dougharps
419 posts
Jul 14, 2013
9:49 AM
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A while ago I bought an old Webcor tube tape deck extension speaker cab that had a power amp built in, but no preamp. The preamp was in the recorder, which was not at the antique store where I bought the cab. It is push pull. I drove it with a Danelectro Fish and Chips EQ until a friend built me a preamp section as a gift.
My friend built in several input impedance choices, so I can use a variety of mics. It now has base and treble controls, gain and volume (though I usually open up the volume and just use the gain). It has a boost switch that uses another half preamp tube for when I need the gain with a weak mic. It is a great little amp now, using an 8" Weber Alnico Sig 8. It sounds good through my 12" Cannabis Rex, too!
The 12AB5 power tubes sound great, and I wondered if anyone into amp experimentation had ever used them in a single ended or push-pull project, or had an amp using them.
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Doug S.
Last Edited by dougharps on Jul 15, 2013 12:14 PM
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HawkeyeKane
1881 posts
Jul 15, 2013
10:53 AM
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This is the first time I've heard tell of 12AB5's. From what little I've read since seeing this, apparently they're mini-novals considered similar to 6V6's. Ideal applications for them were in car radios. Guess that makes sense since yours are in a tape recorder. Other similar tubes were 6AQ5's, 6BW6's, and 6CM6's. I do love it when oddball mini-noval power tubes like that prove themselves in a harp capacity. Is there a manufacturer stamp on either of them? Looks like tubedepot has them for a steal.
Who's your preamp builder Doug? Anyone I know?
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Hawkeye Kane
Last Edited by HawkeyeKane on Jul 15, 2013 12:14 PM
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dougharps
421 posts
Jul 15, 2013
12:13 PM
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He is a guy who would prefer to not be named as an amp builder. He is affiliated with the University of Illinois and he loves to study oddball tube amp circuits. He is not in the amp repair/building business, and he just did it for me as a friend as an experiment. I just paid for the parts he needed.
It was an interesting powered 3 way cab, but now it is a cool little amp with an 8" speaker, though on a loud stage it might not be loud enough (8 to 10W).
I mic'd it to two 10" powered monitors on one occasion, and it sounded pretty big!
I just wondered if anyone on MBH had messed with these tubes before. The sound does remind me more of 2-6V6s as opposed to 2-EL84s.
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Doug S.
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HawkeyeKane
1882 posts
Jul 15, 2013
12:40 PM
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Do you have a picture of, or model number for the power amp? I think I may have a schematic for you.
I'd bet a Harp Attack or Holmes Commander would sound pretty killer through it from what you're describing.
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Hawkeye Kane
Last Edited by HawkeyeKane on Jul 15, 2013 12:47 PM
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Rick Davis
2110 posts
Jul 16, 2013
4:19 PM
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The 12AB5 is pretty obscure. The only place I know of it being used is in old Conn and Webcor organs. As far as I know I have never heard any amp that used this tube, so I can't offer any opinions about it.
Tim, I have a question: Since you have never heard the tube or any amp that uses the tube, how did you form the opinion that "a Harp Attack or Holmes Commander would sound pretty killer through it"? That seems out of left field.
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-Little Rick Davis
The Blues Harp Amps Blog
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tmf714
1850 posts
Jul 16, 2013
4:48 PM
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The 12AB5 is actually a 6V6 tube. It can be used as such-
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Rick Davis
2112 posts
Jul 16, 2013
4:54 PM
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"The 12AB5 is actually a 6V6 tube. It can be used as such-"
Tom, are you sure about that? I thought the 12AB5 was a miniature 9-pin tube, like an EL84.
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-Little Rick Davis
The Blues Harp Amps Blog
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Last Edited by Rick Davis on Jul 16, 2013 4:56 PM
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dougharps
422 posts
Jul 16, 2013
5:18 PM
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Here is a data sheet:
http://www.tubebbs.com/tubedata/sheets/079/1/12AB5.pdf
On a Hoffman Amp tube forum a moderator claims they are like 6V6 except for the heater, but they are 9 pin, as you noted:
http://www.el34world.com/Forum/index.php?topic=7327.0
I emailed the power section schematic to Hawkeye as I am not presently set up with online storage for posting.
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Doug S.
Last Edited by dougharps on Jul 16, 2013 5:20 PM
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HawkeyeKane
1888 posts
Jul 16, 2013
5:46 PM
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@Rick
Well....considering that he drove it with a Fish & Chips until he had a preamp fabricated by his friend, I don't think it's unfathomable that a harp-specific preamp unit would undoubtedly sound good with it. That, and I got a look at the schematic Doug sent me, and the output section appears pretty similar to that of a 5E3 Deluxe. Doug...do you have the means to test my theory?
And on the point Tom made...yes, they are indeed mini-novals. However, they COULD be made for use in an octal socketed amp by way of adapters, like VHT's or Yellow Jackets.
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Hawkeye Kane
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Rick Davis
2113 posts
Jul 16, 2013
5:58 PM
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Tim, Doug did not say it sounded good with F&C, he said it is a great little amp now that he has a real preamp.
How is the output similar to a 5E3, and how does that that convince you that this amp would sound "killer" with a Harp Attack or Harp Commander? Really, I am trying to understand your thinking here.
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-Little Rick Davis
The Blues Harp Amps Blog
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dougharps
423 posts
Jul 16, 2013
8:34 PM
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Rick, if I could find your email address, I would send you the schematic that was in the powered cab.
I used to have an online account I could use to post audio and pictures, but it mysteriously vanished a while back, and I haven't set up another.
The cab didn't sound as good with the Fish & Chips as it sounds now with the preamp and Weber, but it was good enough that I gigged with it once at a bookstore coffee shop. Harmonica jazz in a group called Groove Yard, with harp, guitar, bass, and drums at relatively low volume.
I don't have the Harp Attack or Holmes Harp Commander, so I can't test Tim's theory, though I do have an input my friend built in that is supposed to bypass the preamp.
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Doug S.
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Rick Davis
2115 posts
Jul 17, 2013
7:34 AM
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Hey Doug, my email address is bluesharpamps@gmail.com. Your amp sounds like a fun little project. I'd like to hear it if you get a chance to record and post it somewhere.
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-Little Rick Davis
The Blues Harp Amps Blog
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Rick Davis
2116 posts
Jul 17, 2013
11:08 AM
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Here is the schematic. Sorry about the size; I left it big to make it more readable:
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-Little Rick Davis
The Blues Harp Amps Blog
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dougharps
424 posts
Jul 17, 2013
2:05 PM
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Here are pics and audio for the Webcor project amp. I used a Brown Biscuit with 520D element first, with the amp at 10M impedance. I switched to 1M for the EV630 and Shure 585SAV. The voice volume is low, the playing is not flawless, but what the heck, it is an amp demo that I put up quick and dirty.
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Doug S.
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Rick Davis
2117 posts
Jul 17, 2013
3:50 PM
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Doug, it sounds great! It has a nice warm tube sound.
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-Little Rick Davis
The Blues Harp Amps Blog
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