three-easy, Good for you that you had the intellectual honesty to pay attention. To complete the experiment, try re-inserting the new tubes after 2-3 weeks using the old set. See if your impressions remain the same.
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three-easy, I would expect that different persons would get different results, if each person were to replace the stock tubes with the aftermarket brand you selected, because to a great extent it depends upon the pre-existing condition of the tubes being replaced. Could it be that your stock tubes were a bit long in the tooth?
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three-easy, Manley, in the Chinook and in my Steelhead, use a hybrid cascode at the phono input. "Hybrid", because the cascode is built out of a transistor (probably a JFET) and a tube (the 6922/6DJ8). That is quite a bit different from using a 12AX7 all by itself to supply all needed gain. A cascode per se will give you more gain and a wide bandwidth, plus shielding from input capacitance (because the JFET drives the 6922 via its cathode), compared to any single tube used in common cathode mode. The trade-off is high output impedance of a cascode. The 6922 is well suited to this topology, because it has high transconductance, needed to take advantage of the drive from the JFET, rather than a high gain factor, in contrast to the 12AX7. 12AX7 has relatively low transconductance and very high gain. You'd not use a 12AX7 in a cascode. Nor would you use a 6DJ8 by itself for the high gain needed at a phono input.
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I far prefer tube-based circuits to solid state, for sound quality, but all tubes do have a finite life span, and as the end draws near, they become noisier, until they reach a final common pathway of being noisy. This malady will affect SLN tubes as well as non-select samples. The variable is time from new to noisy, which is impossible to predict for a given sample of a new tube, but I would agree that for a tube that starts out SLN, perhaps (on average, but there are no guarantees) the time to its becoming noisy will be longer than for a non-select tube. But the point I was trying to make earlier is that circuit design also has a lot to do with "noise" one perceives over the speaker. For one example, ARC stuff was always known for driving tubes very hard, near their recommended limits for voltage and current. In the old days, this led to short lifespan. Circuit layout also has an effect. What tubes are chosen for a given task also has an effect. Someone wondered why Herron(?) didn't choose to use a 6DJ8 instead of a 12AX7 in its phono stage; the simple reason is you could never get enough phono gain out of a 6DJ8, used as Herron uses the 12AX7. The 12AX7 is a standard choice used in common cathode mode for the input of an MM phono stage; all the gain is done at the input before RIAA, as Hagtech mentioned.
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Almarg and Hagtech, Do I conclude from the talk about Johnson noise that (1) MM cartridges with their inherently much higher internal resistance and much much greater inductance will be noisier than MC types? (But of course the noise that accompanies the greater gain needed for a typical MC vs an MM would swamp Johnson noise in most cases.) And (2) what about all those high value Rs often used in an RIAA network, again, IF one is obsessive about Johnson noise?
I've never measured the R across the secondary of a typical SUT but I would never have imagined it could be as high as 3900 ohms. Surprising. I am not an SUT kind of guy, either.
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Thanks. I can’t argue with your logic, and won’t.I might wonder whether one can model a phono cartridge based solely on its internal resistance, in that calculation. But I have no better idea.
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Whether or not you have "tube rush", in my opinion, is much more a function of the topology of the phono circuit than it is a matter of buying super low noise tubes. RAM and Kevin are completely reputable, but any tube will become noisier as it ages, and such aging can occur at different rates for different samples of the same tube type. So even SLN tubes are only SLN on the day they were measured. And one SLN may become only LN, for example, in a matter of several hours of use, while another may remain SLN for a much longer period.
I own both an Atma-sphere MP1 and a Manley Steelhead. The MP1 uses a dual differential cascode input voltage amplifier in the phono section, and the Steelhead uses a hybrid (solid state and tube) version of a cascode. (It’s single-ended.) Neither of these devices ever produces any tube rush at useable settings of the attenuator. The Steelhead, in fact, is silent for practical purposes (meaning you may hear a very faint noise at the sitting position if you are not playing an LP, with the volume control cranked all the way over).
It seems to me that phono stages deriving most of phono gain from the use of 12AX7 tubes at the input might be more likely to exhibit noise at high volume control settings.
Almarg, What formula did you use to calculate the Johnson noise of a phono cartridge? I was prompted by your earlier post to do some reading on it, too, and I would have guessed the noise of a phono cartridge per se would be much lower than what you quote. but I am not sure what are the correct values to plug into any of several equations I found on the internet.
Mijostyn, You can really achieve 110db SPL in your home audio system? I hope you don’t live in an apartment.
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Almarg, You are very kind to answer for Hagtech. Maybe he will also answer for himself. Just in case he was not talking about "Johnson noise". Apparently every conductor creates Johnson noise, It's not a property inherent to transformers only.
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hagtech, In explaining that SUTs can be a source of noise, you wrote, "If they use too thin of wire the resistance gets too high and becomes
the main source of input noise." What??? Where does that idea come from? To begin with, the primary of a SUT is seeing the miniscule output voltage of typically a LOMC cartridge, like 0.5mV of AC, or less.
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