Originally Posted by JMoss1 ➡️

Heterodyning or Intermodulation Distortion can occur when two or more signals are mixed through a non-linear device. LIKE YOUR EAR. Your ear is a nonlinear device where heterodyning (intermodulation distortion) does occur. Messy indeed.

Transient response is one of the performance requirements any Pro Audio vacuum tube design should have built in. A lot of it depends on the audio transformers used. There are many things I had to consider when designing this preamp. LOTS of things. Safety issues included.

Sagging power supplies are sometimes there on purpose, designed in. I thought about doing that, but decided against it. My preamp power supplies are fully regulated, even the HV B+. I use a higher than usual High voltage to get the sound I wanted. A lot of guitar amps use starved plate designs. It is a sound. I just wanted a much more robust design. My design can maintain the proper design internal voltage on the 120 Volt setting anywhere from 80 Volts to 135 Volts. I am sure it will handle higher and lower voltages than that. I just don't claim that. Same for the 240 Volt setting.

I have been recording field festivals where the power line voltages went down to 80 volts. They were then adjusted with a Variac.

I leave the studio on 24 hours at a time with no oscillation, within or outside the audio bandwidth. The proper design of a HV power supply takes that into consideration. That is all you need to have a power supply turn into a UHF oscillator. That is just good design. Also, the ability to operate for days with all the vents closed off and the room temperature at 90F is an important thing to design in. I had great mentors I could go to for advice. They kept me out of trouble. Maybe even had me over build too much, but better safe than sorry.

In some of your comments you seem to be talking about solid state preamps or HIFI systems. I am not commenting on them.

Jim Moss

Originally Posted by wildplum ➡️

the incessant hawking of the gear you sell is getting a little boring.
if you want to talk tech, great. if you want to sell your gear, buy an ad.

Originally Posted by StevenLMorgan ➡️

I have enjoyed the design dialog and appreciate the contributions to the thread. Great information.

Originally Posted by JMoss1 ➡️

In some of your comments you seem to be talking about solid state preamps or HIFI systems. I am not commenting on them.

Originally Posted by M50k ➡️

Classic TF understatement.

To Tony directly - did you ever aquire any of the Decca M50s that were converted to FET?

Originally Posted by Plush ➡️

Tony is a great fellow and friend. I have learned so much from him, primarily about recording and mic philosophy.

TO WIT: I can comment on Decca fet M50 mics.

I bought some M50 Decca fet modified mics and used them for many years. They sounded fantastic. Very quiet and very dynamic. Typical M50 sound except for the tube modus sound which of course they did not have. But having used tube M50's a good bit as well, I would say that the fet versions had 80% of the real M50 sound.

I have discussed this with Tony F. and he offered that he "was sorry for me" that I had to use the fet version. I laughed and said that I agreed.

But any engineer working with orchestra, piano, and other instruments with prodigious bass is VERY fortunate to use any version of the M50 mic.

The unlimited Deutschmark investment in the design of the M50 (with semi-government research involvement), is probably not to be seen nowadays.

But I still hope that Tony will encourage RODE to release the TF50 into the wild.

Originally Posted by ISedlacek ➡️

Oh, my thread Well, (not sure if I mentioned it before), I tried some tube preamps in the meantime on various acoustic instruments and found that comparing to my Forssell SMP-2 preamp they all sound a bit limited, 2D, small and not very natural, simply worse ... So I am not bothered by this option any more ...

Some years ago I tried tube Schoeps bodies (very expensive). They stopped producing them long time ago. I was also happy that I have CMC6

But yes, for vocals it is good (like U47 I have here). I tried even stereo pair of U47 on acoustic instruments ... Quite bad comparing to Schoeps

Originally Posted by M50k ➡️

It's always puzzled me how M50/49s and U47s are kept alive when the tubes go - is there that much AC701 and VF14 NOS still out there?

Originally Posted by kludgeaudio ➡️

The thing is, those microphones were designed for exceptionally long tube life. Part of what makes for the coloration of the U47 is how they run the tube with low filament voltage for low emission and long lifespan, so tubes really do last a lifetime. That's a requirement in the broadcast world.

