So you're saying 25-31v or something to that effect?
YesSo you're saying 25-31v or something to that effect?
Revisiting a fascinating thread for a question about the ST-35 variant and similar dual triode front ends lacking the pentode gain stage screen grid correction you describe here.In the ST-70 and the MK II/III amplifiers, the screen grid is power the same way, with the exception of different tube choices. In the SCA35, the screen is powered from the cathode of the phase inverter. This approach solves the problem of pentode tube variables, allowing the triode phase inverter section to always be properly biased: If the cathode voltage is low in the triode inverter section, then so is the screen voltage in the pentode section. This causes the pentode's plate voltage to rise, causing the triode's cathode voltage to rise, along with the screen voltage of the pentode. The value of the screen resistor used causes the arrangement to stabilize at the ideal operating voltages for the triode splitter section. It really works quite well.
I hope this helps!
Dave
If the heater filament is hotter than the cathode sheath it is heating, wouldn't the normal course be the electron fllow tendency be from hotter to cooler? So the reverse bias to counter would be to make the heater positive relative to the cathode?
Revisiting a fascinating thread for a question about the ST-35 variant and similar dual triode front ends lacking the pentode gain stage screen grid correction you describe here.
How do these stay linear with variances in the triode sections between examples, or do they?
The 6L6 family is fairly broad.
6L6 - metal, 360v max, 19w max
6L6G - glass shouldered tube. Same ratings as metal 6L6
6L6GA - glass straight bottle, same ratings as above
6L6GB - same as above
5881 - 400v max, 26 watts
6L6GC - 500 volts, 30 watts
KT66 - 550 volts, 30 watts
EL34 - 800 volts, 25 watts
6CA7 - 800 volts, 25 watts
KT77 - 800 volts, 25 watts
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Dave, is the fact that the 6GH8 has a triode u of 46, yet the 7199 triode has a u of 17 any concern?Yep -- and I've extended the testing to my own ST-70 as well. The 6GH8A with an adapter works just fine in the stock driver board/stock design of the ST-70. Would that it could be such an easy option for everything that used the 7199!
Dave
In most cases it will work just fine.On this driver variant tangent, there are adapter plugs available to allow use of more common tri/pent small signal alternatives to the oop & qualitatively unreliable 7199. The question is whether ideally circuit component parameters should be altered to accommodate specific replacements for best operational results, rather than leaving them as-is?