My Old Bogen CHB10A Conversion Resurfaced on E-bay

[Platefire]

I was doing a e-bay search on Bogen CHB models and was surprised that my CHB10A I sold several years ago is for sale. It appears it's still in its original form with no changes as it was when I had it.
I had to chuckel a little bit when I read the part about it being perfessonally converted.          Glad it's still in service!! Platefire

 http://cgi.ebay.com/Bogen-CHB-10A-Champ-Style-Guitar-Amplifier-/250795923540?pt=LH_DefaultDomain_0&hash=item3a649a1854

[alerich]

I just sold one of my "professionally converted" tube pa to guitar amp projects to a young man on Craigslist. It started life as a late 60's Knight KN3220A made by Allied Radio in Chicago. 1 12AX7 and 1 6AV6, two Mullard 7189 power tubes and a 6CA4 rectifier - the rest were old RCA tubes. After my basic conversion I ended up with this:

http://www.alerich.com/7189.gif

I wasn't particularly wowed by the preliminary results. Great clean tones but I have decided that am not a generally a big fan of EL84 distortion. I am more of a 6V6 kind of guy. Sounded really good when he tried it out, though. Suited his "What the hell are you playing through?" jones to a T. I hope he enjoys it.

[Platefire]

That's a clean looking amp head. I'm a little confused though as the power tubes are 7189 and you are referring to EL84 distortion? Anyway, if your like me you'll probably start wishing you had it back before long. I had an old Webcor reel to reel conversion I used for recording a lot and sold it and have regreated it every since. Don't think I could build another amp to sound like that one even if I tried.

I have a SE EL84 amp I like a lot, I call the Little Bear. I've just recently aquired a Baldwin Organ amp that has two EL84's and Two 12AX7 that I think I will eventually make into a Stout. I kinda like the EL84 sound. Platefire

[jojokeo]

Good story Plate! Is it selling for the same price as before? What's the special switch for clean, med breakup and full distortion - bypass caps on a switch or different bias resistors? Inquiring minds want to know, lol.  

Thanks for sharing. That's kind of cool actally to have one of your amps advertised and spoken of like that w/ such praise. I know that Hoffman's resident beloved narcissist (and self proclaimed ex-member) would be doing cartwheels right now.  

[alerich]

That's a clean looking amp head. I'm a little confused though as the power tubes are 7189 and you are referring to EL84 distortion? Anyway, if your like me you'll probably start wishing you had it back before long.

Aside from max plate voltage the 7189 and the EL84 sound the same to me. Not my cup of tea. Now, the little guy below I will never sell. I still can't wrap my head around the fact that I built a tube guitar amp that sounds this good. Those Sylvania 6V6 outputs are as old as I am. Of course, so are the filter caps but that is my next project.

[jjasilli]

 

[Platefire]

Well once you sell an amp you really can't tell where it will end up. The little Bogen CHB10A had a surprising amount of clean headroom for a SE. Those clean, half clean and distorted as I recall was:

1-Clean was with NFB
2-Half Clean was without NFB
3-Distorted was with a cathode bypass cap added without NFB in on the second stage 12AX7 preamp after the tone stack.

I thought I had a schematic on that and tried to look it up but couldn't find it. It might be one that I actually didn't do a schematic on? Anyway that's what I remember. It actually worked pretty good because kicking that extra bypass cap in distorted more exagerated for some reason on that amp than any other SE I tried that on.

