When were the best tube amps made?

not sure whether Zero Autoformers are really zero cost. I used them on a Graaf GM20 about 15 years ago and while bass was significantly tightened, the overall SQ lost pizzazz. It felt a bit like a Ferrari on Qualudes…

@antigrunge2 The result depends largely on the interface of the amp to the load. In our case, it works really well if the load is lower than the amp is comfortable with. There are tradeoffs if part of the load (like the mids and high) are easy to drive while the woofer isn't. However sometimes in that case, if the speaker can be bi-wired, you can run the ZERO just on the woofer to correct the impedance and run direct on the mids and highs.

If the amp uses feedback, as in the case of the Graaf (IIRC) then its a good idea to have the feedback at the output of the transformer, which would take some work... So its not a cure-all solution, but it is one that can be very effective if the stars align.

Transformers are the key element for both SET and push pull.  Even for OTL the power transformer is relevant.  Likely the best commercial winders were in Japan in the late '90s, companies like HIrata Tango and Tamura.

@rogerh113 Excellent power transformers can be made by a good number of companies here in the US.

If you take an OTL and run the ZERO autoformer on it, you wind up (if you see what I did there) with the possibility of an amplifier with much greater bandwidth than any conventional tube amp. This is because the ZERO has such low turns ratio and extremely low distributed capacitance that its bandwidth is very wide- about 2Hz to 1 MHz- wider than any tube amp made. Usually the bandwidth limitation is in the output transformer.

I have directly compared the Linlai 6sl7 withe the ken rad vt229 and I can say the Linlai elite do sound better, now hopefully they last just as long as the ken rad.

Yes- that's the one thing...

Which modern 6SN7 are better than NOS? I currently use Ken-Rad VT-231 from the 1940s in the input stage of my integrated 300B SET. They are not the most expensive NOS but still to change the pair of these tubes is expensive for me.

The Linlai 6SN7 is really impressive. I'm familiar with the KenRad, which is one of the best vintage types. Our customers report that the Linlai is actually a bit better- combining the best of the vintage 6SN7s with none of the weaknesses, which is a really strong recommendation! We've had a few in the shop and I really couldn't fault them. I didn't like the globe version- the glass itself is resonant so if the tube were to develop microphonics the glass will exacerbate them. 

what are the tubes nowadays like? I keep getting the impression that NOS are the best

@lohanimal That depends on the tube. I think most of the NOS tubes are better than a lot of the new pentode power tubes.... but there are some excellent 6SN7s being made these days that our customers say are better than 'the best' NOS... And there are some very decent pentode power tubes too- so the best answer is 'it depends'.

Would be really interested to hear from an engineer and tube amp builder today who has seen and heard many generations of amps.

@earthbound For the record, you did if you read this thread.

What kind of power supply capacitors did Western Electric use in 59A amplifier? I'm not sure it was already the mass production of electrolytic capacitor then.

@alexberger There were electrolytic capacitors in common production prior to WW2- by the early 1930s (the time of this amp) they were sealed aluminum cans that had all the wires exiting a single central mounting cylinder on the bottom of the part. The cylinder was threaded so the part was held in place by a single large nut. The internals of these things was frightful; electrolyte filled, with metal plate not unlike what you see in a vacuum tube supported in space. When I used to restore antique radios I would open these things up and install the replacement cap inside. From what I've seen of WE though they didn't use such parts. 

The best parts for making tube amps exist now, not 50-80 years ago. There are some pretty cool designs out there although its all too common for newbies to rehash 1950s designs.

I don't subscribe to the idea that older transformers were better. I think that might have been true in the 1980s but not any more. There are too many artisan transformer manufacturers that fly in the face of that.

(I bought a set of Edcor transformers for a little 5 Watt amp project I was working on and they performed great- full power bandwidth past 100KHz. Not proof of anything really but you can get good transformers without having to work too hard at it.)

Of course OTLs allow you to get around that problem. When we came on the scene nearly 50 years ago we had the distinction of making the world's first reliable OTLs. But we were actually pushing balanced operation more than we were OTLs- and so our OTLs were fully differential and balanced.

Balanced operation offers a lot of advantages if the circuitry is fully differential. This is because the distortion signature is so much more benign than you get with single-ended circuits or single-ended combined with push-pull like you see in something like the Dynaco ST70. This is simply because of the kind of non-linearities present in such amps; mathematically, single-ended has a quadratic non-linearity, fully differential amps have a cubic non-linearity (so harmonics are falling off at a faster rate as the order of the harmonic is increased) and traditional push-pull amps have both (resulting in a prominent 5th harmonic, which is why I think most SET lovers don't like push-pull). Balanced allows for lower noise and greater immunity to power supply issues, as well as the ability to reject noise at its input.

There are brilliant amps like the Berning zero-hysteresis design (so-called 'ZOTL', which is an acronym with no meaning; that was something made up by the late Harvey Rosenburg) however such amps are actually hybrid as they rely heavily on a solid state switching circuit running at about 250KHz.

Overall its pretty easy to demonstrate that the best tube amps are made right now. But that's not to say that older amps can't compete if properly refurbished- since there are better parts now, you can easily get older amps to outperform their new specs.