Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Texson on September 01, 2019, 03:57:20 pm
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Hello all. I bought a complete AO-29 that should be here by mid week and would like some advice. First I have to confess that I'm a bit of a noob but not 100%. I built my first amp a few years ago which was a 5F2A. It wasn't a chassis kit. It started with a scrap sheet of 16ga aluminum that I bent, drilled and cut according to the layout I wanted and believed would be noise free. I read, researched and sourced every part because I wanted to improve the learning experience. I even got sucked into the capacitor brand, year, color, smell, taste, tone void and survived.
I'll likely gut the chassis of the 29 and my question is: What would be a good amp to make from the parts? I am aware that changes are needed to tame plate voltage with this PT but I don't know what the OT is capable of or if it's worth using. I'd also rather hammer out and drill a new chassis than reuse the old one, but exactly what it's going to be will remain in the air until I settle on a circuit.
My current plan is to check the parts, pick an amp and see what's left over or needs ordering. I really like the tone of 5E3's and since my first was a 5F2A it shouldn't be too difficult to make the jump. I also like the Vox and Marshall sound but I want to keep this around 12-15 watts max. The 29 will arrive with stock tubes but they don't have to stay married to it. I'll use EL84's, 6V's or 6L's depending on the circuit I end up with.
I'd welcome your suggestions. What would you build with these parts?
Thanks for any tips or advice.
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I typed in AO-29 in the search box right of your avatar, picked these for starters
happy hunting reading, building.....
http://el34world.com/Forum/index.php?topic=20482.msg216950#msg216950
http://el34world.com/Forum/index.php?topic=22224.msg236967#msg236967
http://el34world.com/Forum/index.php?topic=24756.msg267559#msg267559
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I typed in AO-29 in the search box right of your avatar, picked these for starters
happy hunting reading, building.....
http://el34world.com/Forum/index.php?topic=20482.msg216950#msg216950 (http://el34world.com/Forum/index.php?topic=20482.msg216950#msg216950)
*snip*
I just read that thread yesterday after seeing an AO-29 on fLEAbay. In the thread, the AO-29 turned into a Hoffman 6V6 Plexi build, though in the end 6L6s were subbed.
In that thread, Steve Luckey said: I do not recommend using that 380-0-380vac PT from that AO-29. Your B+ will be way too high.
But when I look at the finished photos, the PT and OT look to be original. It's not economical to buy the AO-29 if I can't use the same iron for the Plexi (or some other build) if the B+ will be too high.
I would appreciate any thoughts on this.
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if the B+ will be too high.
If you understand what you're buying and what it's capable of then you can plan for it, but ya, using >400vdc for 'el84s might not be good planning, on the other hand EL 34's or KT88s really like it :icon_biggrin:
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Thanks for the links.
I read about the high B+ and bought one anyway (cheap) hoping I could find a circuit that would work with it, or maybe find a way to bring it down. It's what I have to work with so I'll look at the links in detail.
I searched Google and found some info but Google also returned many other AO-xx conversions. Just not a lot of good ones on the 29.
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https://ampgarage.com/forum/viewtopic.php?f=6&t=24331
Lots of pages of Hammond conversions here
With respect, Tubenit
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https://ampgarage.com/forum/viewtopic.php?f=6&t=24331
Lots of pages of Hammond conversions here
With respect, Tubenit
Looks like a goldmine! I'm in. Thanks.
Randy
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Good timing with this thread, I just picked up my third AO-29 chassis the other day, and I'm currently deciding what to do with it.
My first one I partly gutted, and converted it into a version of tubenit's (?) HoSo56, using the original chassis and using the existing 6AU6 pentode in place of the original 5879. Schematic attached, you might find it interesting. The second one I stripped, shortened the chassis, and converted it into a 6V6 plexi. The AO-29 OT's are really good with 6V6s IMO, definitely use it if you can.
As far as voltages go, only the early ones with the field coil speaker had crazy high voltages. You probably got one of the later versions (they're a lot more common I think?) and while the voltages are still a bit high you'll be fine if you keep the 5U4 rectifier or use a 5Y3. Mine have all ended up with right around 400V on the 6V6 plates using the original 5U4. I've put my as-built voltages on the Hoso56 schematic so that should give you a pretty good idea of the reality.
Hopefully this helps! Let me know if you want any more info. Cheers!
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Here's a link to a video from a guy who did both Plexi and a AB763/Trainwreck conversions from AO-29. I am not cluefull enough to vouch for his science skills, but the voltage reduction by center tap resistor or zener diode seems to be a commonly used method to bring the B+ down. Cluefull people please chime in.
https://www.youtube.com/watch?v=bSMnopIPyVI&t=608s (https://www.youtube.com/watch?v=bSMnopIPyVI&t=608s)
Crying baby in background is real, not a projection of his frustration. :laugh:
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Crying baby in background is real, not a projection of his frustration. :laugh:
Thanks. For a second I thought it was a new kind of pedal.
Good video!
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Texson,
I built a tweed tremolux and a JTM45 using 6L6's on Ao-29 hardware and everything seemed to work out ok. It's a great set of iron but you may wish to upgrade the output transformer. It has a lot of current and NFB amps may be more difficult to dial in properly... but the JTM45 build is on the road with a Nashville artist right now and he loves it.
Youve done the right thing.
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Texson,
I built a tweed tremolux and a JTM45 using 6L6's on Ao-29 hardware and everything seemed to work out ok. It's a great set of iron but you may wish to upgrade the output transformer. It has a lot of current and NFB amps may be more difficult to dial in properly... but the JTM45 build is on the road with a Nashville artist right now and he loves it.
Youve done the right thing.
I greatly appreciate the upvote and info for the AO-29. Seems like much of what I've read is hit or miss with most complaining about high plate voltage. My thoughts at the moment are not so much about converting it but in gutting and starting over with a build using whatever I can.
I knew when I bought it that the chassis would be longer than I wanted but Hammond known for good iron so I went for it figuring I could cut, modify, or make my own once I figure out what to put in it.
I'm not a total noob builder but I'm in fairly deep for my level and have some thinking to do with this jumble of parts. My first amp was a slightly modified 5F2A circuit that started life as a flat sheet of 16ga. aluminum. I wanted a learning experience - not a kit (no offense to kit owners). Did my own design and parts layout and it turned out really good. Very quiet with clean tone until breakup starting at about 4.
What I had in mind a week ago was a 5E3 style or similar 15'ish watt amp so I'll give the Tremolux schematic a look. I don't have much in this so if I can use the PT, sockets, strips,and tubes I'll be money ahead, if there is such a thing. My projects almost always end up costing 4 times what I plan for.
Having a JTM45 clone sure wouldn't hurt my feelings either. It would be hella stout for what I'm looking for but that tone! Wow.
Thanks!
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Having a JTM45 clone sure wouldn't hurt my feelings either. It would be hella stout for what I'm looking for but that tone! Wow.
Thanks!
Its not too bad until you jumper the channels together. Then it gets wild.
Enjoy the build! Document it so we can see it. Looking forward to it!