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Old 12th April 2005, 05:59 PM   #1
Dave W. is offline Dave W.  United States
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Default Rodding Bogen CHB-10A Mono Blocks

Hi, I just picked up a set of two Bogen CHB-10A mono block amplifiers and in an attempt to learn more about my new tube amps was looking through the "tubez for noobz" thread as well as a few of the other tube-centric threads. I read through many of the links within that thread and found them very useful. Thanks much for posting all of those.

In some of the other threads I'd read about people hot rodding tube amps. I did a search for bogen amps and didn't find anything for my model. I was just wondering if any amp can be modded and if anyone knows of any resources for getting a little more power out of these particular amps. I'm trying t drive a set of speakers that use Linaeum ribbons and they seem as though they'd like a little more power.

Thanks.
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Old 13th April 2005, 12:15 PM   #2
SHiFTY is offline SHiFTY  New Zealand
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You could:

1) upgrade power supply with SS diodes and bigger caps to get higher B+;
2)re-bias and re-wire to use EL34s. Might need to add an extra filament transformer;
3)try using fixed bias... Fixed bias EL34s in SE pentode would give you ~15 Watts or so?

However you might not get much of a difference, maybe a few more watts, depending on whether the output transformers can handle the extra current.

It is possible that your power tubes are a bit weak, also... You could try getting a new set first.
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Old 13th April 2005, 02:26 PM   #3
Dave W. is offline Dave W.  United States
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Are those things that I can do one at a time and then check difference or are they dependent on doing them all in a specific order to get the best result?
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Old 13th April 2005, 08:15 PM   #4
PRR is offline PRR  United States
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> Bogen CHB-10A mono block

"mono blocks"??? Gosh, look like cheap PA amps to me.

1) upgrade power supply with SS diodes and bigger caps to get higher B+;
2)re-bias and re-wire to use EL34s. Might need to add an extra filament transformer;
3)try using fixed bias... Fixed bias EL34s in SE pentode would give you ~15 Watts or so?


Not "EL34s": this is a Single-Ended amp!

Pictures of a CHB-10 on eBay

Uses large-Mini tube 7868, which indeed was rated 11 watts SEP. (From 19 Watts plate rating.... they cheat!). (This looks an awful lot like the guts of the original 6L6 in a small bottle.)

No rectifier tube: this is one of the late Bogens with Silicon rectification.

If it is self-bias (Bogen used a lot of fix-bias), and the cost of tube melt-down ($49-$78/pair! On a $50/each amplifier!) is not frightening, converting to fix-bias would give about 7%-10% more plate voltage, 15%-20% more output power (and plate heat). This is at most 0.8dB "louder", which is not going to turn sadness into joy.

Bogen tended to cheap-out on power caps. Myself, I like the Big Stick: go in with 470uFd 450 across the main filter caps. If it blows the 1960s rectifiers, they needed replacing anyway. I once raised some 150W Bogens to 180-200W that way. The increase of power was meaningless, but when run in clipping they didn't buzz so bad. However that was AB push-pull, a single-ended amp won't make much more power with better filtering. (Do it anyway, get the Bogen hummm out.)

Summary and pics of a CHB-10 hot-rod job

Replacing Bogen low-bid parts with fresh new ones may help, maybe a little or a lot. However if 10 Watts is not what you need, any such investment is a waste.

Since it has enough gain for MIC, and we only need line-level input, you could rewire the mini-6L6 as Triode, get a little less distortion, though at significantly less bench-test power (2-3 watts sweet, instead of 10 watts nasty).

> I'm trying t drive a set of speakers that use Linaeum ribbons and they seem as though they'd like a little more power.

Sit closer to the speakers, clean out your ears, learn to enjoy lower levels. Or don't use speakers that depend on vast amounts of cheap power.

Also, if you are not a child of the 1950s, you might not be used to the sound of a naked pentode. This amp will run 15% THD, a mix of 2nd and 3rd, and might sound "strained" when it is really "within ratings". (Bogen may have put a little feedback around it for a better-looking THD number, and so it did not flip-out when all the PA speakers were switched-off, but that doesn't necessarily sound better.)
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Old 14th April 2005, 12:16 AM   #5
Dave W. is offline Dave W.  United States
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PRR - that ebay link you put up kinda confuses be because that doesn't look anything like the ones I bought. I hope it's not a PA amp, but when I bought them I had just started looking into tubes and it seemed like a decent price compared to a lot of others. The guy selling them said he'd been using them to amp his home speakers, of course, being ebay, he could very well have been lying.

Here's the link to the ones I bought:

http://cgi.ebay.com/ws/eBayISAPI.dll...MEWN%3AIT&rd=1

One of my friends here who knows a little advised me they'd make good stereo amps and they did look like nice little (little being the key word) amplifiers. If they are indeed a PA amp... is it even worth trying to amp a set of speakers with them or is it a better idea to just resell them and find something else?
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Old 14th April 2005, 01:31 AM   #6
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I think they could be very nice amplifiers with a good set of output transformers and efficient speakers. The outrput transformers are the weakest link... a good set of Hammonds or One Electrons will cost as much as you paid already. 7868 tubes have become a whole lot LESS rare lately, as you can get new Russian ones for $40 a pair.
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Old 14th April 2005, 12:36 PM   #7
SHiFTY is offline SHiFTY  New Zealand
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I think they are pretty cool as they are.

Try and find a schematic online somewhere, maybe we can give you exact instructions... It is possible that they are limited by the circuit design- how do they sound? Also are the internal components original or new?

They might be designed for PA use which you could improve upon. For example, boosting the power supply caps, increasing the size of the coupling caps for more bass, rewiring the circuit to be more hi-fi.

As another poster has mentioned, they have a lot of gain from the 12AX7. You could use this gain to put a negative feedback loop around the amplifier- SE pentode often sounds a lot better with a little NFB. (Heresy! )

You could always rebuild them from the ground up- i.e. find a new schematic online (say a single-ended EL34 ) and strip the amps right to the sockets. However thats a bit of work... but really fun.
You could use better output transformers if you really wanted to spend some money.

BTW If you look closely at the hot rod link given, they are the same only missing their outer (ugly) cabinets, good riddance!

Those do look in very good condition and were a good buy.

keep it up
H
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Old 14th April 2005, 01:51 PM   #8
Dave W. is offline Dave W.  United States
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Excellent. Thanks a lot for the advice guys!!! I'm going to scour the net for some schematics. If I don't find any I'll draw up a set myself, probobly not quite as nice as original, but it'd get the job done. I'm also going to open them up and see what the wirings like inside. I'll let you know exactly what I find.
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Old 17th April 2005, 04:20 PM   #9
Dave W. is offline Dave W.  United States
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Alright, I've been looking over the net the last couple of days and havn't found any schematics yet. As soon as a have some extra time again I'll sit down and draw out a set myself.
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Old 17th April 2005, 05:06 PM   #10
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If I get a chance, I'll scan the Bogen CHA-10 - single-ended 6L6 - my be similar.
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