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Old 3rd December 2007, 02:21 PM   #1
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Default Guitar amp build...

Hello all, I've got a Bogen CHB-35A here that i'm planning on turning into a guitar amp. I'm thinking i'd rather rebuild the amp completely and fit a Fender rendition in. I don't know if the 7868 tubes are any good or not, because i'd have to recap the amp to find out. Most of the electrolytics in the amp have failed.

I would like to use the original transformers, because that would save most of the cost of a new amp. I disconnected all the circuit components from the power transformer to make sure it worked and i got the 6.7v from the heater winding, and i got 180v from one of the main secondaries. I thought these had a vac-0-vac style supply like solidstate amps (i'm more familiar with SS electronics)... but i got only the single 180vac output.

Next up, i want to use a simple, basic circuit. Just gain, tone and a master volume control. One input. I've looked at a lot of the Fender schematics but i really can't decide. I would like to use one with two output tubes (as the holes are already there in the chassis) and i have holes for up to three preamp tubes.

I'm looking for a metal - Iron Maiden/Judas Priest tone - so would anybody mind putting me in a good sounding direction? Output power is not a big deal really. Anywhere from 5w to 20w as i have a 100db/1w 15" JBL 130a to use with this.

Any help for a toob noob?
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Old 3rd December 2007, 03:56 PM   #2
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Would have helped if i looked around for schematic before i started posting. I was thinking the yellow/grn wire was the ct for the high voltage windings. Heres the schematic.
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File Type: pdf bogen_chb35a.pdf (37.8 KB, 90 views)
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Old 3rd December 2007, 04:15 PM   #3
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Easy enough to mod. The first stages use a 6EU7 which is a 12AX7 with a different pinout so might be worth rewiring the sockets for ease of sourcing a variety of new tubes. I'd also bias the first stage(s) with 100k Ra and 1k5 Rk // 50uF.

You already have a Bax tone stack, which could easily be changed to a Fender; just copy one from a Fender amp. You can get the tone stack generator from www.duncnamps.com and play around with values.

Otherwise, clean it up, fix the PS and off you go.

The 7868 is a 7591 with a different pinout. It's quite a nice little tube in the old Fisher hifi gear I've used, but never tried it in guitar. EH make good versions of both these types and I've been told they have a unique tone for guitar. Personally, I'd give it a whirl, and if you don't like them, you can rewire for 6L6; there's a how to on the Triode Electronics schematic or dta page.
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Old 5th December 2007, 02:46 AM   #4
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Looking at the tube compliment, you get 9 pins everywhere.

If it was me, I'd go to Fender Field Guide and look for a PP amp.

I'd use a Princeton tone stack and a 6v6 PP amp for inspiration.

Or if you are not tied to a Fender, a Matchless Spitfire may work. This is a 12ax7 -> 12ax7 -> EL84 PP.

Or if you need a Fender, use the Princeton front end and the Spitfire outputs.

Since you got the tubes, I'd use the 6eu7's and 7868 outputs (assuming they are good). You may need the 6C4 PI to drive the outputs. I don't know.

Visit
Duncan's TDSL for tube specs.

Should be a fun project.

rick
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Old 5th December 2007, 06:49 PM   #5
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Hello, thanks for the suggestions guys. I have to replace one of the electrolytics and the two power supply diodes and then i will try out the bogen circuit to see if the tubes i have work.

Brett - Did you mean to replace the two 470k resistors coming from the power supply to Pins 7,6 on the 6eu7 with 100k? Where do the 1.5k and 50uf go? Any other changes to make other than the tone controls?

Thanks for the help.
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Old 5th December 2007, 07:57 PM   #6
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Quote:
Originally posted by Stormrider
Brett - Did you mean to replace the two 470k resistors coming from the power supply to Pins 7,6 on the 6eu7 with 100k? Where do the 1.5k and 50uf go? Any other changes to make other than the tone controls?

Thanks for the help.
Yes.
The 470k are the existing anode resistors for the cct. Replace them with 100k. The 1k5//50u is the cathode resistor and cap.

Apart from that, looks like a good starting point.
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Old 12th December 2007, 01:23 AM   #7
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Well, in the garage anyway, i thought it was fairly warm for December in New England so i decided to build the cabinet. It's a 15" JBL 130A alnico driver in a 1.5ft^3 sealed cabinet. 3/4" MDF and braced well. The amp head you see in the photo is a 20w Ampeg G-100. I didn't even know Ampeg made non-bass amps until i found that amp...

Sounds good now. Nice tone on bass or guitar. Warm sounding even with solid state distortion.

After doing the mods suggested by Brett above i found one or both of the 7868 tubes in my Bogen were dead, so i'm going to use the power and output transformers for a clone amp. Just need to decide which one!

http://i120.photobucket.com/albums/o...33/Picture.jpg
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