Bogen Amp Project

[daveyajd]

Hello to all you amp gurus out there. I have a Bogen CHA-75 that I want to strip down and rewire to suit guitar. I have two of these actually so I am keeping one and would like to use the bones of the other to practice my building skills. I have some knowledge with tube amps so I would like to aim for something a little beyond SE style amps to push my skills a bit. There is also a lot of real estate in this chassis. Anyway, the paperwork on this amp is amazing for the age. I wish I could find documentation like this for my Fenders.
 
Schematics
http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/Bogen%20Amp%20Project/bogen-cha75-1.jpg
http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/Bogen%20Amp%20Project/bogen-cha75-2.jpg
http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/Bogen%20Amp%20Project/bogen-cha75-3.jpg
http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/Bogen%20Amp%20Project/bogen-cha75-4.jpg
http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/Bogen%20Amp%20Project/bogen-cha75-5.jpg

And here is the amp:





Tubenit I hope you are reading this because I have come across threads of your CBS builds. I think you did something from an old Bogen. That amp sounds killer and I would love to pick your brain. Thanks in advance.

Aaron

[RicharD]

Replace the filter caps.  Rip out the front end and rewire the 12AX7 cascaded along the lines of your flavorite amp.  Maybe change the tone stack to something more guitar friendly.  585V B+ is pretty big stuff.  Might wanna leave the back end be although 4x 6AV5's is certainly making 18 watts the hard way.  If you're gonna gut it to the bone, you want to measure the transformers.  Figure out the primary impedance of the OPT and I'd assume it to be no greater than 20W.  Measure the offload voltages of the PT and it's DC resistances and plug it into PSUD2 to finger out it's current capabilities.  Obviously you have ample filament current to do pretty much whatever you want.  I'd guess HT is at least 400 - 500mA from looking that the 6AV5 cut sheet.
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AV5GA.pdf

[Platefire]

Wow! 6AV5's are new to me, don't know nothing about them. I always start by removing one of the mic volume knobs/pots and installing a 1/4" input jack along with a fender style input network to a 12ax7 just to see where the amp is at in original state. Most of the time after it's all said and done, I gut it and completely rebuild it to some existing amp circuit that matches the tube configuration. The 6AV5's is a different twist. A lot of options to go--Twin, marshall, voxy?/ Platefire

[PRR]

> Might wanna leave the back end be although 4x 6AV5's is certainly making 18 watts the hard way

It's 75 Watts.

Power supply will run, full-load, 500-550V at 200-240mA.

OT primary is 4K-6K (probably near 5KCT); which is ~~10K per-pair.

What a wacky setup. Three small rectifier bottles. The absurd B+ has to be dropped way down so there is a TV V-sweep bottle making 200V ratio-ed (not regulated). Power section needs 1.5V-2V at V3A grid, so V2 must put out around 15V.

My gut says do as little as possible. You can play guitar into the MIC inputs. You might like it better changed over to 33K 1.5K+10u 100K Fender values. The volume and mixer network is already known-fine Fender values. Bogen output stages are usually musically benign.

For more pizzazz, increase R28 to 33K, 56K. Don't go a lot higher; this amp is high-strung and may hurt itself without NFB. Take the "Phono" (crystal) pot and put it between tone controls and V3A as Master; V2 sure can distort with just a little more loss after it.

R16 R27 look strange; they are semi-Akido networks. (There's nothing new under the sun.)

There's no obvious drop-in "guitar tube" output stage for this OT, even for this PT. If 6AV5 are really hard to get, we could consider pair-6550 (maybe quad-EL34), but they really should have G2 well below 550V more like 300V but the "200V" reg can't go any higher without stressing its heater insulation.

[daveyajd]

Wow! Lots of good info in there guys. Thanks. Here is what I think I want to try to do. Completely gut and salvage. Only going to try to reuse the chassis, octal sockets, and iron if I can make them work. I would like to build an amp that will use more common tube types. Thinking maybe 2-EL34s with 2-3 preamp tubes, tube rectifier. I could do a quad of EL34 if the transformers are better for them. So how do I go about getting the measurements to determine my voltages and impedances for the transformers. I can operate a DMM just need to know where to look. Power up the amp with all the tubes in or out? I have not powered this amp up because the power cord was all shredded so I don't know what will happen. Tone stack wise I think a gain, MV, bass, mid, treble. Maybe a bright switch or something thrown in there eventually. Thanks for the help. Keep the info coming.

Aaron

[DummyLoad]

Completely gut and salvage.

respectfully, but that's NOT a wise path to follow with this amp. at least reconsider fiddling with the power supply and the power output stage. this amp is VERY persnickety because of the output tubes and power supply. if you don't get some tight regulation on the 6AV5 screen supply, things will go south real fast. trust. i've built with sweep tubes. if you do gut, then it would be wise to copy the power supply and output stage exactly as is now. may we suggest that if you must gut, then do so from the phase inverter to preamps.

the bogen topology seems workable and with just a few tweaks would likely make some nice tones. PRR reinforced what i'd do with the exception of of the phono level input converted to master volume; i just didn't think of it. :p  my thoughts were just to snip out the phono level pot to lighten the load of the summing network for more gain.

good luck and happy rosin smoking.

--DL
 

[PRR]

> getting the measurements to determine my voltages and impedances for the transformers.

It's all on the plans or in my notes.

True, I can't make 380VAC agree with 585VDC.... not very worried about the discrepancy.

The iron was designed for that unusual tube compliment.

AND the 6AV5 is a $7 tube.... where else can you re-tube the whole 75 Watt power stage for $28? HALF a tank of gasoline?? I've paid more than that for one Russkie 6550 (and a lot more for one Kentucky 6550) to get 13 Watts.

It's strange, but fine. Leave the big-end alone.

[daveyajd]

Ok. Sounds like the consensus is to rework the preamp stage and utilize the output end. I respect the advice of the board so it is certainly worth a shot. I will start with powering it up to get some measurements. A couple more things, As I said I have 2 of these the other one has 5y3 rect and this one has 5v4. I understand those are pretty much the same tube except for a higher plate handling in the 5v4. This one also has a 6fw5 where a 6av5 should be. Not familiar with that tube so...any issues? Also the cap cans on this are riveted to the chassis. Seems like a PITA to replace. Thanks for all the help.

Aaron

[Willabe]

I will start with powering it up to get some measurements.

Aaron do you know about a lamp limiter for testing for shorts on old gear and new builds? It can save your PT and filter caps, etc.

This is a _ must _ have _ must _ use tool for amp building/amp repair work.

Here's a link to it;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


             Brad        

[DummyLoad]

This one also has a 6fw5 where a 6av5 should be

sort of similar... but different enough electrically, higher plate dissipation & higher slope so sucks more current - one 6FW5 mixed with 6AV5 will be out of balance - IMO you should install what's specified for both rectifier and output valves.

--DL

[daveyajd]

Willabe thank you very much! I have heard of these but haven't put one together yet. I will do that before I get to far. I will also pull a 6av5 from the other amp.

[Willabe]

Willabe thank you very much! I have heard of these but haven't put one together yet. I will do that before I get to far.

     And thank you to Sluckey.


                 Brad