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Post by topshelfmg on Feb 1, 2017 22:59:11 GMT -6
I have a heavily modded stereo Ampex 601 preamp, and since one of the channels has gone noisy, I've decided to replace the tubes on both channels starting with ef86s I just bought, but want to gather some experience about NOS 12au7s. There is a Cinemag input transformer, the original output transformer, a 12au7 and ef86 in the signal path of each channel. It uses a tube rectifier though I know that hardly matters. I figure the ef86 is what went noisy due to their nature, so I just picked up 3 NOS (tested NOS, and actually NOS), mesh plate Mullard ef86s made in the Blackburn factory, branded as Amperex bugle boys based on the recommendation of my tech. Got a great deal on eBay, and I figure having a 3rd tube in the bunch reduces the risk of eBay tube shopping.
ANYWAYS... if this does not fix the noise, it has to be the 12au7's, so I figure I'd start gathering some info since I'm not super knowledgable on tube mojo. There are currently NOS RCAs in the 12au7 position, though I couldn't tell you what type, since they came in the unit when I bought it and most of the markings are faded. Anyone have tonal favorites for NOS 12au7's? I honestly may go ahead and replace them for the heck of it, even if the noise is solved with these ef86s just for fun and to experiment with the tone a little bit.
Cheers
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Post by Johnkenn on Feb 1, 2017 23:02:30 GMT -6
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Post by bowie on Feb 1, 2017 23:46:51 GMT -6
Don't discount the possibility of an EF86 being noisy. Those early mesh Mullards sound sweet but are very noise-prone. I find few of them that I can use for sensitive stages.
Try swapping the 12AU7s in their respective positions and seeing if the noise changes. I don't have a schematic in front of me but I thought the 12AU7 was used in an output, or some later stages in those units. Anyhow, that's usually the case with the 12AU7. Because they're lower in gain, you'll rarely find them in input stages. In those situations, the 12AU7 can contribute but you don't want to spend a ton on a 12AU7 for a couple reasons. One being that, unless it's your end-all preamp, it's better to put the money into the more influential tube (EF86 in this case) and just get a competent 12AU7. The other is that there are a lot of nice 12AU7s that get overlooked, but rival the quality of some of the sought-after names when used in an output stage. I usually steer my customers toward some of the more reliable and cost-effective 12AU7 options like certain versions of Sylvanias, RCAs, etc. Some prefer to go all-out and put Mullards and Telefunkens in every stage. But, in many 12AU7 stages, the difference between a Telefunken and a good Sylvania are very subtle.
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Post by topshelfmg on Feb 2, 2017 0:20:54 GMT -6
Don't discount the possibility of an EF86 being noisy. Those early mesh Mullards sound sweet but are very noise-prone. I find few of them that I can use for sensitive stages. Try swapping the 12AU7s in their respective positions and seeing if the noise changes. I don't have a schematic in front of me but I thought the 12AU7 was used in an output, or some later stages in those units. Anyhow, that's usually the case with the 12AU7. Because they're lower in gain, you'll rarely find them in input stages. In those situations, the 12AU7 can contribute but you don't want to spend a ton on a 12AU7 for a couple reasons. One being that, unless it's your end-all preamp, it's better to put the money into the more influential tube (EF86 in this case) and just get a competent 12AU7. The other is that there are a lot of nice 12AU7s that get overlooked, but rival the quality of some of the sought-after names when used in an output stage. I usually steer my customers toward some of the more reliable and cost-effective 12AU7 options like certain versions of Sylvanias, RCAs, etc. Some prefer to go all-out and put Mullards and Telefunkens in every stage. But, in many 12AU7 stages, the difference between a Telefunken and a good Sylvania are very subtle. I greatly appreciate the quick feedback. My tech said he used to use the type of EF86 I just bought in U67s when there wasn't a Telefunken EF86 on hand, and that next to the Teles, they are his second favorite EF86. He went on to state that putting a tube in a mic was the ultimate "noise test", so I am really hoping that I get as lucky as he seemed to with Amperex branded tubes, that two out of three are quiet enough. They tested extremely strongly, and all have matching values. After reading your response, if the EF86 solves the noise problem, which I have a feeling I will, I'll most likely stick with the RCAs in the output stage that I have now. Thanks! If the I lose those odds and those three Mullards are not quiet enough for this application, leaving me with an unneeded EF86 cache, what would you recommend? I spent $65 after shipping total for the three so luckily it won't be a catastrophe, just an inconvenience. |
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Post by EmRR on Feb 2, 2017 7:27:20 GMT -6
Give the new tubes burn in time to quieten down, day or two.
12AU7 in output stage should contribute no appreciable noise. Earliest stages are always the largest contributor because of cumulative gain.
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Post by svart on Feb 2, 2017 8:46:59 GMT -6
I use JJ in my U47 and it's hella quiet. Quieter than an RCA NOS that I had, and quieter than a few others. Tone isn't appreciably different either. When I let Shannon mess with it a while back he mentioned it was a good sounding clone too, so I feel that I accomplished something!
I bought a box full of tested NOS baldwin brand tubes as well as a couple Gold Lions. I do like the gold lions better in the preamp of my orange rocker to reduce the gain slightly and give it a little more richness, although I could use just about any non-noisy/microphonic with some changes in gain structure.
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Post by topshelfmg on Feb 3, 2017 1:20:44 GMT -6
Thank you everyone for the info! I believe there are JJs in the EF86 position now, so I'll let you know if these Mullards fix the problem. It is channel two, and it passes signal but there is a static-y sound that goes up and down with the gain. Since the unit was originally built in the late 50s I would love to see how these NOS tubes affected the sound, even if it is very subtle. Also, I trust Shannon's word and ears more than anyone else in the world when it comes to microphones just about, along with Dave Thomas at Advanced Audio (who recently joined the forum), so that definitely is a compliment with some meaning to it!
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Post by EmRR on Feb 3, 2017 8:26:04 GMT -6
Do you mean the noise moves with respect to gain setting, or it's noisy when gain is in motion? I'm assuming the first of those.
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Post by Ward on Feb 3, 2017 8:56:38 GMT -6
12 aU7(s) are pretty solid for the most part and tend to last a lot longer than X's or Ys. Tonally, I haven't found much variation when it comes to NOS units
I have used old Gold Lion, RCA and GE 12aU7s and not found a lot of variation!
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