Looking for a tube : 12AU7/6C4 vs 12AX7/???

[CB Amp]

Hello,

I'm looking for a single triode which could be used as one of the triode of a 12AX7, with the same specs (gain, etc) but half the heater current (0.15A). Just as the 6C4 (used in Hammond organ amps for example) which is half a 12AU7.

I know the 6AV6 which has the gain and specs of a 12AX7, but 0.3A heater current.

Do anyone know such a tube ?...
Thank you.

[David Root]

Your best bet might be a small signal pentode wired as a triode e.g. 12SJ7, 12BA6, 5879, 5899, etc.

You'd have to find one that could match a 12AX7 which would typically get 50-70 gain in a conventional grounded cathode circuit.

[Firestorm]

Probably the closest thing is a 7C6, which is a duodiode plus a triode with an amplification factor of 100. Unfortunately, this is a loctal type, so even if you could find one, you'll also have to find a socket to use it in. Another possibility would be the 6AQ6 (edit: this is also a duodiode/triode), which needs the slightly more findable 7-pin miniature socket. These were only 70 mu tubes, but you can compensate for that with a larger plate resistor. Forgive my asking, but why do you need to do this? If 150mA makes a difference, the power supply is way underdesigned to begin with.

[dave g]

Well, you could just use one half of the 12AX7 and only wire one of the heaters (pins 4+9 OR 5+9) for a 150mA current draw...but like Firestorm said, if you're worried about 150mA then you really do need a larger PT.

[Phil_S]

Take a look at the 6AQ6. If biased appropriately, you can probably get the gain you would like to have. Most pre-amp tube circuits do not produce the max amount of gain (mu) the tube is rated for.

IMHO, it will be much simpler to buy a 6.3V filament transformer to take care of what we are guessing is your problem. These often go for around $10 or less on eBay. Here's one that will work:
http://cgi.ebay.com/NEW-12-6-volt-ct-3- ... 286.c0.m14
This is the kind of monster I'd avoid:
http://cgi.ebay.com/NOS-TRIAD-F-21A-6-3 ... 286.c0.m14
You might pick through this guy's inventory of used transformers and get what you need:
http://www.oldradioparts.com/pg2a23b.htm
Radio Shack used to make one that's good, but I don't know if they still do.

[Wayne]

Well, you could just use one half of the 12AX7 and only wire one of the heaters (pins 4+9 OR 5+9) for a 150mA current draw...

I think you need the whole heater to run whether you use both sections or not.

W

[dave g]

Well, you could just use one half of the 12AX7 and only wire one of the heaters (pins 4+9 OR 5+9) for a 150mA current draw...

I think you need the whole heater to run whether you use both sections or not.

W

Nope, you can actually connect only one heater for half the current draw. You can think of the 12A_7 series tubes having two 6.3 volt heaters internally connected in series but with the common node at pin 9...this allows you to use a 12.6 volt heater supply across pins 4 and 5 with a 150mA draw, or a 6.3 volt heater supply across pins 4+5 and 9 with a 300mA draw. When you bridge pins 4 and 5 you are putting the heaters in parallel - each individual heater wants 6.3 volts across it and 150mA through it, and it doesn't care how it gets it, series or parallel. In parallel, the heater for triode 1 doesn't care how much current flows through its partner as long as he's getting his 150mA...it's just KVL/KCL/Ohm's law, draw a diagram for yourself...

Try it if you don't believe me...

Now, if I was only using 1/2 of a 12AX7, I would still connect both heaters for the benefit of reduced noise due to the hum bucking nature of the filaments.

[CB Amp]

Thanks a lot for your replies. I will try these tubes.
I wanted to add only one triode on a Marshall 4210 combo (I build a new amp in the chassis, with the original trannies).
So, I didn't want too much load on the PT. It will certainly handle 300mA without problem, but I'm a fussy guy... and I don't like non-used things in an amp...

For those interested in Marshall amps, this 4210 combo doesn't have the "normal" 2205 circuitry as written on the schem, and it sounds really trivial. All the 4210 I see here in Europe have this circuit, never seen on any schematic on the web. I don't know what Marshall did with this amp, but it was a bad idea...

[tictac]

a 6AT6 will also work as a single triode, I believe the old Fender Tweed Harvard uses one in the input of the preamp. It is a 7 pin base.

TT