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Noisy Preamp

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Noisy Preamp

Postby jimh76 » Fri Jan 31, 2020 11:35 am

Hi,

I own a Presonus Tube Pre (v1) and have noticed that it's quite a noisy preamp. I think it's been like this for a while. There are 2 knobs on it, 1 for drive and 1 called gain. I think that the 1 for gain is just standard gain and the 1 for drive passes the signal through the valve/tube. Even with the drive at the min the preamp is noisy.

My question is this, can tubes get noisy over time or is it a case of a tube either works, or it don't....?

Would replacing the tube with a nicer 1 get rid of some of the noise?

I know that tube preamps are generally noisier than solid state, and I know that it's not a high voltage tube, so the effect is not going to be creamy, but the noise is very noticable and I was wondering before sticking it on Ebay whether changing the tube for about 20 quid and keeping it would be worth it

Thanks in advance.

Jim
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Re: Noisy Preamp

Postby Wonks » Fri Jan 31, 2020 12:21 pm

Yes, valves can get noisier, so a swap of valve may give you an improvement. But it may be another component causing the noise.

You'll find that the 'drive' knob is basically the first volume control with 0-20 dB of valve gain, and 'gain' then controls the overall volume level with 0-40dB of solid state gain. For a dynamic mic, you will probably need both up a fair bit, but for a capacitor mic, you can use the drive knob to dial in as much valve overdrive as the unit lets you, and control the output level with the gain knob.

Cleanest results will be with the 'drive' set to 0 and the gain control set as necessary.

Unless you want to go expensive and look for a NOS Mullard or GEC valve, then Sovtek are the brand my amp tech recommends. Valves are basiclly made in 3 locations: Russia (Sovtek), Czech and Slovak republics (JJ) and China. They are then often rebranded for the likes of Electro Harmonics, Groove Tubes etc. Sovtek seem to be the most reliable in quality at the moment.
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Re: Noisy Preamp

Postby ef37a » Fri Jan 31, 2020 12:28 pm

Hi Jim, without a schematic or at least a block diagram of the pre amp we can't know where the valve is in the signal path, the 'drive' control might bypass it completely or not.

Pre amp valves usually don't go 'hissey' or at least not JUST hissey, usually get a bit 'Rice Crispies' as well! For start take it out and give the base a spray of contact cleaner and buff the pins with a fibre pen if you have one, if not a VERY light touch with the finest emery paper you can find (car wet and dry Silicon carbide paper)

If no improvement I have a spare ECC83 you can have. No idea how good it is, ex test stock but I doubt it will be exactly the same for noise as the incumbent?

N.B. If folks suggest a different valve from the 12AX7/83 fitted do NOT do that. I have no idea of the circuitry (suspect it is low voltage) but the wrong valve can burn out anode loads.

Five years ago the TAD 7025HG was a good, low noise exact equivalent to the 12AX7 at a reasonable price but firms tend to source valves where they can these days so you never know!

Dave.
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Re: Noisy Preamp

Postby ef37a » Fri Jan 31, 2020 12:36 pm

Hi Wonks, another amp tech will say "touch nothing but JJs"!

But yes, valves are rebranded to death and it can be hard to find the gold among the dross. (I have a pair of Russian "Mullard" EL84s! Might be ok, who knows?)

I know nothing of the pre amp. The 'proper' way to build a valve pre is with 300V of HT and nice Sowter mic transformer feeding the valve but I will bet odds to sausages that the front end of that Presonus is solid state feeding the bottle!

Dave.
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Re: Noisy Preamp

Postby jimh76 » Fri Jan 31, 2020 12:58 pm

Cheers peeps.

I was just wondering if it was worth spending 20 quid on a new valve, for a preamp only worth about 30 quid.

The noise is definitely only a hiss, not a crackle.

I'll try the cleaning (would cotton buds be ok to use?). I've got some contact cleaner.

If not Dave, I might take you up on that offer of your valve (give you some dosh though).

Cheers.

Jim
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Re: Noisy Preamp

Postby jimh76 » Fri Jan 31, 2020 1:00 pm

I think the valve is a ec803s. Is this the same as a 12ax7?
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Re: Noisy Preamp

Postby Wonks » Fri Jan 31, 2020 1:07 pm

ECC83 (UK name) is the same as a 12AX7 (US name).
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Re: Noisy Preamp

Postby Wonks » Fri Jan 31, 2020 1:14 pm

You can get a 12AX7 for about £15 from Amazon. The factory fitted valve is probably of the cheapest Chinese variety, and they can be pretty noisy.

And Dave, I was told Blackstar dropped JJ and went to Sovtek a couple of years ago. But things may have changed yet again.
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Re: Noisy Preamp

Postby ef37a » Fri Jan 31, 2020 1:32 pm

Wonks wrote:You can get a 12AX7 for about £15 from Amazon. The factory fitted valve is probably of the cheapest Chinese variety, and they can be pretty noisy.

And Dave, I was told Blackstar dropped JJ and went to Sovtek a couple of years ago. But things may have changed yet again.

I was not advocating JJs Wonks, just an example that various guitar* amp techs have their favourites but that can change when they get a duff batch! At one point we thought the TAD 12AX7 caused rare parasitic oscillation in the Artisan 15 and 30s but it was eventually proved not to be valve specific.

Then a respected brand of KT88, named for a jewel, lasted all of 20 seconds in a 1/2 power life test in a Series One 200! It seems there ARE no 88s with GEC specc' anymore?

* The valves used in proper pre amps and microphones are generally a cut above those in guitar amps. The ECC83 is the wrong valve anyway, you want high gm and low resistances not high mu and 100ks!

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Re: Noisy Preamp

Postby jimh76 » Fri Jan 31, 2020 1:43 pm

Sorry Dave. Not sure what you are saying. Have I got the wrong valve in my preamp?
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Re: Noisy Preamp

Postby Wonks » Fri Jan 31, 2020 2:50 pm

No, you have the right valve for that preamp. Dave is just saying that a 12AX7/ECC83 isn't a good design choice for a low-noise valve mic preamp.
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Re: Noisy Preamp

Postby ef37a » Fri Jan 31, 2020 4:06 pm

Wonks wrote:No, you have the right valve for that preamp. Dave is just saying that a 12AX7/ECC83 isn't a good design choice for a low-noise valve mic preamp.

Yes! Sorry Jim, just some old fool droning on. As you were, you want an ECC83/12AX7.

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Re: Noisy Preamp

Postby jimh76 » Fri Jan 31, 2020 4:31 pm

Ah right then Ted.... :thumbup:
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Re: Noisy Preamp

Postby jimh76 » Tue Feb 04, 2020 5:27 pm

Hi,

I removed the preamp tube and gave the pins a clean and it did seem to help with the noise. I then bought a JJ 12ax7 tube (I know we say valve, but I prefer to say tube) off Amazon. Fitted it and it does sound good. whilst I had it open, I tried to see if I could work out how to change the bulb behind the VU meter, but it seemed like a sealed unit.

Anyway, I have also purchased a GAP Pre73 MKiii in the meantime off eBay, so my preamps at the moments in order of good to not quite as good are:

Warm Audio WA12 Mkii
GAP Pre 73 Mkiii
Presonus TubePre (newly serviced)
2 x Art Tube USB Preamps

Thanks for everyone's input on this.

Jim.
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Re: Noisy Preamp

Postby Martin Walker » Wed Feb 05, 2020 9:46 pm

Sounds like a happy ending Jim, and we all enjoy those :thumbup:


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