Fender Twin Reverb Reissue Tube Help!

[cyclone221]

First off, I know nothing about tube amps and am new to this site. I seems like people really know what they are talking about here.

I just recently bought one because I came to the realization that they own solid state.

So, I bought a Fender Twin Reverb Reissue and short after it started to break up, cut out, and distort at a low volume. I replaced the the four power tubes with a matched quad of Sovtek 6L6-GB's. It still distorts and breaks up but not as bad.

I happened to be at a Gibson factory store and asked a tech there. He said that I should just replace the preamp tubes and it should fix it and that they do this to amps with the same problems all the time.

Today I took out my preamp tubes to see which ones to replace them with and get the model numbers, well there are three different kinds of tubes in there. One Fender/Sovtek 12AX7, three 12AX7s made in China, and two Fender 12AT7.

From what I understand, the 12AT7 isn't the best one to have in there and they should all be 12AX7's. Is this true?

Also, there are two sets of three tube holes*? Is there a difference? Or is that just how they are spaced?

Please help me get my amp sounding correctly again!

[tmarra]

A Fender Twin Reverb uses two different preamp tubes. 2 -12AT7's for the Reverb and Phase Invertor, and the rest are 12AX7's.

There should be a tube chart inside the cabinet to help you put them back in the correct position.

But if you don't have one start with the tube closest to the 1st input jack:

(V1) 12AX7, (V2) 12AX7, (V3) 12AT7, (V4) 12AX7, (V5) 12AX7, (V6) 12AT7

-Tony

[dementedchord]

whew... where do i start?? wish you woulda asked before you replaced those 6l6's you saved them right? crackles generally end up being in pre's and or socket issues...typically what most of us do is tap lightly on the tube till you find the flaky on... or you could do it the right way and just learn to service your own amp in general... and a good place to start is the sockets.... turn off the amp and let the supply dissapate...maybe play a chord as you turn off to drain it... make sure in any case that the supply is down cant stretch this enough there are lethal currents here yeah i said current its not the voltage(pressure) its the current(volume) that kills ya... anyhow.... pull the tubes and lay them out in order so you know where they were.. and take small screw driver (jewlers size) i use a dental pick.. now look inside and you'll se a little clamping thing set in the bottm kinda loose and insulated from the socket... what yya want to do is close those up alittle to make sure the pins are gettin grabbed tight.... when you put the tubes back in its a good idea to spray them with some cleaner (nonlubed)... next... the at7's are being used here as a function tube....alright another day perhaps... in short here we want extremely clean and little or no gain...not to say you cant use ax's and some times personal tastes approach fetish proportions...they call use slutz for a reason....

[cyclone221]

wow, great reply's guys! keep em coming! this is all greek to me, but im picking up some of it and i think im getting the idea! i really appreciate the help!

[michaelepstein]

here you go.

plucked from an archive i made for my own personal ref.
hope this helps.. its pretty much accurate in in layman's terms


[IMG]******//i38.photobucket.com/albums/e148/arenicproduction/AFENDERSUPER.jpg?t=1265665184[/IMG]

the only thing i'd add.. rather is v6 is more technically --- the phase inverter tube. 12at7
-- i like a nice military spec tube for this spot --- it's not directly dealing with gain so much
as it's dealing with actually inverting phase of the signal before output stage... Check out
any tubes labeled as JAN or anything labeled 12at7wa

FYI: 7025 is just another way of saying "12AX7" or low-noise 12AX7 -- (*low noise)
v1 v2 v4 are going to be where you want to put your BEST tubes you can find..
if you never use the normal channel and plug into the vibrato input --- Then v1
really won't matter as much.. and i'd save the spots of v2 v4 for the best tubes..
then again, if you only use the normal channel.. then v1 is certainly your most important
tube in the pre-amp section.. mix and match.. experiment, you can pull v1 out of the socket
all together and push v2 harder for earlier breakup (but obviously it will only work on the vibe input).

