Stand-alone tube preamps

[martin manning]

What you're proposing is exactly what is in the schematic posted above from Hoffman. Note he recommends a 1A wall wart, and I would agree. A 500mA unit would be on the edge. For the backwards transformer I would consider a dual primary/dual secondary flat-pack such as this: https://www.mouser.com/datasheet/2/410/ ... 068097.pdf

[M Fowler]

I have used the Button single 12AX7 preamp power transformer no problems.

It is short so clears a 2" rack.

Mark

[roberto]

Buying 3 to 5 units of toroidal transformers, you can get lower prices than standard transformers in shops.
And you can fit in 1UR even with high wattages (so if you want to add some power to the preamp.

[megatrav]

I know this thread is a bit older.. what does "filtering" when it comes to building a preamp or other tube pedal refer to?

[jarrodthebobo]

I know this thread is a bit older.. what does "filtering" when it comes to building a preamp or other tube pedal refer to?

Filter capacitors. IE the big caps that get rid of the ripple/switching noise from the power supply. Typically encompasses if you're using a choke/include the stepdown resistors and such that form filters with the capacitors

[ChopSauce]

I know this thread is a bit older.. what does "filtering" when it comes to building a preamp or other tube pedal refer to?

http://www.valvewizard.co.uk/smoothing.html

(would you like to read some more on the subject)

[megatrav]

Thank you both for the reply.

One other question-

For builders who make tube preamps/tube pedals- is it typically just taking the amp schematic and then recreating that or are there other things that would be changed?

[jarrodthebobo]

Thank you both for the reply.

One other question-

For builders who make tube preamps/tube pedals- is it typically just taking the amp schematic and then recreating that or are there other things that would be changed?

You can very well just chop the preamp section from before the PI out of a generic amp schematic and make that into a standalone unit, sure. However, there is an argument to be made that there is quite a big benefit from adding a CF to the end of the preamp that you've built in order to drop the impedance some to better drive whatever you're running the unit into.

I have a little preamp unit I've been messing around with; initially I opted to only use 12ax7s in this build which utilized at minimum, 3 gain stages. I wanted the 4th stage to be yet another gain stage placed after a tone stack in order to somewhat simulate the gain a PI imparts into the amp. I was finding, however, that there was some significant high frequencies lost when using any length of cable longer than "short". I've since added a 3rd tube to my unit; including 2 additional CFs (one for the tone stack, and the other for the output in order to drive low impedance) and have been much more satisfied with the results.

Also, building a standalone preamp typically means you can go a bit more lax when it comes to power tranny selection as you'll be drawing much less current with just 12ax7s and the like. The only other suggestion I can think of is varying the dropping resistors between filter stages in order to get more appropriate values on your preamp unit; without a PA section, there will be less voltage drop and you may be running your preamp tubes hotter than they would normally in the original amp you're basing your circuit on.

[megatrav]

Thanks for the tip.

I’m sorry, I’m an amateur.. what does CF mean?

[10thTx]

cathode follower http://www.valvewizard.co.uk/dccf.html

[megatrav]

Thank you for the response!

A lot of great information here. You all have been very kind and helpful

[megatrav]

Not to derail this thread at all, but I have a technical question.

A buddy of mine has been making tube pedals using 6N21B tubes running at high voltages (around 200 volts). They sound and react very similarly to a 12ax7 except they are soldered in.
He stated that when he builds pedals he can do a single 12ax7 design (like the Alembic FB-2, which he has cloned and put into pedal format) but whenever he has more than one tube in a pedal he get oscillation. Any idea what might cause that or what might be a cure for it?

[didit]

Any idea what might cause that or what might be a cure for it?

Several possibilities. Posting some detailed photos & the schematics would be good place to start.

Best .. Ian

[megatrav]

Sorry, I don’t have any schematics. As far as I know, he follows the schematic from the amp. He uses the power supplies that allow it to be powered by 9 volts. I can’t remember what they’re called but the tube itself is running at high voltage.

I did mention the cathode follower. He said a few of his designs have it, but not all of them and that adding one in can change the sound.

Lastly, I have a couple pedals from him that use 6n21b tubes.
One is a Fender style, the other one is the clean and drive channel of an Orange Rockerverb. It has 3 of them in it.
It sounds amazing, but gets very hot. It also has an issue where when the gain is turned up really high it starts feedback and squealing. It seems like the issue is because of the tubes themselves. They start feeding back and create a high pitched squeal.
Noise gate pedals don’t seem to take it away.

I’m not a builder at all, but I do have a great appreciation for the craft. Sorry for being a middle man of sorts but I’m also very curious.

[tony hunt]

megatrav, in a pedal enclosure the layout of the components will be much more crampt than in a typical guitar amp. The use of miniature tubes helps a liitle bit. But at high gain, I would expect sqealing and oscillation.

The most sensitive parts of the circuit will begin to pick up the signal from a later part of the circuit. Circuit layout is a subject for itself and worth looking into. I am not saying that your friend made a mistake, I think the sqealing is a result of physical limitations when pushed here into high gain. To me, it is logical.

If it was me, I would accept that limitation and either avoid the real high gain setting or musically implement it somehow.