Considering switching from Audio Research to PrimaLuna, troube with VS115 amp

fsonic, I don’t think upscale suggested buying amps by their looks. He set up a listening session comparing 2 PrimaLunas to an AR integrated that was more than twice the price of the PLs, after all. He invited everyone to attend. Those who did attend listened and selected the PrimaLunas as sounding better. A single listening session is not enough to determine which component will sound best in the long run, of course.

He also suggests that you look at build quality as part of your buying decision making process. Good advice, I think, but far from suggesting that you buy with your eyes.

Thom_mackris : I like your thinking. What’s your take on Rogers then? Cathode bias, and I suspect they run the tubes hot. Yet they say this on their website. It’s all true, except I’ve not seen a cathode bias amp with "the longest tube life". :

• Auto Bias Circuit– For long tube life and ease of operation. Self Bias circuit for adjustment free operation and longest tube life. Customer does not need to adjust tube bias. As the tubes age, they adjust for bias changes automatically. The customer can change tubes at any time or replace a single tube rather than the full set of 4 and the amp will automatically re-bias for the change.

Hi Kevin (@upscaleaudio),

The first box I look to check off when considering a product is reliability and ease of service. My earlier rant was partially based on this prerequisite. Obviously, great construction doesn’t mean you have a stable design, or good sound, but any component that doesn’t pass this test is immediately eliminated from further consideration. It’s a big trigger point for me.

For a vacuum tube product, you should be able to go to any reliable tube vendor and pick up replacements that won’t blow up your gear. Others are free to disagree.

People tend to forget that back in the day, vacuum tube gear wasn’t considered unreliable – when our “greatest generation” depended on military radios working reliably in the field because lives were at stake. Unfortunately, a segment of our industry has gone off the rails with respect to this, and it hurts all of us – the consumer as well as the manufacturer who honors this tradition.

Some manufacturers will have you believe that a component being on the knife edge of instability is an entry point into the inner sanctum of world class sound – that theirs is a finely tuned, thoroughbred circuit and this is the price of admission. My opinion about this can’t be repeated in polite company.

I haven’t had the opportunity to listen to any of Rogers’ products but it’s clear that he checks off this very important (to me) box. His thermal stress testing is impressive. One could conclude (correctly?) that his circuit designs are stable. Great engineering doesn’t necessarily mean great sound, but that’s what auditioning is about.

Of course, cathode bias means that you run a slightly higher B+ in order to achieve the same plate to cathode voltage, but it’s the quiescent current in conjunction with the plate to cathode voltage (not plate to ground voltage, which includes the voltage drop of the cathode resistor) which sets the plate dissipation of the tube. I’ve never heard anyone relate tube life to choice of biasing (fixed vs. cathode), but rather to operating points (plate dissipation).

This reliability pain point reminds me of a customer’s experience – something I think all too many people can’t or won’t admit to (a bit of Stockholm Syndrome, methinks) …

I had a customer who was looking to break in a Hagerman Trumpet phono stage. Jim Hagerman used to supply a small, inverse RIAA circuit that also dropped the signal by 40dB. The idea was that you could run your digital source into this small board and then into your phono stage. Playing on repeat, you could quickly log 25-50 hours on your phono stage.

He called me up to ask: “do I have to run my power amp in order to do this?” Well, his big amplifier (4 KT-88’s per side) had a penchant for blowing screen resistors, and every time he powered it up, he experienced angst. It’s that sort of thing that (in my opinion) is ruining the industry.

People shouldn’t be afraid to power up their gear. This hobby is supposed to be fun. Now, it’s my turn to start chasing kids off my lawn ;-)

Cheers,
Thom @ Galibier Design

Of course, cathode bias means that you run a slightly higher B+ in order to achieve the same plate to cathode voltage, but it’s the quiescent current in conjunction with the plate to cathode voltage (not plate to ground voltage, which includes the voltage drop of the cathode resistor) which sets the plate dissipation of the tube. I’ve never heard anyone relate tube life to choice of biasing (fixed vs. cathode), but rather to operating points (plate dissipation).

Many of the cathode bias amps I've seen come through here run hotter than hell.   I think people are attracted to them being"auto bias" when they are "self bias" and the fact that so much power goes up in heat isn't well-known.  

This come back to you statement about tube amps not being considered a problem back in the day.  Squeezing the last bit of dissipation out of a tube wasn't something that Marantz would have done in the old days.  
 

Hi Kevin (@upscaleaudio),

John Atwood (One Electron, Artemis Labs, etc.) has a fairly concise commentary on some of the design considerations you mention (post #2 of this thread in DIYaudio: http://www.diyaudio.com/forums/tubes-valves/132233-fixed-vs-cathode-bias.html).

I'm thinking that the hot running cathode-biased amps you've experienced were more of a design choice for maximum power than anything else.

There are certainly different issues at play with a cathode biased amp from that of fixed bias.  For example, the quality of the cathode resistor's bypass capacitor can have a noticeable effect on the amplifier's sound.  Typically (due to the required value), they're electrolytics, although bypassing them with a high quality cap can work.

I'm not arguing in favor of one technique over another, but one serendipitous attribute of cathode bias is that you might loose fewer (or no) parts if a tube shorts out (a plate or screen to cathode short).

Of course you can build protection into a fixed bias amp to address this.  I think the Prima Lunas do this.  It's a nice feature.

I think that sonic considerations for fixed/auto bias are more evident in guitar amps where the output stage is pushed very hard.  At this point, the time constant in a cathode biased amplifier (the recovery time) John refers to in his post comes into play, and cathode biased guitar amps have a slightly softer edge to them as a result.

My thinking about audio in general, is that as design architectures "mature" (move higher up the sonic food chain), their sound converges ... toward reality.

And so it goes ...

Cheers,
Thom @ Galibier Design

Hey folks I'm getting on a plane now and may be out of touch for a bit. My wife is cool and loves audio but I better be present if you know what I mean. Vacation time.