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Old 4th April 2008, 01:55 AM   #1
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Default Bass guitar amp...

I still have a gutted Bogen CHB35a here that i haven't done anything with, frankly because i haven't decided what to do with it yet...
Schematic for the Bogen :

http://tdsl.duncanamps.com/pdf/bogen_chb35a.pdf

Up 'till now i've only been thinking about regular guitar amps, but really, i'm a bass player. I thought it'd be cool to have a guitar amp at my house for friends to jam on when they come over... but, i stumbled on some Ampeg schematics and now i'm thinking about building an SB-12 type amp. Schematic is here :

http://www.drtube.com/schematics/ampeg/sb127868-jp.gif

I have the power and output transformers, and the pair of 7868 tubes that were in the Bogen. I'm not positive the tubes even work, but if they don't i can just rewire for a pair of 7027a's.

I have to use solidstate rectifier because of the lower voltage transformer the Bogen uses. So, i'll just copy that basically.

The thing i'm unsure of is the bias voltage. It isn't labeled on the SB-12 schematic. The bias voltage for the Bogen was -23vdc, should i just start there and adjust it while looking at the output on a scope, looking for crossover distortion?

Any changes i should make to the Ampeg circuit? Thanks for any suggestions, and thanks to those who have helped me so far.
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Old 4th April 2008, 04:23 PM   #2
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What you propose sounds feasible, the one potential issue I see is whether or not the existing OPT can handle low frequencies well or not.
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Old 4th April 2008, 07:34 PM   #3
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Dumb question but, where would i put the pot for the bias control? In place of the 56k in the bias supply?

Another question... What does the "balance" pot between the power tubes in the SB-12 schematic do?
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Old 4th April 2008, 10:17 PM   #4
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The balance pot is used to balance the bias between the output tubes if they are not exactly matched.
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Old 6th April 2008, 04:08 AM   #5
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So, any ideas on the bias voltage needed for the SB12?
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Old 6th April 2008, 05:16 AM   #6
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A pair of 7868 in PP pentode will give about 35W, nowhere near enough unless simply jamming quietly in a house without a drummer. Even with an efficient cab, probably not enough to accompany a jazz/blues guitarist in anything but a small club.

Could make an interesting studio or home practice amp though.

Look to 7591 designs like Fishers to get design ideas.

I've contemplated making a hifi amp using Fisher OPTs, 7591s and a slight modification of SY's Red Light District amp. A friend has the original amp and is going to restore it, so I never did the mod.
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Old 6th April 2008, 11:57 PM   #7
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Quote:
Originally posted by Brett

Could make an interesting studio or home practice amp though.

Exactly what this will be used for.
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Old 7th April 2008, 01:05 AM   #8
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Quote:
Originally posted by Stormrider
Exactly what this will be used for.
Sounds like a plan.
The SB12 schem above looks good at a glance. One suggestion would be to add a switchable Fender tone stack as well as the Bax for more tonal options. Cost is a few pots and passives and a switch.

I'd also ditch the 12AX7's for some 6SL7's or possibly 12AT7's. With most basses, you won't need the gain. The Ampeg B15N has a nice preamp.
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Old 7th April 2008, 01:58 AM   #9
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I have a bunch of 12AX7's, AU7's and AT7's so i can mix and match later.

The Bogen doesn't have the space on the the front panel for that many pots, but i will definitely play around with the tone stack to see what i want there. The tone stack in the SB12 looks to be the same as the B15N.

I'll be doing the work on the chassis tomorrow. Bogen used rivets for everything so i have to drill them all out. Once i have the chassis completely stripped i will paint it, probably black. Then, i await the boxes of parts to build!
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Old 12th April 2008, 12:02 AM   #10
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Yet another dumb question...

Should i put a stand-by switch in this circuit? The Bogen didn't have one, and the Ampeg doesn't show one on the schematic. If yes, where do i put it?

The power supply will be almost identical to the Bogen power supply so do i put the switch after the series diode in the rectifier, so the bias voltage gets to the power tubes, or right at the beginning where the ac comes out of the transformer?
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