skyboltone wrote:It depends an awful lot on the way the transformer is made also Phil. You've got a "soft" transformer. I know for a fact that the Hammond Organ AO-35 power transformer, 300-0-300 unloaded, will, when using a SS rectifier and a pair of 6BQ5s, burden to around 345 VDC. When you measure the HT winding from end to end it measures 265 ohms. A comparable "hard" wound transformer will measure about 70 ohms. It will yield the 1.4 voltage.
This is new information for me, so thanks very much. It just hadn't occurred to me. I don't recall the ohms on this particular PT. It is from
www.musicalpowersupplies.com. If I get it on the bench, maybe I'll remember to check.
Once again, use the Duncan software and experiment with the transformer impedence variable. It will tell the story.
I have it.
I've got a "new" old tranny on the bench now, trying to figure what to do with it. HT is at ~675vac on the outer legs without a load; ~178 ohms. I'll be putting in into my test "apparatus" to see if I can estimate mA rating. The Duncan suggests it is abotu 100mA. That seems low by the eyeball method, but I see how the winding R affects the whole thing. Very interesting.
Oh, and how well do the 6AQ5's hold up at 335? What do you run on the screens? These tubes are supposed to max out at 275VDC
Dan
Well, Dan, now you have forced me to check my recorded results. Va=341, Vg2=335, Vg=-26.9; I guess I'm getting almost 18W out of them? Or is it half that? Anyhow, I don't play out and this amp probably doesn't see more use than 4-5 hours per week, if that much. I'm well aware of the design center spec. I confess that I don't know how they will hold up, but the voltage doesn't seem to bother them. I've got a small pile of 6AQ5's (maybe 50 of 'em), so I'm not concerned about toasting a pair or two. The Sylvania brand tubes seem to be the most robust. GE and RCA are in second place. If I were gigging this amp, I'd get a different PT and try to knock down Va to around 300V. I've got a Gibson Scout (GA17-RVT) that operates in this range and it is a great sounding amp.
As an aside, you recently chided us (no offense meant) in the discussion about dropping B+...I bought an IRF840 to install in this amp as a cheap way of dropping B+, but never got around to it. I will probably just leave it alone. I completed the amp in late February and my interest in tweaking it has gone south.
The TONE on this amp is just like what I hear from the other samples, though I'd guess more discriminating ears might say otherwise.
Incidentally, cutting the supply ladder 18K2 resistor to 9K1 brought the pre-amp voltages right in line with Francesca.
