12AX7 tube amplifier design question

Thread Starter

James 26

Joined Mar 26, 2020
3
Hello all,

I am attempting to build a simple tube amplifier with a 12ax7 tube which I believe has a maximum plate voltage of 300v, I am using a design from america that is working of a 120v rectified to dc which equates to 170v.

I am in Australia, we work at 240v, this rectified to DC = 340V too high for 12 ax7.

If I use a resistor in K ohm range to feed the plate this should work? What value would I use to get 170V?

Your input much appreciated.
 

LesJones

Joined Jan 8, 2017
2,598
You would normally use a transformer to produce the voltage that you require and to provide ISOLATION fro direct connection to the mains supply. Many years ago some manufacturers did not use isolation transformers but they ensure that no one could touch any part of the circuit as thy could not be sure that users would ensure that the chassis was connected to the neutral side of the mains.

Les.
 

Alec_t

Joined Sep 17, 2013
11,119
170V would be a reasonable anode voltage, as it's never a good idea to run things at their maximum rated values. I suggest you get a 240V-to-120V isolation transformer with a suitable power rating, which should be easy to source.
 

Marley

Joined Apr 4, 2016
347
Absolutely must use an isolation transformer. These voltages are dangerous! This valve (tube) only uses a small anode current (a few mA) so insert a 1kΩ to 10kΩ power resistor close to the power supply so that if something shorts, the current is limited to a safe value. An anode voltage of 100 to 150V should be OK for most applications. Power supply 200V max. DC of course!
 

crutschow

Joined Mar 14, 2008
24,708
thy could not be sure that users would ensure that the chassis was connected to the neutral side of the mains.
I remember repairing a mains operated 5-tube clock radio to replace bad filter caps (a lot of hum in the speaker), and noticed that the chassis was connected to the main's circuit common side through a capacitor. Thus the chassis was at AC ground for the signal frequencies, but the cap was not large enough to carry lethal current at the main's frequency.
 

schmitt trigger

Joined Jul 12, 2010
196
In America they could barely get away with transformerless radios.
As long as the Bakelite knobs and wood chassis held, they were somewhat safe.

But in the rest of the 240V world the resulting hot chassis would be so dangerous that I would simply not recommend it.

Additionally, these transformerless radios used special filament voltages in the tubes, connected in series, such that the total heater voltage would be roughly 120 volts: 12BA6, 12BE6, 12AV6, 50C5, 35W4.

Obviously, because you are using a 12AX7, depending on how you connect the double heaters, you will require either 6.3V or 12.6V at several hundred milliamps. You DO require a transformer. And if you are going to purchase a transformer, get one which also has a high voltage winding.
 

schmitt trigger

Joined Jul 12, 2010
196
Actually, and idea came up which would be lower cost than the two winding transformer discussed above:
- Procure a 12.6VAC transformer
- Connect the 12AX7 heaters in series to power them up at 12.6VAC.
- Rectify and filter the 12.6VAC to obtain roughly 16.5 VDC.
- Apply this DC voltage to one of the many "Nixie" power supplies avaliable from the web. Those will step it up to the required 170 VDC, as long as your current requirements are modest.
 

dendad

Joined Feb 20, 2016
3,425
Definitely DO NOT use rectified mains!
That is extremly dangerous.
If you cannot buy a 2 winding transformer, look for an old valve radio and grab one from it.
Or, a pair of Jaycar transformers, the first a
https://www.jaycar.com.au/12-6v-ct-7va-500ma-centre-tapped-type-2853-transformer/p/MM2013
and use it to power the filaments.
Then use a
https://www.jaycar.com.au/30v-ct-4-5va-150ma-centre-tapped-type-2855-transformer/p/MM2007
with the 30V secondary connected to the 12V of the first transformer.
Now, you rectify the second transformer's 240V winding, ( it will be about 100VAC I think) to give you your HT supply.
It is 4AM here in OZ and I cannot generate the mental energy to actually calculate the voltage ;)
 

Thread Starter

James 26

Joined Mar 26, 2020
3
Definitely DO NOT use rectified mains!
That is extremly dangerous.
If you cannot buy a 2 winding transformer, look for an old valve radio and grab one from it.
Or, a pair of Jaycar transformers, the first a
https://www.jaycar.com.au/12-6v-ct-7va-500ma-centre-tapped-type-2853-transformer/p/MM2013
and use it to power the filaments.
Then use a
https://www.jaycar.com.au/30v-ct-4-5va-150ma-centre-tapped-type-2855-transformer/p/MM2007
with the 30V secondary connected to the 12V of the first transformer.
Now, you rectify the second transformer's 240V winding, ( it will be about 100VAC I think) to give you your HT supply.
It is 4AM here in OZ and I cannot generate the mental energy to actually calculate the voltage ;)
So what you are saying is that the 30v transformer will work in reverse as a step up Transformer?

Does anyone know of any good output transformers I could use to drive an 8 ohm load?
 

Reloadron

Joined Jan 15, 2015
5,337
The 12AX7 dual triode is more suited as a pre amplifier than an amplifier. The Maximum plate voltage is 300 Volts and the maximum plate dissipation power is 1.0 Watt. Typical operation has a plate voltage in the 100 to 250 volt range and the plate current in the 0.5 to 1.2 mA range. The tube like many of its time has a center tapped filament so you can run the tube on either 6.3 VAC or VDC or you can run it on 12.6 VAC or VDC the first places the two filaments in parallel and the latter in series.

One later trick which was used for isolation was find a pair of 12.6 VAC (One CT) and pair them up like this;

Low Level Safe B+.png

Plate transformers can still be salvaged from old radios and Hammond still manufacturers them including audio out transformers.

Ron
 

LesJones

Joined Jan 8, 2017
2,598
I agree with Ron's comments. For the output stage something like an EL84 or 6AQ5 (These numbers are just from memory from about 60 years ago when I was a teenager.) There were also dual valve consisting of a triode for the preamp stage and a pentode or beam tetrode for the output stage that were used in cheap record players. At that time I also used heater transformers (240 volts to 6.3 volts) as output transformers when I did not have a proper output transformer.

Les
 

dendad

Joined Feb 20, 2016
3,425
A 12AX7 is a preamp, not a power amp.
My first amplifier I built around 50+ years ago had a 12AX7 preamp, EF86 driver and 6V6 output.
The 12AX7 was shared between both channels, half each as it is a dual triode.
Oh, it may have been a 12AT7 but that was a long time ago. In a weak moment, I gave a lot of stuff away and my first amp was one of those, and I have regretted it ever since.It would just be nice to have my first electronic construction for a memoir.
 
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