...Baby tube Guitar amp...

[James D.]

I think people who want crazy overdriven distortion at bedroom volume levels need to really consider amps with a Master Volume control. Otherwise you need to limit yourself to an amp that hits the wall at milliwatts of output.

Actually both my boys use a Boss GT6 if they want that.

But to do the hard crunch of late 60s rock you need to overdrive the power tube and OPT - a Master Volume control works before the power tube so no good - although it does work well on ss amps.

I'm trying relatively inefficent speakers to keep the volume levels down too...

Maybe I should use an EF86 as output!!!

James

[fdegrove]

Hi,

Maybe I should use an EF86 as output!!!

Philips MINIWATT ?

Ciao,

[Arrrg it Bit me]

Joel,
which Gibson is your favourite, from which amp?

[MIKET]

Try this on for a mini amp
http://digilander.libero.it/paeng/hi...ubes_part3.htm

You can get the 6C45pi from Handmade Electronics
http://www.hndme.com/

Good Luck

[James D.]

This last weekend I was asked to knock up a quick guitar amp for a friend and all I had othe shelf was a 6N6P and a matched pair of 6C45pi. I had a Ra-a 11K OPT and a small psu trannie so I was away...

I used the 6N6P as a twin triode pre-amp with gain and tone controls and used the 6C45s in PP LTP as OPT. B+ was 300V. It worked really well having a super clean tone with excellent response to fingering. The pre would push into overdrive starting to crunch up nicely and the output stage would also push through into overdrive (6C45s where biased at -2.5V and I had 16Volts of drive available before the master volume!). I'm a bit worried about how positive the 6C45s grid gets driven but it has survived several hours of abuse so far. These valves are tough!!!

I guess I should put a diode on it to limit positive drive to 0.7V as it is well behaved up to about that point but it is such a nice overdrive sound that I might leave it and see how long the valves last...

It's quite different from my sons 2A3 guitar amp or the ECL82SE guitar amp. It's closer to 60s Marshall sounds when overdriven...

I refer to the PAEng site a lot. Stefano really knows his stuff

ciao

James

[Bandersnatch]

Hey-Hey!!,
I am putting a simple one together for a friend of mine. Simple and easy. A 6Y6 as power tube, U-L connection, fed by a 6J4 with a DN2540 as input gain stage( no filament hum from that puppy, eh?). Power from a 261G6 Hammond and SiC diodes into a CLC filter, RC decoupled for the gain stages. Huge SE-UL hammond OPT and may run anything form 6K6,6W6 up to an EL34, though for reasons of economy the 6Y6 is likely going to be the final answer...
regards,
Douglas

[PRR]

Get a 1940s-1950s "5-tube" kitchen AM radio. VERY Preferably the kind with a power transformer, not one of the Hot-Chassis "AC-DC" affairs.

Ready-made, just cut the wire to the top of the Volume control, wire a jack to the top and bottom of that pot, and put a high-gain booster-pedal between guitar and jack. (Yeah, I know ready-made is no fun: but these antiques ARE cute and anyway there are lots of mods you cna do.)

They make 1 watt as single-ended pentode. The "better" ones might make more.

Add a triode switch for a less nasty tone.

They usually have a hefty treble-cut capacitor on the output transformer primary. That's needed to cut AM static but most guitarists like another octave or two of treble response. Try with that cap, try without that cap, and split the difference to taste. 1/2 to 1/4 the original value should be sweet.

The sensitivity at the top of the volume control is about 50-100 milliVolts, so you need another gain of 10 to get guitar-pickup sensitivity, plus enough to cover whatever losses your tone-stack gives. You can yank all the wires from the two IF cans to the tube between them, which is a Pentode, and re-wire as an audio voltage amp. Without tone-stack, it might work as Triode. With tone stack you will have to work it as Pentode (or bodge the RF/Conv tube into another audio stage). 100K on plate and 1Meg for screen should work. Cathode resistor will be -about- 1K; change that until the plate sits about 1/2 to 2/3rd of that stage's supply voltage. Or just yank all the wires and use a 6SL7. (If this is a MINI socket set: 6AV6 is 7-pin and sorta-like half a 12AX7 only different. However I think you'll be happier with an OCTAL set.)

Super-size the filter caps: 30µFd got the 1955 price down so your kid's grandma would buy it, but today's caps are so cheap you may as well go 100 or 220µFd everywhere. Typically the RF/IF stage ran on fairly raw power; add a 22K/47µFd decoupling section to clean-up the power to the low-level stages.

You can try this with the original 4-inch speaker: cute desktop amp. However if the kid rocks-out the full watt, the speaker may croak.

If you go with an larger external speaker, you may want to up-size the coupling and cathode caps 2X or 4X stock, to pass the bass that a larger speaker can handle and needs to justify its floor-space.

6EM7 is a classic US TV tube with a hi-Mu volt-amp and a low-Mu Power amp in one bottle, both triodes. It can push way over a watt of audio as SET within ratings. It is very available as NOS. I once sketched it as a 1/2 watt headphone amp, using dirt-cheap parts, but I think you'd be able to see how it would work as a speaker amp and get it up to a walloping Watt. Again it needs preamp gain, and surely two stages.

Your pot-sweep triode/pentode plan: it may barely work. The screen impedance may be 10K or less, so at mid-point on a 50K control the screen may feel isolated. If you reduce the pot to give the screen a lower source impedance, it steals significant power from the load. Since this is not a power-critical application, that may be fine. But I suspect you will have to play with it to find an optimum set of values. If you could find a center-tapped winding (oversized P-P tranny?) you could do a 3-way Triode UltraLinear Pentode selector, which is probably plenty of mode-control.