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Sag resistor
Sag resistor
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Old 27th June 2013, 03:06 PM   #21
kevinkr is offline kevinkr  United States
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Sag resistor
I've always understood the technically inelegant resistor approach to be more desirable sonically speaking in a guitar amplifier application because of the effect of variable sag depending on how hard the amp is pushed. Poor power supply regulation is really part of the sound.
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Old 27th June 2013, 04:24 PM   #22
GoatGuy is offline GoatGuy  United States
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Yah... I am pretty familiar with that. In fact, "chained sag" between sections is also the norm. Ostensibly, each resistor in series is both lowering the mean B+ voltage for the next section ("more to the left" on most conventional schematics), but with the use of very small filtering caps ... the net result is RC constants that are pretty small. Seconds.

If the "resistor sound" is what is desired, then just go with a resistor - and heat-sink the thing. If it gets "awfully hot", remember that for a power resistor, +100C to +200C is within its design spec. The ceramic ones ... could get much hotter, and stay that way for decades of hard service. After all, there ain't much removed between a power resistor and a heating coil in a toaster.

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Old 27th June 2013, 06:29 PM   #23
fabioab is offline fabioab  Brazil
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Originally Posted by GoatGuy View Post
Hey... There is a MUCH simpler way to do this than resistors!

1N5339 - a 5 watt, 5.6volt zener diode. (industry uses 5 watt zeners a lot in motor and panel work). 850 ma working current. Can take quite a bit more than that 'pulsed.'
Hi all and thanks, my objective is reached! 5W 12V Zener (1N5349) diode (this was value I found at the electronic store near my house). So, setting the current 32mA per tube (El34) I got +B=468V and the sound is perfect to my ears! Living and learning. "Some times you loose several bucks to learn things in practice!!!" 480V to 468 with only one diode working "cold"!

Thanks all!
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Old 27th June 2013, 07:36 PM   #24
Struth is offline Struth  Canada
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Hi Guys

The diode is not going to be "cold" but may be cooler than the resistor while playing.

If you are pushing the amp to full output, then you still have the same maximum current through the zener as you had through the resistor. The only difference is that the zener provides a fairly constant voltage drop where the R provides a variable drop.

At the 500mA quoted above, the 12V zener dissipates 6W. The device should heatsinked.

With any component that operates hot, it is important to move the heat out as quickly as possible. Even tubes do not like being hot despite the fact they need the cathode to be hot to function.

The sag in the supply is caused by wiring and component resistances. Enhancing that with further resistance allows the sagged compression effect to occur at a slightly lower power level. On the other hand, using the zeners to provide a constant drop of supply voltage does not change sag at all but does very slightly reduce the maximum power from the amp - maybe 5% if you're lucky, which represents only 0.22db sonically.

Have fun
Kevin O'Connor
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