schwimbo
03-31-2009, 04:38 AM
I recently bought a Pittbull 45 (from 1996), and it sounds great (in both AB and A operation). I biased it in AB operation so that I have around at idle 20ma cathode current with a plate voltage of 430V. This gives me roughly 70% of the recommended maximum dissipation for EL84 tubes (12W).
In Class A, the plate voltage is 375V, and the cathode current is around 55ma. If there is 5 to 10ma of screen current contributing to this amount, it still leaves at least 45 to 50ma of current. If I multiply by the cathode voltage, I get between 17W and 19W of dissipation in the EL84 tubes, which seems to be pretty high. I haven't heard back from Steve Fryette Designs (new VHT name) about this yet. Has anybody (who has experience taking bias probe readings on their Pittbull 45) gotten similar readings? Any ideas on how to reduce the Class A dissipation (assuming it is too high for any "reasonable" length of tube life)? Can the Class A idle current or plate voltage be adjusted downwards by an amp mod? Any ideas about this are appreciated.
In Class A, the plate voltage is 375V, and the cathode current is around 55ma. If there is 5 to 10ma of screen current contributing to this amount, it still leaves at least 45 to 50ma of current. If I multiply by the cathode voltage, I get between 17W and 19W of dissipation in the EL84 tubes, which seems to be pretty high. I haven't heard back from Steve Fryette Designs (new VHT name) about this yet. Has anybody (who has experience taking bias probe readings on their Pittbull 45) gotten similar readings? Any ideas on how to reduce the Class A dissipation (assuming it is too high for any "reasonable" length of tube life)? Can the Class A idle current or plate voltage be adjusted downwards by an amp mod? Any ideas about this are appreciated.