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In Reply to: RE: 6B4G Single End amp in a new suit ? posted by pix on April 02, 2017 at 23:01:29
It appears from the circuit that the 6B4G grid bias is positive, but everything else (including that the tube does not immediately melt down!) indicates that it must be negative. So, while I'm confident the circuit is mis-drawn, I am not confident aboout what the actual circuit really is.
Michael Samra, the 300v/50mA operating point is a good one; I use it in the Bottlehead Paramour/Stereomour - the basic design is 17 years old now and has never been a problem that I know of. The important thing is to use a higher load impedance - I use 4000 ohms. I don't know what impedance is used in this circuit.
Yes the label "bias" in this schematic is a negative voltage even though I have miss to wright it down.
The LL1664 is configured as a 3000:8 ohm load.
The amp sounds fine, no question about that. But that does´nt mean it could not be make sounding even better.
My curiosity was pointing at the input and the E280F driver, with the aim of increase sensitivity and decrease the source load.
Is it adviceable to try the driver as a pentode to lower the miller cap?
Change the 25K volume control for a 100K dito?
Increase (or remove) the grid leak resistor?
I know you can do it but I always get a little nervous when you are at full dissipation of the tube's rating.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Yeah, that can be an issue. The classic RCA 2A3s are pretty tough (and/or conservatively rated), and many people now use the modern mini-300B single plate ones which are often quite happy at 20+ watts. I've used television tubes at full spec as well; they were developed to be reliable in a hot environment with many tubes in an enclosed space.
WE rated the 300B at 40 watts dissipation, but buried in the spec is the caution that this would significantly reduce their lifetime - they are not rated as conservatively. So while I don't think there is a universal rule, I do agree with the idea that it's an issue worth looking at closely.
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