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Old 5th October 2010, 05:26 PM   #1
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Default 12ax7 Bass Preamp Power Supply Question.

I have spent time on this forum and others and I have a few questions that I can't seem to find answers to.

You all have seen these types of questions before so, if this is redundant then please just ignore this thread and I will keep searching.

I am trying to build an integrated bass amplifier. I settled on the power amp section and now I am having trouble with the preamp.

I have read a ton about the Alembic FB-2 Preamp. I am getting stuck when I get to the power supply. There seems to be two basic power supply designs that work (using 2 walwart style transformers or one dual secondary transformer) both with a voltage doubler.

In my internet research I was reading the Hoffman Amps page and I noticed for his tube preamp he uses this transformer:

Mojo Transformers GA-5 Reissue Rocket/Reverb Unit Power Transformer

In this configuration:

http://www.el34world.com/projects/im...pschematic.gif.

This appears to be a completely different design.

I am looking for the simplest PSU because I plan to put this preamp inside an amp chassis that will already include a transformer for the power amp I plan to use:

HPA-nxV201PS 180 watt Audio Amplifier PSU Module

I like the idea of using the single transformer for the preamp because I should be able to use a single power cord to provide 120v to both trannies. Instead of having a power cord AND and wall wart. If this will not work for any reason please let me know.

Now, I have built tube circuits before (5e3 style) but, they have always been well spec'd and easy to follow.

This is the first time I am just building a preamp. At first I was gonna build a preamp based on the fender deluxe (I love the simplicity of this design) and just use the standard deluxe style power supply. I soon realized that this would be overkill and started looking for people who had already done a 5E3 stand alone preamp. I couldn't find any which led me to use the Alembic preamp but, even this has me pulling my hair out.

Questions:

1. I guess my main question is, is whether or not either of these power supplies are "BETTER" than the others?

2. Are there any pro's or cons to the PSU used in the Hoffman preamp.

3. Would there be any way to power my mosfet amp module (with built in power supply) and tube preamp from the same transformer. It will have 35v secondaries.

4. I have also downloaded the Duncan power supply designer and I can't figure out how to use it. What am I missing?



Thanks for reading!

Last edited by bill mountain; 5th October 2010 at 05:42 PM.
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Old 5th October 2010, 07:29 PM   #2
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So I read a little about chokes here:

Chokes

The Hoffman style power supply seems like a viable option for my project. Sure, I have to buy a choke but it would still be a smaller part count than either of the other two PSU options.

Thoughts?
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Old 5th October 2010, 08:44 PM   #3
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Ok Your idea should be workable. The Hoffman preamp schematic shows two identical channels each with 4 gain tube sections and a tone stack. Thats a lot of gain. I doubt that you need all of that. There may be a reason Hoffman disigned it that way. But that dosent mean that it is best for what you need. The transformer that you found from Mojo is likely to be OK to power the preamp. Or any normal tube preamp design for this application. Forget about using a wall wart, or anything in the way of a voltage doubler. Wall warts are generally not avaiable with high voltage secondaries, and voltage doublers have several inhearant drawbacks, like low current capability, and unique filtering considerations. Just use the transformer you found. Sounds like you want to build a tube front end with a solid state output. Perfectly valid approach. I think the off the shelf power section is a good answer, but you might consider a diferent pre-amp. Why not just use part of the 5e3 style that you have already worked with? Do you really need two channels? Anyway- sould be do-able. Let us know.
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Old 5th October 2010, 08:57 PM   #4
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I should have mentioned that I only want 1 channel of the Alembic preamp.

I just want a solid power supply to drive a single 12AX7.

Power supplies are way over my head but, I'm starting to understand them a little.

I just didn't want to beg for an answer. I wanted to show that I had done some homework.

I'm just in over my head and I'm still trying to sort it out.

Thanks for any help you can give.
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Old 6th October 2010, 01:31 AM   #5
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Another option I forgot to mention is the power supply kit here:

http://www.stf-electronics.com/Page0015.Html
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Old 6th October 2010, 03:06 AM   #6
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So you need around 320V at VERY LOW current - say 5mA for 2 off 12AX7.
That is about 2.5 VA (Volt Amps).
You also need 6.3V AC at 600mA for the heaters for the 2 off 12AX7
That is about 4 VA

Get yourself a pair of 10 or 12VA Dual Primary (115 + 115VAC) to 6V AC transformers.
In this size you can even get them as PCB mount style which could go on some vero or Matrix Board.
Wire up the first tranny to run the 6V AC heaters.
Then wire the 6V AC secondary of the 2nd tranny to the 6V AC output of the first tranny (that is in parallel with the heaters). That will result in 115 + 115 = 230V AC appearing across what WAS the primary of the 2nd transformer. Bridge rectify that to get around +300V High Voltage. A choke will NOT be required. Use say 22uF/450V cap then a 2K2 2 Watt resistor (instead of a choke) to a 2nd 22uF/450V capacitor. BINGO that is your complete preamp supply.

Just a suggestion - a million ways to "skin the cat" (and none of them pleasant for the cat).

If you want a ready made solution them that $79 job in your post #5 is a bit of an "overkill" (you will use about 5mA of its 80mA High Voltage Capability) but will certainly do the job.

Cheers,
Ian
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Old 6th October 2010, 10:16 AM   #7
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The picture I have of an F2B's internals show a stancor PS-8415 transformer. If you're planning on stuffing this into a 1U rack case like the original, I would try to find one of these. A quick google search led me here:

Leeds Radio - transformers

They list it at 22$.
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Old 6th October 2010, 11:19 AM   #8
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Thanks for the suggestions. Yes that SFT kit is over kill.

Now you see my dilemma. I now have 2 great posts (from Gingertube and Stormrider) giving me two different options.

These are the two prevailing PSU designs that I see for this project.

I guess now I will just have to see what parts I can acquire cheaper and easier.

Thanks.
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Old 6th October 2010, 11:34 AM   #9
Ian444 is offline Ian444  Australia
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Stancor PS-8415, 117V primary, 125V@15ma DC, 6.3V@0.6amps $22.00

Voltage may be a bit low, even with a doubler, borderline?
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Old 6th October 2010, 12:00 PM   #10
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I'm at work now, but when I get home I'll post the pictures and schematics I have. The original F2-B used that Stancor transformer and a doubler supply. The filament winding is rated right on the edge for two 12AX7's, but will work fine, as it did in the Alembic. It would run cool if you only needed one channel, one 12AX7. I hope Leeds Radio has more than one or two, as I wanted to get one for an Alembic clone myself.

As I said before, this is probably the simplest option if you are planning a stand alone preamp in a 1U case like the original. If you have more space, I would probably do the two transformers back-to-back thing if it was cheaper.
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Last edited by Stormrider; 6th October 2010 at 12:03 PM.
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