Quick Mark III tube question (a tube has become microphonic)

[Scottyyy]

Hey guys. I noticed my amp started to make random distortion noises even when I wasn't touching my guitar, so I did tested the power tubes by tapping them lightly with something. The outer left power tube distorted whenever I tapped it, the rest didn't make a sound. So this tube is microphonic and needs to be replaced right?

The outer pair of power tubes are 6L6 GC STR 440, and the inner pair are 5881-6L6 STR 425. My question is - can I use any 6L6 power tubes in these sockets? Or should I stick with STR 440s in the outer sockets and STR 425 in the inner sockets?

Mesa tubes are very expensive so I was just going to replace the outer tubes with something like this: http://cgi.ebay.co.uk/Electro-Harmonix-Platinum-Matched-Pair-6L6-Valves_W0QQitemZ350232003757QQcmdZViewItemQQptZUK_Consumer_VintageAudio_RL?hash=item518b7444ad&_trksid=p3286.c0.m14

Would that be a good idea or should I stick with Mesa tubes? They're more than twice the price of those tubes I just linked to.

Sorry if this has been discussed before, I'm still new to tubes.

Thanks

 

[silentrage]

I talked to a tech recently about power tubes and he said it shouldn't matter what kind of 6L6s you put, I have a pair of GBs for my inner, and GCs for the outter, there is a very SLIGHT difference in bias current on the GBs, but it's insignificant.

Make sure you get a matched pair.

 

[Scottyyy]

Just what I wanted to hear. Thanks dude.

Hmm, after doing some research about those Electro-Harmonix tubes I'm going to forget them and get some Mesa tubes.

 

[silentrage]

Keep in mind Mesa tubes are most likely sovteks, they are supposed to be "tested" by mesa but I don't think anyone knows how they go about testing.

It can't hurt to consider all your other options.

 

[JOEY B.]

If your MkIII is a simulclass, the outer sockets are biased hotter and need a RED or YELLOW color code MESA 6L6GC (or equal heat range) in the outer sockets. They are biased hot for 6L6's.

 

[Scottyyy]

Keep in mind Mesa tubes are most likely sovteks, they are supposed to be "tested" by mesa but I don't think anyone knows how they go about testing.

It can't hurt to consider all your other options.

Ok, thanks again dude.

If your MkIII is a simulclass, the outer sockets are biased hotter and need a RED or YELLOW color code MESA 6L6GC (or equal heat range) in the outer sockets. They are biased hot for 6L6's.

My Mark III has a 60w/100w power switch, so it's non-simulclass. What does this do exactly? I vaguely remember reading switching to 60w disables 2 of the power tubes? If so, is it the outer or inner ones that are disabled?

 

[Scottyyy]

Ok, problem mostly solved. I moved the outer pair to the inner sockets and vice versa and switched it to 60w. The distorted sound is almost gone, but I still get quick distorted noises every so often, like every 10 minutes or so. Even though the microphonic tube is disabled (I assume it is as when I switch to 60w the inner tubes go a lot dimmer), could the distortion still get through? Could I actually take the two inner tubes out when I'm running in 60w?

 

[JOEY B.]

Could I actually take the two inner tubes out when I'm running in 60w?

Yes, you can for testing purposes. One of the manuals suggests doing this if you have 1 bad tube. Just to get by for the evening. 8)

 

[lshred365]

@ a plate of 478 VDC 6L6GC 43.9 mA, 6L6GB,G,GA,or WGA=27mA, 6L6WGB/5881=33.7. So beware what tech you talk to, nun of them have a BA or Master degree in electronics. There are a couple of online Bias calculators, here's one. http://www.dreamtone.org/Calculate_Bias_Form.htm

 

[Scottyyy]

@ a plate of 478 VDC 6L6GC 43.9 mA, 6L6GB,G,GA,or WGA=27mA, 6L6WGB/5881=33.7. So beware what tech you talk to, nun of them have a BA or Master degree in electronics. There are a couple of online Bias calculators, here's one. http://www.dreamtone.org/Calculate_Bias_Form.htm

Sorry mate I'm not following.

Anyway I got a pair of JJ tubes to try out, and it has fixed the distortion sounds completely. I guess it was a bad Mesa tube after all. The amp also sounds quite a lot better to my ears now with these new tubes. Or maybe I'm just getting better at dialing it in.

 

[edgecrusher]

I have another question on a related note. I replaced all my preamp tubes with brand new JJ's and the amp is a good bit noisier than it was before on the lead channel (I had JJ's in there before, I figured it was time for a new set though). I'm thinking one of the tubes might be microphonic from the factory. If I can only notice the noise on the lead channel is it most likely v1, v2, or v3 is the culprit? There no point in replacing v4 or v5 right? Which tubes affect the lead channel?

Also if I don't use reverb, can I just take out the v4 tube completely or will the amp not work? Thanks

 

[MrMarkIII]

This is why you change tubes one-at-a-time. (As put forth in the manual).
Why pull the reverb tube? To save on your electric bill?
Which tube does what is described in the manual.
Pre-amp tunes generally don't "wear out" like power tubes. Many people spend lotsa dough on 40-50 year-old pre-amp tubes because they sound better than new production. But it is a good idea to have spares.