250k ohm recommended load impedance on a 12au7 cathode follower?

[AardvarkBry]

I have this old Ampex 1260, and found an old service manual, and it says “recommended load impedance of 250k ohms or greater.” From reading whatever I could find, it seems a 12au7 cathode follower usually has a very low impedance, like below 600ohms. I tried a variety of different transformers for the output. First I tried a 600:600, and it was pretty light on the low end. Then I tried a 10k:600, and it was a little better, but still light. Then I tried a 600:50k, and ran it through a hi-z input, and there was no bass. Then I tried the that same transformer backwards, 50k:600, and this was by far the fullest bandwidth, but it cut so much signal, it introduced a lot of hiss. What do you guys recommend for getting the strongest, cleanest signal out of this? This thing sounds like magic, but I’m having trouble getting a strong, consistent output into modern gear. Sometimes it sounds wonderful, then other times there is a ton of noise, and sometimes the signal gets choppy, and cuts in and out. It’s very unstable without an output transformer.

 

[fazer]

Sometimes it sounds wonderful, then other times there is a ton of noise, and sometimes the signal gets choppy, and cuts in and out

Sounds like electronics intermittent. Maybe a corroded tube socket. Have you wiggled the tubes when this happens? IMO it’s circuit fix first then check for a impedance issue.

You could build a 250k input with an opamp. You might need to construct a tube line amp if you want to drive to +20 DBm transformer balanced out and keep it all tube.

Old tube hifi amps like Fisher and Scott had inputs for this so as to not load the output.

 

[fazer]

Also you could use a direct box with active input to mic pre

 

[AardvarkBry]

Sometimes it sounds wonderful, then other times there is a ton of noise, and sometimes the signal gets choppy, and cuts in and out

Sounds like electronics intermittent. Maybe a corroded tube socket. Have you wiggled the tubes when this happens? IMO it’s circuit fix first then check for a impedance issue.

You could build a 250k input with an opamp. You might need to construct a tube line amp if you want to drive to +20 DBm transformer balanced out and keep it all tube.

Old tube hifi amps like Fisher and Scott had inputs for this so as to not load the output.

I changed out all the tubes. I’ve cleaned all the pins and sockets with deoxit. I feel like it’s most likely an interfacing problem. It works flawlessly with my old hifi Dynaco Pam1 preamp, through Dynakit Mark iii. It also works perfectly through my Ampeg b15. It’s only when I try to print tracks into a daw through modern recording gear, where it starts acting up. Even then, sometimes it works perfectly. I’ll print a track, and it will sound spectacular. I’ll come back the next day, and try to print another track, and it will be acting up.

I just picked up this old Valley People hh2x2b line matching interface that seems to have been made for this exact scenario. I hope this is the answer to all my frustrations.

 

[ruffrecords]

This the old, old story that output impedance is no guide to drive capability. The two are quite different. A cathode follower does indeed have quite a low output impedance, typically equal to 1/gm. In the case of the 12AU7 this is 2.2mA/V so the output impedance will be about 500 ohms. However, it is important to remember that this is the theoretical small signal output impedance. What determine drive capability is the quiescent current through the tube. Looking at the schematic you posted I see the cathode voltage is 40V and the total cathode resistance is 15.6K. The quiescent current is therefore 40/15.5 which is about 2.5mA. Not matter how you drive this tube you can only drive the current down to 0mA and up as far as 5mA. Beyond that one half of the waveform will clip. So 5mA peak to peak current which is .177mA rms. This means the maximum signal into a 600 ohm load is 1.77mA x 600 ohms = 1.062 volts rms. In practice you cannot swing the cathode current this far so the maximum output level is even lower.

Bottom line is cathode followers do have a low output impedance which is good from the point of view of diving longish lines without picking up interference but their drive capability is limited so for "undistorted" output you need a fairly light load.

Cheers

Ian

 

[Tubetec]

I often match cathode followers into 1M, even with 15 feet of cable frequency response is very good
Thats what Bruel& Kjaer reccomend for their cathode follower output mic preamps , and around 250k is the minimum .

