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Recently, a comparison was made between a resistor and a diode dampener as a means of providing bias for an output tube. Here is how it was done with an accompanying description of the amplifier that served as the test bed for the comparison. First the description of the amplifier as the comparison began. It is a two stage amp stereo amp using the 5842 RC coupled to a 46 which is AC heated. A hum balance pot is used and the wiper connected to the cathode resistor and bypass cap. The cathodes of each respective stage are shared. The power supply consisted of two diode dampeners used for rectification and a pi filter. The amp was well known.
To convert to the biasing the output tubes with the diode dampeners the following was done. Two hexfreds replaced the diode dampeners serving as rectifiers. The cathode resistors and their bypass capacitors were removed. The wipers of each hum balance pot were connect to the diode dampeners that were then wired in series to ground. Bias and operating points were pretty close, though there was 20 volts of B+ increase.
The sound improved a bit with the diode dampener bias. Nothing huge though, just a bit more juicy. Not really sure one could tell much without the back to back comparison done in this fashion. However, the turn on was much improved. Total silence up to operation. Absolutely beautiful.
Follow Ups:
I never considered damper diodes...great idea! That would really give you a slow start on the output tube.I noticed a huge reduction in hum using a rectifier tube (DC heated) on the cathode. I also noticed a loss of low end. It's also strange that the tube, a 2A3, came up to the correct current draw but the voltage drop across the rectifier was not what I expected. I was shooting for 50 volts and measured only 35.
In your case, I wonder about the changes the Hexfreds have made. Could it be the sound of them in the power supply or the additional B+ that contributes to the change in tone? Only one way to find out-keep the dampers in the PS and then do the comparison with another pair on the cathodes.
Tom
If ya think about it, it really shouldn't matter where the vacuum diode is.
Nah.
Frankly there is not enough of a change to make a difference.
Tried this before, no difference.
Interesting post. Thanks. Experiments trump opinions.Which damper diodes were you testing?
6dm4a
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