Looking for Bogen CHB 20A Schematic (Bogen CHB20A Project)

[Platefire]

DummyLoad requested a sound clip

This my first attempts at you tube. I had the amp turned way down low, wife was talking on the phone--so I did something to keep from doing nothing

! No longer available

[DummyLoad]

thank you! very nice and laid-back tone. it's a keeper as-is me thinks. i'll put a finger on my NOS suppliers if you wish; please send me a PM if so.


--pete

[Platefire]

Thanks pete

I've actually just purchased some on e-bay today. There were two NOS/NIB GE & Admiral 6GW8's for $49.99.
I offered $30.00. He came back with a counter offer of $45. He had $3.95 shipping on them, so I came back with a counter offer of $45 to include shipping, so I actually got them for $41.05. There is one hitch though, MIJ!! But the NOS Lendel that I put in there to replace the bad Bogen tube was also MIJ--so they will be in like company. I may of did bad! but I did something If those tubes made for Bogen had lasted all the years with that intense plate voltage, maybe these will to? Here is a picture of them. Platefire

[DummyLoad]

Thanks pete

I've actually just purchased some on e-bay today. There were two NOS/NIB GE & Admiral 6GW8's for $49.99.
I offered $30.00. He came back with a counter offer of $45. He had $3.95 shipping on them, so I came back with a counter offer of $45 to include shipping, so I actually got them for $41.05. There is one hitch though, MIJ!! But the NOS Lendel that I put in there to replace the bad Bogen tube was also MIJ--so they will be in like company. I may of did bad! but I did something If those tubes made for Bogen had lasted all the years with that intense plate voltage, maybe these will to? Here is a picture of them. Platefire

you've done well. the MIJ tubes are likely either hitachi or matsushita/national and either are as good as euro or USA made stuff. i'll query my contact anyhow, i'm curious to see what he has, if anything. he mentioned that they are used in some HAM radios and i remember him telling me that they were a prize tube for HAMs as well.

--pete

[Platefire]

Thanks, it makes me feel better that I might of made a good selection. If you look at the construction of the tubes very carefully in the picture, they look identical even though they got different brand names. This is the first time I've used the "Make an Offer" feature, kind of like it. Platefire

[Platefire]

PRR

I hear you on the plate voltage. A 100V over is more than I would purposely design/build an amp. Makes me wonder what Bogen was  thinking in their design? But also, how did these vintage tubes last all these years? most of them still have their original tubes in them. Platefire 

[Platefire]

I found a guy who modded his CHB-20A by lowering the voltage 110V using diodes. Attached is a picture of what he did. Is this a better approach than using dropping resistors> Never seen this used for this purpose before. He said he drilled some extra ventilation holes by the terminal strips, so must be considerable heat involved.

I'm not sure yet if I want to do this on this amp because I'm wondering how much that would effect the clean headroom I'm presently getting. But on the other hand, if I end up using this amp a lot and tubes start going down the drain I may need to do something. Just a consideration that I'd like to understand better. Platefire

[DummyLoad]

that amp ran reliably for decades with 400V with those tubes. i'd leave it be. just my 2cents...   


--pete

[Platefire]

Amen! I think I will take it to Church tonight and hook it up to my Boogie 4-10 cab and if all seems good, use it in Church services tomorrow with that cab. Platefire

[Platefire]

I did take the Bogen to Church and practiced with it hooked up to my Boogie amp(see Pix) with a 4-10 cab. It has Jensen Special Design Alnico speakers and are wired for combined 8 Ohm load.

The Bogen sounded great through the Boogie cab and handled the 4 10's fine. I left the MV wide open and just used the pre-volume as my only volume with my LP. The amp has wonderful round tone just set flat on the tone stack. I tried boosting the treble a bit but it was too much, much better flat. When I got through and was shutting down I noticed the volume was on only "3". It took my pedals fine---had to do a little pedal tweaking on them but that's normal for any change up. I practiced with the wife and my JamMan but looking foward to using it with the full band tomorrow. Very Very Pleased with how it sounded and operated tonight. Platefire

[Paul1453]

I think you got a reasonable deal on those tubes.   

They are not common tubes and I couldn't tell for sure from the pic, but they appear to have the Philips X on the top.

HBP pointed out that MIJ tubes with that X were made on Philips manufacturing gear.
To me, that indicates that these could also be considered premium tubes.

