Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: nandrewjackson on November 03, 2018, 01:41:22 am
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OK, third times a charm I hope. I've written and tried to post this 2 times already, losing my text in the process both times.
I bought the stage right 5W tube amp from monoprice for $120 to rebuild. I rebuilt it to 5F2 circuit, using diode rectification.
heres pics. amp sounds good.
https://drive.google.com/open?id=1f46Bkcgx9FeGLXAaozKP8LmQ9UW6gZfV
https://drive.google.com/open?id=1OH5dsA6WrNVFskaCqY2DLYBFmph8dSCO
https://drive.google.com/open?id=12WsYyYgNXRqzYUV3ZgHrffJaqqxQ95Op
https://drive.google.com/open?id=15XUGYjUSfhROM58Pdco7YSkOHWok1eJV
https://drive.google.com/open?id=1RpRgZlEALHAoOu7kWJW-49ixYDJFrroJ
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feel free to ask questions. I typed out a bunch of details of this project, only to lose the text when I hit POST because of file too large error. I'd really like to throw my laptop through the wall right now, so I'll get back to you on the details of the project later.
:cussing: :BangHead: :cussing: :BangHead: :cussing: :icon_biggrin: :icon_biggrin: :icon_biggrin: :icon_biggrin: :icon_biggrin:
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Looks great.
It's good to write long posts to notepad or wordpad first and then copy/paste it here.
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Nice job! What size speaker is in that cab?
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it comes with an eight inch celestion "super 8" speaker.
All 3 PCB's had some completely cold solder joints out of the box. I kinda feel sorry for whoever designed and got this amp to market. I'm sure those cold solder joints are gonna bite somebody in the ass, if they're bad throughout the production of the amp. I traced out the circuit, It is very close to fender 5F2 / 5F2A princeton, except it uses diode rectification. It should be a nice little amp, especially for the money, but those cold solder joints are bad, bad, bad. It's a stout little cab, the speaker sounds good, transformers, chassis, etc. It looks like the power transformer is also wired for 220? or whatever the rest of the world uses. It has a few unused leads.
The circuit is very close to 5F2/A , the main differences I could find were: 1) Diode rectifier. 2)The volume/tone circuit ties into the cathode of the second stage, at the "top" of the cathode resistor, where the NFB also gets tied to. I'm guess they didn't want to exactly steal the 5F2/A. I didn't draw it out, I don't remember specifics about it. 3) it has a front panel button for 5W/1W operation. This button switches in and out a cathode bypass cap on the first triode stage. I didn't include the button on the rebuild. 4) there's a switching jack on a small PCB at the secondary of the output transformer, providing an external speaker jack. I kept that in for the rebuild, not really a circuit change, but a neat Idea overall.
The octal socket on the stock PCB should've been able to be reused because it has eartabs to mount to chassis, but I pulled one of the pin sockets out and couldn't get it to stay in afterwards so i had to use a whole new socket. The 12ax7 socket doesn't have ear tabs to mount to chassis, so I had to use a new one.
I kept the small PCB with the power switch and LED lamp. The LED burnt out the first time I fired up the amp, so I installed a new LED, and added some low resistance in series with it. The LED is fed off of the heater wires, it didn't have any resistance in series as stock.
The small PCB with the extension speaker jack and OT secondary wiring also has connections for the NFB and ground. Again, cold solder joints here. Their ground wire was, maybe, 24ga, running from there to the main PCB. this is the ground for the OT secondary/speakers. I ran a 20ga ground to the filter caps main ground.
So for $120 shipped, plus 2 tube sockets, 6 terminal strips, 3 filter caps, 2 pots, and handful of small components, I'm fairly pleased with this project.
I keep feeling "regret" for not even turning on the amp before I pulled out the stock PCB, but I bought it specifically to rebuild it, and that's what I did.
Voltages after rebuild; 360, 320, 260 down the main power rail. 7.1 AC on the heaters. A bit high on the heaters???
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Man, you bridged between far ends of the spectrum - mass production vs. boutique build. I'm imagining Monoprice coming out with an up-market hand-wired model
If you could find someone with the original it would be interesting to do a side-by-side comparison.
It would make a great YouTube video.
I don't think I've ever seen a side-by-side of the same circuit in PCB vs PTP.