This Week on the Breadboard: A Mini Tube Pedal

[Chuck D. Bones]

6418 tubes were originally designed to be used in portable, low-power (2mW) amplifiers. Tube part numbers that consist of four digits are hi-rel, ruggedized tubes for military applications. The datasheet is dated July 1, 1963. The date code on the box is Dec 1978.

I think it sounds great. Smooth transition from clean to mild distortion to fairly crunchy as one turns the GAIN up.

A have a few other ideas for using these in pedal circuits.

Turns out that bypassing the filaments to ground is unnecessary because the AC resistance between filament and ground is only a hundred ohms or so.

 

[jubal81]

Low voltage and no heat makes them very attractive.
It'd be easy to fit a pair in a 125B - just mount them on the backside of the board and put the rest of the components "over" them.

 

[Chuck D. Bones]

The directly-heated cathode limits the circuit choices. A cathode follower requires an isolated filament supply. Basically, the cathodes (filaments) need to be at, or near ground.

 

[Chuck D. Bones]

Here's a RangeMaster circuit. The JFETs boost the gain & headroom. As shown, the high-pass corner freq is 720Hz. For variable high-pass, replace R1 with B1M in series with 100K. At the moment, I have R3 jumpered and there is still plenty of gain.

 

[fig]

Cool! I already had the power stage built (and most of the rest with some replacments).

 

[fig]

Sounds great! I raised C1 to 10nF. It made quite a difference. I may try something between to mellow it a bit.

Edit: (yes Fig, explain how you loved it so much you immediately changed it)

 

[Chuck D. Bones]

Sounds great! I raised C1 to 10nF. It made quite a difference. I may try something between to mellow it a bit.

Increasing C1 to 10nF moved the corner freq from 720Hz down to 72Hz. You've basically turned the RangeMaster into a Superheated SHO.

I'd try 2.2nF, 3.3nF or 4.7nF.

 

[KreoPensas]

Waking up an old thread...

I have *dozens* of these 6418 tubes...

These tubes were used in *hearing aids*, among other uses.

The microphonics are legendary, thump the board and it'll ring for twenty seconds.

Ive used them in usb headphone preamps, and theres a pair warming up my entertainment center audio. I've also made an overdrive circuit, but that never tolerated a footswitch so it's an always on effect.

 

[Chuck D. Bones]

Good info. I hope that the click from a momentary stomp switch driving a relay won't trigger ringing. Guess we'll find out.

 

[BuddytheReow]

Could this work if I utilized a MAX1044 or TC1044SCPA charge pump instead of your TC7762B or bench supply, pinouts aside?

 

[Chuck D. Bones]

Absolutely. Pinouts are the same. I have a couple of mini-tube breadboards; one has the TC7662B, the other a TC1044S.

 

[fig]

Revisiting the Fender, I ran it straight into a SS power amp and it really shines. I have a Hartke Kickback 12 enclosure I gutted and put a Jensen Special for the time being, but I'd like to maybe get a Celestion Blue in there as the cab is pretty tight. The head slot is begging for this kind of setup, and I've got several power amp chips around I can build around, so thanks again...more fun!