K6JRF's Page
formerly W6FZC
Groove Tube Test Page
(Updated: Sept 21, 2008)

This page details the tests performed with the GT Electronics 12AX7R-2 tube against the original SovTek 12AX7 tube used in the Behringer MIC 2200 Preamplifier. SpectraPlus plot of three (3) tubes and pictures of the test setup are shown.

Why Test?
I guess the answer is, why not? It's a rainy and cold Saturday, so this is the ideal day to do this since I'm stuck inside! The test vehicle is the Behringer MIC 2200 preamp with its 12AX7WXT tube made by SovTek. The GT tube retails for apx $25 at various places including the Guitar Center. The SovTek retails for $14.50 at PartsExpress. The Sovtek brand are military grade vacuum tubes made in Russia and are considered to be among the finest currently available.

Take it Apart!
You may not want to try this if you are not adept at getting 'things' apart. The MIC2200 is a good example. In order to replace the tube, you must completely remove the pc board from the front part of the housing, since it is mounted on the pc board itself!

First remove all of the screws (3 on top; 3 in back) holding the top cover cover. Next remove the bottom screws (3) holding the front brushed aluminum section. Next cut the cable tie on the ribbon cable running to the back assembly. Remove the HOT GLUE from the connector mounted on back assembly so that you can disconnect it at that point.

Next remove the white strips on the inside of the front section. These are used as spacers to keep the pc board in place. Use a small knife to pry up the 'moulding' and then remove both pieces. Next remove all KNOBs but pulling them straight out. Also push all push-switches IN to their furthest position. Then just slide the whole pc board assembly out. Spreading the 'lips' of the front section so that the pc board drops to the lowest position makes its removal very simple.

GT Electronics 12AX7 mounted in MIC2200 Preamp With the pc board out, the tube is next to remove. First, loosen the two screws the hold the RUBBER holder for the tube. After loosening the bracket, remove the rubber holder. Now the tube can be removed. It is in this position, that I tested the new tubes.

The replacement tube is the GT Electronics (also known as Groove Tube) 12AX7R-2. Inspection of each tube shows very different construction. The SovTek tube has square, open plates whereas the GT tube's plates are larger and flatter. The filaments can be seen to glow in the SovTek and not so in the GT tubes. It appears that the filaments are completely inside of the plate structure and no glow is evident.

Test Setup
Test Setup for GT vs SovTek tubes The test setup employs my Sony MD with a Pink Noise (20hz to 20Khz) track. This is sent to the MIC2200 and its output taken directly to the input of the sound card in my laptop computer (SENS 810).
Each tube was tested after 10 minutes warm-up. All settings were bypassed in the MIC2200 so that it output represents the preamp's response curve. The output level pot was adjusted and then not changed for the remainder of the tests. The 'open' setup allows you to quickly change tubes.

Sweep Results

GT vs SovTek Tube Response

Well, like things in the world, they sometimes disappoint! There doesn't appear to be a major difference between those tubes except that the GT tubes are a touch better for low frequency response thus making them a warmer tube with the GT #1 tube being the best. However those differences could be due to measurement variance. On the other hand, the SovTek has a bit more 'warmth' at 100hz. Or is that muddiness? But overall, they sure look very close to me!

So unless you have a Mullard or some equally old, exotic tube, then I'd would not advise taking the MIC2200 apart for differences as small as what I'm showing here. Instead, take your wife to a movie!

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