Bogen Deluxe

[blvd]

Hi there. I have a question about converting a public address amplifier that I picked up a local thrift store for use with guitar. The amp is a Bogen CHB 35a which seem to be pretty popular with people doing these kinds of things with them. I'd like to convert it as close as possible to a 6G3 or AB763 fender deluxe circuit. The powertubes in the Bogen are 7868 vs 6V6 in the deluxe circuit and I've already rewired the 6EU7 in the Bogen for a 12AX7 pinout

My question is simply which parts of the Bogen circuit I should replace with the components from the Deluxe. How much of the power-amp section needs to remain intact for the amp to work, impedance to match up, etc? Any advice would be appreciated. I know enough about safety to be working on this, but feeling a little bit unsure about whether I know enough not to make the amp stop working!

http://www.schematicheaven.com/hifiamps/bogen_chb35a.pdf
http://www.ampwares.com/ffg/schem/deluxe_6g3_schem.gif
http://www.ampwares.com/ffg/schem/deluxe_ab763_schem.gif

[imaradiostar]

I have a Bogen 35 that I bought many moons ago with a bad power transformer. I've used it as a prototyping chassis with three or four different circuits over the years- blackface fender, "dumble" and most recently a pentode pre circuit come to mind.

If you haven't already switched out the sockets for the 7868's I'd definitely recommend keeping them. They're affordable and really an excellent sounding tube. I've not experimented with it much but many people prefer the sound of the phase inverter in the Bogen to the long tailed phase inverter in a Deluxe. Your mileage may vary. If you want to change the bare minimum of components then I would leave the PI and power section alone. You could leave V2a as it is and just configure V1a&b to be a blackface preamp and blend them using a 220k resistor at the grid of V2b like a Fender. This way you can change the minimum of components and you'll still have the real estate available if you want to change the phase inverter.

I removed the tiny socket and 6c4 from mine an installed a 9 pin socket instead. If you went that route you could build a blackface Princeton reverb circuit. You could use the 6eu7 as the normal blackface preamp and the 12ax7 as the reverb mixer and phase inverter. Use the extra 9 pin socket for a 12dw7. You can use the high current side of the 12dw7 to drive a high impedance junker reverb tank from a solid state amp (using a 4-5k plate resistor and .47-1uf cap ala magnatone or ampeg) and use the high gain side as the reverb recovery amp. It won't do Dick Dale surf verb sounds but it'll sound great for anything short of that. If you mod the phase inverter to have a little more voltage and do the "fixed bias mod" as some guys do to their princetons I think you'll capture the blackface fender vibe but with a little sweeter sound and possibly higher output thanks to the 7868 tubes.

One other note- some people think that triodes like this tend to sound better when drawing a little bit more current. You might try some more Fendery or Marshally values for the cathode and plate resistors (100k plate resistors). Use smaller value cathode caps to trim the flab if you don't like the sound when it's cranked up.

That's my opinion, take it with a grain of salt!

jamie

[stevesamps]

The phase inverter sounds ok stock but the deluxe you may end up with
ir more tweed style..
The main thing to fix first is the voltage doubler caps
its a good idea to just rebuild the power supply
then remove all that crap related to the "remote control?

you only have two 12ax7 tubes to work with
ab763 would be a stretch
also this thing will not be as clean
with those tubes and the stock inverter

The idea of a more simplistic reverb is a good one traynor
has some examples as well

the idea of no reverb is a better one :)
to get closer to that ab763 you may want to go with 6v6 and replace the 6c4 socket with
a 9 pin so you can go Long tail pair...

but I dig those 7868's also
they just seem to drive easier so mo distortion
of course I always clip the NFB which explains why they
rock out more..
the last time I rewired one I built a hybrid of plexi and 2203
and it was a chb 100 it was radical..
see yah
Steve



Steve

[imaradiostar]

Yeah, he's right. You should definately check the caps in the voltage doubler because it'll make a huge difference in the output power and sound.

As much as I like a Fender circuit you'd be better off making a plexi or bassman kinda preamp and keeping the output section as-is. You could try removing the negative feedback or even running the output tubes cathode biased for different sounds- like a tweed deluxe output section with a marshall pre.

The 7868's don't require as much input as a 6L6 type tube to provide a decent output. This means less gain required before the output tubes...once again, experimentation will be the key. You might find that output distortion with an inexpensive Bogen output and power TF isn't as pretty as preamp distortion.

jamie

[hearforever]

Which capacitors are the voltage doublers? Would it make sense to clip everything between the plate resistors on V1A/V1B and the remote control? In other words, to just wire from the plate resistors of V1A/V1B to 240V on the power filter caps?

[stevesamps]

http://hyperphysics.phy-astr.gsu.edu/Hbase/electronic/voldoub.html
the filter caps are in the can and there is one in series there is usually one on a metal clip 350V 40uf is something..
the power supply rail can be modified to be like any normal
tube amp I think they have like 470k redsistors that go to the
back of the amp on the speaker terminal strip
some kind of way to lower the plate voltage to
turn the volume down i dunno
that whole thing can be clipped yes
just wire up maybe another 10k and a 22uf 450 v cap
so you have a filter for the preamp

just copy a guitar power supply after the doubler
they also dont have screen resistors
not a bad thing to add,,

[blvd]

Thanks for your reply steve
I'm still not totally clear where to wire the power supply caps after the voltage divider. Let's use this ampeg circuit with a similar tube complement for example. On the attached bogen power section, would the 40uF caps stay put and then I would copy the ampeg values for the 20/10/10 caps?

[J Martin]

I would restore the power supply by replacing the two 40 mfd caps with two axials. Take out the old can and replace with a JJ or F&T 32/32 can. Replace the final 10 with an axial located near the preamp circuit. Do the bias cap also.

[stevesamps]

Mr Martin has it down

I often leave the can in place and just rebuild the doubler
with caps under the chassis, (because I have tone of radial caps like 33uf 450v
this does require adding a terminal
strip sometimes but if you spring for the can cap
it makes life a lot neater..

there are two caps in series that form the doubler
one is part of the can cap and the other is the 40uf axial on the clamp

after that is resembles a normal tube amp supply

Steve

[66merc]

I've done several of these and I think it's best to leave it 80% stock. The stock circuit with the 7868s has a great clean tone with amazing distortion when cranked. Definitely take out the "remote" stuff since there could be a shocking surprise to someone feeling around the back of the amp at a dark gig!

I usually take out the mic1 pot and use that hole to put a long barreled input jack. I use a grinding bit on my drill to enlarge the metal case hole so the guitar cord will fit. I then somewhat fenderize the input for mic2 with typical cathode bypass resistor/cap. Change the 6eu7 plate resistors to about 200k or so. I also usually take out the Aux circuit and wire up a variable negative feedback knob for a little extra grunge--I use the stock Bogen pot for this along with a 20k resistor.

I usually replace all of the filter caps and doubler caps. Sometimes I'm lazy and just do the doubler caps.

I remove the Bogen aluminum case plate and paint the case scientific gray or blue sparkle. Put on new rubber feet and a black fenderish handle.

The only thing I would like to do different some day (I have a stack of these 35 watters in my garage) is to figure out a good master volume scheme since the master and the remaining volume knob essentially do the same thing.

http://s155.photobucket.com/albums/s315/spammeup/Bogen%2035%20watt/