Output Stage Modifications -- Pt 1
With the issues surrounding the output stages laid bare, correcting them then becomes rather easy once the new OPTs are installed. They are key to implementing the rest of the improvements, as improving the performance of the stage requires increasing the standing (quiescent) current through it. Since plate current passes through a single ended output transformer in an unbalanced fashion, it leaves the core subject to saturation if more standing current flows than the transformer is designed to handle. If that happens, that leaves no magnetism left to transfer the audio portion of the plate current in the primary winding into the secondary winding. Since the standing current needed to be increased in the neighborhood of 150%, this was almost certainly going to outstrip the capabilities of the original smaller transformers. So besides correcting the impedance mismatch of the original units, the new transformers also allow for more power to pass as well -- necessary if the little 8600 is really going to sing.
With the transformers replaced then, here are the remainder of the changes necessary to add some kick to the output stage:
1. In the power supply, the 4.7K 1W resistor should be change to a 1.5K 1W resistor. This resistor primarily sets the relationship of the screen grid voltage to the plate voltage. With the modified amplifier, the plate to cathode voltage is ~ 250 volts, while the screen to cathode voltage is now just 5 volts below this. The screen grid ultimately determines how much current can flow in a pentode tube, so it acts in a big way to match the plate load requirement of the tube to that offered by the OPT. Operating the screen grid at 245 volts optimizes that relationship in the modified design.
This change also increases the B+ voltage to the AF amplifier stage which is of no concern, as the bias for the stage is automatically adjusted to account for it. The net result is that the stage is simply capable of greater undistorted drive to the output stage than in the stock design.
2. The common cathode resistor should be removed and the cathode connection between the two output tubes separated. Also, the connection over to the common cathode bypass cap in the power supply can needs to be removed as well. Locating the cathode bypass cap in the power supply can cap was a major source of hum in this amplifier, since the can cap was grounded at one end of the chassis, and the CT of the HV winding and the common cathode resistor was grounded at the other.
This is a terrible arrangement, allowing significant power supply ripple current to be conducted through the chassis, which of course caused a slight voltage drop to occur across the face of the chassis. With the output tube common cathode resistor and HV winding CT lead at one end, and cathode bypass and power supply caps at the other, this meant that the voltage drop across the chassis was then injected into the cathode circuit of each output tube via the cathode bypass cap, amplified by the tubes, and reproduced as hum in the output. This is the essence of what a ground loop is. To resolve this, extend the HV CT lead, and ground it directly at the can cap. This prevents any ripple current from flowing through the chassis. Then, use the ground lug between the two output tubes for all output stage ground connections for both tubes.
3. Each output tube cathode terminal (pin #3) should have a 120 ohm 1/2 resistor connected between it and the ground lug. There should also be a 100 uF 16V cap connected between each pin #3 and the ground lug as well. The negative terminal of these caps should connect to the ground lug.
This is a much lower single tube cathode resistor value than most followers of these type amps are used to, where normally, a single tube resistor value would normally run in the 200 to nearly 300 ohm range. Such values typically over bias the tubes, and cause the unequal clipping previously discussed.
There is a common wisdom prevailing in this hobby that reducing output standing current draw is always good. It extends tube life, doesn't hurt sound quality, helps with today's higher line voltages, etc, etc, etc. That wisdom is often very sound (pun intended) when it comes to push-pull amplifiers. But when it comes to single ended flea tube amps -- where every portion of every watt counts -- that line of thinking couldn't be more wrong. The correct amount of standing current draw is crucial in obtaining maximum power output and minimum distortion. If you under or over bias the tube, the operating point shifts from the optimum position, so that the plate signal swing is cut off on one end or the other, neither of which is good.
A value of 120 ohms properly biases the output tubes in the modified amplifier. This value of resistance produces a plate current draw of 44 ma under quiescent conditions, and produces exactly equal clipping as maximum power is approached and exceeded. It also promotes Class A operation throughout the entire power range: As power output is advanced from zero to maximum, the cathode voltage of the output tubes barely moves if at all. This indicates near perfect constant current operation over the full power range of the amplifier, which is the very essence of Class A operation by definition. It is also why the typical thinking of making for large power supply reserves in the modified amplifier will produce little benefit. The current draw by the amplifier from the power supply is virtually constant, regardless of what level of power is being produced. The total current drawn by each tube is 47.5 ma.
As for the tubes themselves, with a plate to cathode voltage of 250 volts, this equates to a plate dissipation level of exactly 11 watts. Hello more power, at lower distortion. When this is tag teamed with correcting the impedance mis-match via the new OPTs, this is where the 300+% increase in power comes from in the modified amplifier. This dissipation level is well within the capabilities of the 6BQ5 tube family. As judged against the more modern Design Maximum standards which tubes developed after about 1960 were rated under, the tube is operating a 83% of its dissipation rating, or very close to the 80% "standard" that is often used as the benchmark today in setting quiescent dissipation levels. In short, when combined with the voltage levels applied to the tubes, it is a very safe condition to operate the tubes under -- and is required for the Maggie to be all it can be.
Now the power supply really takes this all in stride matter of factly. Total current draw of the modified amplifier is just under 100 ma -- or exactly what the power supply was designed to deliver in the first place.
Pt 2 continues in the next post.
Dave