The amplifier itself is the best tube-tester. If a tube fails to bias correctly in situ in the amplifier, it should be replaced.
Unless you want to spend thousands of bucks on a separate tube tester that can actually test power tubes at their typical voltage and current, you don't need a tube tester at all. This was mentioned earlier. The vintage tube testers, many of which are collected by strange people who collect tube testers, were generally NOT able to test power tubes at meaningful levels of voltage and current. Of those vintage testers, Hickok is the brand worth having, because Hickok testers along with only a few other brands can test transconductance. Testers ARE nice to have around for small signal tubes, used in the front end of amplifiers and in preamplifiers/phono stages. But even that is a bit frivolous. I do own a Hickok Model 533A, one of the older models probably from the post-WW2 era. I do use it to check transconductance (Gm), which is a sign of the life stage of a tube. If Gm is below the accepted range for that tube, you can ditch it with a clear conscience. I paid $75 for the 533A. The grandson of the 533 is the 539, one of the most sought after. A fully reconditioned 539 in top operating condition will cost you more than $1000, easy. 539s come a little closer to being able to test power tubes properly but not really close enough. I would bet almost no one here besides me owns any tube tester. I bought it for my own interest in DIY.
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If one wants to roll tubes, one can do so, but that is not a requirement. To me, it is a form of mental masturbation. Bias adjustment would be per the suggestion of the manufacturer. Some designs run the tubes harder than other designs (closer to their maxima for voltage and current). Such designs would benefit from bias adjustment at more frequent intervals compared to designs that run the tubes conservatively, because the tubes are wearing out faster in the former case. How often to replace tubes is also design-dependent. Vinyl is much more demanding than using a tube amplifier, on a day to day basis, IMO.
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Soko, Read what I wrote. Elliot's Cayin amplifier is the only tube amplifier I ever heard of where the manufacturer actually invites the user to enter inside the chassis, under any circumstances, let alone when powered up. All other (US-made) tube amplifiers I know about will usually carry a warning NOT to open the chassis, ever. Usually the warranty is voided if one does that. Typical tube amplifiers can be re-biased externally with no risk, or not at all if they incorporate an autobias circuit. Got that?
It so happens I am a DIYer, so I often do work on my amplifiers and preamplifiers, usually for fun and to satisfy my interest in electronics. But I didn't even try to do that until I had a solid knowledge base, in part to keep me safe.
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In my long experience, tube amplifiers that require periodic bias adjustment typically provide both external test points (female jacks where you stick the pos and neg meter probes) and externally accessible potentiometers with which to do the bias adjustment. Some owners of ARC amplifiers have already mentioned this. Some brands (like Atma-sphere on at least some of the model line) even provide a built in external meter, so you don't need to own a DVM, etc, in order to adjust bias. Another large set of tube amplifiers have an autobias circuit built in, which automatically senses a drift in grid bias voltage and compensates for it, until the tubes themselves are worn out. Your Cayin amplifiers are unusual in that they apparently do require manual adjustment of bias, but you seem to have to open up the chassis to get at the adjustments. That may reflect their "made in China" provenance, where perhaps the liability laws that provide legal recourse for persons who are injured by electrical shock whilst biasing are less stringent. (But perhaps the US distributor could be made liable in the US.) So just take the warnings seriously and proceed carefully. I always keep one hand in my pocket when working with power on, to remind myself never to grip the amplifier with two hands, which could send a shock across your heart muscle. It doesn't take much to be lethal. I don't think you need a non-conductive screw drive blade so long as the handle is non-conductive. But it couldn't hurt.
Maybe Ralph can chime in, but I have no idea what is being adjusted to 0.4V in your amplifiers. Surely that does not represent grid bias voltage (between grid and ground or grid and cathode) for an output tube. Perhaps you are adjusting the voltage across a fixed value resistor which in turn affects bias current. Also, you need a fairly good quality meter to be accurate in that under 1VDC range.
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You may have learned A theory of relativity, but it has nothing to do with Einstein's Theory.
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Elliot, I see no reason why you should be surprised that there are no products currently for sale that mimic your Macintosh MX 110, with an FM tuner built into a preamplifier essentially. FM radio is on the rocks. Not many audiophiles even try listening to FM radio for musical pleasure these days. Except maybe in their cars. Back in the heyday of the MX110, every major city had FM stations that actually competed with each other for the quality of their signal and prided themselves on their classical and jazz music content. That’s all gone, pretty much. So that accounts for the absence of a built-in tuner. As for the tone controls, those fell out of fashion in the 90s, except for some notable made in Japan preamplifiers from Accuphase and Luxman. Privately, I think they fell out of fashion because not incorporating tone controls saves a bit of cost on build and development. Audiophile publications often carried articles suggesting that tone controls add distortion, which of course they do. But it’s a question of trade-offs. As far as I am concerned, I would not consider a preamplifier with tone controls, unless they were switchable in and out of the circuit. Apparently you do enjoy FM radio broadcasts, in which case the MX 110 was made for your needs. It’s great that you like it so much. Now I think of it, Accuphase and maybe Luxman too may have made preamplifiers with built in tuners of more recent vintage than that of your Macintosh. Those would be all solid state, most likely. Very high quality. |
Sokogear, It's a personal decision based on your own listening to music. One cannot say for sure exactly which tube amplifier you might prefer to your Plinius SS one. There are many types of tube amplifier, just as there are a few different standard topologies for SS. You would have to put in the work yourself. But since you have already stated your bias for SS, it might not be fruitful. It's not anyone else's job to convince you of anything.
