12AX7/12BH7 Hammond Tube Preamp Project

Construct a preamplifier intended to accept a 1/4" keyboard line-level signal and drive a Leslie 147 amplifier. The preamp will utilize a 12AX7 and 12BH7, following the design of a Hammond Organ preamp. The focus will be on the section from C6 to the output transformer, without additional components. There are inquiries regarding whether to use the existing circuit or design a new one from scratch. If opting for the existing circuit, questions arise about the suitability of a 250-0-250V power transformer compared to the 340-0-340V transformer depicted in the schematic. Additionally, there is consideration of whether to eliminate the output transformer as shown in the schematic. The Leslie 147 power amplifier is specifically designed to pair with this preamp schematic. Suggestions are welcomed, particularly concerning the use of a 250-0-250V transformer for cost, size, and weight savings, as well as the choice between a solid-state or tube rectifier. Another schematic of an older Hammond AO-10 preamp is referenced, which employs a 250-0-250V transformer to achieve 280V and 290V supplies, utilizing a 6X5 rectifier instead of a 6X4. Clarification is sought on how the transformer can output 250V while the 6X5 rectifier provides 320V. There is also a question regarding the possibility of using this power supply to energize the plates of the 12AX7 and 12BH7 in the AO-28 schematic.

The proposed preamplifier circuit is based on the Hammond AO-28 design, which features a dual triode configuration using the 12AX7 and 12BH7 vacuum tubes. The 12AX7 is typically employed in the first stage to amplify the low-level signal from the keyboard, while the 12BH7 serves as a driver stage, providing higher current capability to drive the output transformer. The section of interest, from capacitor C6 to the output transformer, is crucial for signal integrity and amplification fidelity.

When considering the power supply, the choice of transformer voltage is significant. A 250-0-250V transformer can indeed be used, as it provides sufficient voltage for the plates of both the 12AX7 and 12BH7. The output voltages can be adjusted through the use of appropriate filtering and regulation. The AO-10 preamp's use of a 250-0-250V transformer with a 6X5 rectifier illustrates that lower voltage transformers can still achieve the necessary plate voltages through rectification and filtering, albeit with different rectifying components.

The decision to use a solid-state versus tube rectifier also affects the circuit's performance. Tube rectifiers, like the 6X4 or 6X5, introduce a certain amount of sag under load, which can contribute to a warmer sound characteristic. In contrast, solid-state rectifiers provide faster response times and higher efficiency but may lead to a more sterile sound.

Furthermore, the output transformer plays a critical role in matching the impedance of the preamp to the Leslie 147 amplifier, ensuring optimal power transfer and tonal quality. While it may be tempting to omit the output transformer for simplicity, it is advisable to retain it unless there is a specific design consideration that necessitates its removal.

In summary, the construction of the preamp using a 250-0-250V transformer is feasible, and the choice of rectification, along with the inclusion of the output transformer, should be carefully evaluated based on the desired audio characteristics and performance requirements.Build a preamp that I want to use to take a 1/4" keyboard line level signal and drive a Leslie 147 amplifier with it. I want to build the preamp using a 12AX7 and 12BH7 based on the design of a Hammond Organ preamp. Here is the schematic to the Hammond AO-28 Preamp I am going to build: I want to build from C6 to the output transformer.

I don`t need anything other than that. Would it be better for me to use this circuit or design a circuit from scratch If I did decide to use this circuit, would it be ok if I used a 250-0-250V power transformer or would I have to use a 340-0-340V transformer that is in the schematic. Also, would it be better to not use an output transformer like in the schematic Here is a schematic of the Leslie 147 power amp that I am driving: Please keep in mind that the Leslie was designed to go with this preamp schematic.

I am open to any and all suggestions. I am mainly interested in whether or not I can build the 12AX7/12BH7 part of the Hammond preamp using a 250-0-250V transformer to save on cost, size and weight. And wheter or not to use a solid state or tube rectifier. Here is another schematic of an older Hammond AO-10 preamp. It uses a 250-0-250V transformer to get the 280V and 290V supplies, but it uses a 6X5 rectifier instead of a 6X4.

How can the transformer be putting out 250V and the 6X5 putting out 320 Could I use this power supply to fire the plates of the 12AX7 and 12BH7s in the AO-28 schematic 🔗 External reference