I don't get what you are saying. If you already have the amp, why do you need reviews?
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Quick, give it away to Goodwill. Otherwise you will acquire an 'addiction', which of course will leave you broke and deeply depressed. Their is no known cure, other then to go 'cold turkey' and give it away, and do it now. Sadly, others have not followed my advice and now they are looking for 12 step programs, but beware, those programs often meet in bricks and mortar audio stores and you hear all kinds of 'sweet' sounds that never cure you.
If your question is what is the characteristic sound of particular brand/year tubes in each of these tube categories, there's lot's of anecotal info on the web if you simply run a search using the tube type. I have 'rolled' various tubes in different equipment over the years, and it is an enjoyable aspect of the hobby that can make a noticeable difference in sound.
Recognize that for any given tube, it may yield different results depending on how it is used in the circuit; thus, one person may 'like' a particular tube in a piece of gear that, to you, in your gear, may sound too euphonic.
Also worth noting that unless you are really knowledgeable, there is much confusing info about the old tubes- apparently, different companies shared manufacturing facilties, so a tube legitimately branded under a well-known name and place of origin may have actually been made for them by another company in another country. It is worth dealing with a legitimate supplier- there are several- if you are going to delve into 'new old stock' since the price of some old, highly sought after tubes is considerable and the potential for fakery is there.
Thanks for all the replies!
@Zd542: I am not looking for an amp review. I have inherited the amp from my brother-in-law (RIP). I do not know what tubes may be in the amp at this point. He and I had similar musical preferences, and a tendency to tinker, so I suspect that 1) the tubes that are in the amp are not stock, and 2) I will like whats in it when it arrives. At the very least, I will have to replace tubes as they go bad and was looking for advice on high-quality replacements, reliable tube vendors, and good sources of background information and in-depth tube comparisons.
@Rodman99999: Thanks for the links!
@Whart: I tend not to seek out the anecdotal, you really need to listen to ______ (fill in any particular component), reviews. Id be more interested in thorough, systematic reviews of a variety of tubes with ratings in a variety of categories. I dont have sufficient background to know what those categories might be. Im on a relatively steep learning curve at the moment.
@Wolf_garcia: Great link! I wish they had my M520 in the Shop by Amp section.
@Buconero117: LOL Hi, my name is Brian and Im an audio-holic.
@Bajaed: You said it much more succinctly than I did above.
Tand- I'm not sure, apart from old issues of the Tube Valley? magazine, maybe Glass Audio, mentioned above, that you'll find much in the way of scientific comparisons of tubes. There are tube data sheets for many tubes that are available for old tubes that compare specs. If you run a search for tube data sheets, you'll see what I mean, but other than electrical compatability, not sure those will tell you much about the sound.
What I was referring to was less of a commercial review, and more about user experience posting comparisons of different tubes in a given component (recognizing that different circuits may affect the outcome). I have posted such comments in various fora regarding comparisons of various rectifier tubes and my experience in rolling tubes in one of my phono stages. Garcia's reference to The Tube Store can also be supplemented by descriptions of various tubes on the Tubeworld website. I've bought a number of NOS tubes from him over the years. Good luck.
I've built kit amps and preamp, owned probably twenty different tube amps and probably even more tube preamps, and I have rolled many different tubes each with many manufacturers. Were I you, I would find out what tubes you have in there and go to audioasylum "tubes." and inquire about the best.
There are new tubes to be had, but in my experience those around the time of WW II were the best, but are almost unobtainable anymore. Forget trying to find Western Electric tubes unless you are very rich and in Japan.
One of the challenges is to find new tubes that have the character of the grail vintage tubes. There is a thread elsewhere here on Audiogon about the rectifier the OP wants info on- and various comparisons among users who have tried different new tubes. For that tube, I am using a Mullard first series fat base, which is one of the grail tubes, but even these aren't the kind of crazy money that some tubes fetch. (And I have a stash of other old rectifiers for this slot, all of which sound different in my particular piece of gear).
Ultimately, a lot of people use tubes like tone controls, to flavor the sound to their liking, within a given piece of gear, within the context of a particular system and listening bias. The whole notion of tube rolling is, in some ways, experimentation (subject to using the proper tube that is a correct equivalent). Unfortunately, as TBG said, some of the most desirable old tubes fetch big money.
Let me take a stab at providing some useful info. As a start, GZ34 and 5Ar4 are different designations for the same tube. It is a rectifier tube. The most popular old tube from the glory days were Millard GZ34 tube made at the Blackburn plant in England. Good ones are hard to find and expensive. A very good new production tube is the Gold Lion 5AR4. They go for about $40.
Is this the kind of info you want.
That's a nice little amplifier - mostly point-to-point wiring, about 25 watts per channel. You should be able to do some tube rolling with it. There are a variety of EL34 tubes, and you might be able to use KT77s as well, which could give you a little more extension top and bottom. There are reviews on Dagogo, Possitive Feedback and Six Moons. I agree with Brownsfan, despite his questionable taste in football teams, don't discount the importance of the rectifier tubes. They definitely can have a positive effect on the sound.
@Whart: Thanks for all the information! I have received the unit and found that it was populated with that appear to be stock tubes. Frankly, they sound awesome with my Spendor SP1/2e speakers (also inherited with the M520). Ill be quite happy listening to them while I find rolling alternatives.
@Tbg: The amp has Eastern Electronic Performance Grade tubes (5 per channel: EF86; 12AU7; EL34 [x2]; 5AR4). Ill be checking audioasylum as you suggest.
@Zd542: Thanks for the link to The Cable Co. They do appear to have some reasonably priced replacements (e.g. Mullard EL34s for $37.12 each).
@Brownsfan: Thanks for the recommendations.
@Grimace: Ill definitely have a look at the resources you site.