6418 Submini Tube Battery-Powered Beltpack Pre-Pre-Amp v1.3

Here is a circuit I drew out for a guitar pedal/beltpack running on batteries and using a single 6418 tube. It has bypass and volume as the only controls. The plan is for the guitar player to wear it on their belt or pocket connecting to the guitar with a short cable. It could also be used on a pedal board as a boost or whatever. Its a simple gain stage that could even be put inside a guitar. Any suggestions? I'm trying to keep it as simple as possible. It's powered by two 9 volts in series with a single AA for the heaters. I'm calling it "The Hearing Aid". Here is the schematic: http://tablebeast.com/schematics/hearingaid13.jpg

Try to add some plate load resistor... Or a choke...

Oh Yeah, duh. Of course I need a plate load resistor. I redrew this quickly from a sketch and just left it out. I'll update the schematic. What size would be optimal there? The 6418 operates down to 15 volts on the data sheet, but I'd still like to get the most I can out of it.

Jesse

snip

OK, now I've fixed the schematic for The Hearing Aid and the circuit at least makes sense. Sorry for the confusion. I think this new circuit will work well for the intended purpose. Any comments on the new schem?

Do you know what the output impedance of the tube is? I would wonder about the HF rolloff into cable C from Aid to amp (although if it is comparable to the rolloff from a pickup directly I suppose it will be o.k.).

I would breadboard and try changing plate resistor's value for better sound.

Do you know what the output impedance of the tube is? I would wonder about the HF rolloff into cable C from Aid to amp (although if it is comparable to the rolloff from a pickup directly I suppose it will be o.k.).

Click to expand...

I don't know the output impedance, but wouldn't impedance be lower at the plate than what it sees at grid1 by the nature of the tube?. I was thinking this device would improve HF rolloff by lowering impedance and bumping the gain a little rather than make it worse. How do I figure out the output impedance from the data sheet? Here is the one for the 6418: http://tablebeast.com/schematics/6418.pdf

500 KOhm tube, 100 KOhm anode resistor. A bit less than 100K as the result.

[quote author="Wavebourn"]500 KOhm tube, 100 KOhm anode resistor. A bit less than 100K as the result.[/quote]

Yeah, 100K is in the range I was thinking. Isn't the impedance of most pickups 1M or more? I'll be prototyping this circuit this weekend so I'll report back with how it worked and an updated schematic if I make an permanent changes.

Jesse

Again, if the output Z is less than the pickup then you'll be getting less HF attenuation than using a direct connection---and that may well be fine. In fact maybe the pickup manufacturer is counting on cable loading to begin with, so you may wind up with something too bright.

> 500 KOhm tube, 100 KOhm anode resistor. A bit less than 100K as the result.

The plan I see shows Triode-wired. Rp is near 50K. Say 33K total.

> wouldn't impedance be lower at the plate than what it sees at grid1 by the nature of the tube?

The output of a standard geetar pickup is 5K + 5H.

The common modern guitar sports a 250K volume pot.

Compute the sum response of pickup, pot, and 10-30 feet of cable. It is NOT a simple matter of "low i,pedance".

> I was thinking this device would improve HF rolloff

Why would you want to do that? Electric guitar strings NEED to be low-passed. They are full of tizzy inharmonic crap. The pickup designer has designed the pole-piece size and the winding parameters TO give a pleasing tone.

It would be interesting to know why that data-sheet shows two "F+" pins.