Can you tell us more? Are you saying that you should use the tube at 80% dissipation or more in order to protect the filaments? Does this happen with other tubes? Preamp tubes e.g. can be run at much less than 80% of their dissipation.
Good question...... Playing around with 4P1L and 10Y, nothing concrete yet. Spookily enough, I'm driving the 300b with a pair of NE5534 opamps in the meantime. Background listening volume only since gain is x5, but it actually sounds very clean and detailed. I've been using opamps in front of the driver tube for a month or so for more gain.
euro21 - do you mean filament dissipation at 80%? I was thinking anode dissipation.....
Interesting observation about the op-amps as input devices. I always thought that the output transformer (transformers used as voltage/current matching devices, not transformers used as isolation devices with low or no turns ratio differences) was the culprit for glossing over some of the detail contained in the music. I notice this with my 300B and PP EL 34 amps as well. This seemed to be the reason why OTL amps have gained so much traction. Thus, OT metallurgy and design is critical to obtain great sound when using tube amps. Even still, OTs likely still rob some information from the music.
Hey Andy or others, have you compared 300B amps (or other tube amps with OTs) with OTL amps and what is your conclusions?
Hey Andy or others, have you compared 300B amps (or other tube amps with OTs) with OTL amps and what is your conclusions?
Sure.
Tube longevity depends of correct filament heating and moderate overall dissipation.
I'm trying to see where anode dissipation comes into it.......
Does Plate Dissipation Affect Valve / Tube Life In Practice ?? << TubeSync Blog
"Tubes operated correctly at around 30% maximum dissipation can have lives in the region of tens of thousands of hours and can last several decades of years. This figure decreases rapidly as the dissipation increases, in some cased down to hundreds or even tens of hours as they approach or exceed their maximum plate dissipation rating."
Was the "below 80%" filament rating or anode dissipation - still not sure which.....?
Does Plate Dissipation Affect Valve / Tube Life In Practice ?? << TubeSync Blog
"Tubes operated correctly at around 30% maximum dissipation can have lives in the region of tens of thousands of hours and can last several decades of years. This figure decreases rapidly as the dissipation increases, in some cased down to hundreds or even tens of hours as they approach or exceed their maximum plate dissipation rating."
Was the "below 80%" filament rating or anode dissipation - still not sure which.....?
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I can second the previously stated experience that a decent pair of 300Bs run at about 28 watts of plate dissipation should last for 5-10 years in normal daily operation.
The idea that a 300B will last tens of hours running at 40 watts of plate dissipation is pretty silly and not all that difficult to disprove. In a guitar amp that may run a pair of output tubes 100V above their maximum plate voltage rating, also running the plate at maximum dissipation may kill off tubes pretty quickly, but that is pushing the limit on two different limits simultaneously.
OMG. :-(
Typical "journalist" blabla.
How you can design power at 30% tube dissipation?
How you can design power at 30% tube dissipation?
Easily, McIntosh has been doing it for decades.
yep in my Vox last only one year ,I lovered the dissipation now but lost as normal some clean reserve...
I don't think I've seen a McIntosh amp that runs output tubes at less than 60%. They were run cool, but if you run your output tubes too cool transconductance falls, since it is proportional to current, and Zout of the amp will get higher.
Yes, I double checked and they aren't as cool running as I had thought.
You can wander further towards class B. The GE datasheet for the 6BQ5 lists a class B operating point with about 2W of idle plate dissipation on a 12W tube. I don't think I would ever run them that way, but the option is there.
You can wander further towards class B. The GE datasheet for the 6BQ5 lists a class B operating point with about 2W of idle plate dissipation on a 12W tube. I don't think I would ever run them that way, but the option is there.
If you run tube "cooler" , it's another operating point.
Not only anode voltage, anode current and grid voltage changing, but potential power swinging too (due to the changed Vg).
