Need some help with a single tube amp.

[RayRay1132]

So I'm working on the tube amplifier project again with a 12ax7a tube and I made this schematic. Does everything look the way it should for a single tube amplifier? (I know I left out the volume pot for the second half of the tube.) Just asking before I have it made into a pcb. Thanks in advance -Ray

 

[audioguru]

A speaker has a low resistance and needs a fairly high current.
But your vacuum tube circuit is 70 years old and can drive a high resistance with a low current but cannot drive a speaker, but it might be able to drive a power amplifier.

Calculating that if the input impedance of a power amplifier is 10k ohms then the 22nF output capacitors of your circuit will reduce sound frequencies lower than 730Hz. No mid-bass and no deep bass.

 

[RayRay1132]

A speaker has a low resistance and needs a fairly high current.
But your vacuum tube circuit is 70 years old and can drive a high resistance with a low current but cannot drive a speaker, but it might be able to drive a power amplifier.

Calculating that if the input impedance of a power amplifier is 10k ohms then the 22nF output capacitors of your circuit will reduce sound frequencies lower than 730Hz. No mid-bass and no deep bass.

So, could I use a high impedance set of headphones or a high impedance speaker and if so what impedance would be suitable?

 

[audioguru]

Nobody makes high impedance headphones or speakers today.
Because nobody makes vacuum tube amplifiers anymore.

Many years ago, vacuum tube amplifiers used a huge, heavy and expensive output transformer to match the high impedance of vacuum power tubes to low impedance speakers.

 

[RayRay1132]

Nobody makes high impedance headphones or speakers today.
Because nobody makes vacuum tube amplifiers anymore.

Many years ago, vacuum tube amplifiers used a huge, heavy and expensive output transformer to match the high impedance of vacuum power tubes to low impedance speakers.

Ok, how would I go about figuring out the transformer to use to match a speaker to the output?

 

[schmitt trigger]

First of all, you would require a power tube. A 12AX7 won't cut it.

Something like the ubiquitous 6L6, but there are others like the 5881, 6BQ5, 6V6, or monsters like the KT88.

Second, you go to a place which manufactures tube-output transformers, like this one: https://www.hammfg.com/electronics/transformers/classic

 

[RayRay1132]

Would something like this be better than my circuit? I've noticed on this circuit they feed the audio signal into both grids at the same time; on my circuit I feed the audio signal into one side of the tube and then reamplify it through the other half of the tube. This also doesn't need to be a very powerful amplifier; just around 1/4 watts.

 

[Nigel Goodwin]

There are hundreds of valve circuits out there - use one of those, rather than randomly connecting a pile of parts and hoping it might somehow work (it won't).

Valve output transformers are freely available still, even RS Components sell them.

You also don't use a 12AX7 to feed a speaker transformer, it's a preamp valve - you need a power valve, such as an EL84. For your specific application I would suggest using a triode/pentode, such as the ECL82 or ECL86.

Something similar to this (rather crude version):

Old ('60) italian ECL82 Record Player on the bench (FINAL - UPDATE)

Hoffman Amplifiers Tube Amp Forum, Old ('60) italian ECL82 Record Player on the bench (FINAL - UPDATE)

el34world.com

 

[MacIntoshCZ]

Because nobody makes vacuum tube amplifiers anymore.

I dont think so. Its still rarely use.

 

[JimB]

Something similar to this (rather crude version):

Old ('60) italian ECL82 Record Player on the bench (FINAL - UPDATE)

Hoffman Amplifiers Tube Amp Forum, Old ('60) italian ECL82 Record Player on the bench (FINAL - UPDATE)

el34world.com

A very traditional single valve circuit.

But beware, there is no isolation between the amplifier and the incoming mains supply.
It is possible for the whole thing to be sitting at 240v above ground. Dangerous.

JimB

 

[Nigel Goodwin]

I mentioned RS Components earlier - that was because I recalled an article on their website about building a valve amplifier. I've now found it:

Building a Valve Amplifier Part 2: Single Channel Prototype and Loudspeakers Assembly

Building a brand new valve amplifier using the RH84 design and off-the-shelf components.

www.rs-online.com

 

[RayRay1132]

There are hundreds of valve circuits out there - use one of those, rather than randomly connecting a pile of parts and hoping it might somehow work (it won't).

Valve output transformers are freely available still, even RS Components sell them.

You also don't use a 12AX7 to feed a speaker transformer, it's a preamp valve - you need a power valve, such as an EL84. For your specific application I would suggest using a triode/pentode, such as the ECL82 or ECL86.

Something similar to this (rather crude version):

Old ('60) italian ECL82 Record Player on the bench (FINAL - UPDATE)

Hoffman Amplifiers Tube Amp Forum, Old ('60) italian ECL82 Record Player on the bench (FINAL - UPDATE)

el34world.com

With this circuit, should I build a preamp with the 12ax7 tube or will this be sufficient by itself?

 

[Nigel Goodwin]

With this circuit, should I build a preamp with the 12ax7 tube or will this be sufficient by itself?

It's already got a preamp - the first half (triode) of the valve.

I would also suggest a somewhat better power supply, with a proper isolated transformer - but back in those ancient days it was fairly common to have live chassis's to save money.