Low Voltage Clean Valve Booster

[psychedelicfish]

Here's the clean tube boost I made a little while ago that I mentioned here http://www.diystompboxes.com/smfforum/index.php?topic=63479.msg908019#msg908019.

As you can see, all that's there is an SRPP amplifier. I normally wouldn't post my random breadboard experiments, but this one sounded really good. It sounded really rich and warm, and provided plenty of boost too. With the cathode capacitor disengaged it was absolutely clean. With the switch engaged, it gave a very slight hint of distortion, and gave the sound a very definite valve warmth, even playing through the distortion channel of my cheap SS amp. Should also work with a 12AU7 at 12v, however I haven't tried this so YMMV. Sounds a lot thinner and more distorted with a 12AX7 though.
There's plenty of other uses for this circuit too, I replaced the volume pot with the 10k side of an output transformer, and connected the output side to some headphones, and it made a nice little headphone amp, and with an increased supply voltage, a decent one valve power amp.  There are lots of possibilities...
Enjoy!
Edward

[duck_arse]

nice work. I still haven't done my valve boost (it's still on the breadboard gathering dust), and have been thinking about srpp-ing it instead. now, I won't have to think. thanks!

[alpine]

Resurrecting an old thread I know. I've been experimenting with a valve preamp for a Leslie speaker where I'm using a transistor PA. It's made from the remains of a real one I got without any electronics.  I don't want massive distortion, just that growly crunch which makes the magic Hammond organ sound. I've tried the Valvemaster and the double valvemaster in various configurations, and then I came across this thread. The other ones, even with a 12AU7, were too high gain, but this one makes a really good sound which can be clean, or just the right crunch! I'm driving it via the balanced output from a real valve Hammond.

Thanks for the circuit.

[Ben N]

Nice!

[alfafalfa]

That's something I'm defenitely going to try out. Do you know how many mA this tube draws?

Thanks for the schematic.

[gutsofgold]

Would love to try this out, any chance of a bigger schem? Can't see the part values. 

[Kipper4]

Excuse my ignorance but what is the portion marked 4 5 9 all about.
As your may be able to tell I'm not familiar with tube stuff?
Thanks.

[samhay]

The valve filament/heaters. You want about 12V across 4-5. 9 is a centre tap so you can alternatively run 6V across both halves.

p.s. Looks like a nice circuit.

[alfafalfa]

And you need that big 66 ohm 5 watt resistor to bring down the voltage from 24 volt to 12,6 (as it should , I think) which is standard for the heater filaments connected in series.

[alpine]

That's something I'm defenitely going to try out. Do you know how many mA this tube draws?

Thanks for the schematic.

On the heater, I drive it from a 12vdc supply that I use elsewhere for motor switching. The spec sheet says 150mA. If I was using the 2 heater halves on 6volts, in parallel they would take 300mA.On the B+ anode supply, I'm just using a tap off the +40v supply to the power amp with a multiturn pot to adjust it and its drain is less than 1mA.

[alpine]

Excuse my ignorance but what is the portion marked 4 5 9 all about.
As your may be able to tell I'm not familiar with tube stuff?
Thanks.

On the circular B9A valve/tube holder, the pins are numbered 1 to 9. 9 is the centre tap pin of the 12.6v heater to allow it to run from a 6.3v supply. There is reasonable tolerance on the heater volts, though it's better to go a littler lower than higher.