Building the Tiny Giant amp

[PRR]

> use unregulated power without dire consequence

Yes.

The power chips is made to run 12V-14V un-regulated in a nasty environment (car).

Respect the MAX Voltage of the power chip, but this is 18V steady and >24V blips, so a "10V" supply is unlikely to ever get close to danger.

This power chip is very good at rejecting crappy power. Alternators whine and kick-out, fuel injectors CL-TIK-ing constantly, some relay in my O2 system chatters. None of this gets in the radio, so wall-power buzz won't either.

Power output will be near 10 Watts in 4 ohms and less in 8 ohms. Bigger than a Champ, smaller than a DeLuxe. Plenty to annoy the neighbors. Not enough to play the Palladium. Going to higher voltage might double power, but that's not a big difference.

[musicisphysics]

Thanks Taylor, and thank you too PRR (for that detailed explanation).

This is what my build is looking like...

[Taylor]

You know, originally my PCB was laid out like that, but then I changed it at the last moment...

[Ben N]

That may not be tiny, but it sure is purdy.

[bcalder]

Holy moly. I thought I was a minnow before ...

[Flemming]

This weekend i finally found time to stuff my Tiny Giant in a standard stompbox enclosure. Brief testing performed on one Jensen C12Q 35W 8Ω speaker inside a cheapish cabinet using this 16V 2.2A laptop PSU. It looks very ordinary by now, so i'll wait with the pictures for later in the process.

Status report so far; It sounds lovely, and very loud! This is exactly what i was looking for I of course ran into the 2 vs 3 prong PSU issue. Ensuring proper power makes any noise dissapear instantly. My tests lasted only minutes, so i didn't notice any real heat from the box. I hope to test further in the next few days, beefing up to combine this with some tube preamp.

Cheers

[musicisphysics]

Touché Taylor,

I have since cleaned up the layout a bit. I completely bypassed the TL072 buffer, and the TDA7240A sounds so good on it's own (even on unregulated power), that I don't really plan to use any preamp with it; just plug in the guitar and play "as is". Does anyone here foresee any issues which might arise.

[Taylor]

Pretty crazy! I don't think you'll have any problems running it that way.

[waltk]

What sort of volume control are you planning to have?  Without the preamp acting as a buffer, you could get some tone-sucking effects if you just stick a passive volume control in front of the amp.

Awesome build!

[musicisphysics]

waltk,

I'm not quite sure what I'm going to do yet. I may attempt to use it as a reverb recovery stage, from the following driver...



through the following Doepfer tank...

http://www.analoguehaven.com/doepfer/springreverb/

[musicisphysics]

waltk,

Relative to volume control, I think I've figured out what I want to try (note the 560k pot)...

[waltk]

Relative to volume control, I think I've figured out what I want to try (note the 560k pot)...

I don't know enough about tube circuits to comment on this.  I've built a ValveCaster and a Dual Valvecaster and tried them both in front a Tiny Giant.  They worked great (but I just left the stock TG buffer/preamp in place).

I would say the only thing you need to be concerned about is getting an appropriate output level from your buffer.  If it's truly a buffer, you'll be fine.  If it adds any boost, you should limit the gain to about 2. The TL072 in the Tiny Giant has a gain of about 2.  You definitely don't want to overdrive the TDA7240 to clipping - it's ugly.

-Walt

[musicisphysics]

Good point waltk,

I'm thinking that the schematic which I posted would give a gain slightly under unity, but your comment got me to thinking: "Why not connect the both halves of the E88CC in full parallel operation (with both using a shared plate and cathode resistance at half the value of the single triode)?" Then I could implement a half-gain option by sending the input signal to only one grid, to obtain an approximate gain of 0.4 with less noise and lower output impedance.

Something like this...



I'm just wondering whether I should ground the unused grid and/or the internal shield, or let one or both of them float.

[PRR]

> note the 560k pot

I've never seen a 560K pot; perhaps 500K?

However, look both ways. The source is tube and a 5K6 pull-down resistor (6K3 with bias). The load is, I assume, the power-stage of the Tiny Giant, which IIRC is 20K input.

As a first guess, the pot could split-the-difference between 6K source and 20K load. 10K9. You could do a detailed analysis and prefer some other number, however "10K" is such a common part that you'll probably end up with 10K.

> connect the both halves ... could implement a half-gain option

Connect two cars together. Push both gas-pedals. They go full speed. Now push just one gas-pedal. Will it go half-speed? Well.... it's complicated, but likely the two cars fight each other, get unstable, skid, flip, or just overheat.

If you are going to heat both halves, you may as well use them. You can run two triodes on the same 750r+5K6 resistors you use for one, or you can go 375r+2K8. If you want "half-gain", divide the output with resistor voltage-divider, not with a tube-fight.

[musicisphysics]

Thanks again, Paul. I appreciate your input.

I've never seen a 560K pot

Yeah, they're not extremely common.

http://parts.digikey.com/1/parts/1963290-pot-560k-ohm-1-4w-20-side-flat-1624199-7.html

"10K" is such a common part that you'll probably end up with 10K.

I'll keep that in mind; thanks for the advice.

fight each other, get unstable, skid, flip, or just overheat ... a tube-fight

The act of connecting two triodes for parallel operation, has been widely implemented and documented (i.e. Matchless does this -- and others); as has the act of grounding one of the grids while allowing the second triode to draw its quiescent current (i.e. Merlin has written on this -- and others). I'm unaware of any mention of tube-fighting within these scenarios (even with the triodes relatively unmatched): it's not as though I'm attempting to run two thyratron oscillators in parallel.

http://www.springerlink.com/content/q051218341mt4661/

[musicisphysics]

The datasheet shows that the input resistance of the TDA7240A is 70k minimum, so shouldn't the volume control be from 10k to 100k?

http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1469.pdf

[PRR]

Simple parallel is fine if you have budget.

The Springerlink citation covers _plate_ loaded stages.

Tying two cathode followers together is different. Bredbord and see

[kinski]

Hi, this seems like a great little project. I'm gonna order a kit pretty soon.

One question, how would I add a on/off indicator light?

[Taylor]

You could use a DPDT switch as your power switch. Use one pole to cut power to the amp and the other pole to light your LED. Or, just tap your LED from the power on the board. When you switch the amp on it will light, when off it won't. Make sure to put a resistor in series with LED as usual.

[kinski]

Thanks! Also, is there a power supply I could get from Small Bear Electronics that would work? Would be great if so, cause I am about to place a big parts order from there.

I have bad luck with wall warts. I smoked a circuit once using what I thought we be the right supply.