retube or not?

[Dethska]

How do you know if you need to retube your amp? I have a Sovtek Mig100 that I've owned for about 10 years. I don't play it very much and when I have I never drove it all that hard. But I started thinking about it, and I have never replaced the tubes in it, and I bought it used. Would it be beneficial to replace the tubes? How does this change or improve the tone?
If I did replace the tubes, is it better to buy a matched quartet, or is buying tubes singly all right? What kind of tubes are good for what applications? And is it true that by only replacing half the power tubes, I can get it to break up faster?

I know, so many questions. Thank you for all the help you can give me.

 

[gilwe]

For the amp to function, replacing tubes is not needed unless they are burnt out.

As for the sound - the difference in sound and sound quality is HUGE (!!!) between different tubes and tube manufacturers !
You definately gonna improve your amps tone if you use new-old-stock tubes (tubes that where manufactured in the 60s and 70s and haven't been used at all) from manufacturers as RCA, Generalelectric, Phillps, Brimar, Mullard and many more.

I have tried so many tubes in my Twin Reverb till I sticks with a cetrain tube setup.

Keep inmind you will have to bias the amp when replacing power tubes.

 

[Dethska]

How do you bias the amp?

 

[Major Tom]

To bias the amp, you need to take it to an amp tech, unless you are an electronic tech, in which case you can find a few methods available on the 'net.
With 100 watt amps, you can pull out a pair of power tubes (either the 2 inside, or the 2 outside ones), making the amp 50 watts. This will reduce the volume of the amp some, although not as much as you might expect; the reduction will be about 3 db, so it will break up at little lower volume. Some amp gurus say that your speaker load will need to be doubled, others will say that it doesn't. I have done this with a 100 watt Fender with no problems.
Gilwe is right about the New Old Stock tubes being the best available, but they are PRICEY! To retube your MIG with NOS will cost more than you paid for the amp. The quality of new tubes has improved a lot over the last decade, some reviewers claim the gap has almost disappeared- you can read the reviews on the net. The top rated 6L6 power tubes are Svetlana, JJ/Tesla, Sovtek's new 6L6WXT+, and the Electro Harmonix, all 4 are about $13-15 per tube. The top rated new preamp tubes are the Electro Harmonix 12ax7 EH, and JJ/Tesla 12ax7 both around $10 per tube. All the ones I've mentioned have been compared vary favorably with the "holy grail" NOS tubes.
The sound of the amp will be different if you re-tube, I think the amount of improvement will vary with the condition of yer existing glass, and the tone you desire. I'm assuming that your Sovtek amp has Sovtek tubes, which in general aren't "bad" sounding, but are usually categorized as a little dull sounding. They do however, have a reputation for longevity and low microphonics, I can personally attest to that. I have switched out some of the Sovteks in my Hot Rod Deville, with some old used-but-functional RCA's and others that I have laying around, and I don't find the difference to be "huge", although I did not re-bias the amp, or change all of the preamp and power tubes at the same time.

 

[gilwe]

I find Sovetek (=EH=Fender=GrooveTubes) to be the CRAP of the crap of the tubes, soundwise.

Any NOS I have tried so far (mainly GeneralElectric and "Lewis and Kaufman") sounds dozensXdozens times better in my Twin.

Biasing should be done be a tech, not ANY tech. Get a recommendation for one. Biasing is crucial as it determines how your amp will sound and how long the tubes are gonna live.

 

[Major Tom]

dozensXdozens...hmmm...interesting...according to my calculations then, having an amp with nos tubes, and running that formula backwards, substituting new tubes like Sovteks in the same amp would roughly be the equivalent, sound-wise, to playing an Affinity Squire Strat strung with rusty bailing wire thru a $29.00 boom box using a whoopee cushion as a speaker......cool !

 

[Dethska]

I appreciate all the help so far. It's been great. I think the thing has Sovtek tubes in it. Good thing I asked, I thought they were decent tubes. Are 5881 tubes the same as 6L6's? Because I believe it has 5881 tubes in it right now? Also should the tubes be matched, or does it matter?

I'll probably skip the NOS tubes. I don't think I care that much about the tone. I'm not as professional as I wish I was, ya know.

I figure I could probably figure out how to bias the tubes, if I had a decent website to use for reference (electronics being my main profession.) I work with an old engineer who worked with tubes in the old days, so he could probably help me out too.

If I pull out two tubes, Is there anything I need to do with the open sockets?

Don't worry about me, I'm not going to be digging around in my amp with the power cord on and a big screwdriver. I'm basically just gathering information right now. I may end up going with a pro guitar tech, but right now, I'm curious as what it would take to improve the sound of the amp, and maybe make it easier to deal with (the thing is too freaking loud!!!)

