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

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?

I am also looking for a lab/bench symmetric power supply of up to +/- 100 volts DC at about 2 - 3 amps (so about 500 - 600 watts) and having a very difficult time finding it.  The very rare ones I have seen are in excess of $1500.  I need such a supply for testing purposes.  I do not want to build one.

Does anyone know of one around a max of $500 ?

Thanks

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. .

I've certainly seen the page for the 6AS7 :)  What I am telling you is the the 6H13C is a different tube (the Russian variant). Once preconditioned, they can hold up better than an American tube. Far less likely to see the cathode coating falling apart, at least until the tube gets weak.

Regarding bias stability, the amp has no need for a servo. If the DC Offset is unstable, its likely noise in the driver tube as the output tubes obtain their bias from the driver tube. Normally the DC Offset is the sort of thing that you set or at least check once every 6 months or so. IOW, its **very** stable!

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

There is! I regard the audio versions as common myth, often bandied about inappropriately:
http://www.atma-sphere.com/Resources/Common_Amplifier_Myths.php

Although we disagree on many things I do appreciate your Gentlemanly approach, unlike that other fellow who left us.

Thanks - let's hope he stays away. His approach produces so much noise, its impossible to have an actual conversation, not to mention his creation of an entirely new wing of physics (or at least alternate meanings to words to which no-one was previously aware)...

Just to be clear, I have a lot of respect for you as I do Nelson Pass, John Curl, David Berning and a number of others. There's a lot of snake oil in this business so its refreshing when we don't have to deal with that. Like you, I've been at this a long time but went down a different path a long time ago:http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpand while I understand completely how the Voltage Paradigm works (IOW I don't make amps to be 'tone controls'), I'm not at all convinced that the Voltage Paradigm is the only way to achieve the most neutral presentation. For me, the reason was best expressed by Norman Crowhurst, who pointed out a good 60 years ago that while feedback of course suppresses distortion, it also introduces some of its own, which tends to be entirely higher ordered harmonics. Its not that I'm against feedback, but its inappropriate or inexpert application does bother me, and for that I'll use the current ARC amps as an example.

The problem is that the ear converts all forms of distortion into tonality (and the ear/brain system has tipping points where that tonality can be favored over actual frequency response), and the the most egregious problem in audio IMO/IME is brightness (and its twin brother, harshness), which in transistors is entirely caused by distortion; also in many tube amps that use feedback. This is because the ear uses higher ordered harmonics to sense sound pressure, so is really sensitive to them as a result (moreso than most test equipment due to the range that the ear has to cover)! The line I draw in the sand is I want it to sound like real music as opposed to a just a good stereo. To this end, I do my best to vet every customer's system and expectations in order to make a sale. This limits my sales for sure, but it also results in really excellent results both in sound and customer loyalty when everything is set up right. 

Regarding the different gain in the M-60s, its entirely possible that the CCS is damaged. Clio9 was an early adopter of Mk3.3 and I suspect he has earlier CCS boards in his amps. IIRC that is... If you can send a photo of the CCS board to my email (found on the atma-sphere.com website) that will tell me a lot.

Although we disagree on many things I do appreciate your Gentlemanly approach, unlike that other fellow who left us.

Thanks - let's hope he stays away. His approach produces so much noise, its impossible to have an actual conversation, not to mention his creation of an entirely new wing of physics (or at least alternate meanings to words to which no-one was previously aware)...

Well, I lied. I have to respond. Ralph-I can't believe that you said that about me after I have done nothing on this Board (in multiple threads) but praise you and after you agreed that preamps are not easier to design (well) than amps. Yes, there were a couple typos in my last post due to being busy and typing in haste, but my points should have been clear. I could easily recite ten typos and misspellings in the various posts of Mr. Modjeski, were I to feel the need to resort to that. 
It is also shocking to me that Mr. Modjeski criticized the hell out of your M-60's biasing circuit and claimed your RIAA curve in your preamp is not up to snuff and you are doing nothing but kissing his arse.