Your amp sounds great to me!   

[PRR]

> I left the MV wide open

I suspect the Bogen was intended to run with Master around "5". It's not a strangler but a gain-balance.

Then you would use more of your Preamp knob.

For guitar, you might try that and then disagree.

[Platefire]

Paul,  thanks for another conformation on the tubes. I looked at the pix and didn't see the "x" you were referring to. I found myself looking in the back of the amp as I played it this weekend even feeling them with my finger tips. No red plating or outlandish heat. The sound was great, so I think we nailed the bias, of course with a little help from my friends. So hopefully these tubes will hold me for a while.

PRR----I'm so impressed with the pure clean tone on this amp is the reason I set the volumes that way. I will get around to dialing in more feel by rolling back the MV but that clean & my LP is sounding so good I'm just smelling the roses a bit. I do have some good dirt boxes to stomp some in as needed, but yeah I want to get around to dial in a response where I can use my guitar vol to go between clean and slightly edgy breakup. I've already sampled it a bit and it's good. Platefire

[Platefire]

I've got s schematic cooking for this one. I think it's a pretty good recipe worth documenting. Based on past track record, there will probably be unforeseen changes down the pike. I'll post it when I'm done and request you look over my shoulders, sometimes it's hard to see your own mistakes Platefire

[DummyLoad]

the "X" are seams in the molding of the glass. you see seams that appear as crossed lines where the glass bulb was melted together to seal the top of the bulb.


--pete

[Platefire]

pete--I'm assuming you are talking about that little tit at the top of the tube? I can't see it. Maybe when they come in I can see what your talking about Platefire

[DummyLoad]

run your fingernail around the top of the tube next to the tip. if it's an amperex, you'll feel four lines on the glass. look at it close you can see the lines they form a cross across the top of the tube.



--pete

[Paul1453]

pete--I'm assuming you are talking about that little tit at the top of the tube? I can't see it. Maybe when they come in I can see what your talking about Platefire

Take a look a the tops of these tubes.

Old Mullards, Bugleboys, Amperex also have these lines.

I forgot that Uki had a great picture of that X.  Thanks Uki!

[uki]

Lines on top of the tube like making an X? See if this is what you are talking about ?

[Platefire]

Thanks uki

I can definitely see the X on the pixs you linked. I can't tell on the pix of the 6GW8's I ordered but now I know what to look for. I'm learning a lot about NOS tubes lately. I also been watching the discussion going on about
12AX7's, which ones is best. I have quite a few used vintage USA tubes but very few NOS anymore. NOS's will be going up and up as they have been and any good deal you can get on a NOS/NIB I think is a good investment. With that said I just threw away a whole hoard of NOS/NIB Television tubes that I couldn't sell or even give away. So you would need to chose and pick which NOS/NIB to invest in and that will take a little NOS education which I'm getting little by little. Platefire

[Platefire]

My NOS 6GW8's came in today. No X seam on the top. Platefire

[Platefire]

Well I finally got the schematic finished. I've already located a couple of mistakes but I'm sure there is more.
Please check it out and let me know of any errors you find. When it appears it's done, it can go in schematic library.

I've attached the SCH file, also the original Bogen schematic and also sluckey's schematic of the Ampeg Gemini
II preamp. Bare in mind some of the values on the caps in the preamp, I didn't have the exact value and used what I had. The ones I actually used is what recorded as an as built schematic. Thanks, Platefire

Note: On V1A I've got the cathode marked pin 1 and plate pin 3 incorrectly. I have corrected that.

[Platefire]

UH OH! Left the standby switch off. So here is the new version. Platefire

[sluckey]

Crap! I thought you had come to your senses.   

[Platefire]

Dream on 

[AmberB]

I've been reading this entire thread from a few years ago, I just got my hands on 2 of these Bogen CHB-20As, one of them is in a 19 inch rack mount case.  I'll start a new post on this once I get into the amp, hopefully soon.

[Platefire]

Well My! It's been ten years since that mod. That pre-amp mod sure made a world of difference for the better in that amp. I highly recommend that mod for that amp. Hope all you old ampers are doing good. Platefire

Edit: Sorry, Just saw mod was done in 2015---only 6 years old

[AmberB]

Once I get into my CHB-20, I'll have to decide if I want the preamp configured like the Ampeg Jet 2, or if I want to put the tone controls between the first and second stage triodes.  I'm not sure which would be better...