Thorpej -- If I may, I would offer a few points for your consideration in your project:
1. As long as the bias control circuit in total (that is, including all the controls associated with it) has enough current flowing through all of its supply points (where the grid return resistors attach to) under any combination of control settings, then the complete bias circuit can be considered as being "invisible" with respect to your concerns for the total amount of grid no.1 dc resistance for your output tubes, as in effect such current flow provides a virtual ground relative to any reverse grid current the tubes might develop. A virtual ground will be achieved as long as each supply point is flowing at least 1 ma for any control setting. The bias circuit of the HF-89 does not normally draw that much current, but in that case, the total grid no. 1 resistance including that of the bias adjustment circuit was still within the recommendations of the tube manufacturers for the EL34, so the concerns were minimized. The best way to alter the circuit for use with 6L6 class tubes is to up the current through the bias adjustment circuit appropriately, and then set your grid return resistance as you will within the recommended range.
2. 6SN7 class tubes are perfectly capable of driving 6L6 class tubes to full power output under conditions of high bias in the output stage, and with low distortion too -- but for the most practical results, should use 47K plate load resistors to do so. Eico used the same inverter values as found in the HF-89 in their HF-22 design as well. In that circuit, the 6L6s operate with cathode bias with lower average bias voltage at full power output than you will be using under quiescent conditions in your project, and the inverter as originally designed struggles in that model under full power conditions at low frequencies. Oh the original Eico design can do it if you want to seriously up the current flow through the inverter tube. But all you'll really be dong is shortening the life of the tube, whereas raising the value of the plate load resistors coupled with a modest increase in current flow will accomplish the same thing much more effectively, and the tube will last much longer.
3. If you are planning the operate your output tubes in UL, then the optimum plate-to-plate load for 6L6 class tubes when using fixed bias is 6600 Ohms. Any lower value -- even a value of 6000 Ohms compromises HF performance. You will often see these tubes with this plate load value operating with cathode bias -- but if such designs are performance tested, their THD and IMD numbers will be found to be quite elevated. However, using fixed bias with these tubes and this loading condition, in this mode of operation, will produce very low levels of distortion, and produce wonderful performance across the full audio spectrum. The optimum UL tap is 43%, although 40% is certainly acceptable. With a capable OPT and about 450 vdc of B+, the tubes can develop about 30-32 watts RMS into a secondary load, and deliver very high quality performance.
Good luck with your project!
Dave
1. As long as the bias control circuit in total (that is, including all the controls associated with it) has enough current flowing through all of its supply points (where the grid return resistors attach to) under any combination of control settings, then the complete bias circuit can be considered as being "invisible" with respect to your concerns for the total amount of grid no.1 dc resistance for your output tubes, as in effect such current flow provides a virtual ground relative to any reverse grid current the tubes might develop. A virtual ground will be achieved as long as each supply point is flowing at least 1 ma for any control setting. The bias circuit of the HF-89 does not normally draw that much current, but in that case, the total grid no. 1 resistance including that of the bias adjustment circuit was still within the recommendations of the tube manufacturers for the EL34, so the concerns were minimized. The best way to alter the circuit for use with 6L6 class tubes is to up the current through the bias adjustment circuit appropriately, and then set your grid return resistance as you will within the recommended range.
2. 6SN7 class tubes are perfectly capable of driving 6L6 class tubes to full power output under conditions of high bias in the output stage, and with low distortion too -- but for the most practical results, should use 47K plate load resistors to do so. Eico used the same inverter values as found in the HF-89 in their HF-22 design as well. In that circuit, the 6L6s operate with cathode bias with lower average bias voltage at full power output than you will be using under quiescent conditions in your project, and the inverter as originally designed struggles in that model under full power conditions at low frequencies. Oh the original Eico design can do it if you want to seriously up the current flow through the inverter tube. But all you'll really be dong is shortening the life of the tube, whereas raising the value of the plate load resistors coupled with a modest increase in current flow will accomplish the same thing much more effectively, and the tube will last much longer.
3. If you are planning the operate your output tubes in UL, then the optimum plate-to-plate load for 6L6 class tubes when using fixed bias is 6600 Ohms. Any lower value -- even a value of 6000 Ohms compromises HF performance. You will often see these tubes with this plate load value operating with cathode bias -- but if such designs are performance tested, their THD and IMD numbers will be found to be quite elevated. However, using fixed bias with these tubes and this loading condition, in this mode of operation, will produce very low levels of distortion, and produce wonderful performance across the full audio spectrum. The optimum UL tap is 43%, although 40% is certainly acceptable. With a capable OPT and about 450 vdc of B+, the tubes can develop about 30-32 watts RMS into a secondary load, and deliver very high quality performance.
Good luck with your project!
Dave
).
and the only one in stock at Mouser.
