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

@atmasphere  We get pretty high figures and we do it with a tube.... As a hint, look into 2-stage CCS circuits. You aren't going to get good numbers without a decent CCS, a resistor or a single-stage CCS won't hack it.

Of course i use good CCS, what concerns me is the variations in the gain of the two sections of the 6SN7. Since I have the M-60 on the bench I will measure that. What kind of numbers should I expect and at what frequency do you measure it?

I guess this really depends on what is meant by 'current' (since the word has become a charged term in audio)! 

Current is well defined by science. I didnt know there was a special audio definition.???

And a lot depends on the 6AS7 in question too- the GAs don't hold up; most American tubes have problems in our circuit as well since they really aren't intended for fixed bias operation. We prefer the Russian variant; they hold up the best of any we've seen.

The 6AS7 is a pass tube in a DC power supply. Heres a link, one has to scroll down a bit and read the application paragraph at the beginning. I dont think brand is going to make a lot of difference as they are all made for the same application as stated clearly here. . 
https://frank.pocnet.net/sheets/191/6/6AR11.pdf

Speaking of bias I note the M-60 offset, which is entirely based in bias stability or lack of. The offset wanders  +/- 100 mV at 0.5 to 5 second rate. Perhaps its all power line related, but nevertheless its quite easy to see on a scope. The DCR of a quad 57 is 0.5 ohms so there is considerable current if the offset gets large.

Having my hands on an M-60 for several weeks now has given me a better opinion of it than I had before. My customer does not want to be adjusting the offset all the time or at all. He has requested I design a servo. I now have the offset down to about 1/10 of the original.
 
Although we disagree on many things I do appreciate your Gentlemanly approach, unlike that other fellow who left us.

@iaelas The solder joints just fall apart from vibration.Interesting. My Force is still going strong after 15 years. Pre ROHS.Two good questions for sub makers :)Another is "Why provide such useless controls?" Many subs are clearly not designed to be integrated in any meaningful way.Anyone interested in the effort expended, visit http://ielogical.com/Audio/SubTerrBlues.phpMost 'non-ap' listeners comment on bass definition rather than quantity

ROHS may be the death of reliability. However people are getting used to short life, recycleable electronics. 

Yea the controls are pretty lame and hard to see the markings if any. 

My sub is designed to work with ESLs as I like ESLs and they present a challenge to blend. An ESL is a resistance (air not ohms) loaded driver My woofer in its tiny box is also resistance loaded (air spring)

@bdp24 

Yow, things are warming up around here ;-) . I like opinionated people, it requires and reveals passion. That’s true even if I see things differently; no two people agree on everything, and everyone develops their own priorities and tastes in music and it’s reproduction. I, not being an engineer, love reading about hi-fi design by those who are, and find discussions of design philosophy and styles very intellectually stimulating.

Like many Americans, on Thanksgiving day my assembled family expressed, member by member, what they are thankful for. I’m thankful for having Ralph, Roger, and all the passionate Audiogon music and hi-fi enthusiasts contributing to this forum!

Thanks for your kind words. I like strong opinions as long as they can be supported. So many audio "truths" are more rumors than truth. Who said what and when. I strive to quote members accurately, check spelling and grammar. (where is the spell check on this forum?)

Passion sometimes brings out the best and the worst in people. Somtimes an honest answer from someone skilled in the field is not appreciated. Sometimes no matter how well one can back up his point it is never taken. A lot of this would play miserably in a formal (Harvard style) debate.

Why do people who dont design preamps or even understand them want to say which is harder to design. Thats a very personal thing. I don't go around saying things like that. Each design, pre or power, is its own challenge. One picks his challenge and goes for it. I like to do things that have not been done before and I feel the few designs that I choose to produce and sell are my best work. Others are just studies. Thats the art part of it.

Thanks again ramtubes.  Sorry did not get back to you in time.  I was the one asking about the reliable electrolytics.  The amp I am building is about 300 rms watts/channel int 8 ohms.  I usually like over engineering things mostly for purposes of reliability and longevity rather than sound quality since after a certain point sound difference becomes indistinguishable.

I am therefore using 1800 VA toroidal and a 270 amp rectifier.  Caps are 4 x 47,000 uF but s I mentioned, at 85 degrees C the caps are rated at 2000 hours.  Therefore they are the weakest links in the amp.  I saw some Nichicons with 105 degrees C at 10000 hours but they were about $250 EACH (phew).  The rectifier, altho likely to be used for welding applications, has soft switch characteristics AND it is quit fast, so would be suitable for audio.

I over sized the rectifier so that it would easily handle the initial short circuit of the caps when the amp is first turned on.  I am also using an inrush current limiter, composed of Airotronics MC1004531J (rated to 25 amps AC) and a flame proof 33 ohm 25 watt wirewound resistor as the current limiter.

I could not decide between a thermistor and a resistor so decided to go with a resistor.  My concern was that if the relay did not engage due to a fault, the thermistor would run at about 150+ degrees, which in my opinion is NOT safe in a solid state enclosure.  The resistor would most likely burn out open circuit and I would either smell it or notice it the next time I switch the amp on, when the lights dim :-)

Thanks

It doesn’t make sense to me that some people get so angry/defensive/personal/nasty when discussing audio equipment! It’s fascinating in a way, but doesn’t do much to educate anyone, and most certainly impedes oneself from getting an education. I listen to music because it makes me feel good. I don’t want a discussion about the equipment needed to play music to piss me off! I believe we can agree to disagree on matters of opinion. This thread has much in the way of fact, but also with opinions masquerading as facts sprinkled in. Back on topic... What differentiates a passive “preamp” from a “tube buffer” with level and switching control? I really like the idea of a simple level control with switching capability, but as I stated earlier, my first try at using a passive, albeit a very inexpensive example, was less than stellar. Specifically, I’m looking at a Schiit Saga to try as a replacement for my tubed preamplifier. I’m weighing options for use with a tube amplifier, but I’d also like to understand what a tube buffer does differently than a tube preamp. Also, would the gain of a preamp “help” a lower powered amp sound more muscular, or is that more “made up” information?