Dear RTF671, I made quite some progress the last few days on getting the screen grids back in line and prolonging the life of my new tubes. You know the grids were glowing under heavy conditions. Heavy conditions as in driving the 6550's well into clipping. Not a condition a tube would normally see in any hifi amp.
The heavy clipping, amp design and the fact that the 6550's are beam tetrode makes the screen grids susceptible to glowing. The grids are protected with mere resistors, know as sliding grids and were just 2 Volts below the plate voltage. Plates drop below 0 Volts when clipping, so the screen grids start conducting heavily. I wish I knew that earlier! The design of the tubes, amp and conditions all contributed to the problem! It seems that all Marshalls, 50 and 100 watters, exhibit the same problem, but it is worsened with beam tetrodes. I even read that power tubes in these harsh and heavy conditions only last as much as 6 months!
I took your suggestion of increasing the screen grid resistors in consideration. The 2 major drawbacks would be: 1) lots of heat dissipated, 2) added compression as voltage drop is current dependent. That was not my preferred route, but I am very thankfull for your help and suggestions. Made me think harder and consider everything
The OT is 3500 Ohms on the primary side. PSU is 420-430 Volts loaded. Datasheets specify screens should be around 310 Volts then for 77 Watts at onset of clipping. I lowered the voltage on the screens extremely simple by placing a string of 100 Volts (total) 5W zeners in series with the screen grid resistors of both output tubes. Voilà, problem solved! Screens are still drawing 40 mA max at full power (clipping) but that seems pretty typical for these tubes. Fact is that I can only see a very faint screen grid glow when severely overdriving the power tubes. In the dark that is.
Although the tubes seem a lot safer now, I still plan on making a variable high voltage regulator for the screens. Seems to be key to safer operating conditions for the power tubes in amps that overdrive them regularly and could be a nice contribution to this forum from my side. I' also going to experiment with different values of bias feed resistors. They are 150k now, the standard Marshall value for 6550's, but according to other resources 82k or 100k seems to be a more reasonable value.
The heavy clipping, amp design and the fact that the 6550's are beam tetrode makes the screen grids susceptible to glowing. The grids are protected with mere resistors, know as sliding grids and were just 2 Volts below the plate voltage. Plates drop below 0 Volts when clipping, so the screen grids start conducting heavily. I wish I knew that earlier! The design of the tubes, amp and conditions all contributed to the problem! It seems that all Marshalls, 50 and 100 watters, exhibit the same problem, but it is worsened with beam tetrodes. I even read that power tubes in these harsh and heavy conditions only last as much as 6 months!
I took your suggestion of increasing the screen grid resistors in consideration. The 2 major drawbacks would be: 1) lots of heat dissipated, 2) added compression as voltage drop is current dependent. That was not my preferred route, but I am very thankfull for your help and suggestions. Made me think harder and consider everything
The OT is 3500 Ohms on the primary side. PSU is 420-430 Volts loaded. Datasheets specify screens should be around 310 Volts then for 77 Watts at onset of clipping. I lowered the voltage on the screens extremely simple by placing a string of 100 Volts (total) 5W zeners in series with the screen grid resistors of both output tubes. Voilà, problem solved! Screens are still drawing 40 mA max at full power (clipping) but that seems pretty typical for these tubes. Fact is that I can only see a very faint screen grid glow when severely overdriving the power tubes. In the dark that is.
Although the tubes seem a lot safer now, I still plan on making a variable high voltage regulator for the screens. Seems to be key to safer operating conditions for the power tubes in amps that overdrive them regularly and could be a nice contribution to this forum from my side. I' also going to experiment with different values of bias feed resistors. They are 150k now, the standard Marshall value for 6550's, but according to other resources 82k or 100k seems to be a more reasonable value.