The truth is there aren't very many VF14s out there anymore, even though they were picked originally because they were plentiful and cheap on the war surplus market. For a long time people would just swap the tubes in the mikes when they suspected anything, because the tubes were cheap and easy to replace. Now they aren't so plentiful and they sure aren't so cheap.

Worst case: when the VF14 fails you put the nuvistor kit in there and the mike sounds better with the nuvistor anyway...
--scott

Originally Posted by Jim Williams ➡️

The common 12AX7A has a 12v/us slew rate, similar to a TI TLO72 opamp. Driver tubes like 12BH7 are slower. 12AY7's are faster due to less stray capacitance.

Some rf tubes are very fast but not suitable for audio usage. Transistor designs here go up to 7000V/us slew rate, that is very fast.

Transistors are faster than tubes.

Originally Posted by kludgeaudio ➡️

From intermodulation effects that you don't see. From slew limiting. From transient effects like power supplies sagging. Maybe it breaks out into oscillation after running for an hour but you never notice that on a short test. What you describe is a very good test, but not a universal test. There are a lot of tests that can measure that stuff, but you need to know to use them first.

--scott

Originally Posted by JMoss1 ➡️

The 12AX7's don't have the current drive capability provided by other tubes of that series like the 12AU7. These tube were developed for 5 tube radios. Not quality electronics. They work great where the users are not that picky.

Originally Posted by cathode ➡️

complete nonsense.
12A*** tubes are found in time tested, highly regarded audio equipment.
manufacturers include McIntosh, Ampex, Marantz, Scott, Fisher.
5 tube radios used a variety of tubes, but not from the 12A family.
https://en.wikipedia.org/wiki/All_American_Five

Originally Posted by M50k ➡️

It's always puzzled me how M50/49s and U47s are kept alive when the tubes go - is there that much AC701 and VF14 NOS still out there?

Originally Posted by JMoss1 ➡️

Isn't there any alternatives that can be used as a substute? I seem to remember when I bought my U67 that the seller included a tube that was an alternative. I always liked the U47 fet better. Someone must have a upgrade for the U47.

Jim Moss

Originally Posted by JMoss1 ➡️

I have bags of these I collected back in my teenage years that came out of radios. My point was that with an amplification factor of 100 and only 1.2 watts in dissipation it is not the best for low distortion high gain amps. They can start to sag. They are used in lots of radios like Drakes and Swans to mention a few.

https://frank.pocnet.net/sheets/049/1/12AX7A.pdf

Complete nonsense? This is an angry crowd.


Jim Moss

Originally Posted by dseetoo ➡️

There is nothing wrong with 12AX7 tube if used correctly, even in a high gain circuit. Its high output impedance won’t drive any real-world load, but the spec sheet already tells you that. Combine the two triodes of a 12AX7 together in a cascading circuit can get you more than 70dB of clean, low noise and wide bandwidth gain. If you can’t design good sounding equipment using 12AX7 tubes, you need to read some more textbooks, or reverse engineer some good old great sounding gear. 12AX7 is not your problem, that is for sure.

Originally Posted by JMoss1 ➡️

Now here is an old guy story.
When I went to college we studied tubes/transistors/fets side by side. That was in the early 1970s. This thread started with slew rate and noise. I find 12AX7 tube amps having pretty high THD+Noise. The best I have seen has been a preamp with 1% THD_Noise. That is great for a guitar amp, but for a modern Pro Audio preamp that is bad. My preamp is around 0.02% to 0.05% (20Hz to 20kHz) in final testing consistently.

Originally Posted by dseetoo ➡️

Another way to get high gain, low distortion with high standing current is to use constant current source as your load. As I said before, a single 12AX7 can get you 70dB of gain, with NFB included. Been there, done that.

Originally Posted by kludgeaudio ➡️

Yes, and this is different than a cascode arrangement it's true. But you're still back to two tubes for the stage then, one being the amplifier and the other being the current source. (Which is why we have dual tubes like the 12AX7s and even compactrons.)
--scott