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I've got a Bogen "Challenge"

I was thinking you were running with no feedback at all. If so, connecting it would lower the gain. Adding more feedback will too, but that can lead to stability problems.
 
I've gone through countless schematics online from Fender to Fisher, or anything that uses two halves of a 12ax7, which is basically what two 6sf5's are, and they all seem to have, say, 470k plate and 2.2k cathode. I probably will end up gutting all the components on that switch and playing with the feedback resistor. Maybe start from scratch on the preamp tubes. This will be used for audio only now. Maybe I'll put a 6j5 in the number two spot.
 
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Both of the preamp tubes are already in series. You just need to use your blend circuit in place of the one for guitar.
 
I think tomorrow, rather than using jumpers, I'm going to disconnect all the components from both pre tubes and tack in the components, correctly, see how it sounds. There really isn't much there and in not going to worry about wasting a few resistors. If it's still too much gain I'll rewire the second spot for a 6J5, see how it goes. Appreciate all the input, thanks everyone.

Alan
 
gadget73 said:
I was thinking you were running with no feedback at all. If so, connecting it would lower the gain. Adding more feedback will too, but that can lead to stability problems.
Click to expand...
Gadget, I did try to run it without the feedback, it didn't sound right at all. I tried, also, with the stock feedback resistor and no tone control and it was better, but not great. I know this thing has potential, I just need to figure a way to squeeze it out of it. There's no rush on this one, it's for a '****'s and giggles' mono setup I'm making. It's listenable how it is. I just can't leave well enough alone.
As for your earlier post, I think the schematic has a 330k for the feedback, I'll measure the actual resistance tomorrow, I'm not sure that's what's in there. You think a 150k might sound better?
 
Looking at the schematic, it seems there is a 100k in series with a 47k when its set to position 2, whatever that is called. That one seems to have the least meddling done with other caps and whatnot, so I was thinking that may be a reasonable starting place for feedback without the nonsense. If that doesn't match what is actually in there, then who knows. I'm just guessing at it based on the schematic.
 
RUmad,
If you look at the underside shot of my amp, you will see that I made a modification that may be of some interest. I moved the PHONO control to a more useful position. It is an easy mod, that replaces the 470k grid leak resister with the 500k phono pot.
Mod as follows, remove the coupling cap at pin1 of the 6SL7, cut out 470k at pin1 too. Cut out 470k on PHONO pot wiper and attach wire from wiper to pin 1 on 6SL7.
Remove input wire from Phono pot, attach coupling cap that was previously removed from pin1 of 6SL7.

I'll attach my schematic, but it is geared to guitar use, not Hi-Fi.

Challenger-CH18.png
 
I see. I was going to move that 470k to the from spot, and leave the 220k, use the 1meg grid to ground on the second and eliminate one pot and the tone stack. Looks like a killer guitar amp! Mine sounded good as is , but you've got more going on. Nice!
 
I popped the bottom off of mine last night, I was going to take voltage measurements so I could post here as I make changes and HOLY CRAP. the 5w4G was supposed to be a good replacement for the 5y3, using .50 less current. Well, either something's wrong with mine or its just NOT a good replacement. Took me awhile to figure out why, I thought that I biased the 6v6's with that rectifier in place. Guess not, cuz I lost a hundred volts. A HUNDRED! Did the math, plate dissipation was about 6 watts. No good man. Put the 5y3 back in and all is right with the world again. Ok so today I'll start removing stuff and putting the 6sf5's in series. If I still get too much gain I'll rewire the two spot for a 6J5. Less gain, more current, very linear audio tube. We'll see how it goes. If I can't get it to sound good with just NFB and no tone, I can figure out a tone control another day.

Thanks everyone
 
The 5W4 shouldn't be a terrible sub, but it is rated for less current. Datasheet for the 5Y3GT says with 700v in, you get 350 out with 125ma of load. The 5W4 is rated for less current, but with 700v in you get 360v out with 100ma of load. If both were at 100ma, you'd have pretty darn close to the same voltage. I really doubt a pair of 6V6's is drawing enough current to exceed the rectifier limits. Possible yours is just weak. They do drop more voltage when worn out. The 5W4 is rated for a fair bit less input cap, 4uf vs 10uf for the 5Y3. If yours has a big first cap, perhaps its just been damaged as a result.
 
Yes sir, gotta be a bad tube. I had the 5y3 in last night and was listening for a couple hours, worked great. First cap is 8, I replaced everything to spec before I decided to modify.
I'll tell you what else, I'm fixing to give up for the day. I just finished removing the instrument input, putting the audio on the first tube, and the first pot, cathode resistors both 2200. Plugged in, nothing but loud feedback. :(
Everything looks fine, I don't know wtf I did.
 
Did you unhook the output transformer leads? If so, switching them will turn the feedback from negative to positive, and it will scream like crazy. Moving the feedback connection to the wrong place will do that too.
 
No, gadget, I just took out the guitar input. I ran the audio directly to the first tube-100k plate/2200 cathode with the 470k470k on the grid and coupled it to the second one-100k plate/2200 cathode. I don't know wyf happened. May have melted a pot? Maybe it's just too much gain? I figured one step at a time so I didn't mess with the feedback or anything else yet. I feeling a little less angry about it now, I got my Fisher in the living room and it's singing like a bird. So that's satisfying. Did a pretty full resto on that and it works great. Maybe I'll take the Bogen back apart tomorrow. Run from just the one pre tube and see if I can get it back on track. If I do I'll check in. And if it works, I'll just use it like that, pre/phase splitter/ output. Wth, it sounded fine. Shoulda left it alone. Then I'll try messing with the feedback values. Got nothing to do tomorrow anyway besides listen to the Fisher. Thanks for checking in man.
 
Partially sloppy because I was just tacking in. Partially sloppy because I can't feel most of both of my hands. And thaT hideous cap/resistor from plate1 to grid2 was tacked in after it didn't work with just the cap. I was going to desolder it this morning and try going back to just one stage but I'll hold off in case y'all see something. It worked great with these values before I messed with adding a second stage. I'm fairly confident I can get her going again with one, I really think it's just too much gain. It worked very well with guitar. Doesn't seem too happy with audio running thru two. I'll post a pic and I'll go draw up a schematic of what's in there so you don't have to read resistor colors. Thanks
 
I reorganized it so that the first preamp tube is no longer connected. Works great again. Now; do I strip the socket and wire it properly/permanently? And move on to removing the tone switch and playing with the NFB? Or go back to square one and try to add a 6j5 after the 6sf5? Opinions please
 
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