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For my own curiosity, I checked out Mapleshade. Yes, they are still in business, but no they don't say anything about restored vintage Fisher gear on their website. I would still call them if you are interested. Pierre Sprey who is one of the principles of Mapleshade is a great and brilliant guy.
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Check Mapleshade Audio. If they are still in business, they used to sell re-conditioned vintage tube gear, specializing in Fisher stuff. You pay a little more, but you get a completely rebuilt unit probably with a warrantee. Otherwise, if you are not a skilled DIYer, caveat emptor when it comes to very old tube amplifiers. They can be fun to fix, but they can also be a pain in the arse. I would NOT buy off eBay unless the seller is well known and within the USA (for buyers who live in USA). You want to have some recourse if there are insurmountable problems.
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As tubes age, they lose transconductance. As they lose transconductance if the grid bias voltage is fixed (not varying with respect to cathode voltage), you gradually run out of oomph (for want of a better term). The amplifier may begin to sound a bit limp. But this happens so slowly that the user is often only dimly aware that the sonics are not quite as wonderful as they once were, with fresh tubes. And eventually, one buys new power tubes. In this way, I once got 10 years out of a set of power tubes in a Futterman OTL.
However, as the owner of an amplifier with adjustable bias and a built-in method for measuring and correcting bias current when it inevitably goes off, by adjusting grid voltage, I have to say it is a revelation to hear the amplifier restored to normal bias current after a periodic bias adjustment. By "periodic" I mean once a year or so. If you can adjust bias safely and know what you are doing it for, I highly recommend monitoring it and keeping it up to snuff. Also, owners of amps with some sort of autobias circuit need not be concerned.
I have a Hickok 533. I use it only to test for transconductance of small signal triodes. I do not use it to evaluate power tubes because it does not have the capacity to subject such tubes to plate voltage and plate current parameters where such tubes need to operate.
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Elliot, I am not quite sure I understand your question, "Sonically, what are your experiences?" Do I think biasing an output tube makes a difference? That goes without saying; a power tube cannot operate without some chosen parameters: plate voltage, plate and/or cathode resistor values, grid bias, etc, that determine the electronic envelope in which the tube operates. For every tube, there is a data sheet, usually available on-line and/or in published tube manuals. These data show maxima and minima for plate voltage, bias, plate current, etc. The tube has to be set up so its operation falls within these upper and lower limits. And taken with the plate curves I mentioned earlier, you can "see" how different choices within that envelope of values will affect performance when the tube is fed an audio (AC) signal that will have a voltage swing. I use exclusively Atma-sphere OTL power amplifiers, so of course no output transformer, and the power tubes are triodes. In fact, in 45 years, I have never owned a transformer coupled tube amp, only OTLs driving ESL speakers. With Ralph's help, I built my amplifiers so I can set bias current for each of the output tubes separately. This allows me to "match" the tubes for the way they are treated by the circuit. But tubes themselves are unavoidably at least slightly heterogeneous in the way they individually respond to signal voltage.
If you consult with tube gurus, many of them have their own pet theories on where to set bias points for various power tubes, for best SQ and/or for most power, longest life, etc. (Sometimes the settings for best SQ conflict with maximizing tube life.) By the way, the description of vacuum tube operation in my previous post is for triodes only. So called because there are 3 nodes, plate, cathode, and grid. Hence a "tri"-ode, the simplest type of audio tube. Most power output tubes especially on transformer coupled tube amps, will be tetrodes or pentodes. The extra nodes are additional grids that can further control the flow of electrons to the anode, but only the one grid receives the signal (with some very rare exceptions; for example some Berning amplifiers drive the screen grid); normally the one or two screen grids do some of the modulating of electron flow by virtue of the voltage supplied to them. Small signal input tubes and phono and linestage tubes are nearly always triodes.
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I suggest you buy an elementary treatise on how vacuum tubes operate, and then read it. In the accepted model for a vacuum tube, electrons are emitted from the cathode when it is heated. The cathode accumulates electrons and hence is thought of as having a negative charge. Because the anode is positively charged, or relatively deficient in electrons, they flow cathode to anode. If this is all that happened then vacuum tubes would be like light bulbs; the cathode would give up electrons in an uncontrolled manner, and they would flow to the anode. When this process reaches a state of equilibrium, the tube would be burned out and that would be the end of it. But instead there is a "grid". The grid is situated in the path of the electrons on their way to the anode. In a functioning tube, most of the time, the grid carries a net negative charge with respect to the cathode, and thus retards the rate at which electrons get all the way to the anode. That is, when no music signal is present, the grid has a DC voltage on it that is negative with respect to the DC voltage on the cathode. When a music signal is present, think of that as an AC voltage, a voltage that varies with frequency and in magnitude and represents music. So, music enters via the grid which always has a constant DC bias voltage on it, in simplest terms. The music signal modulates the grid bias voltage, and in another story, that’s how we get amplification of the signal across a resistor that connects the anode to the PS voltage. When amplifiers talk about "bias", they are usually referring to this steady state DC voltage on the grid. That grid bias voltage also results in a steady state current (electrons) passing from cathode to anode when there is no music signal. This DC current is referred to as "bias current". The grid bias voltage, the "plate resistance" of the tube, and the power supply voltage between anode and cathode, also called the "plate voltage" together determine the bias current. (Ohm’s Law.) For any power tube, or any tube at all, you can usually find a complex graph depicting "plate curves". This single graph will show current on the Y axis and plate voltage on the X axis, and a set of curves depicting what that tube will do at different acceptable grid bias voltages for different changes in plate voltage or current. And....
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