Old school Bogen Tube PA (viable Guitar amp?)

[Michael.E]

So... my father passed away not too long ago..
I recently had a chance to go through some of his old stuff
I found this Bogen Amplifier at his house while stopping by the other day.
it's really beat up, and dirty! but curious, I brought it home and took apart
the chassis and decided to take a few pics.. Any ideas?

I had this weird idea, that this might acually be a cool project to experiment
with -- one day, if i ever get the time! Not too worried about ruining it
(since it's already old and busted anyway) and perhaps maybe it'll be a fun
little guitar amp or dirt machine (if I ever do end up deciding to fix it up)
could have some kind of sentimental value too. Any thoughts ideas?

PS: I already removed some of the crud and cleaned the contacts (not pictured)
however, no power it seems.. could the the transformer be busted? or
just in in need of some new tubes and bit of love? Anyone know anything
about this model? Not expecting much, but figured it might be cool to
fix up, since my dad was kind of one of those people who never throws
anything away.. he prob would of dug the fact I fixed this junk up for him

PDF MANUAL
http://www.bogen.com/support/discontinu ... HB100m.PDF

SCHEMATIC
http://www.diycustomamps.com/images/sch ... chb100.gif

SLIDESHOW LINK
http://s38.photobucket.com/albums/e148/ ... 55f81e.pbw

[RefD]

why not?

back in the 80s i would sometimes play my Tele through a borrowed RCA tube PA and Kustom 2x15 cab with the roll and tuck torn off and it sounded huge.

just don't get yourself zapped.

[Michael.E]

I really didn't get much of a chance to research this yet
or whatever.. I just did a quick google search and posted
the results above.. I will most likely look more into later.
but in the meantime, if you or anyone on this board has any
ideas or suggestions, it'd be greatly appreciated

How do I convert this into a guitar amp?
I think it needs a few modifications first (correct?)
and will prob also need a fair bit of repair work too.

The unit doesn't even power up.. *(i don't think?)
the tubes aren't lighting up or anything.. which leads
me to believe it's one (or more) of these several scenarios.

a) blown fuse
b) busted tranformer or PS?
c) It won't power up if one (or more) tubes are dead inside?

[tubetapexfmr]

OK, first things first. Do you know anything about working on tube amps? While it is simple once you know the stuff it is easy to screw up. Kind of like learning a language or instrument. So, that said I know a lot of people harp on this but tube amps have extremely high voltages that can kill if you are not careful, so really it is not something that would be wise to undertake lightly. If you are willing to learn it may serve as a nice first project, though I would recommend something simpler as what you've got here is quite a complex amp.

One good bit of new is that your two preamp tubes are both Mullards, some of the best ever made. The Power tubes and phase inverter look like RCAs. More than likely the tubes are fine. You do need one more 7868 to get up and running. I may have an extra RCA that would match these if you're interested. These old American and British tubes that have been used a lot and have even been sitting for decades usually read higher and will far outlast the best new tubes.

First of all do your testing with the amp unplugged. DO NOT plug it in again until you know more about the amp. Just running a tube amp with no speaker load can burn up the output transformer. With a simple digital multimeter you can test a lot of things that may be the culprit. You can get one for $20 and up pretty much anywhere online that sells electronics.

My guess would be if it is not powering up start from the power cord and trace out with a continuity meter each connection. It may be as simple as a worn out power switch. The slide switches on these old Bogens get dirty and sometimes break so I would look there first. Next check the fuse. If those check out then check the Dc resistance between the power transformer taps. Check the primary, it should read a few ohms, probably 4-8. Next check the secondaries. The High voltage should read about 15-20 ohms. The heaters should read about .5-.8 ohms. If these numbers check out at least you know the power transformer isn't shorted out. Next check the output transformer. The primary should read about 350-500 ohms. The secondary tape should all read at .4 to 1.2 ohms. If the transformers check out then you're golden. Those are the parts that are like the engine and transmission in a car. If they're good the rest is easy to replace. From there I would replace all the electrolytic caps anyway as they are organic and wear out like tires or rubber on an old car.

The Bogens sound pretty good with guitar in stock form, but with some reworking can sound AMAZING! The EQ especially is not suited to guitar at all and the preamp circuits they use waste too much gain. Plus a proper phase inverter it can take it over the top! The Bogens I modify put modern Fenders and Marshall to shame. They will even give the properly tuned classic ones a run for their money. Those old transformers are gold and even the best stuff made today just doesn't have the Mojo of that old iron. If you decide that you can't get it going on your own let me know. I can rework the amp for you into a real monster for not too much money. Good luck and let me know if you have any questions.

[Michael.E]

thanks for the response!

I will grab a multimeter and test out some of your
suggestions this week, and let you know what I find!

Question: In terms of safety..
if the amp is unplugged for more than 24 hours and
(i only had it on for a minute yesterday btw)
is it safe to operate? I don't want to get killed lol
but I also got a few books on the topic of tube amp
repair etc.. So I'm trying my best to read and learn
all I can.. However, I am still fairly new, so thanks
so much for pointing me in the right direction!

[Michael.E]

If you decide that you can't get it going on your own let
me know. I can rework the amp for you into a real monster for
not too much money..

I most certainly will
(yours look great btw)! thanks!

[tubetapexfmr]

Question: In terms of safety..
if the amp is unplugged for more than 24 hours and
(i only had it on for a minute yesterday btw)
is it safe to operate?

You should be fine. There are a pair of 220K resistors in the power supply that will act as a bleeder resistor and discharge the amp properly after an hour or so. If you want to be REALLY careful, use your multimeter to check the voltage from the output transformer center tap to ground. If it reads 0 you're all set.

