Twin Reverb Suddenly low Volume

[plekto]

Hey,

I need to fix a friend's Twin which is a blackfaced Silverface. It suddenly
went from normal way loud thing to 5 won't cut it in a jazz club
setting. All tubes glowing and both speakers plugged in properly.

Has this happened to anyone else? Kind of half cocked to ask before I've
properly gone through it but trying to get things squared away quickly
for a gig this weekend and work around the In-Laws living in my shop
for the holidays... I won't even get to start looking at it in detail until
tomorrow. (Today by now actually...)

Merry Christmas to all by the way. (I got a nice delay pedal - ho ho ho).

Plekto

 

[Telenut62]

How old are the power tubes?

 

[T Prior]

I guess some of us may want to ask..

why are you fixing it ? Why did he bring it to you ?

A typical routine process for this type of failure would be

#1 pull the chassis, check the rectifier circuit as well as all HI DC voltages TR's historically run in the +450 VDC range


Once it is confirmed that the HI DC as appropriate then you move on to next steps which can be many things, some small and inexpensive some not. If the HI DC is not correct you will need to learn why.

If you do not have a DCV meter and are experienced in looking at +450 VDC I would suggest you only swap a few tubes , see if that resolves the issue. If not, bring the amp to a tech who can pull the chassis and dig in.


These 4x6L6 Amps can run on two output tubes as well with more than enough volume to cut a gig. So if it is a power tube ( or front end tube ) you can figure that out quickly as well.


The point here is someone can get hurt in a hurry if they are not familiar with electronic circuit repairs.

I do very little repair work these days but if someone brings a Fender chassis to me, the very first thing I do, regardless of what the issue is , is pull the chassis, put it on the bench fixture and go thru the Power Supply and record all voltages for the user. Then the real work follows.

be careful

t

 

[Coop47]

Similar thing happened to me after bringing it home from a gig. Brought it to my friend/tech who found a broken solder joint on one of the big blue caps located under a screwed on cover on the chassis. (And my inability to give a better description is a good indication as to why I didn't mess around with it myself.) 2 minute fix - hope yours is as easy.

 

[T Prior]

Similar thing happened to me after bringing it home from a gig. Brought it to my friend/tech who found a broken solder joint on one of the big blue caps located under a screwed on cover on the chassis. (And my inability to give a better description is a good indication as to why I didn't mess around with it myself.) 2 minute fix - hope yours is as easy.

And the problem was, if the description is accurate, the HI DC Power Supply failed. It sounds like an easy fix but required a person with knowledge and experience to make it look that easy. I would expect that the tech, if he was indeed savvy, also measured all power supply voltages to be certain they were proper. It was a 2 min fix once the chassis was out, the tools were in place and the right eyes and mind were in front of it. The fact that it was a BLUE cap has history, someone else did a repair job earlier and perhaps didn't do a great job soldering...

Good fix !

t

 

[BobbyZ]

You can check the power tube on a TR real easy.
Pull the inside two play it.
Now put them back and pull the outside two and plat it.
Should be pretty loud and sound the same both ways.
If one pair works way better put that pair in the empty sockets and try again.
Sounds the same then the socket wiring is good and one 6L6s is bad.
If one pair of sockets sound weak then it's a problem inside.
Rember it has to be the inside two sockets or the outside two sockets.



But before you do that.
Try the normal channel and see what that does. If both channels have low volume then the only preamp tube it could be is the phase inverter.(right next to the power tubes)

If just the trem/verb channel has low volume then try swapping V-2 and V-4 one at a time.

A Twin Reverb well any old Fender Reverb with six preamp tubes will run with a bunch of tubes missing. Good thing to know when one goes bad.
A link with a preamp tube map. http://www.kcanostubes.com/content/fender-amp-preamp-tube-layout-and-recommendations


After the tube swapping if it still don't work then it needs to come apart.

 

[plekto]

Hey,
Thanks very very much for the info - I miss sam's fotofax..

He's bringing it to me because I'm playing Organ on the gig and he's a
great guitar guy and if I can get this thing fixed by Sat we can use
that instead of his Polytone which is (ahem...) not as much to my taste...

I'm ok with 'lectical stuff and building tube amps but have less experience
fixing versus initial debug and no real
experience with TR (I'm a Vibrolux Reverb guy really...)
so high rail voltage great to know, will hit that after the "has it got gas"
phase (tube swap etc). I have scope, signal gen, meters, chop sticks etc.

Thanks again.

p

I guess some of us may want to ask..

why are you fixing it ? Why did he bring it to you ?