A funny thing on that amp if you look at it, has a 1" Dia Pilot light(big!). I found a hand full of those at a flea market in NOS condition that takes a #47 bulb. I used them on several projects. I think I have one left with a clear lens.

alerich--that's a nice looking amp. Was that a conversion or scratch build? Reminds me of my first conversion of a General Electric PA-20 Hi fi amp. It has one 6SC7, two 12AX7's, two 6L6's and a 5Y3. I was feeling my way in the dark, asking a lot of questions but somehow in my fumbeling the sound of the amp turned out great. I can't remember who on the Hoffman site helped me, but they stuck with me and walked me through the whole thing. That's one I will never get rid of either. Platefire  

[alerich]

alerich--that's a nice looking amp. Was that a conversion or scratch build? Reminds me of my first conversion of a General Electric PA-20 Hi fi amp. It has one 6SC7, two 12AX7's, two 6L6's and a 5Y3. I was feeling my way in the dark, asking a lot of questions but somehow in my fumbling the sound of the amp turned out great. I can't remember who on the Hoffman site helped me, but they stuck with me and walked me through the whole thing. That's one I will never get rid of either. Platefire  

It's a conversion. I have converted three amps and rebuilt a Fender Vibro Champ from the ground up. I've yet to tackle a scratch build but I do have a semi-scratch build on tap in a Firefly I am going to build in a little Park solid state practice amp whose cabinet and chassis I saved. The amp pictured above was an old kit amp by a company called Hi-Fi. I bought it on ebay for a song. The PT date code indicates 39th week 1962. The seller even had the original schematics and assembly instructions where the builder checked off the steps in pencil as he went along.

Two 12AX7, two Sylvania 6V6 and a Kenrad 5Y3GT. I replaced the 12AX7 tubes with a pair of pulls from my old Marshall JMP-1 preamp. I think they are EI Yugo tubes or some such. By all rights they should be just cheap current issue (at the time - 1995) tubes but they sound fantastic. The circuit is very closely based on the Marshall Studio 15 combo. So not only is it a conversion but I copped the design but still - this little thing just sings. I traded the original amp about a month ago because the clone sounded way better. I did replace the old paper wound OT with a Deluxe drop in from Magnetic Components. That seems to be the weak point on these old mono hi-fi amp conversions. I'm impressed enough to buy another the next time I need an OT. It has a built in low volume attenuator and the Hall VVR. The face had a multi input selector switch and a pilot light that I removed and used those holes for controls.

I ordered all the parts for it from Doug but since I was using a tried and true design and with a little serendipity I don't think I asked many questions. When I was really studying tube theory I was still hitting alt.guitar.amps on Usenet daily and in the old Deja archives. Back in the day there were some very talented folks who frequented there. Maybe not on the level of PRR's chops but really good. AGA has since deteriorated (not unlike many Usenet discussion forums) and the Hoffman board has replaced that for me. The folks here are awesome. I really don't post much but I read everything. Tremendous learning experiences are to be had here.

As far as the little 7189 Knight amp I just sold it's kind of a gas to know that someone is out there making music with and enjoying a little amplifier that I put together even as simple as this one was. Small beans compared to some of the builders here but a gas nonetheless.

Ok... I'm rambling.

[RicharD]

Ah the pain of seller's remorse.  Hey Plate, I got a question about the Webcor (which I may have asked before.  Did you leave the motor(s) intact?  My flavorite amp ever was an old R2R.  It burned up.  I tried to recreate it twice and neither time did it match the original.  I'm convinced the rumbling of the motor added to it's "special" tone.

[Platefire]

Well I did find a schematic of the CHB10A that can be found on my long ago abandoned Flicker account. Flicker deleted most of my pictures off of this site when they reduced the capacity of the free account and anything over a certain MB had to pay, so I abandoned it-some stuff remains. The schematic is out dated in that I changed the DPDT to a center off and may have only useing one side of the DPDT. Center off being no NFB no extra bypass cap.

 http://www.flickr.com/photos/12623503@N04/

BYW-A picture of the old Webcor and it's schematic can be found on there also.

[jojokeo]

The schematic is out dated in that I changed the DPDT to a center off and may have only useing one side of the DPDT. Center off being no NFB no extra bypass cap.

I like the creativity on the switch where more than one thing or section is affected by it's use. 

[DummyLoad]

After my basic conversion I ended up with this:

http://www.alerich.com/7189.gif

i thought you needed a coupling cap between the wiper of the vol. pot and grid of the SLPI? otherwise the pot would upset the bias of the bootstrapped SLPI?

