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I've been listening to tubed stuff for quite a few years now, but I really don't understand much about them other than changing tubes when they go bad, rolling tubes to make myself even more neurotic than I already am, & adjusting bias.On the subject of bias, what effect does bias have on what one hears from the speaker? I have noted on a previous amp that I used to listen to, that as the output tubes started sounding old & tired, the bias would start slipping downward & I would correct the problem by screwing the bias pot in until I broke down & replaced the tubes. But assuming that the tubes were fresh & serviceable, what effect would
1) less bias current than called for
or
2) more bias current than called for
have on the sonic effect heard from the speakers?Question 2 is: what is the relationship between a push-pull amp's power & the number of output tubes it runs with? This question does seem like a no-brainer (I did title this as a "for dummies" question) but for example, I once owned a pair of mono-blocks that ran with a total of 8 6550s (4 per monoblock) that was advertised at 110 wpc in class AB. I now own a stereo amp that runs with 12 EL34s & is advertised as producing 100 wpc in AB "pentode". There was this amp called "The Mesa Barron" (with a total of 12 5881s) that I once took home to try out that said it made 150 wpc in AB "pentode". Is this all due to the difference between the output tubes, or is there something else that is different going on inside the amps?
Thanks/Matt
Edits: 07/25/20Follow Ups:
Question 1:
Bias is the current that is applied to the plate to make the pentode/tetrode operate-
each tube has a slightly different (or sometimes dramatically different) set of operating parameters.
It is generally accepted that the parameters set forth in the RCA tube Manual or the GE tube Manual are gospel - they are mostly on-line today, but a few of us have original copies...
the tube characteristics for a given tube in a given circuit is presented in both listings, and includes bias voltage and current (in milliamps).
Many designers choose to operate a given set of tubes at a certain set of parameters, but each designer chooses the operating set points for their amp(s)
You, the user, get to maintain those set-points - increase the current, you will get a bit more power, a bit less headroom (dynamic range) and a shorter lifespan of the tube(s).
Lessen the current, and you will have slightly less power, about the same (design) headroom and normal to slightly longer tube life -
2.
The amount of power that a designer/manufacturer gets from a set of tubes is - as indicated above - a direct result of the design parameters that the designer chooses to employ
There was a very well though of amp in the late '80s that used a pair of 8417 to get 60 wpc - but they were run hard and tube life was ~6-9 months, possibly a year....
Another designer made an amp that could use EL-34s, OR 6CA7, OR 6550, or KT-88 - 4 tubes per channel and would get 100 wpc w/ the EL-34/6CA7, and slightly more (~115wpc) with the 6550/KT-88.
It all depends upon the design and the execution...
Happy Listening
Thank you.
I would have thought that more current, therefore more power, would increase headroom?
There is a finite amount of power that can be developed from a pair of tubes -
So, if you want dynamic headroom - you need to run the amp in the middle of the curve- (linear part) but if you shift the operating point higher - you do not get to increase the ceiling....
Happy Listening
okay . . . that helps.
Kind of like losing horsepower when you rev an engine at RPMs above it's power-band?
Edits: 08/05/20
2) examples of wattage per channel vs tube type 6550 vs EL84 vs 5881
Mono blocks, 4 x 6550 at 35 watts each = 140 watts capability per channel
Cary, 6 x EL84 at 17 watts each = 102 watts capability per channel
Baron, 6 x 5881 at 23 watts each = 138 watts capability per channel
Wattage may vary slightly between tube brands.
Specs provided by the manufacturer can reflect peak, RMS, power supply or other enhancements combined with measuring conditions that will not always equal the exact tube data sheet specs. But this should give you the relative gist of it.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Thank you! I am tired & I am starving, but I am coming back to this.
1) I Googled this article. It nicely summarizes bias settings. For guitar amps actually, but applies to audio as well.
See link.
2) 6550's and EL84's have different output wattages and tonal quality. In fact tonal quality can be quite different amongst brands, even though they meet spec. (example, all 60 watt light bulb brands give off identical light).
Furthermore, circuit design and parts can maximize efficiency. (example, the same Boxter engine used in many auto brands will react differently based on the parts around it. More horsepower, response, gas mileage.)
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Various power tube types are rated for differing anode (plate) dissipation. The KT88/6550 group is rated higher than the EL34/KT77 group. So, 4X KT88s will comfortably deliver 100+ W., while 4X EL34s will be overtaxed and 70+ W. is about right.
IMO, some technical knowledge is needed to obtain max. pleasure from your equipment. I suggest you acquire a copy of the 1st or 2nd edition of James Brophy's "Basic Electronics for Scientists". Both tubes and SS are discussed by Brophy. You will encounter differential equations.
Eli D.
Not in this lifetime!
Nostalgia's not what it used to be
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