[Teacher's Pet]

I have one of the 35-Watt Bogen Challengers.

It would be great if any/all of you posted back here with repair and/or modification updates; I'd like to follow along on my own.

I posted some photos here:
http://flickr.com/photos/powerrecordings/2209439471/
There are more if you search for "bogen challenger."
Since I shot these photos, I have cleaned the unit off, replaced a fuse holder & fuse, and the thing powers up and passes sound. I need to (at least) replace the input/output jacks with 1/4" in/outs. That's about as far as I can get on my own. I stopped messing around with it because I got a little freaked out about discharging the caps. I need a *really* easy and simple run-through / how-to on discharging caps and measuring output voltage.

The tips posted earlier in this thread are very helpful. Thanks!

Any like-minded folks in the NYC area should PM me if you're interested in meeting up and teaching/learning any of this stuff. The Brooklyn Amp Builders Union is always looking for new members, especially teaching members.

- Robin

[tubetapexfmr]

I'm actually working on one of those CHB-35A amps this week (and another waiting in the wings). I am almost completely gutting it and converting it to a Marshall Plexi style amp. Of course it has a completely different power supply and different output tubes, but the preamp/eq/feedback is the same and it sure has a very Marshally sound. I will have pictures, modified schematic and a complete description of its construction later in the week when its ready for evilbay.

[Michael.E]

I'd love to see a youtube video or audio clip of
some of your amps btw.. they look straight fire!
Good luck on your sales, Thanks for chiming in too

[Dubious]

I have a CHB35a

$25

werks as is but its def a bit on the funky side, somethings not right. One its not very loud, and there's also a weird mettallic / ring mod type overtones when you play. .a super cool tone nonetheless.. but im thinking about takin it to the local tech and have it gone over

I love old Tube PA heads

i have a Bell And Howell Pacemaker that KILLS (destroyed my buddies Orange Tiny Terror in a shootout)

and my main practice amp is a 20 watt Mcgohan head thru a 4 x 10 cab

i love that all these heads have multiple taps, lots of cool variations by switichin to diff cabs.

has anyone ever used the mic inputs for as a pre?

the bogen has two, and my mcgohan has 2 with balanced transformers, ive been tempted to mod it for direct outs.

here's the herd minus the Mcgohan:

[Electricide]

I've been modding one of two newcombe tube PA's that I found. rodC has the other, and he's been walking me through converting it to a guitar pre and mic pre combo thingy. Can also use the power amp section to drive a cab.

So it's a good idea, granted you have someone to do all the technical work for you

[tubetapexfmr]

my main practice amp is a 20 watt McGohan head thru a 4 x 10 cab

Is it the model M204? I have one of those and I can tell you whatever you want to know about it. First of all it is almost identical in design to the Bogen CHB-35A. The power supply and some values are different but it uses the same tubes and design. AS for mic pre duty? I would say you can really only have a good guitar amp OR a good mic pre from that thing. The problem is impedance matching and gain staging. They are all way out of whack for a mic pre with a line level output. I have used it before as a distortion vocal pre with a SM7 plugged into the McGohan and then hooked up to a 2/10" cab and miked with an SM57. What I would do (and what I AM doing to my McGohan) is to remake the amp as a Marshall plexi style amp. Keep the 7868 Power Tubes as they sound freaking awesome, but replace the 6EU7 with a 12AX7 (or 12AY7) and the 6C4 with a 12AX7 (or 12AT7). Once you do that rewire the preamp, EQ, and driver sections just like a Bassman 5F6-A Schematic. The 7868 tubes really respond well to this design and in my opinion sound better than the 6L6 family. You could still use the mic inputs for FX pres like I described, but direct outs would require a sea change of design.

If yo want to make it into a mic pre then I would think about gutting the whole thing and removing the power section along with the output transformer. Then wire in your favorite fancy regulated tube power supply feeding two channels of your favorite DIY tube mic pre. For inspiration look to this guy: http://www.bottlehead.com/et/music/digitaljimmyhat.htm

[tubetapexfmr]

By the way if anyone has one of these tube PA amps that they want transformed into a guitar/bass/harmonica badass let me know. That is what I do these days. If all you are doing is adding 1/4" jacks to these guys you are really missing out on what they have to offer. I not only replace the jacks with new high-quality switched jacks, but I also clean all pots and connections, add a properly grounded three prong power socket, completely restore the power supply, and relace the preamp, eq, and driver circuits to be more suitable for guitar/bass/harmonica depending on your specific tastes. I also guarantee my work for a year! I can do most amps for $100-$200 depending on complexity with about a two week turnaround. E-mail { tablebeast AT tablebeast DOT com } or PM me.

I'm not trying to discourage DIY here. I'm just offering the option to those who are not DIY inclined theirselves to have a pro do it for them. For those of you who ARE interested in doing it yourself hit me up as well. I am all about DIY and is how I got into working on amps in the first place. I've messed with most of the common American made Tube PAs and have lots of modified schematics to share. My website http://idigtubes.com will eventually cover all the models I've worked on with their accompanying schematics and step by step conversion processes.

[Scodiddly]

I had an old Bogen mixer/amp tube thing I bought years ago, and eventually transformed into a frankenstein Fender/Marshall 50 watt amp. I don't play loud guitar often, so I loaned it to a friend who still hasn't given it back. That was around 10 years ago.

One of these days I need to get back to that Bogen tube turntable/amp thing I acquired from my antique dealer cousin a few years back. That thing sounded great for leads, but I spent more time on the similar product from Califone because the turntable still works.



Lately I've been thinking about building a real rotor (Leslie style) for that turntable.