12au7 600 ohm output

[razorrick1293]

hi there, i wanted to check a 12 au7 cathode follower output stage, what is a good cathode resistor for driving a 600 ohm load? does it matter or should it be quite low? would 100k do? 68k? 47k? 22k? 10k? 600 ohm? sorry if this seems stupid but id like to be sure.

 

[DF96]

You are asking a significantly incomplete question. HT voltage? Quiescent current? Bias method? Peak signal voltage? Why ECC82/12AU7?

The quick answer is that this valve won't drive a 600 ohm load, unless you are happy with significant attenuation and some distortion.

 

[razorrick1293]

HT 200v and its being driven by the other half of the ECC82/12AU7 with a 220k plate load biased for by a 3.3k resistor and 47uF cap, and because its all i have available and i know they can drive the load, and attenuation is fine, distortion is ok depending on how much, 600 ohm is a worst case scenario and its much more likey to be driving 1.2k-5k loads. (some local NFB may also be used)

 

[DF96]

An ECC82 CF driving 600ohms would only put out half the signal put in, so not a very effective buffer. I would say 5K is the minimum load resistance, 50K would be better.

Cathode resistor could be somewhere around 10k? It depends on the anode voltage of the previous stage, assuming you are using direct coupling.

 

[razorrick1293]

halving the signal sounds fine for what im planning, maybe even a bit more, 10k cathode resistor, thought so. would i get away with 20k seeing as i have no 10k 5w resistors around?

 

[SY]

Is this for an MI amp or hifi? If the latter, be aware that the distortion will be quite significant.

 

[razorrick1293]

its kind of a guitar/mic preamp, so im not worried about some distortion, its more for testing bits and pieces, tone stacks, gain stage voicings etc. so i can build stuff without building PA sections to go with it and it can be changed fairly quickly.

 

[razorrick1293]

but im looking for a lowest possible impedance out for some recording interfaces, that seem to have 600 ohm inputs.

 

[pedroskova]

Why not use an ecc88 / 6dj8? Same pin out, better suited.

 

[SY]

Bingo. For low impedance, you want high transconductance. 12AU7/ECC82 might make 2000 umhos on a good day, downhill, with a tailwind. ECC88 is about 5 times better.

 

[razorrick1293]

ecc88? hmm, might have to invest in one of those, how expensive compared to an ECC82?

 

[SY]

Similar.

 

[razorrick1293]

Thanks, il get right on that!

 

[dsavitsk]

Even better, how about a IRF820. It will work as a drop in replacement for a cathode follower, will drive anything, doesn't require a heater, will sound better, and can be had for ~$1.

12AU7/ECC82 might make 2000 umhos on a good day, downhill, with a tailwind. ECC88 is about 5 times better.

IRF820 is about 700 times better than the 12AU7 ... just sayin'.

 

[SY]

Well, yeah. But it doesn't glow.

 

[razorrick1293]

erm i think il stick with the tubes

 

[Miles Prower]

hi there, i wanted to check a 12 au7 cathode follower output stage, what is a good cathode resistor for driving a 600 ohm load? does it matter or should it be quite low? would 100k do? 68k? 47k? 22k? 10k? 600 ohm? sorry if this seems stupid but id like to be sure.

This is one of the most common misconceptions. A 12AU7 as a cathode follower might get you a source impedance of 600R.

However, the cathode follower is a device that can present a Lo-Z output, but only to a Hi-Z load. With Rl= 600R, it really doesn't matter what the tail resistance is: 100K || 600R is still 600R, for all practical purposes. No small signal type like the 12AU7 likes running into a loadline as steep as that of a 600R loadline. The end result will be the same as connecting that 600R load to the plate: very limited output swing, lots of distortion.

A cathode follower can't make any VT into something it is not, and never can be: a low voltage, high current device. You'd be better off with a source follower to drive a load that small.

 

[razorrick1293]

quite possible i got something confused there, thanks for pointing that out, i did think that about the cathode resistor but accepted a while ago tubes can do funny things, some good, some bad in strange conditions... but thnaks anyway, ive gone with a 22k cathode resistor, i looked at some of the loads id be driving and it seems it isnt a whole lot after all, but it seems to be working as intended so all is good.