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Any info on this Linear?
- Thread starter Crambone
- Start date
If memory serves, the two driver tubes are suicidal, and frequently burn the driver plate choke to a crisp.
The reason was the filament voltage. The power transformer feeds 28 Volts or more to those two tube sockets. A 20LF6 tube just runs too hot until it breaks down. The original 27LF6 tube is extinct. We recommend using a 30KD6 tube in the two driver sockets.
It has other shortcomings, including a tendency to oscillate, but for now I'll just point out that the five final tubes have their heaters wired in series, powered directly from the 120 Volt line cord. This would be fine if it had five tubes with 25-Volt heaters, but using five 20LF6 puts 25 Volts onto each of five 20-Volt heaters. They run too hot and fail prematurely.
Add to this the insulation breakdown-voltage rating of the tube's cathode-to-heater insulation and the "blue flash, SNAP!" excitement gets easier to explain. The 27LF6 tube it was built to use was made for color televisions sold in Europe, not the USA. Line voltages over there are typically 240 Volts, not 120 like they are here. A consequence of this is that any tube that has its heater wired directly to the power cord need twice the insulation-voltage rating that a tube used in an american TV would need.
The 27LF6 tube it was built with not only lasted longer because the heater voltage was correct, but because pumping RF voltage from the driver tube into the cathodes of the five final tubes on top of the AC line voltage made american-made 20LF6 tubes break down that heater-to-cathode insulation and roach a tube in a (literal) "flash".
We saw some of these amplifiers with RF chokes added to the heater circuit to keep RF out of the heater circuit, and reduce the breakdown problem. This was a band-aid.
If you're just in love with this particular amplifier, go for it. Just remember, true love is never cheap.
73
The reason was the filament voltage. The power transformer feeds 28 Volts or more to those two tube sockets. A 20LF6 tube just runs too hot until it breaks down. The original 27LF6 tube is extinct. We recommend using a 30KD6 tube in the two driver sockets.
It has other shortcomings, including a tendency to oscillate, but for now I'll just point out that the five final tubes have their heaters wired in series, powered directly from the 120 Volt line cord. This would be fine if it had five tubes with 25-Volt heaters, but using five 20LF6 puts 25 Volts onto each of five 20-Volt heaters. They run too hot and fail prematurely.
Add to this the insulation breakdown-voltage rating of the tube's cathode-to-heater insulation and the "blue flash, SNAP!" excitement gets easier to explain. The 27LF6 tube it was built to use was made for color televisions sold in Europe, not the USA. Line voltages over there are typically 240 Volts, not 120 like they are here. A consequence of this is that any tube that has its heater wired directly to the power cord need twice the insulation-voltage rating that a tube used in an american TV would need.
The 27LF6 tube it was built with not only lasted longer because the heater voltage was correct, but because pumping RF voltage from the driver tube into the cathodes of the five final tubes on top of the AC line voltage made american-made 20LF6 tubes break down that heater-to-cathode insulation and roach a tube in a (literal) "flash".
We saw some of these amplifiers with RF chokes added to the heater circuit to keep RF out of the heater circuit, and reduce the breakdown problem. This was a band-aid.
If you're just in love with this particular amplifier, go for it. Just remember, true love is never cheap.
73
