Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: vampwizzard on June 24, 2020, 11:26:19 am
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With the tweedish gibson stable and functional im turning my attention to my next project. Ive got a blank chassis to fit a gutted blues jr cab. Looking to do an AB763 this time around.
Hopefully I can fit a deluxe vibrato channel with reverb into this chassis. As a testament to his amp chops.. research IMMEDIATELY pulls up Sluckey's TDR https://sluckeyamps.com/tdr/tdr.htm (https://sluckeyamps.com/tdr/tdr.htm) and this is pretty much what Im after. Ill probably condense it down to one input and possibly figure out a different way to handle the bright switch, and possibly relocate the fuse to inside the chassis.
Sluckey: Any opinions on replacing the reverb circuit on your design with a one tube variant? And without the extra channel tubes im coming up with a princeton-ish power transformer as a better match than a deluxe. Is that a fair assumption? Id still probably use the deluxe sized OT. Finally, The tweed cab you used is similarly sized to the blues jr with the blues jr being about 1/2" less wide. Any tips on what to look out for aside from the great documentation on your website?
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What is your concern for going with a less robust PT? Footprint? if so I don't think any PR PT is going to be good enough current wise except maybe the Hammond 290AX.. or the Allen amps TP25. The PR PT from most manufacturers are only rated at 70ma. That is pretty skimpy for a DR even 1 single channel. Also if you're worried about space you could use a stand up PT. I think Hammond would have something appropriate.
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sorry, didnt have reply notification turned on :BangHead:
I was looking at a 4 preamp tube calc coming in around 85ma putting the hammond 290X square in the ball park. Its not so much size but the 290X is a bit smaller but delivers 100ma. Its also about half the height off the chassis (depth to the speaker) than a classictone blackface deluxe xfmr.
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Updates:
Ordered hammond xmfrs from the 63 deluxe reissue. 290CX PT and 1750H OT. 194A choke and 1750A reverb driver. Took Doug's one channel AB763 board and deleted the trem from it. Trying to get it to import correctly into the custom board designer.. right now it doesn't shorten the board despite the project and board sizes being shorter than what its importing.
Anywho. Excuse my component layouts.. it was hard to maintain everything with the grid setup I had. The turrets are good though.
Sluckey: I saw that you had 0.047 coupling caps in your TDR compared to the standard 0.1. Did you prefer that in your build?
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yes. That's a popular mod to any AB763 amp.
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Found a fun bug in DIYLC thats addressed on that particular forum section https://el34world.com/Forum/index.php?topic=26166.0 (https://el34world.com/Forum/index.php?topic=26166.0)
Was able to get a board together that imports correctly and its updated. Long story short: the XML file doesnt work correctly with the dimensions set to "explicit dimensions", but it is certainly fixable in the XML file itself.
Next question regarding Doug's AB763 single channel: the schematic and parts list shows the bright switch, but his build and board/layout diagrams dont have it in there. The deluxe (my aim) has a 47pf hardwired in and Ive heard a lot of people clip it out or converts it like in the Dumble cool mod. C11 on the One Channel AB763 shows a 120p cap to a switch. Its been awhile since that projects been touched (it appears) and it went through several revisions. Not sure whats up with that.
Does anyone have preferences on the bright switch? Sluckey, I'm assuming since it matches your TDR schematic that you prefer that value?
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yep
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Doug was DAMN quick on the fulfillment/shipping! Thanks man! Gonna have a fun holiday weekend.
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Didn't realize how many solder connections were in an AB763 and boy did that take forever! Plate voltage was around 450 with a GZ34 so swapped in a 5Y3 and got it a bit lower. 78% diss. was the best I could hit on the bias pot and it sounds real nice. Used a short tank blackface replacement and its still pretty aggressive.
Things I would do differently: mount the terminal board closer to the center of the chassis line to give me more room at the pots. Also would not recommend trying to cram this into a blues jr chassis... the cloth push back wire saved me here.
Orange lead off the PT to input power ground is a electrostatic shield (ESS) for the PT. ESS's should be grounded at the incoming power ground but in a tube amp I doubt it makes a difference.
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Video and pictures of it in its blues jr cabinet. https://www.instagram.com/p/CC9f5zVJZ7Y/?igshid=5mu20xyq3ewt (https://www.instagram.com/p/CC9f5zVJZ7Y/?igshid=5mu20xyq3ewt)