I sometimes imagine that people think I just make these things up, but here is a description of exactly what I experienced with my valves in a headphone amp:
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So thats three first hand accounts of spontaneous ringing-microphonics from the 6AS7.
Just to be clear here, no other amp I have ever built and designed has experienced similar behaviour driving exactly the same speakers, using the same preamp tubes to boot. Believe me the 6AS7 is the only common factor.
Shoog
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So thats three first hand accounts of spontaneous ringing-microphonics from the 6AS7.
Just to be clear here, no other amp I have ever built and designed has experienced similar behaviour driving exactly the same speakers, using the same preamp tubes to boot. Believe me the 6AS7 is the only common factor.
Shoog
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This tread should be dealing with 6AS7GA as Push pull Output amplifiers.
A headphone amplifier is no reference at all. There are operated at a few volts signal level and in PP beyond 300 Vpp.
The difference is 30...40 dB. Of course this also means 30...40 dB difference in S/N.
In addition there is quite a difference in sensitivity between earphones and speakers.
6AS7G is fully ok as an output tube for PP-amplifiers. There are a lot of "live" examples about this.
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Not wishing to get into an argument here - but what you are saying is that myself and multi are lying about our own personal experience with "very sensitive speakers" and the 6080.
So you have never encountered this behaviour - so all witness statements to the contrary must be wrong. NICE.
Ringing or self oscillation has little to do with the signal been amplified - it is a physical property of the behaviour of the valve and so can manifest at any signal level superimposed over that signal. Let me also reiterate that for most people with average to high sensitivity speakers will probably never encounter this at an audible level.
Shoog
So you have never encountered this behaviour - so all witness statements to the contrary must be wrong. NICE.
Ringing or self oscillation has little to do with the signal been amplified - it is a physical property of the behaviour of the valve and so can manifest at any signal level superimposed over that signal. Let me also reiterate that for most people with average to high sensitivity speakers will probably never encounter this at an audible level.
Shoog
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Have a look at the schematic below for a 6as7/6080 driver + output stage combo:
- that does not burn high wattage in the cathode resistors
- is easy on the Ub (+280V)
- is able to comfortably swing 290Vpp on the drivers, without any exotic parts or phase-bending iron
- gives high output with low thd (~13W, ~ .5%) in pure class A
Attachments
Lets not assume anything of the sort since where I encountered the ringing was in a PP output stage. I cannot see any mechanism where a PP would suppress ringing in a given valve where that ring could not be symmetrically and equal in both halves.
Why would it surprise anyone that these valves would be microphonic, after all they were never intended as audio finals and would never have been optimized for silence in that application. In fact all of the design decisions that went into optimizing them for their intended purpose, series regulation, would make them susceptible to microphonics (think about that massive grid structure and huge undamped cathodes).
This is really starting to annoy me at this stage.
Here are a few more examples of people finding 6AS7's microphonic:
what causes microphonics??? | Audiokarma Home Audio Stereo Discussion Forums
Post 3198
For 6AS7G tube rollers here ..... | Page 214 | Head-Fi.org
Philips JAN 6080WC OTL Amp Project | Mojo Shout
So will all 6080 variant be microphonic ? No
Will many people not hear the microphonics even when present ? Yes
Shoog
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If we take this point of view, we could say that 300B is a tube with hum problems.
We can find hundreds of post telling this. But we know that this is not the case with 300B.
Is the reason that some people can make better performing amplifiers that the other?
I don't know what has gone wrong with those dozen 6AS7G PP amplifiers that my DIY-fellows have built, since those do not have a sign of ringing or microphonics.
We can find hundreds of post telling this. But we know that this is not the case with 300B.
Is the reason that some people can make better performing amplifiers that the other?
I don't know what has gone wrong with those dozen 6AS7G PP amplifiers that my DIY-fellows have built, since those do not have a sign of ringing or microphonics.
Maybe you should address the one issue which has been highlighted here - what sensitivity were the speakers those twelve people were using ? Have you ever applied one of your amps to a 105db/w pair of horn's or like in my case a 100db/w vintage field coil drivers.
Since the power output is in the region of of 20watts - I very much doubt it.
So are you actually talking from relevant experience or just your experience ?
And yes a 300B amp which has audible hum shows that 300B are prone to hum - and that is a common experience.
If you have to apply special measures to tame a valve then that valve has an intrinsic flaw. I would call the need for DC heaters as just such a flaw.
Shoog
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With insensitive speakers is possible to get away with lots problems, hum, micro-phonics ; I have friends who claim there amps are hum free when I connect them to my speakers the hum is so bad all you can do is Shut them down, as fast as possible, same with Micro-phonics no use asking if people using 84DB speakers if they get micro-phonics from 6080’s. Building amps for 105db speakers you have to get everything correct; I have had no problems with any other tube but 6080 & 6AS7.
I noticed that I have already got your .asc at 2016.
I have only used cathode bias according to original suggestion by RCA.
How that "mixed" bias has been working ?
Your taking a very argumentative and bloody minded attitude to all this.There must be. Check again.
its quite possible for any number of people to have non-microphonic or ringing 6080's based amps without it invalidating the general statement that 6080's have a tendency to microphonics.
Two statements are not necessarily exclusive and in need of "a win" in discussions.
Shoog
The idea is based on info provided in the ECC230 philips datasheet. I have not actually build one such amp, since you can build more powerful and better spec'ed EL84 amps with much fewer parts in comparison.
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