Step 3: The Tubes


If we still wanted 50L6 tubes, they are fairly plentiful--lots of radios used these tubes. Same with 12AX7's, they're still being manufactured today and are plentiful (although not cheap.) I already had three for my Ampeg...

But choosing the 6DG6GT for a power tube was truly a pleasant surprise. The tube was standard in many TV sets, and they're cheap and easy to find. I bought 4 from ebay, at a cost of only $3.50 each! (shipping included!) Contrast that with 6V6's--good ones run $20+ per tube, minimum...

And availability is always a concern. No point in building an amp if you can't buy any replacement tubes.

The 6DG6GT tubes are RCA NOS. The 12AX7 is a NOS Raytheon, which I "borrowed" from my Ampeg Gemini II amp (it needs work, anyway--a future project.)

Heater Voltage and Current Requirements

The big concern with all 50L6 variants is the large amount of current required for the tube filaments. A bit of background: most (US) tube names begin with the voltage requirements for the heaters:

Tube name : filament voltage

50L6 : 50V
35L6 : 35V
25L6 : 25V
12AX7 : 12V (they have a split filament, and also run @ 6V)
6V6GT : 6V
6DG6GT : 6V

(Pardon the weird formatting--Instructables yanked the ability to use PRE tags, and screwed up the conversion when they did so. I've tried to fix it the best I can...)

But if the 6DG6GT is to have the same electrical characteristics as the 50L6, the heaters must perform in a nearly identical fashion. The filament wire itself must be designed to compensate -- using more current, at the lower voltage:

Volts X Amps == Power consumption

50L6 : 50V * .15A = 7.5 watts
6DG6GT : 6.3V * 1.2A = 7.56 watts

Obviously, the heater power requirements are practically identical; of course that follows since the electrical characteristics of the tube also match. But 2.4 amps (two 1.2A tubes, and not counting the preamp tube) is a fairly high amount of heater current @ 6V for a small amp... (the total heater current is 2.7A @ 6.3V)

Datasheets for 12AX7, 6DG6GT:
12AX7.pdf(551x700) 291 KB
6DG6GT.pdf(598x780) 603 KB
therocksock5 years ago
would two 6L6GC tubes work?

gmoon (author)  therocksock5 years ago

(You should look at simple push-pull amps like the Fender Super 5D4 instead, if you want to use 6L6 tubes. A push-pull output transformer for a pair of 6L6's will be cheaper than a single-ended transformer for a pair...)
ok thanks
taylorg5 years ago
For the 12AX7 should I be looking for a preamp tube or a vacuum tube?
gmoon (author)  taylorg5 years ago
Sorry my friend--I don't understand the distinction...

A 12AX7 is a 12AX7. They are dual-triode vacuum tubes, used most often in amplifiers as preamp tubes.
taylorg gmoon5 years ago
Awesome.  I had never heard of a difference; just thought I would ask...