Hopefully someone can save me some hassle and indicate a range of neg. bias for a class AB1 amp with ~525V B+, fixed bias and the option for 6550s, KT-88s and KT-120s operating in the 50mA to 65mA range. Thanks
Triode, UL or pentode? Impossible to answer the question withouth knowing the screen grid voltage.
That will need a fairly wide bias range, as a guess -55V to -35V.
But you'll also need a regulated screen supply that is much lower than 525V.
But you'll also need a regulated screen supply that is much lower than 525V.
For fixed bias, and for fixed adjustable bias operation, be sure to read the tube specifications.
The maximum grid return resistance, Rg, is listed, do not exceed that.
Most of the same tubes that are self biased, have a larger grid return resistance, Rg, maximum spec.
Failure to observe the maximum grid return resistance, Rg, may result in Thermal Runaway, and things like red plates, and destroyed tubes.
Some designers are tempted to use larger Rg resistances to make it easier for the driver tubes.
Do not do it that way, instead design the driver to easily drive the correct Rg resistance.
Just my opinions.
The maximum grid return resistance, Rg, is listed, do not exceed that.
Most of the same tubes that are self biased, have a larger grid return resistance, Rg, maximum spec.
Failure to observe the maximum grid return resistance, Rg, may result in Thermal Runaway, and things like red plates, and destroyed tubes.
Some designers are tempted to use larger Rg resistances to make it easier for the driver tubes.
Do not do it that way, instead design the driver to easily drive the correct Rg resistance.
Just my opinions.
I have an old Eico HF-89 that I rebuilt and upgraded 20 years ago. The idea is to redo the amp on a larger chassis with a better power supply (Lundahl LL1650 power transformer). The Class A PPP KT-120 mono-blocks the I built require that I purchase matched octets (output stage is differential with the four output tubes configured as a long tail pair with a CCS tail). They generate a lot of heat and with the Ukraine war and sanctions I don't want to deal with the hassle. Since I have several quads of 6550s and KT-88s and some KT-120s redoing the Eico seems like a plan. I dug out the Eico and checked the bias I worked out when I had previously rebuilt it and it seems like a range from -35V to about -70 should work fine.
