Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Carlsoti on July 07, 2024, 10:25:19 pm
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Hello all. I just picked up a Cordovox Accordian pair; separate tone generator and amp/speaker cabs. It should be no surprise that accordion players seem to take better care of their equipment, and the condition of these was too good for me to pass on. I really wish it had the full schematic inside, though. I've only found a couple hand-drawn schematics of conversions after the fact, and a power-supply snippet someone posted here, but it looks like it may have been altered already.
They've been stripped of all the useful tubes, save for a pair of 12AU7s. It appears as if a ginormous .22/400V Sprague Black Beauty blowtorched one of the rectifier diodes. Additionally, the top cap in the doubler stack is shorted. I've tested the transformers; all seems well and I'm leaning towards a 5F6-A circuit, but that may change due to the small control panel.
In addition to the other damage mentioned, the resistor between the diodes and transformer melted away. Based on reading the few other build threads, it seems this was a 4 ohm/5W resistor. What exactly is this resistor's function in the circuit? How does one go about calculating it's value in instances where other parts of the circuit have changed?
Additionally, the 150uF/250v caps for the voltage doubler seem to be an odd value these days. What changes can I expect if I replace these with 220uF/300V?(or more V, depending on price.)
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I would think the 4R resistor basically acts like a fuse. Sounds like it did it's job.
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Yeah, that's how it seems to have functioned/malfunctioned. I can't say I've seen anything with this exact arrangement before, save for diagrams for voltage doublers, but none of those had this chonky little dropping resistor. I just don't see how it'd be very effective at dropping voltage or ripple with so low a value and/or no cap/choke to help the situation.
I guess I'm looking for validation that this particular resistor is non-essential in a voltage doubler. Now that I think about it, I remember reading that 120 will be the dominant ripple frequency with this configuration, so the answer might be that it IS just a dropping resistor. I wish I was better with PSUD2. I only recently learned, and somewhat by accident, that the load can be changed to a constant current source!
I guess I'll hunt down the other circuit modeler I've seen and see what info I can glean from that.
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If you increase the value of the caps in the doubler it will reduce noise/hum and allow the circuit to regulate a little better. As long as you go over the voltage value they were using, nothing will matter as far as the voltage is concerned except overall cap size.
Check out some other doubler circuits to see what people did. I think the Conn organ amp I did a project with was using 150uF caps in the doubler also. If I remember right I went with 220uF caps instead.
Doubler circuits can sag like a tube rectifier if they aren't super robust, even with a full wave 4 diode doubler circuit. I like them because you can get the sag of a tube rectifier without dealing with the eventual failure of the rectifier.
Greg