12AX7 Output Section?

[parentheticalfact]

Hello there,
I've repaired lot of amps and helped my father design a few amplifiers and I'd like to build a small amp to get started in many other larger projects.
I already have an idea for a preamp with master volume and moderate to high gain capability. The power amp is the section I am pondering over.
I was first going to build an amplifier with 1 or 2 6BQ5 tubes, but I've been considering running a 12AX7 or similar tube as an output section. I've seen this built before but I was wondering if anyone out there has experimented with this as a builder and has found any problems using this tube as an output section. I have not heard any mind blowing results as I might expect from larger tubes using this design, and I don't know if that is due to poor sound clips on various websites or if the little tube just doesn't sound so great as an output tube.
Anyone have any experience with this or have any thoughts or recommendations?

[bob p]

finding a suitable OT will probably be the biggest impediment to using the 12ax7 as an output stage.

[John_H]

Check this out

[parentheticalfact]

finding a suitable OT will probably be the biggest impediment to using the 12ax7 as an output stage.

I figured I'd use a fender reverb driver transformer. They're very cheap and easy to obtain.

[parentheticalfact]

Check this out

Very nice example! I've yet to hear a clip of this type amplifier that had that nice of a sound. Thank you for the link. Looking very encouraging....

[bob p]

I figured I'd use a fender reverb driver transformer. They're very cheap and easy to obtain.

you must be thinking about a single ended circuit then. i think that the overall task will be easier with the 12 AU7 than an AX7 because of impedance matching, but i'd have to check on that.

[Konny]

Hi,

I think an ECC81 / 12AT7 will be the best tube for this application because it can deliver higher currents.

Swen

[Earl]

I've never done this...but would think that the 12AT7 (as mentioned) would be a good way to go.

That reverb transformer is already "in line" with that tube ( in parallel ) to drive 8 ohms. Plus, you have more leeway on power.

[parentheticalfact]

you must be thinking about a single ended circuit then. i think that the overall task will be easier with the 12 AU7 than an AX7 because of impedance matching, but i'd have to check on that.

Yes I was figuring both sides of the tube running in class a as a single ended circuit. I've seen mostly people using 12AU7 tubes for this purpose, so I would imagine that indicates some sort of performance issue with the 12AX7. Probably impedance.

[parentheticalfact]

I've never done this...but would think that the 12AT7 (as mentioned) would be a good way to go.

That reverb transformer is already "in line" with that tube ( in parallel ) to drive 8 ohms. Plus, you have more leeway on power.

Now, I know that reverb tanks in fender amps are supposed to be 8 ohms, so I could naturally assume the transformer is designed for a 12AT7 to match an 8 ohm load.
I know DC resistance is not the correct way to measure impedance, but when you measure a speaker you can get a very close idea to what impedance it is with a DC meter.
A reverb tank rarely reads much more than an ohm from my experience, even though they are supposed to be 8 ohms. Is this a common finding with the rest of you?
That has me curious, but I suppose it would be best to just do some research as to the impedance match I need to achieve and make sure my transformer will give me the proper match up for my desired load amount and that should clear up any doubt.

[J Martin]

Depends on wether you want something novel (12AX7) or more practical. You will get better performance with a 12AU7, 12BH7, 6CG7 or ECC99 tube. I have built a few low power amps like this and prefer the 12BH7. 6SN7 works well too if you have room for an octal tube. Mine were all PP cathode biased amps. One of the Hammond "universal" output transformers would work well in this application and they are inexpensive.

A few years ago I saw a HiFi amp with a 12AX7 output stage. They used about 20 or so 12AX7's to get even moderate output power.

[Joe Bee]

*bump*

I have tried using the ECC83 for both single ended and push pull output, so I figure I could maybe comment on this.
Everything that has been said so far is correct, the impedance issue is what kills you. Basically you are looking at a current swing of only 1mA or so from 250V plate voltage, which puts you in the range of 250kOhm primary impedance. This is not a terribly common figure, so you are basically left to wind your own OTs.
This is what I tried, I butchered a couple of wall-wart transformer in the range of 2VA to 10VA, took out the cores and started rewinding. The problem I ran into is that the small cores need a LOT of turns to handle the voltage without saturating, even the original 230V winding on the (european) transformers is only good for 100V or so without distorting the waveform. So winding the cores with the thinest wire I have at hand, 0.1mm, I ran out of bobbin space long before I had enough turns for a clean waveform.

Now the easy solution would be to use a bigger core, but in my opinion that would completely defeat the purpose of a small, weight-saving output stage. Thinner wire is only an option for those who have access to professional transformer winding equipment.

The simple solution, even if I hate to say it, seems to be to go down in voltage and up in current, e.g. running a ECC81 at 100V and 10mA into an ordinary transformer like the Hammond 123A.

[Steve Conner]

Hi Joe,

That sounds interesting. You would probably have found it impossible to get the leakage inductance low enough for an acceptable treble response, too.

I bought one of these output transformers a while back:

http://rswww.com/cgi-bin/bv/rswww/se...eID=uknetscape

meaning to try it with some small dual triode tubes. It will match up to 15k anode-to-anode, so I thought there was a good chance of it working. I never got round to it though! I thought I had better get one quick before they vanished from the RS catalogue, but they're still there, in the 21st century.

I planned to try a single 12AT7 or 12AU7, and if that didn't work, I'd have connected two in parallel. Those tubes are the ECC81 and ECC82, but I forget which is which.

There are also meatier dual triodes like the 6SN7, and a special tube that JJ(?) made a while back, in a regular 9-pin envelope but about 1.5 times taller than a 12AX7. (Edit: It was the ECC99 and I see someone mentioned it already!) But I didn't have any of these to play with.

BTW, good to see you here! I was a member here for years before I was on 4hv, and I seem to be slowly drifting back from high voltage to audio :|

[Joe Bee]

Hi Steve,
the leakage inductance is infact giving me a headache, but in a different way. For some reason a transformer rated at 230V 2VA, which in my understanding should draw about 10mA at full load draws and idle current of more than 50mA. I don't know whether I should worry about this, or if it is normal for these small transformers, but I know for sure that it is different for "normal" transformers. (EDIT: Ok, this is primary inductance, NOT leakage inductance. I wonder if there is some kind of parasitic air-gap in the core driving it down, but I have been very careful to grind down the core halfs to a flat surface. The leakage inductance is influenced mostly by coupling so interleaving the primary and secondary would probably help with that)

But yes, when I acutally hooked up a speaker to the circuit the treble response WAS poor, just like the bass response!


The link you posted is not working but since RS only has one output transformer I found it anyway. I guess 7£ for that is not all that bad, you should put it to good use. The ECC99 seems to be a very good candidate, with a dissipation of 5W per system it should be plenty loud. The current swing of +-18mA puts it exactly in the 15kOhm range, so it should work well with a lot of common OT's.

[Alex R]

Could one of you post the RS stock number for that transformer please?