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Bogen DB20 - junk?
|Posted on November 18, 2013 at 00:32:39|
Joined: November 18, 2013
|I am wanting to get into tubes soon. I like many kinds of music, rock and jazz mostly. I am intending on getting one of Bob Latinos ST-120 DIY kits when my budget allows. I have been hanging onto two Bogen DB20s monos for years. It dawned on me that maybe there is possibility with them. One works, the other hasn't been tried. Are these particular Bogen amps worth having someone refurbish? I know nothing about working on them. So for now, I will have to send them out to someone to go through. And, I think I read somewhere that some feel certain modifications can be an improvement while they are opened up. Am I wasting my energy and money? Is there any comparison with the amp I have been intending to get from Bob?|
|RE: Bogen DB20 - junk?, posted on November 18, 2013 at 05:59:27|
Joined: June 6, 2002
|RE: Bogen DB20 - junk?, posted on November 18, 2013 at 07:25:00|
Joined: March 20, 2006
Depends on how you're going to use them. What you have basically is two mono integrated amps. If you use a preamp you'll have too much gain. Not to mention some pretty lousy controls and cheap parts in the signal path. Upgrading could get expensive.
If you just want to use them as power amps you will need some means of bypassing the the first two tubes and all their associated circuitry. Not a big deal doing that.
These are also pretty old, so bringing them up to modern electrical standards is important.
|RE: Bogen DB20 - junk?, posted on November 18, 2013 at 07:55:09|
Location: Cape Cod
Joined: September 12, 2008
Bogen constantly updated their products with "series" of the existing product line. Check that the two Bogen amps you have are, if not the same series, at least have the same transformer numbers.
I would love to have two Bogen DB20 of the same series.
|If it were me ...... , posted on November 21, 2013 at 17:58:30|
Timbo in Oz
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
Since: June 25, 2014
Ditch all those pre-amp tubes AND upgrade the splitter driver stage with a 6CG7 or a 12FQ7 in LTP mode and a single FET CCS to load it. You might still need a single twin triode gain tube in front of it an the simplest and best sounding would be a good 12AY7. NOT a 12AT7, IMO.
Upgrade the PSU
- with parallel SS diodes (soft recovery type) around the valve rectifier which then become less $$$ to buy new ones (ELI D has posted here about this a LOT,)
- and put in bigger (> uf) PSU caps, 100degrees c rating and by pass the last two
This will give you a bit more B+ and a bit more power (<3db though!). the amp will be a lot quieter.
For the power amp circuit you now have, upgrade all the small signal R's - double the wattage at least - and use MF resistors - a mix of brands, but the same make/model in each particular spot. This will audibly lower noise.
Use a mixture of good-sounding MF plastic caps. Sprague Orange drops are fine as IMO are WIMA MKPs. again IME double the voltage rating helps relax things sound wise, and use at least two brands -one dullish and one a bit brght. If you must use an oil-filled cap somewhere use a plastic type and for the cathode-bypass on the output stage.
This IS an NFB amp so don't spend $$$$$$$ on parts. EXCEPT for the caps and R's that set the NFB loop/s, where Holco / bulk foil Rs and teflon (commercial ones) caps are worth the money.
Do try less and more NFB. IF you can play with separate NFB on the input and LTP stages - less will likely sound better. esp if you CCS both input tubes, paralleling the gain tube. A bit more around the output stage and transformer might sound better but do experiment.
Skeptical Measurer & Audio Scrounger