Tube Amp Junior Idea

[kt88pppamp]

If any of you have young children, what do you think of the idea of junior tube amp for the kid's room? I have a rough design for one.

It features:

- Durable white painted wooden chassis with metal back and cover with childish decorations (Fisher Price type look and lettering).

- 1/8th" high impedance microphone input for "singing along" with large volume dial.

- Line input for any source with volume dial.

- Two small speakers built into the cabinet.

- Switch to disable the microphone input or line inputs.

The design uses two 6V6 tubes per channel, my universal driver circuit (dual 12AU7 and single 12AX7). The mic stage will use a single 12AX7 feeding both channels. The line stage will use a single 12AU7, each half feeding each channel. Feedback sets the gain for the power amplifier to compensate for the preamp sensitivity. The rectifier will use solid state diodes.

Tube count:

- 2x 12AX7
- 3x 12AU7
- 4x 6V6

To keep it kid friendly, I am trying to use tubes that keep the heat output low. If reducing the tube count really makes the amps cheaper, I could make the output stage class A. Then transformer costs increases. What do you think of this idea? What would you all do differently?

[Geek]

Hmmm, little "Johnny or Susie" with 15W of tube power? Mom and dad might hate you!

What about SE?

[DeathRex]

Ja, you can use a schematic from a German radio, and add a 12AX7 for the microphone. Or it'd probably be cheaper to get a stereo German radio. Should be able to find one on ebay for under $100.

[EWBrown]

I'd go with 6V6 / 6AQ5 or EL84 SE, for 5W use pentode mode, for lower power use UL or triode-strapped mode. Perhaps internally selectable.
B+ 250 to 300VDC, keep the plate / cathode current "reasonable" to reduce the plate dissipation and "blisteringly hot tubes".
The Edcor 5K to 8 ohms would be a very good match for the 6V6s. 10-15 WPC could be a bit too much excessive power for the l'il' kiddies...

About ten or so years ago, Gary Kaufman built up a small 6S4A SET for his daughter, and used what he referred to as "bass shy" OPTs,
He ran them at 250V B+ and 5K OPT and 500 ohm RK which is pretty light duty for these tubes (and used a 6N1P SRPP driver stage).

My favorite operating points use 325V B+, with a 390-400 ohm RK (for approx. 25 mA plate current) and with the tube's relatively high plate
resistance of 3700 ohms, I'd use a 10-12 K primary OPT, or run them in PSE with 5 to 6K OPT. Even with PSE, the 6S4As aren't exactly going to
"rock the house".

6S4As are a very nice sounding low priced power triode. 6CG7 or 12AU7 SRPP driver is a good match, as the 6S4A cathodes operate about 9.8 to 10V
above signal / power ground. use a 12AX7 / ECC83 for any necessary high gain front-end purposes.

/ed B

[soundbrigade]

Get tube shields for the tubes. And while building the amp, give your kids a crash course in tubelectrics!

[DeathRex]

How about using battery portable tubes like the 3V4 or 3S4. They put out very little heat, run on low voltage 67-90VDC, warm up in about a second, and put out very little power. In SE, a 3V4 puts out about 1/4 watt, about that same as a 171A at 90 volts, but with a cheaper tube. I guess maybe 1 to 2 watts in PP.
http://ampbooks.com/home/amp-technology ... ry-amp.gif

[azazello]

Kids like high power sounding.....

[kt88pppamp]

I agree that adolescent males in particular like their earth shaking bass and rap music . Daddy should make him work and save up money so he can buy my PPP amps that in the future I will be selling as kits. Two blocks I anticipate having to sell will cost 3-4 grand (for a pair) for a decent profit margin. Junior gangsta should also be made to put them together as well and learn some soldering skills .

You all are probably right that 6 or 7 year old with small speakers and 15 watts would be way too loud.

The way to go would probably be to use a SE 6V6 which will probably make about 5 watts. Into 3" full range drivers, small SE transformers should not cost too much money.

Daddy had better not buy gangsta a car which he may end up totaling, or spend the 4k on class d car subwoofers that piss of the people in the old folks home!

[Hotsauce]

Build a small 2 channel guitar amp.

It'll be useful for so many things later.

John

[Shannon Parks]

Build a small 2 channel guitar amp.

It'll be useful for so many things later.

John

This is a brilliant idea.

Shannon

[Hotsauce]

If you have more than one gain stage, and knob for each you can play music or voice cleanly, but still crank it up to overdrive for when Dad wants to get loud.

I would use speakers that can not be blown with the power the amp can make. Maybe a pair of EV 12's

I seriously think just a few watts and 2 12's will make you glad it isn't 30W

A stereo 1/8 input, 2 rca ins, and 2 1/4 mono input jacks.

Build it as a combo and at least partially enclose the back so no one can reach the tubes without removing a panel

Something like a Vox AC10 cabinet.

John