v5 is not important.. so don't be foolish and waste a mullard in that spot.. go have fun (hope that helped)

[drtechno]

change the phase splitter tube to another 12at7 , but it needs to be matched triodes.

the 220k grid bias resistors should be the same value. as well the tube bias screener resitor 1.5k and matching all output tubes.

the 1 meg resitors on the phase splitter

check all of the 470 ohm screen supressor restors on the output

[wharfnew]

I had the same problem with my Twin Reverb RI cutting out.It was a preamp tube. I loved the sound of the amp before I took it to the shop even though something clearly needed to be fixed.Now that it is fixed it doesn't sound the same.Amost more Marshally and less Fendery.Maybe a little boxy sounding compared to the 3-D type sound I was getting before.Any ideas?

[MobyTheMinnow]

I would take it to another shop. Its a Fender Twin not a Marshall. My guess is bias is set wrong. Not sure how much it will cost to get fixed but it cant be that much for fiddling with a screwdriver. 50 to 75 bucks a few years ago and that 75 was high.

[fireberd]

I don't know how the newer/reissue Fender's are, but when I was doing amp repair, in the early 70's in Nashville, I found a lot of problems with Fender's were bad solder joint problems.

[The Real MC]

Originally Posted by fireberd

I don't know how the newer/reissue Fender's are, but when I was doing amp repair, in the early 70's in Nashville, I found a lot of problems with Fender's were bad solder joint problems.

That was a product on poor quality control from the CBS era. At that time, final inspection only confirmed that the amp made any sound at all. In fact the new hire assemblers back then were so inexperienced that many amps left the factory with incorrect wiring yet passed final inspection. Many of those amps sounded nothing like what veteran players expected out of a Fender amp. BB King switched to Lab Series amps because he was not happy with the sound of the silverface amps of the 60s/70s which got a bad reputation. Many techs made a living correcting factory wiring issues on silverfaces.

CBS hasn't owned Fender since 1985. And good riddance, they nearly destroyed that brand.

[SAnthony]

I agree with pretty much all of the advise so far. Definately hold onto the old tubes and see if you know someone with a good tester that can check both quality and shorts. If your tubes test good with no shorts, generally the only other thing that can happen to them is microphonic coupling, (ie internallly vibrating components) which would be more of a feedback type of problem not static or distortion. I would first check the pin contacts (after discharging the capacitors) and make sure you have clean snug fitting socket connections. Cold solder joints making bad contact is definately a problem I too have seen on the Fenders, I have seen this more so on chassis grounds or grounds on the base of the tube sockets. After having looked into these possibilities, I would then have someone with an ocilloscope and tube amp experience check and adjust the bias if needed.(kind of like a tune up whenever you do a tube change). Also, you need to determine if this problem occurs on both channels or only on one of the channels.
This is not everyone's favorite idea, and you will likely have a "purist" amp collector want to shun me for saying this, you may be having a problem with leaking electrolytic capacitors. Sometimes this is rather hard to isolate to 1 capacitor, unless you actually see a leak or a buldge in the capacitor itself. If you are having the static in both channels, I would say change all of the Electrolytic "Can" Capacitors in the Power section, (under the metal container). Again, with the amp unplugged, use a screwdriver(holding only the handle) and short the leads of capacitor to ground before touching them. I would suggest obtaining the exact values where possible and you may go for a slightly higher voltage rating on these. Try to replace with exactly the same type of capacitor in an effort not to change the chacteristics of the circuit. If your problem is limited to only one of the channels you may opt to change the smaller Electrolytic caps in the channel circuitry (I think most of these were 25uf 25V). Having said this I will tell you the goal is to make as few changes as possible to maintain the originality of the circuitry, for both tone and collectability reasons. AND Do pay attention to the polarity if you change capacitors, You need to know which lead is positive and which is negative to do this properly.
I hope this is of some help...Good Luck with the Twin!
Shawn