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Re: Reading Voltages

7/8/2005 10:16 AM
Re: Reading Voltages
Actually, you may have gone in the right direction by using an existing chassis with that tube configuration. Reason being, if the OT came with the chassis with this tube set-up, chances are almost 100% likely that the OT is correct and safe for a single 6V6 output like a 5F2A. If the PT came with the chassis, the PT is likely capable of providing the necessary heater and B+ current to power all of those tubes without problems. The PT is also likely providing the correct heater voltages needed for that rectifier and those preamp and output tubes.  
The real question -- as you have figured out -- is whether or not that PT is providing B+ voltages similar to a 5F2A. Voltage differences can mess with a lot of things (like proper bias points and the overall current drawn by the tubes) so you want to make sure you are within a reasonable ballpark of 5F2A voltages. Otherwise, you might need to adjust the value of B+ and cathode resistors to keep your bias, your current draw and your amp safe. The link to Paul Ruby's startup procedure above is a great one. If I recall correctly, it guides you through measuring voltages by first disconnecting certain PT wires, which is probably what you are going to want to do so you can figure out how much B+ voltage and heater voltage your PT is supplying on its secondary side when the amp is not loaded down with tubes. Once you know how much B+ is coming out of the secondary when it is not hooked up to the rest of the amp, you will be able to estimate (through some math) how much B+ voltage you are going to be getting after a 5Y3 rectifier and down to the plates of the tubes. You can then compare your amp's estimated voltages to 5F2A schematics with voltages to see if you are in the ballpark... without even hooking up most of your amp or plugging in tubes! If you follow Paul Ruby's startup guide, you will find it very easy to develop some confidence in firing up a new amp without burning down the garage.  
Since you have caught the homebrew amp-building bug, I would suggest you also check out the AX84 BBS forum if you have not done so already. The AX84 site centers around folks who like building and designing amps of all sizes and types from scratch and turning old parts and amps into something new. Schematics, layouts and clips of the more popular designs become part of the community website. Most importantly, there are literally thousands of present and archived discussions that you can search through on ANY topic related to amps and amp-building, including specifications of Radio Shack wire, adding hi-fi tricks to guitar amps, building amps using only preamp tubes or only octal tubes, using a 12AU7 for a push-pull output section, building self-split output sections without a phase inverter and adding switchable/adjustable cathode biasing for preamp and output tubes. The range of people who post there is also diverse, from complete newbies who have never seen tubes before to amp gurus like Randall Aiken, Doug Hammond, Phil Rowley, Mark Huss, etc. I still learn something new everytime I go there. DEFINITELY worth checking out if you are serious about amp-building.  
Good luck!