Tube Preamp Project: advise and discussion

[johngovan1234]

I want to build a preamp tube that can go into any poweramp but in pedal format. Maybe 1590BB enclosure. Something like kingsley preamp.

My plans are

1. Build a clean preamp using one 12ax7 tube
2. Build a tone stack of Dumble or Vox
3. Use 2 Transformer 12v by 220/240v
4. Rectifier and regulator maybe LM317 to acheive 6.3v for the heater.

Any ideas that i need to? I'll post my schematic soon. Or find schematic that i can use. Thank you so much. Hope someone can help me.

[tonyharker]

I would recommend the Alembic F2-B here http://moosapotamus.net/ideas/alembic-f-2b-preamp/ This will do all you want.

[johngovan1234]

Thank you for your response. I will study this one and i hope i can build this project soon.

[ckilgore]

You may want to check out https://el34world.com/projects/tube_box_3.htm

[wavley]

There's a cat around here that has boards for this very thing.  I have two that I haven't built yet, but it's a nice project.  http://frogpedals.com/index.php/product-category/pcb-products/

[cab42]

Also, look at the Omnibox: https://www.diystompboxes.com/smfforum/index.php?topic=120628.0

I would like to buy one of the frog pedal pcb's but shipping and tax makes them pretty expensive.

[anotherjim]

I would do the F2-B also. Any tone stack variant can go between the triodes, although some were designed to be driven by a low impedance cathode follower stage, it doesn't change things much with the common cathode drive.

Not that making a regulated DC heater supply is particularly difficult or expensive but...
If your transformers are near enough 12v rms, you can get surprisingly good results by running the heaters right off the 12vAC (pin 9 ground, supply across 4 & 5). It's only 150mA current and thinner wire like 7/0.2 can be tightly twisted to deliver low-hum heater power. Grounding pin 9 means you don't need a humdinger, although if at least one transformer has a centre tap secondary, you can ground that instead and leave the tube pin 9 disconnected.
...but if you want DC for auxiliary circuits and will be rectifying the 12v anyway, it may as well have regulated DC heater power.

[johngovan1234]

I thank all of you for your responses. I will try to visit the links you posted. And also try to post a schematic and ask for your opinions and suggestion about my project.

[amptramp]

Not that making a regulated DC heater supply is particularly difficult or expensive but...
If your transformers are near enough 12v rms, you can get surprisingly good results by running the heaters right off the 12vAC (pin 9 ground, supply across 4 & 5). It's only 150mA current and thinner wire like 7/0.2 can be tightly twisted to deliver low-hum heater power. Grounding pin 9 means you don't need a humdinger, although if at least one transformer has a centre tap secondary, you can ground that instead and leave the tube pin 9 disconnected.

Don't connect it like this for 12 volt operation.  If you run the heater from 12 volts (AC or DC), leave pin 9 open and put the voltage across pins 4 and 5.  Pin 9 is the centre tap of the heater.  You only connect pin 9 if you are running from 6 volts (AC or DC).

[johngovan1234]

Here's my initial layout. A rough layout only. Based on two rock clean stage. Any thoughts? Thank you.

[tonight, we ride]

Just a heads up that if you're digging into the Dumble inspired stuff, like the Two Rock, you're DEFINITELY gonna wanna check out what's going on over at the AmpGarage forum.

[johngovan1234]

Thanks for the information. I will check the site and do the research. That's very helpful..

[johngovan1234]

will this transformer work?

[MJ_Sound_Cubed]

Hi man, 

I would not use transformers at all. They are big, heavy and overall its dangerous do work with AC .

I would recommend an SMPS such as this one:



The a Tube based guitar pedal would fit here in 1590C easily like such:

[printer2]

Dangerous to do work with AC? And the dc going to the plate is of no concern? But really higher voltages are not a think to be afraid of, something to be careful of, yes, afraid no.

Now if I were to do a preamp circuit with a tube (and I am) I would use a switching wallwart power supply putting out 12V. Run the heater of it, send the 12V into one of these.

https://www.ebay.com/itm/DC-DC-8-32V-To-45-390V-High-Voltage-Boost-Converter-ZVS-Step-up-Booster-Module/232748063564?epid=1476662431&hash=item3630dd9b4c:g:HpcAAOSwvcVa4plQ



Adjust to whatever voltage you want, no cheesy back to back transformers. It puts out much more power than needed, wish they had a low current smaller model. I took a hack saw and cut off the heatsink flush with the circuit board.

[PRR]

> Dangerous to do work with AC? And the dc going to the plate is of no concern?

Connection to a wall-plug can burn-down the house, or at least scorch the workbench-- current available is very large.

Plate supply is still lethal but less dangerous.

[johngovan1234]

I really like the idea of using an smps. I'll order this smps and try to build preamp as soon as my components are complete. I'll update this post for my progress. thank you so much. Please keep on helping me.

[anotherjim]

Not much love for transformers then...

I don't think it's common practice to use a pair of back to back transformers inside a pedal. Rather, you use an external 12Vac wall wart and make the HT with a back-driven transformer in the pedal. In this case, a connector that cannot make any contact with the standard 2.1mm connector in the ordinary 9Vdc pedals should be found for the 12Vac connection, since many 9v pedals will not like an AC supply up them.
As to the choice of back-drive transformer, a 3VA might be better, although the suggested 1.6VA is just about going to do it, there is very little overload margin. These little encapsulated transformers are pretty feeble creatures, and faced with overload will just be loaded down and do nothing, while something more substantial could start smoking.

Although a potential problem with an AC supply is that you might get some 50/60Hz hum, any smps based supply or boost converter has a switching frequency which might be low enough to be audible, or if ultrasonic, mix with some other ultrasonic source (maybe a BBD delay clock or another smps) and produce audible interference that could be more troublesome than a little hum.

[MJ_Sound_Cubed]

Although a potential problem with an AC supply is that you might get some 50/60Hz hum, any smps based supply or boost converter has a switching frequency which might be low enough to be audible, or if ultrasonic, mix with some other ultrasonic source (maybe a BBD delay clock or another smps) and produce audible interference that could be more troublesome than a little hum.

Well this SMPS is almost silent, because it operates at ~30kHz and with the right capacitor material it almost does not have any harmonics ( which we can "see" on an oscilloscope but not hear). And if you really need to quiet it down, then I recommend using a 100R to 470R resistor between the circuit and the SMPS.

[Jeema]

I agree with the anotherjim, 1.6VA probably won't cut it.  12V x 0.15A (amount of current 12ax7 draws) = 1.8VA.

Actually if you are going to convert the filament voltage to DC, you'll need one rated more than that even - around 3VA for a bridge rectifier scheme.

The power rectification guide on Hammond's website explains it pretty well:

http://www.hammondmfg.com/pdf/5c007.pdf