one of the bad settings but remove the circuit, effectively another way to bypass I suppose
Here's what I would like to do, after MUCH agony trying to decide. Considering I can't actually play the guitar, although I do have a couple, I want to omit the guitar input. Run the audio signal through BOTH preamp tubes (6sf5's) with a two pot tone stack between stages, before the phase splitter (6sl7). Completely gut the selector circuit and add a bass and treble tone stack that I can use as needed. I can read schematics, I can solder and very much do any mechanical work but I'm having trouble designing a circuit, or finding one similar enough to try online.
I'll attach the schematic I found, which is ALMOST identical to the circuitry that was there when I started. I've made some small changes; the blend circuit, biasing the first tube for instrument, and lowering b+ and properly biasing the 6v6 outputs.
Any help would be greatly appreciated. I haven't been a member long but have been reading for years. And since I signed up already ran into some very nice and unbelievably helpful folks. So thanks for make my me feel at home! "Challenge" accepted?
Alan
I've been banging my head on this for over a week. Do I figure out a way to bypass? Or a way to send the signal through one of the bad settings but remove the circuit, effectively another way to bypass I suppose
Here's what I would like to do, after MUCH agony trying to decide. Considering I can't actually play the guitar, although I do have a couple, I want to omit the guitar input. Run the audio signal through BOTH preamp tubes (6sf5's) with a two pot tone stack between stages, before the phase splitter (6sl7). Completely gut the selector circuit and add a bass and treble tone stack that I can use as needed. I can read schematics, I can solder and very much do any mechanical work but I'm having trouble designing a circuit, or finding one similar enough to try online.
I'll attach the schematic I found, which is ALMOST identical to the circuitry that was there when I started. I've made some small changes; the blend circuit, biasing the first tube for instrument, and lowering b+ and properly biasing the 6v6 outputs.
Any help would be greatly appreciated. I haven't been a member long but have been reading for years. And since I signed up already ran into some very nice and unbelievably helpful folks. So thanks for make my me feel at home! "Challenge" accepted?
Alan
