Rodding Bogen CHB-10A Mono Blocks

[planet10]

Originally posted by Dave W.
that ebay link you put up kinda confuses be because that doesn't look anything like the ones I bought

Looks like yours have just been stripped of their outer enclosure -- someone looks to have done some work towards converting them from PA to home-hifi (ie isn't that a choke on the left-hand side (something i've not seen on any of the Bogens thru here).

They can probably be made a decent little amp

(i think i have an original Quad of those output tubes kicking around)

dave

[PRR]

> that doesn't look anything like the ones I bought. I hope it's not a PA amp,

Oh, no! It is the lowest-price Bogen PA amp that year, but not "****". I've had a LOT of fun with Bogen PA amps.

I wouldn't say "I basked in their narcotic radiation", but then I'm not selling one. Fine amps, and NOT transistor.

I'm not sure I would bid over $100/pr (you were not the only one bidding that high), but they are very clean and claimed to be in excellent working order. A clean recently-checked hi-fi amp would sell for a lot more, even if it was really about the same guts inside. Since you are just using the line input, the fact it has a MIC input and a 70V output is a non-issue.

> is it even worth trying to amp a set of speakers with them

Sure!!! PA Watts are fine, you just got 10 of them.

I recently spent a very nice month with a MUCH older sicker PA amp (however with a very efficient speaker, not those meek ribbons). There's a magic in SE, even SEP. My all-time favorite amp is a SE-Pentode with just 1.1 Watts but a bunch of overall feedback.

> with a good set of output transformers

Yeah, but as you say, re-ironing an amp costs as much as another amp. The Bogen outputs were not horrible. Aside from squawk-box and background music duty, Bogens were used as industrial amplifiers. My 150W Bogens spent a decade grinding out 59.9Hz 120V power for film-video conversion (back in the old days when a frame-store cost a million bucks). I recall they would not make full power to 20Hz, but few speakers go to 20Hz. Not much music does either. Better transformers are an idea but I think you need to fix-up nearly everything to enjoy the full benefit.

> Try and find a schematic online somewhere

Bogen used the "same" schematic over and over. Tom's CHA-10 is probably 99% the same, different output tube. While I could trace a 2-tube amp quicker than find a schematic, Dave might find it helpful to have any ~1960 Bogen plan handy for guidance, and just verify any differences.

> It is possible that they are limited by the circuit design

By this time, Bogens were straight-forward and pretty fault-free. The super-6L6 output tube helps (Bogen loved "improved" tubes). It only needs about 10V of grid drive, so even if there is huge drop in the B+ filter to the 12AX7, the driver is not straining.

Bogen did use larger plate resistors than today's fashion calls for. Maybe 470K, not 47K. This clearly reduces bandwidth. But if you wrap feedback around the transformer (this one probably does), you often have to add compensation caps. Just up-scaling the drive plate resistor does nearly the same thing and a dime cheaper.

> isn't that a choke on the left-hand side (something i've not seen on any of the Bogens thru here

Were those mostly push-pull?

I think on SE of that vintage and power, you "have" to have a choke. Yes, 1 Watt radios lost their chokes, but at that scale you put a little more money in the filter-caps and it is good-enough. (Actually, many-many radios leaked some ripple through the output transformer backward for semi-cancellation.) On my 1940 10W PA amp, the choke is as big as the (pretty big) output iron (the choke has more load: it fed the speaker field coil too!). Past 1970, the price ratio between iron and electrolytics reached a point that a C-R-C filter could work OK (especially with overall feedback), but nobody noticed (the 10W amp field was all sand-state by 1970). But look at Fender Champ guitar amp: C-L-C filter to the end of original life, and on nearly all modern clones. You either need much-more than 16uFd, more than 40uFd, to keep the buzz down; or you stick a choke in.

[Dave W.]

This is all extrememly helpful. Thanks Guys!

[Tom Bavis]

eMail me for the CHA-10 schematic - tbavis(at)rochester(dot)rr(dot)com

[pinkmouse]

Dave W, please don't swear. It is very annoying having to clean up behind you.

[planet10]

The "choke" in the attached picture from the auction, doesn't seem to also be in the pics with tubes... be interesting to see the underside.

dave

[planet10]

But there are different (fresh) electrolytic cans, and other tidies... i'm guessing before & after shots.

dave