Build Journal: Bogen CHB35a to switchable Bass/2204

[_Steve]

I bought a CHB35a from Ebay for $100 including tubes!



I'm curious to find out what 7868 output tubes sound like. So my plan is to convert it to my favorite circuit (a bass plexi) as I know really well what it should sound like. Incase the 7868s sound bad, I'm also going to make it switchable into a 2204.

Because I only ever use 1 input on MV amps, and hence just one triode in the 1st stage, I figure I can use a Bass-specced 1st triode (V1b in the schematic below), albeit with a split cathode and ~1.6Kohm, and then use the other triode to have a 2204-spec V1a bypassable on a DPDT switch. V1a's grid would switch between ground and V1b's output, while V2a's grid would switch between V1a and V1b's outputs. I'll tweak things like to mixer resistor/bypass cap etc to be more bass-spec also. Is that workable? (i wish v1a & b names were reversed in that schematic!)



Inside the amp looks easy to work with. I love point-to-point wiring. After checking the transformers work, I've started working on the power section which interestingly uses a voltage doubler. Im replacing the caps with cheap radial ones and adjusting the dropping resistors.



(Note the cap and resistor hanging off the plate of just one of the output tubes!?)

 

[adew1]

Will be following this with interest. The Bogen is a PA amp, right? Looks like it will be a fun project!

 

[_Steve]

Will be following this with interest. The Bogen is a PA amp, right? Looks like it will be a fun project!

Yeah its a PA amp. Enjoying it so far. Happy to be doing point-to-point again after 3 big turret board builds

 

[_Steve]

Yesterday I finished the power section to the first node as well as the bias. Somehow I managed to get a mild shock just after I had replaced all the caps and diodes, but before I had plugged it in! Somehow there was 130VDC at B+1.

I also removed the 7-pin socket at V3, drilled the hole out, and put in a noval socket. Now I need to figure out how im going to wire the LTP and connect it to the power tubes. Also, I need to figure a tidy way to run screen stoppers to the power tubes. It currently has none, and there's no unused pins on the socket to tie them to.



I also drilled out a couple of the old-school connection sockets for the speakers ready to put 1/4" jacks in. Im going to keep the cool wire/screw impedance selector.

 

[Gene Ballzz]

@_Steve
Back in the '70s, several local guys in Syracuse used these amps in generally stock form, into 4x12 Marshall cabs for some very great sounds! Did you try it before digging in to modify it?
Just Curious?
Gene

 

[_Steve]

@_Steve
Back in the '70s, several local guys in Syracuse used these amps in generally stock form, into 4x12 Marshall cabs for some very great sounds! Did you try it before digging in to modify it?
Just Curious?
Gene

Unfortunately no. Didn't even think to try it I just assumed it would sound like doggy doo!

Next time

 

[Guitar-Rocker]

Speaking of flipping preamps, here is a scaled back version of a 1959 / 2203 preamp with a flip of a switch. Maybe something along these lines? The schematic has a dual input that easily could be pared down to a single input.

 

[_Steve]

Speaking of flipping preamps, here is a scaled back version of a 1959 / 2203 preamp with a flip of a switch. Maybe something along these lines? The schematic has a dual input that easily could be pared down to a single input.

Excellent - thanks for posting this! I can use it as a reference.

It looks pretty much the same as what I am planning except it has a separate gain pot for each V1 triode. I think i'll be fine just sharing the same pot and having one 270k/560pF mixer. I don't care so much if the 2204 part of the circuit isn't exact.

I should probably not be so lazy and actually draw a schematic

Does it matter in the above schematic that V1b's grid is left floating when in 'plexi' mode? In my design I was planning to ground it when not in use.

 

[Guitar-Rocker]

It hasn't been an issue

 

[_Steve]

I got the PI done today I'm not sure yet which tail resistor to go with for 7868s.

 

[thetragichero]

i purchased a couple of these (just chassis, missing the metal top) along with a chb50 some months back for a very good price. the iron from one is intended for a plexi-type build (i put three jacks: normal, bright, and both which is like jumping the channels in a convenient switching jack) on a brand new hammond steel chassis. have all the parts just waiting for the fella to send me some $$$ for the deposit to start working on it (that and i am easily distracted so have many many builds/repairs strewn about. but i'll blame it on the money )

 

[_Steve]

i purchased a couple of these (just chassis, missing the metal top) along with a chb50 some months back for a very good price. the iron from one is intended for a plexi-type build (i put three jacks: normal, bright, and both which is like jumping the channels in a convenient switching jack) on a brand new hammond steel chassis. have all the parts just waiting for the fella to send me some $$$ for the deposit to start working on it (that and i am easily distracted so have many many builds/repairs strewn about. but i'll blame it on the money )

Awesome! Do you know if the Bogen TX's are decent or not?

 

[_Steve]

This morning I fired it up with just the PI and power tubes. I wanted to see if the 7868s I received with the unit work or not because I've realized none of the US retailers have them in stock - so I'd need to either buy Russian/Ukrainian equivalents off Ebay or switch out the sockets to octals to run 7951 tubes which are the same tube with a different base.

Thankfully the 7868s seem to work as I was getting output signal. However my LTP PI is not working correctly - passing very little signal. It feels like ive spent the last 3-4 months wrestling with LTPs so I just couldnt be arsed and put it aside to investigate later.

I wired up V2 ready for the tonestack and V1 tomorrow. With a bit of luck I might finish by Friday.

Spot the oopsie:

 

[thetragichero]

i have no idea on quality of the transformers but i'm assuming good things, if anything because using a voltage doubler required more ingenuity than full wave or bridge rectification. i'd like to think that means something

 

[_Steve]

Today I realized i'll need 500V filter caps all the way down the preamp to cover the startup voltage. So I had to order some more parts which will delay finishing it. Not sure why I didn't realize that earlier

I managed to get the tonestack done today. It took way longer than expected trying to limit the solder nodes to the existing terminal strips. Because the faceplate only has labels for Treble and Bass I decided to do a TMB with a hardwired middle at 50%, which is where I always have it when im playing anyway. I'm thinking of experimenting with a NFB-based additional tone control as well, so I added a lift switch to the TMB so it can be bypassed.



Really all that is remaining now for a first-cut is V1+input, the final power section nodes, and fixing the damn LTP.

 

[thetragichero]

are you sure you'll need 500v caps throughout? i mean if you have em use em but i'd think 450v would be sufficient

 

[_Steve]

are you sure you'll need 500v caps throughout? i mean if you have em use em but i'd think 450v would be sufficient

Pretty sure... Startup voltage is 503VDC, and there's no standby switch. With no current flowing there will be 503VDC all the way down the filter nodes no? Feel free to call me an idiot if appropriate!

 

[thetragichero]

with no tubes maybe, but i don't see you starting up the amp with no tubes besides initial testing (i still do on a lightbulb limiter because an extra minute or two is worth the price of a blown fuse). definitely for the first node but you've already got a pseudo totem pole there in the voltage doubler. have a play around with duncan amps power supply designer

 

[thetragichero]

just a rough idea, a little difficult for me to wrap my head around modeling it but max voltage for C4 (phase inverter) well below 450v

 

[_Steve]

Thanks for that! Does "R4" exist when it is just switched on? My understanding was that its effectively an open circuit before the tubes start conducting? I thought until there was current passing through the dropping resistors wont drop any voltage..