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Old 1st November 2006, 08:32 PM   #1
5u4 is offline 5u4  United States
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Default Dropping filament volts/ L-W ramblings

Would like to drop AC filament voltage for a pair of SRPP 6SL7's driving my Fi-Primer Loftin-White. It runs about 6.75V + or - depending on line voltage. A series resistor of about .5ohm would probably do it (.6A X .5ohm = .3V drop) but thinking about another way, which would be a pair of resistors across the line forming a "CT or hum balance of sorts" as in the drawing. How do I calculate the resistors value for .3V drop?

Using formula for hum balance pot: example: 6.3V X 6.3V / 20ohm pot (or 2-10 ohm resistors) = .315A = ? V Drop.

Some L-W ramblings that have little or nothing to do with my question, but tweaks:

The 50VDC bias is taken from one of the 2A3 cathode resistor strings. I had that tied to the 6.3VAC filament supply at the 6SL7 furthest from the transformer & found it pulled the volume down or something weird with that channel, so eventually tied it to the 6SL7 closest to the transformer & all is well?

A note about rectifiers: I first built the amp with Cree zero recovery diodes & thought the amp could sound better, so tried Harris Fred diodes & they seemed to be better at first, found them warmer but eventually 2 dimensional.

Then tried the tube rectifiers (2 different 5AR4's, a 5V4 & 2 different 5U4's, while optimizing the B+ by changing C1 value (Solen PP), so differing voltages from different tubes wouldn't be part of the sound. The 5U4's voltage were a bit too low & were not really considered.

Then back in went the Cree's. I have come full circle with my opinion of these. With all the tubes, the sound was weak in the bass & overall sound was weak. When going back to the Cree's, the sound is of a more powerful amp & 3 dimensional. There's no going back to Fred's or tube rectifiers. The L-W seems to sound more like a PP amp. Kind of reminds me of a Mac MC225 I used to have, not your warm fuzzy sound, but fast & tight, whatever, I like it.

Another area which changes the sound a lot are cathode bypass caps: Eventually tried every combination of 5 different caps I had on hand & settled on Panasonic FC for 6SL7's & Panasonic NHG on 2A3's.
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Old 1st November 2006, 08:41 PM   #2
SY is offline SY  United States
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You'd want resistors in series with the heaters, not shunting it. Total R should equal the desired voltage drop divided by the heater current. Then use two resistors, each at 0.5R on each leg of the heater to maintain balance.
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Old 1st November 2006, 08:59 PM   #3
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Adding resistors as you propose in your diagram will not lower the filament voltage for you. However, if you made them low enough to load down the transformer, then it would. But it would do so at the expense of needlessly straining the transformer.

You first thought of adding a series dropping resistor is the correct way of doing it. Using two, one in each leg, is nice but unnecessary.

Making a voltage divider between legs to add a DC level is still possibile after the series resistor. But I'd use higher values like 100 ohms or more so as not to load the transformer. You're only adding potential and not dropping anything.

Victor
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Old 1st November 2006, 09:15 PM   #4
5u4 is offline 5u4  United States
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Thank you, now I understand.
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