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I've got a Bogen "Challenge"

I'm not sure if this will apply to either of you since I have modified my amp for Guitar use.
But, I also wanted a way to "tame" the gain, and play cleanly at a louder volume. So, I added back in an Adjustable feedback and presence control.
I initially used too low a value for my Adjustable feedback resister, and had to add another resister to prevent the tubes from going into oscillation.
The Presence adjustment is a nice addition, as it has the effect of boosting highs.
Like Gadget said earlier, 100K and 47K are in the original circuit, so it makes sense to use those values.

I do believe that I only had oscillation problems because of the presence control.

CH-18-Feedback-mod(1).png
 
You got it bro. As soon as I get the preamp sorted that's the first thing I'm gonna do. Yank that whole whacky NFB tone crap and jumper in a 150k, take it from there. I've used adjustable NFB on guitar amps too, actually just added a pot to the 22k resistor that fenders schematic called for.
 
RUmad said:
PS, series pots? I'm only using the one pot, unless you mean the pot in series with that 470k resistor. Otherwise it's wired exactly how you stated it should be
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Series meaning its in series with the signal. As it is, with the volume pot all the way up, so minimal resistance, you've got 470k between the input signal and the tube, and another 470k to ground. At the grid, you'd have 50% of the input signal. With the pot all the way down, you'd have 970k between the signal and grid, and 470k to ground, so the grid would get basically 33% of the input signal. It will never go to "zero".

If you wire it so the signal feeds into the top of the pot, the bottom goes to ground, and there is a 1K between the wiper and the grid you'll have a range of 0% input signal to 100% input signal, so the pot will work like a normal volume control. You'll need a log pot rather than a linear one to work like you'd expect a volume control to work, but electrically its the same either way.


The only down side of that is your grid to ground resistance varies. May not matter much, but if that sort of thing bothers you, you can put a 470k from grid to ground, a 1K from grid to a 0.01uf cap, and the other end of the 0.01uf cap would go to the wiper of the pot. That will leave the tube always having 470k of grid leak, and you'll have DC blocking to prevent any nonsense leaking through from the preamp or whatever is in front of it.


RUmad said:
Ok, so we keep the 500k pot, which I thought was rather high, keep the 470k to ground, ditch the 470k to grid in favor or 1k? And lose the .01, because I got no need for excess high end. That what you're saying Gadget?
And what about the idea of a 6j5 after the 6sf5? I've seen those tubes together in n a number of circuits. Just want to get the amplification all set before I hit the feedback/tone loop.
Appreciate all the input guys. Got a couple reasons for wanting to use two pre tubes. But I want to get her done correctly.
Click to expand...

The 0.01 may not bump the HF in any appreciable way, you can always try it in or out to see if you can hear any difference.

Don't know about the extra tube stages. Guitar amps ain't my thing.
 
You're right, we're mixing two different conversations.
I'm going to try that 1000 ohm tomorrow, first thing. Right now I've got my audio signal straight from the RCA jack to the top of the pot (red wire) bottom straight to ground and signal from wiper through the 470k. No resistor before the pot.
Let y'all know how it goes tomorrow. And this is now an audio only amp, no guitar.
Thanks guys.
 
RUmad said:
I don't understand your problem with oscillation. Are you sure that's what caused it? I've actually built several champ clones with a 22k resistor and a pot. Worked great. Of course, it sounded best, in all cases, with the pot turned all the way down. Just the 22k seems right in that circuit, but never an oscillation problem.
Click to expand...

Didn't want to crap up your thread with yet another amp!! Its a 5F1 clone that calls for 22k in the FB loop...however its running a Stancor A3852 OPT, so I put a pot in to try and find the sweet spot,,, it sounds the best, (actually, sounds amazing), set at full volume and controlled by the guitar, with no added FB, and breaks up too early with about 2k added... I rewired the preamp for a 12AT7 to reduce gain, and it help a lot,,, but I haven't been able to get it happy with an 12AX7 and 22k FB resistor, as designed... I'll dig in again after the Challenger/Bogen adventure is done...
I'm guessing the guys that have clones working with the original Fender schems are using the proper output txs...I got the 5C1 clone running great, with a universal OPT, but that doesn't use FB...
I'll put a decade box on the CH618 today, and see how it responds to a little FFB with the goofy tone stack cut out...
 
"But, I also wanted a way to "tame" the gain, and play cleanly at a louder volume. So, I added back in an Adjustable feedback and presence control."

+1... I'll try this schem, but I'm not using original txs,,, so we'll see how it responds...
 
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Yep,, I get em by the pound at hamfests,, a couple pair of Edcor also, but its hard to follow a schematic, with a different OPT... I'll post the out come of the FB mods... you may be able to modify or improve them for your amp... We'll get it...
 
Hey, if the guys in the Fisher forum got me through restoring an x-101-c. This Bogen oughtta be a dang breeze! One more cup of coffee and I'm gettin to it.
 
Ok. Thanks Gadget. I've removed the 470k and replaced it with 1k from the pot to the tube. Got some more volume for sure. I think, rather than mess with a second stage today, I'll mess around with feedback resistor values, jumper it, before I remove all the tone switch components. Listening to Miles from the Bogen while I type. Be back!
 
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Yeah no. 150k sounds terrible. I don't know if it's that, or the fact that all those caps are still connected to tubes. Could that be effecting the sound? Sounds thin. Tinny.
I'll try some more values.
 
Ok, so, more (about 200k) made it worse.
Edit; getting there! 100K sound pretty decent. A little top heavy still but not terrible.
Edit2; 82K was a step in the right direction. In fact, it may be 'it'. Going to try a 68K.
UGH! 68K sound awesome. Dammit. Do I go even lower??
Edit; 56was too far. 68 was nice, all I have in between is 62K. So how can I not try it.
68K seems like the magic number. Gonna listen for a while, different genres. See what we've got.
 
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You can always series or parallel a couple resistors, or use a pot, to get where you want... I'm finding with my set up, 47K is closest without the 100k pot, taking FB from the 16R or 250R winding... But the low notes gets the speaker buzzing... seems it sounds best with no FB at all...
 
Ok. I'm switching between 68k, 150k and zero now. Carefully. Zero sounds terrible, iPod as a source. Now I may like 150k better but I think I need to go a bit bigger on the grid resistor. Not 470k big, but I think I've got too much signal.
 
The easiest way to do it, if you are that close is to use a pot (200-300k or so),,, dial it to it best sound, and measure it... the you don't have to clip different resistors in and out, and cuts down on noise from long leads...
 
Yeah, I can do that. Still listening. Liking the 150k feedback. Cal Tjader is sounding ridiculously good. So did Miles. So did Mumford &Sons. Need to try something bass heavy now. Once I get that feedback resistor soldered in, and the tone crap out, I'll work on the input value.
Holy crap, Sarah Vaughan sounds amazing. Vocals, bass and piano. All frequencies sound about even.
 
Pin 5 on the 6v6's is the grid right? Why are there two tiny capacitors to ground from it on one tube? That part of the tone switch? I'm hesitant to remove.
 
Caps to ground on the grid will work to reduce high frequencies. Depending on the value, it may be beyond the hearing range. If thats the case, its for stability.


Have you pulled the caps off the output tube plates that fed into that tone control?
 
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