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In Reply to: RE: Sovtek 12AX7LPS posted by dandaroy on October 23, 2020 at 19:27:13
Define better, please. Properly culled specimens of the 'LPS are clean, quiet, and reasonably priced. What's not to like?
As 12AX7s run at plate currents in the 900 μA. to 1 mA. range, heat is, most definitely, a non-issue. Refer to TFK "smoothplates" with over 20 years of use still testing good. Tube coolers combined with 12AX7s strikes me as bullshit.
Hmm.. not sure why project would put them on stock tubes.
Anyway, when I meant better I was talking of a smoother most natural tone. Everyone seems to rave about GLs. I am just saying what I have read and heard. I have no personal experience except the Mullard RI 12AX7s. If the Sovtek LPS is really good for a fraction of the price of GLs, then I do not need to spend extra cash.
I'm not familiar with your phono stage (other than hearing the name), so I did a search. Are you talking about the protective discs in the above picture, or do the tubes actually have Pearl type heatsink coolers on them, as shown in the picture below?
Another possibility is damping rings. I tried some Audioquest sorbothane tube rings on a pair of Telefunken smooth plate 12AX7s years ago. While they were removable, they left a mess, and the rings certainly weren't reusable. Beyond the mess, they didn't make a damn bit of difference. I've never had a problem with microphonic 12AX7s.
A picture would be helpful.
Those discs are just cosmetic. The tube have those pearl tube coolers like stuff around them.
As has been discussed previously on the Asylum, the 12AX7s themselves may be largely cosmetic as well.
How does Pro-Ject properly run 12AX7s, which generally use a plate voltage > 200 VDC with but 18 VDC fed in?
Someone I trust told me that it sounds good, but he couldn't explain the voltage mystery either.
Yes, it sounds good, but I also suspected that the 12AX7 tubes are just cosmetic, so I decided not to do any tube rolling here.
I have no idea if the circuit in question is truly tubed. However, it's easy enough to get the 250 VDC or so needed for B+. Voltage multiply that 18 VAC .
The attached 1/2 wave parallel setup would certainly take care of the few mA. 4X 12AX7 sections draw. Even the less efficient Cockcroft-Walton configuration might be good enough.
The PS input to the unit is 18vdc
My guess, they're just running the 12ax7 at super low plate voltages like a lot of guitar effects boxes do.
Have Fun and Enjoy the Music
"Still Working the Problem"
As I just responded to Tre', the last time this issue came up no one had any suggestions, so I was left wondering if the low voltage might do nothing more than light the heaters for listening room tube glow.
Depends on your definition of cosmetic. I think they are just buffers to add some tube warmth which is typical of products in this range.
However, it certainly does not sound tubey in the old fashioned way. Bass is tight, well defined and overall the sound is crispy. Sound has enough detail for my ears. Users have reported that it does respond to tube rolling.
I have a Parks Audio Puffin phono and this is almost twice the price. I am letting it break in while I decide if the audible improvements are worth keeping it over the Puffin.
Before buying the ProJect Tube Box DS2, I had (and still have) a Budgie phono preamp. In my opinion, the ProJect is better.
This phono preamp doesn't show any tube warmth - it is neutral and dynamic, but musical at the same time.
It's hard to think of a worse type for buffer duty than the 12AX7. The triodes in that type "can't fight their way out of a wet paper bag".
My hunch is triode voltage amplification and SS buffers. FWIW, I did that in the tweaked RCA setup.
I claim total ignorance on tubes or their use in a audio circuitry. Here is a snippet from a magazine review of the unit. May be you can translate in layman's English:
"The Pro-Ject Tube Box DS2 phono preamplifier has two 12AX7 dual triodes. It is designed with the first triode as the input stage and is cascaded with a 2SK170 J-FET transistor. The gain of this stage is about 70. The second triode is used as a classic gain stage biased into pure class A."
I need to see an accurate, honest, schematic.
Frankly, I don't trust the magazine. Of course, the 2nd gain stage is Class "A". All Class "A" means is that the device conducts 100% of the time. That sort of operation is mandatory for single ended audio work. The "aroma" of marketing BULLSHIT is intense.
The late Allen Wright is known for advocating SS/triode cascodes at the input of phono preamps. However, cascode topology and 12AX7 sections are not what I expect. The 'X7 triode exhibits very low transconductance (gm) and that's opposite of what's desirable in a cascode.
Yes, there's a HUGE difference between cascade and cascode. That tweaked RCA setup I previously provided employs cascaded triode gain stages, with passive RIAA equalization between those 2 gain stages. The output is buffered by a MOSFET voltage follower.
I want to know how the vendor meets the requirement of driving the 10 Kohm IHF "standard" load. Commercially sold items must (IMO) comport with the IHF standards, as well as complying with local legal standards (FTC, etc. in the U.S.)
You can run signal tubes on low voltages in a phono circuit where the signal is very small.
Have Fun and Enjoy the Music
"Still Working the Problem"
The last time this topic came up nobody had any answers (even speculative ones), so we were left scratching our collective heads.
I've sold 12AX7s for years. I've never had one person, or myself say....Hey, this 12AX7 is microphonic.
Screw the dampers, rings, etc. Save your pennies.
Your interest may vary but the results are the same (Byrd 2020)
I can't compete with the dead. (Buck W. 2010)
Cowards can't be heroes. (Byrd 2017)
If a mildly microphonic tube is encountered, silicone rubber "O" rings, available from McMaster-Carr, mounted on the bottle at the level of the mica wafers can be beneficial.
Silicone can take the heat, unlike the sorbothane previously mentioned, and there's absolutely no reason to pay audiophool prices, when industrial stuff is quite adequate.
"I know just enough to get into trouble. But not enough to get out of it."
they probably put them on as some kind of selling point, which is technically stupid. Even if used for damping or heat dissipation it's a stupid waste. Maybe it's a way to get owners to go back to Project for tubes which some owners will bend over to do.
I've seen a few people on AA who are wrongly critical of Russian tubes. We all can only approve what works for us. Obtain different samples, sit back and listen over time, and YOU decide which sound best in your house.
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