Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Seegs on July 22, 2012, 06:40:48 pm
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Hey guys, I recently aquired a Bogen CHB 20A tube amp with a bad tube. i've moved the mic volume pot over to the aux and put a 1/4 input in instead of the mic input on the back and took out the aux input circuit. My Question is while i am waiting for the new ecl86 tube to get here i read that i need or should bias to the tubes. and also found out that these tubes with this wiring are run very hard. I was just wondering with this amp is it possible to add a Bias trim Pot to adjust the Bias on the tubes because i wont be getting a matched pair. and i saw but couldn't find out how to drop this amp from 20w to 18w to try and be more kind to the tubes.
Thanks
Seegs
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Bogens were run 24/7 for years without matched tubes or bias adjusts.
Tube it up and play it.
http://www.emailtom.org/images/bogen-chb20a.jpg (http://www.emailtom.org/images/bogen-chb20a.jpg)
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Being kind to the power tubes for me is having a little colder bias for tone reasons. Smoother tone and later breakup. If the amps bias is good and solid then you wont need to change the bias coupling caps or bias resistor out for new ones yet. If you just want to hear the difference in tone between a Hot vs Cold bias then raise the value of the cathode resistor to the power tubes and you will have a colder bias and slightly less wattage as well. If this amp has that. I havent looked at one in a long time. Like PRR said: Plug and play. No reason to do all the work to put in a bias trimmer pot. Unless your as hard headed as i am anyway. Live and learn/
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Thanks for the responses, Good to know that i could run this without a matched pair. these 6gw8 tubes are a decent amount of money for a matched pair. can't wait to see how this sounds.
Seegs
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Is that amp cathode biased? If so, you can use separate cathode resistors and bypass caps for each tube and get away with less than ideal matching. I read about that in Mullard's book on audio circuits for tube amps and I've put it in two of the amps I've built so far. It does seem to work, although it is slightly more complicated and uses a few more parts and a bit more space if you are building with a board.
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not cathode bias - fixed bias; with -16V, which is a deep bias for that tube and 400V exceeds the mfg. max. ratings by 100V. bogen knew what they were doing. the OT's are bare-minimum spec it does seem, as small as they are for the power rating and as fragile as they are. a significant percentage of bogen PAs that have come across my bench had/have fried OTs. the CHB series with the FW SS doubler PS and fixed bias are my least favorite. the CHA and K series with tube rectifiers and cathode bias rock! the CHA20 with push-pull 6V6GT operating cathode bias and 6AX5 recto is my favorite so far - i think tubenit has similar opinion. i gave that amp away - somebody please kick me! ;)
--DL
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not to hijack "much" but I've got an M330A that I picked up about a year ago, never got around to messing with yet. Been thinking about replacing my solid state (and really cheep) amp for the stereo with it. Any issues I should be aware of? It's in almost pristine condition, I'd hate to just make another guitar amp out of it.
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john, looks like a spin of the CHB series with the same type low frequency challenged output iron. if it were mine, i'd hack another guit amp out of it. bogen did make some hi-fi stuff, but it doesn't seem to be as "common" as the PA amps.
link to a bogen integrated stereo amp here, (http://www.ebay.com/itm/VINTAGE-1960S-BOGEN-AP-35-TUBE-AMP-AMPLIFIER-WORKS-/350581982964?_trksid=p4340.m263&_trkparms=algo%3DSIC%26its%3DI%252BC%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D15%26pmod%3D260981952307%26ps%3D63%26clkid%3D834640017747208845&_qi=RTM1062691#ht_500wt_1180) notice that even for a 35watt/ch unit the OTs are smaller than used on other brands with similar output power. link here (http://www.ebay.com/itm/Working-Dynaco-ST-70-Stereo-Tube-Amplifier-Dynakit-/360475269513?pt=Vintage_Electronics_R2&hash=item53ee001989#ht_25041wt_1396) to a dynaco stereo 70 for comparison.
--DL
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DL, thanks for the info. I see what you mean about little OTs. :laugh: And when I was posting, I wasn't thinking that it's not stereo (doh!) and I even knew that!
A gittar amp it is then (at some distant time)