What about tubes?

fuzzsniffvoyage

fuzzsniffvoyage

Well-known member
I'm thinking I need to replace the tubes in my ART MPA Gold mic pre. One of the channels quit working. ART site says it has 12AX7A tubes.

Also going to replace tubes in ART PRO VLAII while I'm at it. It has 12AT7 tube.

I kind of looked around a bit on the net. Very confusing, hi-gain vs. low gain, now old stock, new.

So I could use some advice.

Thanks
 
I'm not much of a pro about tubes, my experience with them is almost entirely in guitar amplifiers.

I wouldn't get too caught up in the NOS tubes. Some guys swear by them, and I'm sure that there are some subtle differences that an old American-made tube will produce versus a recently-produced Russian/Slovakian tube. But for the price, I don't think that they will make a big enough difference to warrant the expense. There are good modern tubes being branded as JJ, Electro-Harmonix, Groove Tubes, Mesa, etc. The vast majority of these are produced at the same factories, and each company buys them, tests them, and slaps their own brand name and classifications on them based on the subtle differences of their behavior.

If it were me, I'd just go to Eurotubes, buy some JJ tubes, and be done with it. You can chat with the guy there (Bob, I think) and let him know that these are for ART preamps, and he'll probably have some good suggestions for you. Maybe some of the other guys here have found that "perfect" tube for their VLAs and MPAs, but beware of confirmation bias. Sometimes things sound "better" because we want them to.

I'm not sure what aspect of the tube's performance they're referring to when they say high-gain versus low-gain. I'd say that it refers to the clean gain headroom for that tube; high-gain would produce distortion at lower drive levels. Low-gain would afford more clean headroom before producing distortion. At least that's how I would interpret it. I'm not sure how much of that is a factor of the tube, and how much can be attributed to the circuitry surrounding the tube. I would think that in a preamp, you'd want the most clean headroom you can get. Otherwise you'd have a fuzz pedal.
 
fuzzsniffvoyage said:
I'm thinking I need to replace the tubes in my ART MPA Gold mic pre. One of the channels quit working. ART site says it has 12AX7A tubes.

Also going to replace tubes in ART PRO VLAII while I'm at it. It has 12AT7 tube.

I kind of looked around a bit on the net. Very confusing, hi-gain vs. low gain, now old stock, new.

So I could use some advice.

Thanks
Click to expand...

The bllx talked about valves is probably greater than any other area of audio (if you neglect the Russ A nutters, and PLEASE DO!).

For any given valve type the gain or amplification factor "mu" is very closely fixed being virtually completely dependant upon mechanical spacing. Thus "quality" valves of the ECC83 type (aka 12AX7) will all have a mu of 100* (their other parameters, ra and gm will also be closely controlled)

That leaves noise, hiss and microphony as problems you might encounter. Sadly this is a lottery , even buying batches of 100 83's at a time I used to find maybe 10% good enough for the first stage of a high gain 30W guitar amp and 10% of the rest were almost useless for anything else!

JJ valves seem to be very good. For a good quality 12AX7 try to find a Sovtek 7025 high grade. Very good and about £10 here so not a rip off.
Beware of NOS! Lot of worn out old rubbish about!

Fortunately the ART pre amp does not rely on a VERY low noise, high spec valve since there is an IC driver a'fore it.

12AT7: I am always sad to see these in audio circuits. They were made as "synch separators" for tellies! Mullard class them as a "short grid base, RF triode" . I can only assume some company had 10s of 1000s of the bloody things and decided to use them!
I have always found the odd sample I have tried as very microphonic, not a problem in a video circuit of course!

* FYI: 12AT7 MU=60
12AU7 MU=17 Note also that mu does not give the actual circuit gain, can be a lot less but the very best you will get from one 83 stage is about X 70.

I know it is "stable door" advice but the above is the reason I always advise new comers to recording NOT to buy a valve pre amp. I do understand why people love them, especially in gitamps but for small signal work especially, the valve has had its day.

Dave.
 
NOS telefunken and mullard ARE the best, well worth the price, JJ/Tesla's are a good compromise, they are probably the best sounding valves that are still being manufactured as new valves. Depends how much you are willing to spend really and how much it means to you, personally I'd probably get the JJ valves and only use the NOS valves in something that's extremely high end to start with...like a vintage neumann mic or something of that quality...that U67 that we all dream of.

Also avoid high gain valves, especially for the mic pre-amp, you want it to colour the sound but remain clean, I'd recommend low or medium gain valves only, high gain ones were designed for metal style 100w heads so that it breaks up early without needing to have it at 130dB+ SPL and going deaf in the process.
 
I would like to explain that changing valve "types" e.g. a 12at7* for a 12ax7 will almost certainly lead to an increase in distortion and lower headroom.

This is because the circuitry around any particular valve is chosen for THAT valve type. Notably the anode load and cathode bias resistors. These are calculated, in "linear" circuits anyway, to optimize performance in the above areas, at least this is generally the case for devices such as pre amps or capacitor mic impedance converters. Of course you MAY like the distortion that results from fitting the wrong valve type but that is a whole other issue.

*It is also possible to damage the valve, the circuit or both in certain cases. Early Fender amps used anode loads that were only just high enough rated and fitting a different valve type resulted in over dissipation and resistor burn out. The ECC82 is often used as a reverb tank driver, fit an 83 at its peril. There are several guitar amps that use the 82 as a low impedance driver to the output valves. The 83 is a high Z valve and will sound ***t and not last long.

So, best to stick to type unless you really know what you are doing.

Dave.
 
Thanks for the info. I just going to swap out same for same.

the tubestore.com will definitely be the place to go. Thanks.
 
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