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Old 12th April 2007, 06:47 AM   #1
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Question Bogen H-30 guitar mod

This is my first tube amp project and I'm a chemistry major not an EE so bear with me here...

So I picked up this Bogen H-30 from the local thrift store for $1.98 (yes, the decimal is in the right place).
It's got three 6sf5's for the preamps (mic1, mic2, phono), a 6sl7 PI, two 6l6's OP, and a 5u4 rectifier. The circuitry of the phase inverter, output stage and tone control is still a little over my head (hell, three weeks ago I didn't even know what a phase inverter was!), so hopefully someone can help me out...

Thus far, I've added my 1/4" jack and connected the output of the first microphone stage to the grid of the second mic stage. I am already in love with the distortion those octal preamp tubes produce, however, the frequency response is all wrong.

I'd like to modify the tone control to be more responsive to the guitar frequency range. The amp has a 6-position knob with filters ranging from natural, mellow, bass, deep bass, bright, and crisp. The only two remotely suitable for the guitar are 'bright' and 'crisp.' I would like to retain the 6-position switch but using RC combinations appropriate for guitar amplification (i.e. 6 different hardwired bass, middle, treble configurations). Could someone please help me with this? Any other comments/suggestions are welcome as well.

Thanks!

I've attached a copy of the schematic. Big thanks to 'ivegotthat' on ebay for working with me to find it.
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Old 13th April 2007, 07:15 AM   #2
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Default Re: Bogen H-30 guitar mod

Quote:
Originally posted by tgrizzle

I've attached a copy of the schematic.

um... where is the schematic?
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Old 13th April 2007, 08:17 PM   #3
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errr... I thought I did anyway..

try that:
http://i16.photobucket.com/albums/b1.../schematic.jpg
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Old 13th April 2007, 10:45 PM   #4
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Old 14th April 2007, 02:47 AM   #5
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That tone control is in the feedback network, unlike most guitar tone 'stacks'. The closest thing is the presence control in a Marshall or Fender.

Usually, guitar amp tone controls are passive, and cause quite a bit of signal loss, so you might need the extra gain of the first stage if you put one in. Look at how trad guitar preamps are wired.

If you're new to this, just remember....the voltages in there can KILL YOU,... know exactly what you're doing before you jump in...
Pete McK
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Old 14th April 2007, 10:50 PM   #6
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Thank you kindly for the responses!

Again, I apologize for my n00bishness; however yes sir, I am quite aware of the dangers associated with electricity inside this amp.

Yes, I figured something hairy was going on with that tone switch being connected to the output transformer like that.
So could I replace R20 with a 220k pot wired up with the resistor and capacitor of position 6 (crisp) and call it 'presence?' Playing through the amp on this setting does in fact sound like playing through a guitar amp with the presence cranked.

Is the best place for my tone stack right before the decoupling (?) capacitor, C8? I still want to use the 6-position switch for this. Could I simply take the tone stack circuit for, say a Marshall, and replace the pots with different value resistances to acheive the hard-wired filters?

Thanks
Alex
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