Does anyone care to ask an amplifier designer a technical question? My door is open.

@bdp24 

@ramtubes, Roger, I know you consider the 6DJ8 superior to the 12AX7 for use in a phono stage, but let me ask you this anyway ;-) : how do you feel about substituting a pair of 5751’s in place of a phono amp’s 12AX7’s? I know the 5751 exhibits lower gain and noise (in the 1980’s I had a Dynaco PAS 2 that was "modified" by Frank Van Alstine, and he put in 5751’s. That pre was very quiet.), but are there trade-offs involved between the two tubes? Is the 5751 a true direct replacement for the 12AX7, regardless of application? Or are there circuit considerations? If used in an RIAA moving magnet phono stage (gain of 42dB or so), will the 5751 automatically provide a little more headroom/freedom from overload than the 12AX7?

The 5751 is slightly different froma 12AX7 and close enough to be tried. We have plenty of very low noise ones. CJ has used them for years. I use one in the integrated I am developing. I could have used a 12AX7 but like the 5751 better. it also happens to be a "Premium" tube which means they took more care on consistancy and long life. It falls under the "industrial" category where failures are not appreciated. Headroom is more of a circuit parameter than a tube parameter. 

For EVERYONE,

My new/old/previous technician Ben arrives today and I will be spending most of my time with him and not so much here. I will continue to answer valid questions. We will be making and posting videos at the Berkeley hi fi school site. http://berkeleyhifischool.com/ and on You Tube. Videos are what I want to spend my time on.

In the last week we have gotten off track. This thread is intended to answer questions and not to have people give opinions of my or others answers. I am always willing to debate a point that can be supported. That means supported from a recognized authority. For instance, concerning tube characteristics, ratings and applications that means data books from RCA, GE, Sylvania. Take your pick, they all agree with each other because RETMA made them. Did you know that all brands of tubes had exactly the same price lists? In otherwords you could carry a RCA price list in the 70s, as i did, and a 12AX7 was the same list price even if Telefuken made it. One list.

In some cases I have offered to build a A/B box for those interested in really hearing what is going on. It is my firm belief that is the most revealing way. Others may differ and continue to differ.

I thought the other day, Why not build a switch box with several IEC inlets and one standard outlet and a relay or switches. Let people listen to power cords on the fly. Two would be enough, or more doesn’t matter. Fuses too. What is wrong with that test? Lets do some real research.

I am not interested in de-bunking pseudoscience. Established science needs no de-bunking. To those who write up pages of pseudo science and create paradigms to make excuse for bad specs I no longer care to see here. Skilled people in this industry have come up with some minumum standards for noise, distortion and output impedance. I happen to agree with John Atkinson on these. Best wishes if you care to minimum standards. There are always outliers and they will be most adament.

One thing I have learned in this popular and intense thread it that many audiophiles have created a religion with various belief systems. I’m sorry but that interests me not at all. You can’t design a good ampifier on religious principles. You may get one to work, to make some music, but there will be much missing. Sorry.

I thank all for making this thread the most popular for two weeks on Agone. Im not leaving, just shifting my focus back to my real work.

Short questions without a lot of unnecessary information will get my first attention. Most questions can be asked in one or two sentences.

@roberjerman   I have found more data on the 6N30 tube. Same pin-out as 6DJ8 and 6CG7. Plate dissipation 7 watts max. Plate voltage max at 250. Looks like the 6CG7/6FQ7 will work!

The Russians copied many of our tubes as did the Chinese. What you want to do next is compare Mu, Gm and Rp of the tubes at similar operating points. Let me know what you find out. Great question.
 
Low noise 6CG7s can be a little difficult to fine but I have found some.

Love the idea of a switchbox to test various power cords and fuses!

@atmasphere   Roger, you might want to do some reading at the links I posted in my prior post. Most of this post (except perhaps the comments about subjective listening) is incorrect. As I pointed out earlier, the load is not for the cartridge's benefit- its about the preamp.

Please explain. Im all ears.

There's plenty of old school audio electronics that used the 6SN7. As you know, its geometry is similar to that of the 6CG7/12BH7 and 12AU7 (the latter being the same as the 12BH7 but with the entire structure sawed in half)

My dad built the WM-2 in 1956 so I am very aware of 6SN7, as was Heathikit who used them everywhere they could. Are you aware that most of their early products were built largely from WWII surplus of which there was tons. They would buy tons of surpus and then figure out what to do with it. 6SN7 were in great abundance. I think a 6SL7 might have made a better choice and I snuck one in a Williamson, changed a few resistors, and really liked it . Much more linear tube (low distortion). Why, because it was made for audio, not B&W television.

There are some great books on the history of Heath written my people who were there.  

This other stuff about sawing tubes in half make no sense. Ive taked to tube designers and they dont talk that way... at all.

As to it being lower in microphonics, the tone is of course different as the structure is different. But I would not say they are less microphonic than a modern 6922.  But this is just my experience of testing over twenty thousand 6922s