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previous: Greg P. If I was to drop the voltage of my ... -- 7/30/1999 6:03 AM View Thread

Re: voltage drop?

7/30/1999 7:14 PM
Carl B.Re: voltage drop?
Probably OK. But to be sure, check the specs on your output tube's data sheets for operation at close to the voltage you're proposing. Each tube type has varying load resistance for varying voltages, some change quite a bit, some don't.  
Look at the "load resistance" figure in the specs. If it changes significantly (more than 25%, say) for the plate voltages you're proposing, then consider if a different tap on your OT would be a better match.  
Output impedance on tube amps seems to be a compromise among many factors, among them output power and frequency response. For example, consider that the load resistance value isn't anywhere near the plate value figure - doing that *should* maximize the output power from what I know. But tube plates are never loaded that heavilly in practice. (Perhaps the impedances are quite different than the resitances for the plates vs. the plate loads respectively, that would definitely be factor).  
Also any given physical tube may exhibit quite a variability from it's published spec.  
As such, you don't have to match exactly the figures given in the data sheets, just don't go more than maybe 25% outside of "spec" for impedance matching.  
Well, maybe more than that. I've seen a 6L6 sheet specify the same operating condition excepting for a 6600 plate load vs. 3800 (a 40% difference). The resulting difference seemed to be a difference in the max plate and screen currents and the ouput power.  
Others may have a different "tolerance" figure there. My experience isn't as deep as some other's who post here.  
As an aside, a good rule of thumb I've been given (it seems to pan-out against the data sheets):  
SE Triode: Plate load = Three times the Plate Resistance  
SE Pentode: Plate load = 1/10th the Plate Resistance  
PP Triode: Plate to Plate load = Three times the Plate to Plate resistance  
PP Pentode: Plate to Plate load = 1/10th the Plate to Plate Resistance.  
That might be useful for "filling in the gaps" on some tube data sheets.  
No doubt someone will correct me here somewhere, so keep an eye open for subsequent postings ...  
- Carl B.