Need to make 3 more nights of gig!

[justbelieve]

Have Fender Twin Pro Amp. Noticed it distorting through clean channel. One of the 6L6 tubes was dark, so I figured there's the problem. Need to wait till I get home from road to fix, though.

Other guitar player suggested removing the bad tube along with the working one next to it, since the working one might be compensating for the bad, hence the distortion. (There are 4 altogether in amp). Didn't help w/problem, so just cut volume of amp down to 2 (from 4 where I had it), most distortion went away so I could finish out the gig with somewhat usable tone.

My question: I didn't put those tubes back in amp, so it's only running on 2 power tubes at "2". Will this be okay for 3 nights until I can get home to replace tube (or at least replace 2 tubes as they're biased in pairs)?

[Chuck H]

You would be much better off putting the working tube back in and pulling a tube from the other side. ie: "two outside tubes" or "two inside tubes". If it's the tube socket that is the problem, and not the tube, you will need to avoid that socket. So on the side that had a failure, put a working tube in the other socket. Then use one working tube in the other side. One working tube per side.

As soon as you can you should get new tubes. A matched quad. If that doesn't fix it get the amp serviced.

Chuck

[tubeswell]

Yeah what Chuck H said

The output tubes work in opposite pairs. Therefore if you are going to pull a couple of them for a little while, you need one of each of the remaining ones feeding one side each of the output tranny primary for it to continue working in push-pull mode. If the amp is fixed bias, you won't need to re-bias.

But, if you are pulling two of the tubes, the load resistance will be affected. Therefore the amp will run slightly better if you change the load resistance supplied by the OT and speaker(s) - it ideally needs to be doubled if you are taking out two of the output tubes. Therefore if you have 4R, 8R and 16R OT taps, then move the speaker impedance up a 'knotch' (i.e. e.g.; either put an 8R speaker load into the 4R tap, or a 16R speaker load into the 8R tap).

[justbelieve]

Thanks for your replies, guys.

I did the gig tonight w/a working outside tube and another one next to it. Wow, sounded awful, especially noticeable w/in-ear monitors.
I will change it over to "two inside tubes" as you suggested, Chuck. 2 more nights to go!

Tubeswell, a little over my head! I admire your expertise though.

[tubeswell]

Sorry 'bout that, what I should've said was if you are going to run it on two output tubes, it might sound better if you double the speaker load. So if you have 8 Ohms of speaker's resistance in total plugged in at the moment, then take that out and plug a 16 Ohm cab in. (or alternatively if you have 4 ohms in total, take that out and plug an 8 ohm cab in)

[Amp Kat]

Hate to say this but sounds like you are only running on one side no matter what and have blown screen resistors on that other side so your only using one side anyway getting crossover distortion. You may be better off just running one tube on the good side and cranking it for what it's worth if it sounds ok.

[Dave Curtis, dB AudioTech]

Hate to say this but sounds like you are only running on one side no matter what and have blown screen resistors on that other side so your only using one side anyway getting crossover distortion. You may be better off just running one tube on the good side and cranking it for what it's worth if it sounds ok.

Reread #4; he was using one side with 2 tubes.

[Gtr_tech]

Yeah what Chuck H said

The output tubes work in opposite pairs. Therefore if you are going to pull a couple of them for a little while, you need one of each of the remaining ones feeding one side each of the output tranny primary for it to continue working in push-pull mode. If the amp is fixed bias, you won't need to re-bias.

Pulling a pair of tubes in a 4 tube output section will upset the bias....plate supply will rise. Prob'ly nothing to really get worried about if you just need to get through a night or 2....unless theres redplating going on.

[justbelieve]

I don't know about redplating, just teeth-grinding!
Fortunately, in an Eagles tribute, there is plenty of acoustic playing to get away from my "toy guitar" sound.

Will definitely take it in when I get home just in case. Christmas present to me.

[Chuck H]

The guy only needs to get through two more nights. It's a standard rough mod to simply pull two tubes even without changing the speaker load. My advice included:

"As soon as you can you should get new tubes. A matched quad. If that doesn't fix it get the amp serviced."

Amp cat, the Twin Pro Amp uses individual screen resistors for each socket. I also said:

"If it's the tube socket that is the problem, and not the tube, you will need to avoid that socket. So on the side that had a failure, put a working tube in the other socket."

No reason in the world that shouldn't work.

