300b lovers

But many of these classic PP still sound fine! Why? Because feedback or because PP has PS noise cancelation?  

Feedback allows the amplifier rejection of that which is not the signal, so generally speaking, yes.

I really liked the 3 6sn7 tube version that tubes4hifi used to sell and perhaps still does.   It makes the amp considerably better. 

The problem with any mod that adds tubes to the circuit is the extra load on a power transformer that might already be 65 years old. Dynaco strikes me as being pretty precious about their transformer ratings- I don't like to take chances with them, especially in light of their age.

This is just me of course but if I'm going to modify a vintage piece I follow two simple rules. The first is don't add any extra load to the power transformer. The second is don't do anything that does not fit very easily into the existing chassis. Violate these and you're likely better off doing the whole thing from scratch.

The really glaring weakness in the ST70 is it should have been designed with dual rectifiers; as a result the 5AR4 is the most likely tube to fail in the amp. Triode Electronics of Chicago has a beefed up power transformer that is a drop in replacement that allows you to add a second 5AR4, thereby keeping the correct B+ operating point and so not stressing the output transformers as well. But you have to find room beneath the chassis for some 500V filter caps. It starts to get a bit ridiculous- at that point why not just do your own chassis so you can lay out things properly?

If an ST70 is properly refurbished but pretty well the stock circuit, it can be surprisingly good against a lot of modern PP and SET amps. Since it really does not have enough feedback, you have to help it along with good quality coupling caps and resistors in the voltage amplifier and driver circuit. The second thing to understand about this amp is because of its power supply weakness, you really should not push it hard (which is better for sound but also keeping that 5AR4 alive). CE Distribution in Arizona makes a drop in replacement filter can that features an 80uf section, which should be deployed after the choke, for the plates of the power tubes. That's about as much extra capacity as you can safely add to this amp without stressing the 5AR4.

The octal boards for ST70 are direct fit and do not stress the power supply.  You can use the diode replacement for rectifiers, but it sounds different.  Basically, the amp is not bad, and a great deal for the money when restored and updated.  A decent preamp and some reasonably efficient speakers and a person can have a very pleasing stereo.  That said, an ST70 is nothing special because there really isn't room to make it special.  I think we agree on that.  You might as well start over and do it right without the compromises.

Back in 1993 when I was trying many different amplifiers on the newly completed Ariel speakers, I came to the conclusion that a stock ST70 was the minimum acceptable standard for hifi. Most transistor amps fell below this mark, and most tube amps exceeded it. Restored Mullard designs did very well, as anyone might expect.

What I did not expect was the performance of the Ongaku and the Herb Reichert Silver 300B. I expected all SETs to be terrible, but those two were the best sound ever on the Ariels (which are 92 dB/meter efficient). After hearing several other SETs, I was struck how variable they were. A few were superlative, almost otherworldly, but many others were pretty bad. One was so terrible that Karna and I just burst out laughing ... it sounded like a 1960 transistor radio left out in the sun too long. That’s how variable SETs are ... all over the place.

By contrast, a competently engineered Mullard with a decent power supply is almost guaranteed to sound pretty good. And the best ones are superb.

And of course a part-Mullard circuit is perfectly acceptable for a PP 300B amplifier. Unlike a PP pentode amplifier, though, you need about two to three times as much voltage swing in the driver, so a Dynaco circuit is definitely not the right choice.

A Mullard PP 300B works as follows: input tube direct-coupled to a long-tail pair (or CCS) of triode-connected 6V6 drivers. These in turn are connected to a PP interstage transformer with a modest step-up ratio, between 1:1.4 and 1:2. The interstage then drives the PP 300B grids. This would be a non-feedback amplifier, so good power supplies are required. I would imagine a number of the PP 300B amplifiers already on the market use this topology.

@lynn_olson

I expected all SETs to be terrible, but those two were the best sound ever on the Ariels (which are 92 dB/meter efficient). After hearing several other SETs, I was struck how variable they were. A few were superlative,

Lynn,

What do you attribute to this wide spectrum of SET performance from superlative to terrible amongst them? Given their relatively simple circuit, is it primarily part quality (Output transformer?) or power supply quality and design?

Charles