--DL

[Inca Roads]

The schematic is out dated in that I changed the DPDT to a center off and may have only useing one side of the DPDT. Center off being no NFB no extra bypass cap.

I like the creativity on the switch where more than one thing or section is affected by it's use. 

I built an amp for someone that is very similar to Platefire's design - tweed Princeton with the same kind of 2 stage boost setup using a SPDT on-off-on ; it does work pretty well to offer 2 different levels of gain increase.  I thought it was an original idea at the time, haha. Here's the schem :

[Platefire]

Tubenit IMHO has always been the "Switch King" when it comes to switching cathode bypass resistors and bypass caps. He has done a lot of expermenting not only with different switch combinations but with differnt value resistors and caps and can tell you about what it will do before you hook it up. I got most of my switching ideas from him and of course there are plenty others on the Hoffman site with plenty of experiance in that too.

It's funny back in the 50's and 60's when Leo was creating amps with his help, they were going through a process to make the amps cleaner and cleaner. Now we are doing everything to retain their clean advances they made but also take them back to the tweed days at the flip of a switch. So we take old tweed circuits and add blackface features and take blackfaces circuits and add tweed features. Ain't it Fun!  

[alerich]

After my basic conversion I ended up with this:

http://www.alerich.com/7189.gif

i thought you needed a coupling cap between the wiper of the vol. pot and grid of the SLPI? otherwise the pot would upset the bias of the bootstrapped SLPI?

--DL

I believe you are correct. Since the amp is sold I cannot verify whether I left something off the schematic or if it is indeed wired that way. Sounded really good, though - no obvious issues if that cap was indeed missing.

[tonewood]

Platefire, Do you still like the sound of the Dukane to 5F6A conversion? Should I just copy your conversion or do you have any second thoughts?

Also i have a Bogen CHB-50 to convert. Does anyone have a conversion for that that they like a lot?

THANKS!

[Platefire]

I built a CHB50 and the Dukand both into 5F6A circuits. The Bogen still had the Bogen Doubler power supply. I ended up building the Dukane because I wanted a more pure 5F6A example. The Bogen was built old style point to point and Dukane with a turret board. I found out the Dukane PT bias supply was inadaquate for the 5F6A circuit in that it came up short of voltage needed. I had to tap secondary output of PT and build a Marshall type bias in order get the bias voltage needed.

I never played over a real 59 bassman so I don't have any real reference to know if I made the mark?
Both amps are very loud and clean. I added a extra switchable bypass cap on the Bogen50 on the gain stage on the PI before the cathode follower that gave the amp a bit more flex. Also the Bogen has a Hoffman OT that gave it more bottom. Overall I like the sound of the Bogen better than the Dukane but the Bogen has a bit of a ground hum probably because of the p to p that's irritating to me that needs to be corrected. The Dukane is a bit tinny with its original OT that you have to try to make up for on the tone stack but is quieter. On hind site I would prefer a master volume on both of them. Platefire

BTW-My Dukane layout drawings had a couple of mistakes on them as I recall. You would need to reference the original fender drawing against them for correctness. As far a layout for location of everything, yes, it will work.

[tonewood]

Platefire- Thanks for all the good info!

[Platefire]

Your Welcome! Would like the "The School of Hard Knocks" I been through on these conversions be of some benifit to someone.  Platefire

[Platefire]

Well the Bogen finally sold for $120.00 after having to be re-listed after it didn't sell the first time.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l2557&rt=nc&nma=true&item=250799764849&si=%252Fbwd0SwkkpkcLODXR5OlqBpfZAg%253D&viewitem=&sspagename=STRK%3AMEWAX%3AVRI

[alerich]

That's exactly what I sold my Knight conversion for. Even managed to make a few bucks on it (not counting my time which I do not count since this is a fun hobby).

[Platefire]

Oh my Gosh!!!! I just realized that I was the one who won the bid on this one