Manley massive passive tubes....

[chrisD]

Hey JJ! The root of your problem is that the Mastering version of the MP is designed for 12AU7s, and you're definitely getting a boost of somewhere around 6 to 8dB by using 5751s instead. There IS a gain trimpot near the back of the unit for fine adjustment at unity, but it won't help you here. You'll need to get some 12AU7s like it came with, and that'll solve your issue.

If you MUST use 5751s (perhaps you're partial for some reason?), there's a mod that involved swapping out a bunch of resistors throughout the unit, but I wouldn't recommend that...best to just get a pair of 12AU7s and call it a day.

[lucey]

12AX7 is a gain of 100
12AT7 is 60
12AU7 is 17

[jordanstoner]

The 5751 has nearly the same topology of the 12AX7 (see below) - both of which are higher gain than a 12AU7. Be careful when selecting tubes for this reason - just because the sockets are the same and these two tube types can be interchanged, their topology may not be the same (due to different plate voltages, etc).

Here's a gain comparison chart for such socket-types from Torres Engineering:

Gain comparison chart:
12AX7 =100
5751 =70
12AT7 =60
12AY7 =55
12AU7 =20

Here's some more from VacuumTubes.com:

5751:
This is the "milspec" version of the 12AX7, but is also slightly different. The 5751 has the same plate resistance as the 12AX7, but the gain factor is only 70 compared to 100 for the 12AX7. The 12AT7 also has a gain factor of 70, but since the plate resistance is different, the 5751 is closer to the 12AX7. The older "real" 5751's have extra thick mica spacers and extra support rods to minimize microphonic response. 5751's also have matched triode units. Late versions produced by Philips/ECG don't have the extra support and don't perform as well as the older 5751's.

12AX7:
The earliest ones were all made in the USA by the usual manufacturers like RCA, Sylvania, GE and Tung Sol. Very early ones have rater large size plates. The "plain" version without the "A" suffix can only be used in parallel filament circuits, not series. This usually isn't a problem as series circuits are only normally found in TV sets. All of the tubes will have odd voltages other than 6 and 12. If all of the tubes in your set start with 6 or 12 (except for the 5 V rectifier) , then it is a parallel circuit

12AU7 / 12AU7A:
The 12AU7 is a medium mu multi-purpose double triode in a 9 pin miniature package. The only difference between the 12AU7 and the 12AU7A is the "A" version can be used in series string filament circuits due to it's controlled warm-up cathode.

If you want to replace your 12AU7's with similar gain tubes, I'd suggest looking at Amperex's 7316 or CBS/Hytron's 7730. On the other hand, if you want to experiment with higher gain (more headroom) tubes, check out the ever popular NOS Amperex "Bugle Boy" 12AX7. But I'd first recommend you ping someone at Manley and get the opinion right from the horse's mouth - they know their tubes, believe me. EveAnna is also a moderator on the Geekslutz forum, so you might want to post your question over there.

Tube retailers:
The Tube Depot (I've ordered from them - they are incredibly nice and knowledgeable, and will match tubes at no extra cost).
Vacuum Tube Valley (never ordered from them, but they have an incredible stock list and a very informative website.

[JohnyJoe]

Thanks bros,
definitivamente.... I've a mistake.
I'll go to buy a good matched pair of 5814a/12AU7 at the tube report or maybe at tuberules if I buy a complete retube kit.
In the other way i've notice the aleatorius sibilances was not out
with the replacement of this pair of tubes....
maybe i have to replace the 6414s also?

Big thanks for the help

jj

[chrisD]

The extra gain of the 5751s would have contributed to your microphonics issues, and makes me think that perhaps your initial issue was with one or more of your 6414s (rather than the 12AU7s).

If you've still got them, put the stock 12AU7s back into the unit and run it with the cover off, if you feel comfortable doing that...be mindful of VERY high voltages!...then using a pencil (or the like), tap each of the tubes individually and see if one of them rings more than the others. One bad 6414 could potentially affect both channels, especially if you were using those 5751s and had up to 10dB more gain than you should...

See if you can isolate which tube is complaining the most. Could very well be a 6414.

[JohnyJoe]

Thanks all,
I'll try to put the originals GE's again and try to find the
bad 6414 one/ones As you say Chris...

It's great find this big help in all of you

sincerely thanks