4 Tube amp Questions for noob

[Egyptiansong]

HI. 1st post!
Bought my first tube amp since I started playing about 3 years ago.
I dont know much about them, so here it goes...

I have an Ampeg VT 120. About 9 months ago I had it checked out. A resistor was replaced, jacks resoldered (dont know why he didnt replace them) and he replaced the power tubes with new quad matching sovtex 6L6GC. He also biased them.
It appears the original 4 12AX7 and 1 12AU7 are still there. I bought this amp from a pawn shop, and I have seen it in there for atleast a year, so I dont know how old those tubes are. Amp is probably from the mid-late 90's.

Was thinking about replacing the power tubes with new matched JJ 6L6GC tubes. Read good things about them.


My questions are:

1 - Should I buy a matched quad set of 12AX7 preamp tubes too? I was looking at JJ's also. Does matched matter in this case?

Also, my power tubes were new 9 months ago. I dont gig, but I do practice about 10 hours a week on just this amp at band practice.
2 - Should they be replaced already?? To me it just seems too soon.
Im thinking they need to be replaced because the sound has changed drastically within the past month. Highs are almost gone. Before I could do some nice reggae on it, but not now. Also, it sounds like power chords are "mushy" and dont sound right. The low end is sounding funky. My guitars are all setup nicely with SD PUPS and are intonated, etc. so the problem is the amp, for sure.
I have little idea on knowing whether a tube is bad. They all glow reddish/orange.

3 - Are sovteks a good brand?

I am fairly decent at reading schematics and my soldering skills are good and I sort of know how to use a DMM.
On the back of the amp it has a control for HUM, and has 2 test points that allow you to hook up a meter and witha small screwdriver allow you to bias it.
I think once I figure out whether or not I need tubes, I will ask for more help on biasing.

4 - I am thinking it would be a good idea to bias since I am using a different brand of tubes?

[Enzo]

1. There are no matched sets of 12AX7s. The idea of matching them is meaningless anyway. The power tubes all work together doing the same thing - imagine a set of matched tires for your car. The 12AX7s each do totally different jobs, and since they are two section tubes, each half of each 12AX7 does completely different jobs. So it would be like matching your brakes and your battery in that car.

Power tubes wear out doing their job, just like tires do. The 12AX7 and other small tubes can last years and years - like batteries or starters. You can replace thiose when they fail. No one would replace a car battery because it had hot 5 years old if it still worked. Same with 12AX7s.

The orange glowing spot in the tube is its heater - also called filament - when you see it glowing it tells you one thing and one thing only - that the heater works. Beyond that, it is no indication the tube works.

If the output sounds crappy, new power tubes is worth a try. if they turn out not to be needed, then you have spares. Never a bad idea to have spares.

SOvtek and JJ are fine brands as are various others.

It is a good idea to adjust bias when installing new tubes. It doesn;t matter if they are the same brand or not, tubes of a given brand are not identical, so ANY new set of tubes should get an adjustment.

And your guy resoldered the jacks instead fo replacing them because the jacks were OK, it was the solder that cracked, not the jack.

[Gtr_tech]

Don't worry about matched preamp tubes. That's just for hi-fi geeks where they simply *must* have perfect balance between the 2 triode sections. You may look and see whats in there....maybe someone here is up on that amp and can recommend what works best in what position.

Sovteks for the most part are just generic tubes anymore. Mathews and Co. put the EH name on the upper scale offerings. I like the JJ 6L6s. If you find a good deal on them, the Sovtek 6L6WXT+ is a nice sounding variant. We don't know how your output section has been running so I doubt anyone can really advise here as far as replacement. You might just need a bias touch up.....pwr tubes change characteristics slightly with age.

[Egyptiansong]

1. There are no matched sets of 12AX7s. The idea of matching them is meaningless anyway. The power tubes all work together doing the same thing - imagine a set of matched tires for your car. The 12AX7s each do totally different jobs, and since they are two section tubes, each half of each 12AX7 does completely different jobs. So it would be like matching your brakes and your battery in that car.

Cool! Thanks, thats all I needed to know.

Power tubes wear out doing their job, just like tires do. The 12AX7 and other small tubes can last years and years - like batteries or starters. You can replace thiose when they fail. No one would replace a car battery because it had hot 5 years old if it still worked. Same with 12AX7s.

Ok, so without knowing the ages of these tubes, how do I know if any of the 4 are bad? I assume they control the tone on the "high gain" channel? My high gain channel is not really high gain at all compared to reviews I have read about this amp doing metallica, etc. Mine does not reach that and sounds like crap with a distortion in front.

The orange glowing spot in the tube is its heater - also called filament - when you see it glowing it tells you one thing and one thing only - that the heater works. Beyond that, it is no indication the tube works.

If the output sounds crappy, new power tubes is worth a try. if they turn out not to be needed, then you have spares. Never a bad idea to have spares.

If its not the power tubes causing this sudden change of tone loss what else could be causing it? Im not trying to sound like a smart ass, Im seriously confused and am trying to learn a thing or two about my tube amp, cause I do love it! I appreciate your replies, too!

And your guy resoldered the jacks instead fo replacing them because the jacks were OK, it was the solder that cracked, not the jack.

He said it was a "temporary fix, and would in time begin to cut the complete volume cut off problem" on my amp that I was having and said that "in the future when the problem reappears, I would need to replace it" Even says so on the receipt... Im not saying he is sheisty (I did ask for the old tubes back, and he said he threw them out, but oh well) but I took the amp in because the volume would just drop. 4 weeks later, I got it back witha 200 dollar bill and it had the same problem.

[Egyptiansong]

Don't worry about matched preamp tubes.

Cool, I wont, thanks!

Sovteks for the most part are just generic tubes anymore. Mathews and Co. put the EH name on the upper scale offerings. I like the JJ 6L6s. If you find a good deal on them, the Sovtek 6L6WXT+ is a nice sounding variant.

I see some on Ebay go for about 65 bucks, quad matched of JJ Tesla 6L6GC. Perfect feeback...that good deal?

We don't know how your output section has been running so I doubt anyone can really advise here as far as replacement.

Do you mean how hard and long Ive been running the amp? Well, about 10 hours a week, atleast 15 minute warmup on standby, air conditioned house.
Usually have it set on the 120watt pentode selection, cause it sounds amazing there. I can lower it to 20,40 or even 60 watts, but the tone gets a little dark on the clean. A little. So, new power tubes after about 9-10 months of this might be required? Just seems very soon.

You might just need a bias touch up.....pwr tubes change characteristics slightly with age.

I have taken the tubes out and put them back in. Could not remember if they were in the same order, but I thought it didnt matter since they were matched...
Thanks for your help!

[Gtr_tech]

Well I meant how the output section is biased. Cooler bias extends tube life but can sound a little thin on alot of amps. HOtter bias tends to sound better (YMMV) but wears the tubes faster. I haven't had one of those cross my bench so far so I can't say how the factory set the bias.

[acorkos]

there's a lot of things that can go wrong with power tubes. i have seen new tubes out of the box go bad in a few days. they don't make them like they used to. get yourself a new set and learn how to adjust the bias. if your problem persists, then at least you've likely ruled out the power tubes as the problem.