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X, Various, MK-121 interface

Read more on the Acoustat X in our home audio section

Neal Vito2004-02-23 22:21

Hi, I need some help...please

1. I have Acoustat X loudspeakers....can I use any MK-121 interface versions...which will match my X speaker connections... Or do I have to mod it???

2. One of my servo charge amplifiers started humming for about two weeks, then one tube fire up and crack and fuse blew...should I re-tube the amps ?

3. Can I use 6ge5 also beside 6hb5

Thanks you, Neal Vito

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Andy Szabo2004-02-23 22:21

There are two areas of concern in mating an MK-121 interface to the Model X: electrical and physical. Electrically, any style MK-121 can be used with the Model X. Obviously, the best sonic performance will be had by using a late model MK-121, or an older style interface that has been upgraded to the Medallion blue label or Medallion ”C” red label. These interfaces have a three terminal output, same as the original servocharge amplifier. You\'ll need to remove the original three-prong banana plug from the speaker. The white and blue wires are secured to the MK-121 with thumbscrews, and the red bias wires use a small (0.090”?) pin-plug. These plugs are available from electronics suppliers, or you could probably fashion one from solid wire.

Physically mounting the MK-121 interface is not as simple. Depending upon case style, the mechanical modifications will range from little or none, to quite extensive.

The Model X was available with either the servocharge amplifier, or the original version of the MK-121 transformer interface. This means that there are interfaces available that will actually fit into the same mounting holes as the servocharge amplifier. However, few of these were made some twenty years ago, so you might have trouble finding a pair. I believe these were called MK-121, MK-121-1, or MK-121-1A. The various ”dash” numbers refer to internal electrical upgrades, but I believe any of them would be a good physical fit.

Perhaps the best choice would be the MK-121-2 or MK-121-2A. These were manufactured in much greater quantity, so the chances of finding a pair are good. Although these would require some mechanical modification to mount to your speaker, it would be fairly simple. You may need to drill some holes, and perhaps use some small mounting brackets, but no major sheet metal work would be required.

Both of the above-mentioned case styles are box-shaped, and are enclosed on five sides. The sixth open side is intended for mounting down against the speaker\'s base.

The MK-121B or MK-121C represents the final case style used for the MK-121 series. Unlike the other case styles, it is a small, ”L” shaped chassis. The sides of the chassis are separate metal pieces, which also form the mounting brackets for the panel to the base. The speaker\'s base forms the bottom of the box, and the back of the speaker forms the front of the box. Unless you can obtain the side panels with the interfaces, this version would require extensive mechanical modification to mount to your speaker.

I would be cautious in merely replacing the tube(s) on your servocharge amplifiers. You may have other damage that caused the tube(s) to fail, or conversely, the failing tube(s) may have caused other damage. It would be best to have a qualified technician look at these.

I am not aware of any acceptable substitutions for the 6HB5 vacuum tube, but there may be some that I am not familiar with. You may see alternate numbers listed in a tube-substitution book, but that does not imply suitability for all applications (the 6HB5 was originally developed as a horizontal output for small b&w televisions). Does any one else have a response to this question?

Mike Savuto2004-02-23 22:21

1. I think Andy\'s response (below) is on the nose here.

2. Most often a tube fails in the Servoamps for two reasons, age or overheating. Both of these causes usually result in the same affect, some other parts in the high voltage circuit fail at the same time. Most often the 5w 10k resistor, parallel to the tubes is the culprit. If the amps are very early and still had 3w resistors in that position, they need to be replaced with 5w parts. You don\'t say whether the buzzing was a mechanical noise or electrical and heard through the speakers. If it was heard through the speaker and something like a crackling, you were hearing the tube going into thermal runaway which is why the tube failed. Often one of the two tubes closest to the output fails because of a problem in the part of the circuit in front of it. Sometimes just replacing both tubes will make the problem go away.
You could try to reinstall fresh tubes, replace the fuse, and plug it back in, but be prepared to unplug it quickly if any of the tubes begin to glow brightly. It is possible that the T1 failed, but that usually gives no warning and no other side effects, it just blows the 5amp fuse since the transformer is usually shorted internally. If it continues to blow fuses, I would suggest sending it to Sounds Like New for repairs. They do great work.


3. The 6GE5 is a direct pinout replacement. It has slightly less current capability, but should work. I have not tried the tube personally, both have a customer who has and was satisfied with it\'s performance. First get the amp working before trying them though. I would still suggest using 6hb5\'s. They are not hard to find (try ebay), are fairly inexpensive and when the amp is working correctly, the are stable and long lived. Both of these tubes were available when Jim Strickland designed the amp and he chose the 6hb5 for a reason. I have very little trouble with this tube and have found that the limiting factors in the servos are not the tube choice at all.

J.k. Risher2012-01-15 19:09

6HB5 is a rugged tube ..used them for years.

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