![]() To 75, the base current could be half to a bit more than one ma, so the bias Transistors in a Class AB setup are just about perfectly biased at a collectorĬurrent of 25 to 35 ma, and typical output transistor gains for the time were 20 Idling will not change the bias string current much. The current in the resistance string is set to be some 30 to 60 times theĮxpected idling base current so that variations in the base current needed for Millivolts, and for silicon this is about 450 to 550 millivolts, whichĬorresponds to the barest beginning of conduction for the respective materials. The junction of the two resistors sets the bias voltage to which theīase of the transistor is held. Stretches from the transistor's collector to the outer end of the emitter Stabilizing local feedback, and each (top half or bottom half) has a biasing string which Each transistor has an emitter resistor to provide some Both transistors are the same polarity, PNP for early ones, and The output sections of all the TV amps is the stacked "totem pole" This will be a great help when we start servicing Set up properly even if the driver circuit is not attached to the primary of theĭriver transformer at all. The secondaries of the driver transformers. Transistor biasing that is done in the TV amps is done purely by the bias resistor strings on Transistors on the output side of the driver transformer. Transistors are carried out solely through the resistances attached to the ![]() Transformers simply cannot transform DC between windings, so all biasing concerns for the output It's important to note that the use of a driver transformer makes a completeīreak between the output transistors and driver transistors in the DC sense. So that one output transistor is driven on while the other is driven off. It'sĪ phase inverter and complementary driver. The driver transformer is there to do two things isolate the output transistorsįrom the DC conditions in the driver transistor circuit, and invert the signal Polarity tube to a single polarity transistor was a natural one. The first power transistors were only one polarity as a practical matter, Transformer is a clever solution that simplifies thing a lot. To have one tube pull up and the other down, a driver There's only one polarity of charge carriers that are easily mobile in a vacuum. These stacked tubeĭesigns used driver transformers because a practicalĮquivalent to a "PNP" tube is forbidden by the laws of physics. Widely viewed as a serious impediment to getting good sound. Power tube circuits were devised to avoid the output transformer, which was The driver transformer is a legacy of the vacuum tube. Inįact, I can find almost no reference to this circuit in most of the design ![]() Rapidly as suitable complementary devices became available in the early 60's. Transistors worthy of the term appeared in the mid to late 50's and died out ![]() This style of power amplifier design was only current from the time power Transformer style power amp, which places the date of the designs very closely. Kinship is much, much more than similar styling on the external covers.Īll of the TV solid state amps from smallest to largest use a driver The Thomas Vox (which I usually abbreviate "TV") amps are allĬlearly of a single design philosophy and share certain elements inside. Use your own judgement, and do as you best see fit with your amp.įor those of you who are all ready to go, there is a table of reliability upgrades at the end of the article. In this article, I'll mention some things that are trivial updates, and some that might be seen as defiling the temple. This, I found that all Thomas Vox amps all share some similarities in theirĭesigns, particularly in power supply and output power amp sections that can be improved a great deal. Royal Guardsman, a Viscount, a Cambridge, a Pathfinder, a Pacemaker and a Nova. I own a solid state Berkeley II, a Berkeley III, a Buckingham, a Beatle, a There are also a lot of tips here on how to get a dead one running again. I love the way these amplifiers look (and sometimes the way they sound) and in fixing a couple that I have, I came up with several things that should make them vastly more reliable, and possibly sound better. The Thomas Vox solid state amplifiers are notorious for their poor reliability. Replacing the 3P3T Rotary Power/Standby Switch Getting the Chassis Into and Out of the Cabinet ![]()
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