If you change with Vg remains the same, the original operating point (which calculated for optimal output power AND distortion) would slide down to curved section of Ua-Ia curve (where Vg curves spacing -depending of the load line- would not near equal), thus distortion would growing.
Usually in the SE tube "A" region (within moderated overall power) the "hotter" operating point results "lower" distortion.
Not only anode voltage, anode current and grid voltage changing, but potential power swinging too (due to the changed Vg).
If you change with Vg remains the same, the original operating point (which calculated for optimal output power AND distortion) would slide down to curved section of Ua-Ia curve (where Vg curves spacing -depending of the load line- would not near equal), thus distortion would growing.
Usually in the SE tube "A" region (within moderated overall power) the "hotter" operating point results "lower" distortion.
With the caveat that if you choose an optimum point where clipping is symmetrical, any deviation either way will cause clipping to occur at a lower power level.
But hotter bias will usually improve small signal distortion.
As you know I use -more than a decade- 5k:8 OPT in my 300B SE (about 400V B+, 385V, 70mA fixed bias operating point).
This amp capable even 10W (due FET source follower, which drives 300B grid directly up to about +20-30V to clipping).
p.s.
It's a good 5k loadline for 300B.
Requires 180Vpp grid driving, but operates within A1.
This amp capable even 10W (due FET source follower, which drives 300B grid directly up to about +20-30V to clipping).
p.s.
It's a good 5k loadline for 300B.
Requires 180Vpp grid driving, but operates within A1.
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Hi Andy et al,
I'll begin by saying that I've never built a 300B amplifier, but I have been planning one or two for at least five years...more on that in a bit. In the meantime, actually 12,13 14 years before 2011, I started on a path of system building with Lowther PM6A A.T. Spezials, in DIY Lowther Fidelio enclosures. The first amp that I built was a Chokeless Monkey, a dc coupled (Loftin-White) amplifier that Thorsten Loesch designed one Friday afternoon on the JoeList...it was/is an EL84/2A3/MQ FS 030, and plays in-systeme to this day, even as I write. I built the Chokeless Monkey because I could not find or afford the choke required for a Full Monkey version. And then I built a Loftin-White using 24A/45/James JS-6123H OTs wired 5K, according to Thorsten's London Live Diy Circle article way back then. So, that brings me up to date except to say that I've read, and re-read this thread, several times.
So, back to the 300B and Andy's inquiry about OT primary impedance, and thus to design points. In Jan 2011 Martin Seddon posted to the JoeList a Full Monkey design that he picked from the design point tabulation of the 300A/300B data sheet: Va-k 350V/Ia 50mA/5K load. It's one of the lowest distortion points on the sheet, with 6.2W output power...it's also below the 22W plate dissipation locus.
As it happens, there is more to find in the 300A/300B data sheet package ...there are curves included that allow extrapolation beyond 5K primary OTs...so lets one consider 5K, 6K, 7K and for the real explorers, beyond.
I'm collecting OTs in the 5K-7K range...O-netics, Hashimoto, Tamura. Probably will sell the James.
I'd just like to add a footnote from Sakuma from his Sound Practices Issue 13 article: "It is only after much cut-and-try that I find my best matching point for audio...
To get the best sound from a tube, even if the proper match is 5K, we will
try 7K. And we try these "mismatched" values with input transformers and
other parts."
Very Best, hoping to keep this thread going and going!
Robert Chambers
Corinth, Vermont USA
P.S. I think it would be informative for people to indicate/footnote which 300B tubes they are using...not as an incentive to subvert the subject
Corinth, Vermont USA
The first pair I bought was Genelex 300B tubes. I really liked them but didn't have alot to compare them to. I had one of the heaters in one of them break a wire and die and I bought a pair of mesh plate PSvane tubes thinking it was an upgrade since they cost more. The bass was no where near what the genelex were. Not as punchy nor as deep and rich sounding. Since then I have heard a few more brands but not in my amp. And so far the Genelex are my fav..
Oh and my output is an LL1623 / 60ma running at 5.4k
Jeff