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jordankersten 4th April 2009 07:18 AM

12AX7/12BH7 Hammond Tube Preamp Project
Hello all this is my first post!

I am about to 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?

Thanks for all replies!

SY 4th April 2009 10:52 AM

You should be able to get the voltage you need with a solid state rectifier and a 250-0-250 transformer. You'll probably want to use a CLC filter to minimize voltage drop. Just to be sure (you didn't mention the current ratings or winding resistances of the proposed transformer), I'd spend an hour or two learning how to run the Duncan Amps power supply simulator program (PSUD II) and then sim the supply.

On the receiving end, it looks like the input is configured to take the output of a power amp. So if you don't intend to do that, you can get rid of the two 8 ohm resistors and run signal to the top of the 10k pot; the drive requirements are much easier.

Now, you'll still need the output transformer- you need a low output impedance from the preamp and its gain would otherwise be too high. If you have the circuit chops, you could get rid of the OPT by changing the 12BH7 stage to a cathode follower. That lowers the source impedance and drops the gain. Depending on how long the cables are and the environment, you might still need a 1:1 output transformer to use a balanced connection. If the cables are short, this probably won't be necessary.

jordankersten 4th April 2009 05:11 PM

Thank you for your reply. The current draw from the 12AX7 and 12BH7 will not exceed 14ma (or 30ma if using a 12AX7EH). Unless I need to include any current from the 6X4? The heater current with all three tubes will be just under an amp (around 900ma). I am thinking about using a Hammond 261G6 Transformer found here:

If I end up using a 340V transformer, I will use a Hammond 273X found here:

What would be the pros/cons for using either a 250V or 350V transformer...other than the 250V transformer will be cheaper, lighter and smaller. What are concerns I need to worry about electronically?

m6tt 4th April 2009 08:35 PM

350 should be fine, but it won't sound the same necessarily as 250v. With the higher voltage, you should have more headroom for playing say the bass pedals with other notes at the same time. I find for 12ax7 in MI uses, that often the higher the plate volts, the cleaner it gets, within reason (say 300v at anode or so). The differences aren't that dramatic, though. I think SY is pointing out that the Hammond uses a tube rectifier, so using a solid state rectifier plus a 250v transformer will put you in the same ball park for 350v with a 6x4. The part of the schematic you are using wants 280v, which is entirely feasible with 1n4007 (or UF4007) diodes and a 250v transformer with the appropriate filter caps sizes.

jordankersten 5th April 2009 02:49 AM

I figured out the power supply but my new problem is the output transformer is extremely expensive and hard to find. So I am wondering if there is a way to omit the output transformer using that circuit maybe by making the 12BH7 a cathode follower? If that is not feasible, I am considering building this circuit using a linear power supply:

How could I add a 3rd gain stage to this circuit, particularly a 12BH7 gain stage? Does anyone have or no where to find a schematic to a circuit using 3 gain stages using either 12AX7/12BH7s or 6SN7/6SJ7. Thanks for all your help!

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