I can't be sure, but the schematic does not show an impedance switch. So, assuming the amp is wired for a 4 ohm speaker load, it would be good to do as Tubeswell said and plug into an 8 ohm cabinet until you get the amp fixed and are running all four tubes again. I have to assume that you don't have anything but your amp with you by the way your first post reads. In this case you can rewire the cabinet to use one speaker instead of two. I'm assuming (again) that the cab is two 12" 8 ohm speakers wired in parallel. If your lucky enough to have the little speaker clips on the terminals, just unplug one terminal and tape the dangling end so it cannot contact anything. If your terminals are soldered you will need to unsolder a terminal or clip a terminal. In this case I would not bother. I don't think it will harm the amp to run it into the normal speaker load until it can be properly repaired.

Chuck

[justbelieve]

You all's, or ya'lls info is highly appreciated.

Okay, I've switched to 2 inside power tubes in prep. for tonight's shows. The clips on the 2 speakers I could not budge by hand. Didn't want to get too into their removal thereby making prob. worse, so leaving speaker load as is.

The tubes I will untimately replace seem to be Groove T/Fender (w/Slovtek name) GT6L6 sv.

Should I buy quad at, say, Sam Ash, or is their a better recomendation? And then keeping w/ Groove Tube to match pre-amp tubes, etc.?

What would be best for me presently, is replace quad for Dec., and hoping that fixes problem, replace the rest of the tubes and full service (on it's 4th year of use from brand new w/no service history) in the next month or two.

Wow, I'm starting to sound like I know what I'm talking about!

[Chuck H]

Okay, I've switched to 2 inside power tubes in prep. for tonight's shows. The clips on the 2 speakers I could not budge by hand. Didn't want to get too into their removal thereby making prob. worse, so leaving speaker load as is.

By the time you read this you'll probably be at your show. But did you test the amp after the change? Did it work better?

Should I buy quad at, say, Sam Ash, or is their a better recomendation? And then keeping w/ Groove Tube to match pre-amp tubes, etc.?

There was just a thread here about this. I will never again buy Groove Tubes, or any other brand, from a music store. The problem I've had is that while Groove Tubes sells some good tubes, they also sell "budget" tubes. They all have the same GT logo on them. The music stores buy the cheap GT tubes because they can get them for less than the good ones. By the time they mark them up the cost to the customer implies that they must be of reasonable quality because you'll spend the same amount for the music store GT tubes as you will for good quality elsewhere. But they are not good quality tubes. They are cheap Chinese trash made from recycled metals on ill adjusted equipement by employees that may have been making paper boxes just last week. But with that big shiney GT logo they look very similar to what came in your amp. DON'T BUY THEM. I have never met a clerk or manager in a music store that knew enough about tubes to recommend anything, even though some of them would be offended by that statement because they think they do. Decide what kind of tone your after and do a Google search with quotes using phrases like "6l6 shootout" and "best 6l6", etc. You'll surely find something to buy. You shouldn't really just pop tubes in there. You probably won't have a problem. But you will get the best performance by rebiasing the amp with new power tubes. Your tech will know what to do. If you plan to do it yourself just come back here and someone will help.

As for preamp tubes, same thing. Lots of really mediocre stuff out there for a good price. But if you spend a bit more you'll be rewarded with better tone. The preamp tubes are self biasing so in this case you do just pop them in. I heartily recommend NOS (new-old stock) RCA tubes for an amp like yours. Really big, open sound and the top end rings like a bell no matter how hard you drive them. If you go NOS try to buy from a dealer that rates the microphonics of the tubes. Microphonic tubes will ruin your night really fast. Most sellers will rate them or give you a return option if you ask. If a seller won't do either don't buy from them. They know something you don't about the tubes and they don't want to tell you. Shinanagins. As far as current production tubes I have read good things about the Sovtek STR. I have a couple and they sound just fine. But it's been a long time since I bought currently produced preamp tubes so I have no reference for comparison.

Chuck

[justbelieve]

Didn't seem to sound better, but making it through alright.

Thanks again for all the great info!

[Chuck H]

I don't know which socket had the tube failure, but you shouldn't use that socket in case it was a socket failure. I tried to imply that in an earlier post. With two working tubes in two working sockets your amp should sound about like it's suppose to but not quite as loud. Maybe try the two outside sockets. Nothing to lose.

Good luck

Chuck

[HTH]

I really like Groove Tubes' GE-repro 6L6GC, however it is quite pricey because of the final assembly in the US.

The jj 6L6GC is also very nice and a very robust tube that won't hurt your wallet. These will most likely be your best bet.