by EWBrown » Sat Aug 30, 2008 12:47 pm
The 75THs would be more suitable for a Class B P-P circuit, with an interstage transformer handling the phase splitting duties. These tubes, as mentioned previously, do draw considerable grid curent and need a few watts of drive power to their grids. I'm going on somewhat "fuzzy" memory from reading my 1950's vintage ARRL ham radio handbooks, they cover a lot of this type of tube in both RF and (mostly Class B) modulator circuits.
Now for a bit of health and safety advice:
These tubes, with their exposed anodes, can, and will be a definite shock hazard, unless they are well=protected. Add in the 1000V plate voltage, and there is no such thing as a "little jolt"... THis kind of voltage can be extremely dangerous, and even lethal. The amp in the photo is pretty, but definitely needs to be kept out of reach of casual observers, kids and household pets. ...
Unforch, I can't just grab the books for a quick reference look-up, they are currently about 1050 miles SW of here Yellow_Light_Colorz_PDT_02
Granted, in a proper guitar amp, the tubes will be under cover, and therefore, some air circulation would be a good idea.
Class B typically runs with a zero volt, or positive grid bias, and some amount of grid current, unlik Class e A or AB, which consume no, or very little, grid current.
Class B does inherently have more crossover distortion than Class A , AB1, or AB2. But then for guitar amp use, this can be a good thing...
Good 807 stand-ins are the 1625 (12V filaments, and UX-7 bases)) and the 6BG6G, (octal bases).
I know Ed Sawyer, and he has some amazing designs, and he is a very accomplished builder and woodworker, far beyond my meager capabilities. I had lunch with him last week, (along with a half-dozen other NNETG'ers) and as usual we discussed all things tubular Yellow_Light_Colorz_PDT_03 He uses the 75TH as a driver stage, which is definitely a non-standard application, and at the relatively low B+ and plate current, it should last about forever...
/ed B
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