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08-16-2019, 06:42 PM
Hi
In the bias circuit, we are not adding a resistor back unless you meant that you removed all four 30k1s? There has to be a base current path for the fixed-bias position and the R between C and B for each transistor does this. The bias control is split between tube pairs and each BJT switch has to be independent, so maybe for you this means adding back two Rs.
How you measured Rk during the full power test is correct and it is dissipating the power you calculated, so yes it is going to get hot fast and should be a higher wattage part. Maybe the TUT5 Rks should be 10W, but the original player this amp was designed for rarely used the cathode-bias mode and likely never full tilt since Power Scaling is on-board.
I am not actually much of a fan of cathode-biased power stages and use it only for a tone difference. I've used Rk values up to 1k for standard tubes and this certainly keeps the heat controlled for both the tube and Rk.
The unsnubbered coil allows maximum speed for the relay contacts to open and close but places high-voltage stress on the coil control element. The standard diode is a brute force damper and does not allow the fastest relay transitions. Too-slow open/close causes the welding effect if switched under load. The currents in the Standard are low, but it is enough to hold the contacts together.
In the bias circuit, we are not adding a resistor back unless you meant that you removed all four 30k1s? There has to be a base current path for the fixed-bias position and the R between C and B for each transistor does this. The bias control is split between tube pairs and each BJT switch has to be independent, so maybe for you this means adding back two Rs.
How you measured Rk during the full power test is correct and it is dissipating the power you calculated, so yes it is going to get hot fast and should be a higher wattage part. Maybe the TUT5 Rks should be 10W, but the original player this amp was designed for rarely used the cathode-bias mode and likely never full tilt since Power Scaling is on-board.
I am not actually much of a fan of cathode-biased power stages and use it only for a tone difference. I've used Rk values up to 1k for standard tubes and this certainly keeps the heat controlled for both the tube and Rk.
The unsnubbered coil allows maximum speed for the relay contacts to open and close but places high-voltage stress on the coil control element. The standard diode is a brute force damper and does not allow the fastest relay transitions. Too-slow open/close causes the welding effect if switched under load. The currents in the Standard are low, but it is enough to hold the contacts together.
