Dear Audio Circle Readers,
I purchased 4 Sovtek EL84M’s (a quad) from THETUBEDEPOT (thought I'd save a few bucks) a while ago for my Music Reference RM-10, and recently sent them into Roger to check them out for me because I was a little suspicious about the tubes glowing at different intensities.
THETUBEDEPOT advertise that they sell their tubes in matched pairs. I requested a matched quad set, and when I spoke with the guy on the phone, he said "No problem." I don’t know how THESTORESTORE’s tubes are; I ordered from THETUBEDEPOT because their website provides a phone number (I was able to talk with someone on the phone), and THESTORESTORE does not.
This is how THETUBEDEPOT advertise their Sovtek EL84M tubes:
A military-spec version of the standard Sovtek EL84 - the Sovtek EL84M's extended voltage tolerance - improved plate dissipation - and rugged construction make it equivalent to the RCA 7189. Chosen by Matchless for their designs - the EL84M also features higher transconductance - more power - and longer life than the less expensive EL84 - making it perfect for hi-end audio applications. However - many musician's prefer the regular EL84's warm distortion to the EL84M's tighter - cleaner tone. For all Hi-Fi users - the EL84M is definitely the tube of choice. Price is for a Matched Pair
The following are Roger’s findings.
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Steve,
Yours are EL84M. I applied the same grid voltage to all four tubes. Since they are a quad they should match. If they are two pairs then we should see that also. Here are the actual measurements made across equal resistances. I Number the tubes 1-4 going left to right. For actual current in Ma mulitply by about 2.5. For matching purposes the raw numbers are totally adequate.
Here are the numbers.
4.5 5.4 6.0 8.8
The first two numbers are sitting in the left channel sockets, the second two in the right. At worst case, if the 8.8 and 4.5 ended up in the same channel that would be very hard on the higher current tube. I would have twice the dissipation of the lower current tube which would put it over the 12 watt limit. There would also be 50% unbalanced primary current which would cause output transformer iron saturation and poor bass. When these things are not known, the listener will likely draw incorrect conclusions about how a tube sounds. It is akin to assessing the handling of your car where one tire has half the air pressure of the other.
Given these numbers, 1 and 2 make the best pair. 3 and 4 make the Best other. It's the difference within the pair that matters. Given That pairing, the bias voltage would be different for each pair. We can plainly see these are neither pairs or a quad. The ratio of the lowest to highest is almost 2 to 1. One could likely do better picking 4 tubes at random.
The problem with tubes being so far out of match is that one is carrying 1/3 the load and the other 2/3. The 2/3 one gets too hot and I can see the damage by he brown coating inside the glass. So I could take the two good tubes and possibly add to match them. I like both tubes. The M's have a bit lower distortion and more detail. Since yours were not matched, you haven't really had a fair chance to hear them. The noise selection I do for RM-10 customers is not posted anywhere though it is in the manual. One has to know how the sections are used to take advantage of these details. These are not things other designers even think of from what I see.
Roger