 

[Major Tom]

So you're not going to drop $500-$600 bucks on a set of NOS tubes, eh?
5881's I believe were originally a mititary spec version of the 6L6, they are interchangeable. You don't have to do anything with the open sockets.
Here is a web page with bias info. As I understand, there are different methods of setting and/or calculating bias, and there is a range of settings that will work, you can adjust the tone somewhat by changing the setting.
http://people.cornell.edu/pages/mt24/Amp/mm/biasmeas.html
As far as matching tubes, that's the way most guys go, although there are some techs that will tell you that you might like the sound of unmatched power tubes better - more harmonic distortion. In fact you can download schematics for mods to add 2 bias pots, one for each half of the power tube compliment, so that you can purposely unmatch the bias on each half to your taste.
I have been playing around with a couple of my amps and have done a couple mods to each. Here are a couple easy ones that I have done that cost next to nothing if you DIY. And both only affect the tone if you engage them, the original amp tone is still available.
Add a presence control (if you don't have one) - adds a little distortion, a more "focused" sound as you turn it up.
Add a "fat switch" I copied this from Mesa-Boogie schematics. This adds upper midrange, again, a more focused, bright sound, adding an old Marshall or Vox midrange tonality. I used a SPDT on-off-on switch for a total of 3 settings - original, crispy, and extra crispy.
I added a 100 watt L-pad with an in and out connector so I can disconnect it if not needed. This keeps a constant 8 ohm impedance load, but cuts down the volume right before the speaker so you can get a more full-volume, power tubes-starting-to-sweat sound at lower decibel levels. this is roughly the equivalent of a Power Brake, or similiar expensive attenuator devices. Partsexpress.com has them for like 4 or 5 bucks, they also have pretty good prices on tubes.
If you hunt around on the net, there may well be some mods out there for your MIG that you can download.
A less efficient speaker can result in a dramatic reduction of volume. Speakers differ drastically in tone and sound output, more so in my opinion, than different tubes.
If you decide to buy new tubes, check out the ones I listed in my previous post, I have done quite a bit of research lately - all the ones I listed have gotten excellent reviews. I'm getting ready to order enough for 3 different amps myself.

 

[Dethska]

Major Tom- THANKS! That is really great information. I've already got a presence control, and I don't really need it to be "fatter," the thing is already really chunky with the right EQ settings. But the power brake mod is real interesting. A 100W L-pad? What is this? And will it hurt the amp at all to use it?
I appreciate the recommendations for tubes. I really don't have the money or the patience to experiment with multiple choices for tubes. I figure I'll read reviews, and then give it a best guess. Probably anything will help the tone of the cabinet out. I really think I'll go with two tubes in the power section, and maybe that L-pad thingie. I really want to be able to drive the tubes hard, without losing my hearing or my roommates.

As far as speakers, I have a generic cabinet with four celestions in it. I can't remember which kind, (not greenback). I think they are the "whiteback(?)". I figure they will do the job.

Okay, now I've thought of a new question. Does the impedance of the speakers cabinet affect tone at all. I mean, I have a 4ohm and 8 ohm out on my head, and I could rewire the speaker cabinet for either of these. Would it change the tone at all to change the load? Just curious.

 

[Major Tom]

An L pad is used to reduce volume at the speaker without changing the load on the amp. They are typically used in commercial sound system applications to set the volume level for each speaker. They look like a big potentiometer, the 100 watt deal I'm using is maybe about 2 1/2" in diameter. Unlike a regular pot, it has 2 separate wirewound resistive elements; when its wired up 1 element is in series and 1 is in parallel, so as you turn it down it maintains a constant 8 ohm (with an 8 ohm speaker and 8 ohm L pad) load. You have to buy one that is rated at the correct impedance for your speaker system, both the amps I use it on have an 8 ohm load so I have the 8 ohm L pad. The most common is the 8 ohm, they do come in other impedances.
I would recommend not using it in a 100 watt amp unless a fan were blowing on it - I use it with my 60 watt Hot Rod Deville, and it gets pretty hot after 20 or 30 minutes of playing, but it has not failed after a few weeks. I don't know of any reason why it would hurt the amp, just be ready to shut the amp off if the sound disappears, meaning it burnt out and is no longer providing the proper impedance.
As far as a difference in tone using the 4 vs. 8 ohm out, I have never read anything to that effect. All thats happening with the 4 ohm setting is a few extra windings are engaged in the secondary of the output transformer to compensate for less windings in the voice coil(s). If it did make an impact, I would guess that it would be very subtle or undetectable to most ears.

 

[Dethska]

Sounds cool. If I remove two tubes out of a 100 W, then I have the equivalent of a 50W, right? I'll check 'em out, and see what I can see. Hopefully it won't get too hot. And it seems like it will work. Cheaper than a Power Brake for sure.