 

[AardvarkBry]

This the old, old story that output impedance is no guide to drive capability. The two are quite different. A cathode follower does indeed have quite a low output impedance, typically equal to 1/gm. In the case of the 12AU7 this is 2.2mA/V so the output impedance will be about 500 ohms. However, it is important to remember that this is the theoretical small signal output impedance. What determine drive capability is the quiescent current through the tube. Looking at the schematic you posted I see the cathode voltage is 40V and the total cathode resistance is 15.6K. The quiescent current is therefore 40/15.5 which is about 2.5mA. Not matter how you drive this tube you can only drive the current down to 0mA and up as far as 5mA. Beyond that one half of the waveform will clip. So 5mA peak to peak current which is .177mA rms. This means the maximum signal into a 600 ohm load is 1.77mA x 600 ohms = 1.062 volts rms. In practice you cannot swing the cathode current this far so the maximum output level is even lower.

Bottom line is cathode followers do have a low output impedance which is good from the point of view of diving longish lines without picking up interference but their drive capability is limited so for "undistorted" output you need a fairly light load.

Cheers

Ian

Thank you. This clears up a lot for me. Do you have a recommendation for a simple, fairly low cost stereo preamp I could build, that could comfortably drive this machine into an Apollo x8p? This machine sounds fantastic when it’s working right. I a/b’d it against a revox and an otari, and in my opinion it sounded light years better. My plan is to send my mixed tracks out to the r2r, and then bring them back in to the daw.

 

[AardvarkBry]

I often match cathode followers into 1M, even with 15 feet of cable frequency response is very good
Thats what Bruel& Kjaer reccomend for their cathode follower output mic preamps , and around 250k is the minimum .

I tried that, with mixed results. I sent it into an FMR RNP hi-z input, and it usually sounded good. A couple times the top briefly dipped out on me though. After I sent the machine into a Sowter 600:50k backwards, I realized that the hi-z option had a little less bottom end in comparison.

 

[fazer]

Your interface is chip based input. I 1meg input should do the trick. Using a high quality chip fet input should do the trick. Any transformer is going to have a tone to it that could be good or bad depending on your program and what tickles your ears for the given source. You can also go capi for a 2520 style opamp using the summing amp kit. Then plug in any 2520 pin based discrete or hybrid style using 8 pin traditional. Also the output transformer can be an iron lam or nickel for distinct different sound quality.

https://capi-gear.com/catalog/index.php?
cPath=22_46

https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-123-bmcf.pdf

 

[abbey road d enfer]

Actually, the cath-foll output stage could drive a much lower load, probably about 20-50k without any issue, except the limiting factor is the 0.1uF coupling cap.
I would first replace these caps with 1-2uF and connect direct to the interface without any xfmr.

 

[AardvarkBry]

Your interface is chip based input. I 1meg input should do the trick. Using a high quality chip fet input should do the trick. Any transformer is going to have a tone to it that could be good or bad depending on your program and what tickles your ears for the given source. You can also go capi for a 2520 style opamp using the summing amp kit. Then plug in any 2520 pin based discrete or hybrid style using 8 pin traditional. Also the output transformer can be an iron lam or nickel for distinct different sound quality.

https://capi-gear.com/catalog/index.php?
cPath=22_46

https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-123-bmcf.pdf

Thanks. Would you say the capi stuff is the best of all those kits ie: AML, Sound skulptor, hairball, etc…?

 

[AardvarkBry]

Actually, the cath-foll output stage could drive a much lower load, probably about 20-50k without any issue, except the limiting factor is the 0.1uF coupling cap.
I would first replace these caps with 1-2uF and connect direct to the interface without any xfmr.

Wow, thank you. If it ends up just being the cap, that would be amazing. Your suggesting swap out that cap, and plug it into the hi-z right?

 

[abbey road d enfer]

You can also go capi for a 2520 style opamp

2520 DOA is optimized for 10 kohms source impedance. It will be noisy here. Indeed a FET opamp is a better solution.

 

[fazer]

Thanks. Would you say the capi stuff is the best of all those kits ie: AML, Sound skulptor, hairball, etc…?

It’s just a base PCB that allows you to build a summing amp but can be adjusted to any line amp and as I said capi has hybrid 2520 that is an 8pin socket with a buffer to drive a transformer. This allows you using a fet input amp or any standard single style op amp. It’s easy with there pcb and amps just add a power supply and an enclosure. Keeping the impedance lower with a change of the cap to 1 or 2 uf allows lower noise like Abby’s great suggestion. And it’s pro all the way.