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Old 8th October 2012, 10:16 PM   #1
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Default Bogen CHB-35 for Music...

Hi, I got a pair of Bogen CHB-35 amps and my plan is to use them for music listening from line level sources (closer, I suppose, to their original purpose than the thing people in the year 2012 usually want them for, which is for guitars.)

http://makearadio.com/schematics/ima...n-chb35a-6.jpg

A quick look at the schematic lead me to believe I'd get the cleanest sound by using an Aux input as opposed to the mag input or one of the microphone inputs, since there is one less tube in the path that way. Tinkering with the different inputs verified that guess.

I'm wondering if I can bypass the 12AX7 section, which is the EQ stage. (In my case, there is already a dedicated EQ in the chain). Would it work if I just jumped from the Master Volume pot output (note that it's relatively early in the chain) all the way to the .01 Cap just before 6C4?

There is no lack of gain from the source feeding the amplifier, so if the 12AX7 in the EQ section is also contributing gain in addition to driving the tone control, then it's not a problem for me to make the drop up elsewhere.

Thanks for any hints or suggestions, rs
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Old 8th October 2012, 11:54 PM   #2
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6C4 has gain of 1 as a split-load phase inverter, so you'll need to back up one stage to get any gain (and to keep the negative feedback). Disconnect the grid of the 12AX7 from the treble control and add a 220K resistor ground, and connect that as your AUX input.

I've always added a bias pot to these - change the 15K resistor to a 10K in series with 10K pot - it'll be close to class B otherwise. Add 10 Ohm cathode resistors to measure cathode current, set for 250mV (25 mA) or so.

All in all, a better guitar amp than audio amp...

Last edited by Tom Bavis; 9th October 2012 at 12:06 AM.
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Old 9th October 2012, 12:52 AM   #3
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Thanks for the tips.

If I may ask, why do you say it's better for guitar than audio?

rs
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Old 9th October 2012, 01:11 AM   #4
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Output transformer is too small for much bass at the rated 35W, probably doesn't have very good coupling for high-frequency response. There's negative feedback, but probably not enough for much reduction in distortion or output impedance. Guitar amps are deliberately limited in frequency response - this has a head start in that direction.
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Old 9th October 2012, 01:29 AM   #5
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That is true: it feels more like a 10W-15W amp to me in terms of clean output I can get from it. (I'm auditioning a lot of newer rap music on it, so the bass almost literally could not be louder.) Luckily this "15W" is just right for my particular application. rs
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Old 9th October 2012, 04:27 AM   #6
Enzo is offline Enzo  United States
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It isn't that they are particularly good for guitar as they sit, but the power amp stages of a tube amp are all similar, so an old Bogen - or similar amps from their competitors - needs only a little rework of a preamp stage and you have a nifty little guitar amp. A stock one has all the tube heater wiring already in place, as well as a lot of stage wiring, transformers, etc, so a lot less work that building from scratch.l
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Old 11th October 2012, 03:41 PM   #7
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Well, it's official: there's something wrong with me because I actually *like* the way these amps sound with music.

rs
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