Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: a_e_a on January 29, 2025, 09:47:16 pm
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Hey guys,
Just getting a bit lost figuring out the wires for the drri in my 2 channel ab763 build. In Australia so we are 240v here.
I did try a search and couldn't find the info. If this has been asked often I'd be happy to be directed to another thread or get some advice.
I've added a pic which shows the ptand the excess of wires.
Cheers.
AA
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This explains all...
https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_manual.pdf
See page 6, coordinates B1.
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This explains all...
https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_manual.pdf
See page 6, coordinates B1.
Your link ;
Not the same wires colors
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Home built AB763 ; where this power transformer comme from ?
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Thanks for the replies. I well give them a look and see how I go.
Sorry for the confusion, it's not a scratch build. It's a drri conversion with the hoffman 2 channel ab763 board. So the transformers are being utilised for the time being.
It's a big learning curve and a bit of a departure from previous straightforward builds I've done.
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Look here ; many hoffman and other transformers.
And show schematic and all information about this Hoffman 2 channel ab763
https://el34world.com/schematics.htm#Transformer%20wiring%20diagrams
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Hi a_e_a
Successfully did this with Doug's board.
See attached photos
Made one change, removed C31+C32, used 47u 500V F+T Electro, more reliable :laugh:
Regards
Mirek
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C31 in series with C32 = 1/2 capacitance
2 X 47 mfd = 23.5 mfds . IMO it is very small, why not two 100 mfds ?
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C31 in series with C32 = 1/2 capacitance
2 X 47 mfd = 23.5 mfds . IMO it is very small, why not two 100 mfds ?
No. C31 is 220µF and C32 is 47µF. Put those two caps in series and the total capacitance is 39µF. You must use the reciprocal of the sum of reciprocals formula to solve for total capacitance when caps are not equal values.
Notice the two balance resistors are not equal value either. Seems illogical to me. Who can say what the Fender engineers were thinking. Maybe they were shooting for 32µF as found in the original AB763 and were working with common cap values that they had on hand. Could be a bean counter decision.
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I'm talking about what Glass54 do ;
To replace C31, C32 by 2 X. 47 mfd
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Your link ;
Not the same wires colors
Are you sure? What looks different to you?
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Your link ;
Not the same wires colors
Are you sure? What looks different to you?
Where is the violet wire I see on the transformer primary picture
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Where is the violet wire I see on the transformer primary picture.
It's on the top side of the picture.
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C31 in series with C32 = 1/2 capacitance
2 X 47 mfd = 23.5 mfds . IMO it is very small, why not two 100 mfds ?
The Fender schematic shows 396dcv at the 1st B+ filter cap. And the amp uses a slow warm up rectifier tube, 5AR4/GZ34.
I think glass54 used just 1 ecap at 47uF/500v. His hi-lited scemo says "Fit 47uF 500v." Doesn't say Fit 2 x 47uF 500v. Should work.
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Where is the violet wire I see on the transformer primary picture
I clearly see a violet wire on the top side of the PT. The only primary wire I can't positively identify is the BLK/RED, but I bet it's there. Need a different camera angle or perhaps a_e_a will verify the colors match the schematic.
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Wire that DRRI PT for 240V mains like this. Connect the BLK/GRN and BLK/YEL wires together. Connect the VIO and BLK wires to the mains circuit. Tape off the three unused primary wires, ie, BLK/RED, WHT, and WHT/BLK).
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Thanks Willabe. See attached photo of final Dogbox. 500V was sufficient. and replaced the second Electro as well (47u 500V F+T)
..and Sel49: Fender's DRRI PS with 220uF and 47u in series seemed to be a common point of failure here in Aussie.
so better solution was the 47u 500V F+T
I also built a Single channel "DR style" with custom toroidal Tx and SS Bridge into C1with 100uF 500V (Gives a fairly stiff PS but not everyone's cup of tea) I used Doug's Turret Board for that project. (Fantastic result Doug :laugh:)
Kind Regards
Mirek
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Violet and blk/red are there.
Thanks for all the info guys. It's so handy to have such knowledgeable people here. I'll keep you updated with how it goes.
I had also replaced c31 and 32 with a 47uf before I gutted it. I was going to run the caps inside rather than use the doghouse this time.
Here is a pic
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Be sure to keep the two pole filtering on the bias supply (you can just add a 100v 8-22μF cap to the wiper and shell of the pot, with correct polarity) with these PTs. For some reason, the 100μF filter cap isn't enough with these.
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As Strato said.
Best performance is with a 2 stage RC Filter, Not recommended to go too big with C values.
See attached Sluckey's schematic (typical 6V6 application).
Adj R38 and R42 to suit application ie AC supply voltage and DC Bias range :laugh:
If you're keen, you can read more http://www.valvewizard.co.uk/bias.html (http://www.valvewizard.co.uk/bias.html) This explores failsafe Bias mod in case the trimpot goes open circuit
Kind regards
Mirek
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Thanks guys. All worked well. Hooked it all up now and the only problem I have is the reverb not working. Couldn't expect to have everything work right straight up though. haha. So the project continues...
Thanks again for all the help guys!
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Just to inform you all, I’m an idiot. I left the foam in the new reverb tank when I installed it, hence the no reverb. That could’ve been a lengthy exercise if I hadn’t checked everything!
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Some photos. Happy for some feedback. It sounds pretty spiky and bright. Figure there is something going on somewhere. Gonna have to run through the circuit again to check it all.
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More
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And more.
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Here's a great way to double check a build;
https://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630
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It sounds pretty spiky and bright.
You forgot to connect the feedback wire from the speaker jack to the 820Ω on the board.
EDIT...added pic
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Thanks!
I did go through the layout but the output jacks are on another page so I missed it. You just saved me hours of going through the layout before checking the other docs.