A typical routine process for this type of failure would be

#1 pull the chassis, check the rectifier circuit as well as all HI DC voltages TR's historically run in the +450 VDC range


Once it is confirmed that the HI DC as appropriate then you move on to next steps which can be many things, some small and inexpensive some not. If the HI DC is not correct you will need to learn why.

If you do not have a DCV meter and are experienced in looking at +450 VDC I would suggest you only swap a few tubes , see if that resolves the issue. If not, bring the amp to a tech who can pull the chassis and dig in.


These 4x6L6 Amps can run on two output tubes as well with more than enough volume to cut a gig. So if it is a power tube ( or front end tube ) you can figure that out quickly as well.


The point here is someone can get hurt in a hurry if they are not familiar with electronic circuit repairs.

I do very little repair work these days but if someone brings a Fender chassis to me, the very first thing I do, regardless of what the issue is , is pull the chassis, put it on the bench fixture and go thru the Power Supply and record all voltages for the user. Then the real work follows.

be careful

t

 

[plekto]

Hey,
Thanks very much for all this - sorry to lean on everyone for what should
be my crisis but this saves me tons of research time and for that thank
you all very much again - gives one hope for the human condition...
That preamp info is great.
p

You can check the power tube on a TR real easy.
Pull the inside two play it.
Now put them back and pull the outside two and plat it.
Should be pretty loud and sound the same both ways.
If one pair works way better put that pair in the empty sockets and try again.
Sounds the same then the socket wiring is good and one 6L6s is bad.
If one pair of sockets sound weak then it's a problem inside.
Rember it has to be the inside two sockets or the outside two sockets.



But before you do that.
Try the normal channel and see what that does. If both channels have low volume then the only preamp tube it could be is the phase inverter.(right next to the power tubes)

If just the trem/verb channel has low volume then try swapping V-2 and V-4 one at a time.

A Twin Reverb well any old Fender Reverb with six preamp tubes will run with a bunch of tubes missing. Good thing to know when one goes bad.
A link with a preamp tube map. http://www.kcanostubes.com/content/fender-amp-preamp-tube-layout-and-recommendations


After the tube swapping if it still don't work then it needs to come apart.

 

[Coop47]

And the problem was, if the description is accurate, the HI DC Power Supply failed. It sounds like an easy fix but required a person with knowledge and experience to make it look that easy. I would expect that the tech, if he was indeed savvy, also measured all power supply voltages to be certain they were proper. It was a 2 min fix once the chassis was out, the tools were in place and the right eyes and mind were in front of it. The fact that it was a BLUE cap has history, someone else did a repair job earlier and perhaps didn't do a great job soldering...

Good fix !

t

Yep, all good points. And while it was a two minute fix, the diagnosis was considerably longer. And yes, my friend suspected it was a previous repair job that was the problem. Thanks adding some context, t!

 

[andyfromdenver]

Both speakers working? Got a battery?

 

[T Prior]

Yep, all good points. And while it was a two minute fix, the diagnosis was considerably longer. And yes, my friend suspected it was a previous repair job that was the problem. Thanks adding some context, t!

Many times that is the case . Quite frankly the time it takes to make a good solder joint with a good mechanical connection ( pre-solder) is about 30 seconds longer than not doing it at all ! It really is amazing how "things" need repair because of previous service !

How many times have we heard in our lives that it only takes a few seconds longer to do something really well rather than hurrying the job and saving a minute here or there...

Sounds like a good tech on duty with your amp...

 

[plekto]

Hey - update just for the fun of it:
Replaced PI, front end ax7 on the Trem channel (very badly microphonic) and removed
a pair of power tubes that had at least one bad egg as we say. Tried it with
a couple of jj 6l6's I had lying about (in my vintage VR...) and things got much
much nicer but no idea (yet) as to bias or balance of same. Running vol way down
and listening to each speaker in turn one is
horrible (bad break up) even at whisper level and the other is just bad.

Plan at this point is:
run next two gigs (jazz gigs, low vol fine) with the new small bottle tubes and only
two power tubes and after that get a set of power tubes and look into new speakers.
At that point I'll have the thing open for bias check and will check the DC rails and
such. Sadly this may get to the point that he no longer wants to get rid of the thing
but on the up side that will make my wife happier... (she's the drummer in the
band hence was right there for everything).

Thanks again for all your help - turned a panic into In-laws commenting on
the goodness of this tube over that etc.

Any advice on what to replace these stock like but completely dissimilar looking
speakers would be welcome - all the 12" speakers I see have really unfamiliar
names (i.e. not VR stuff...)

Regards and I get to say MC until NYE...
p

 

[T Prior]

Also remember that by pulling two power tubes the B+ rises to some degree on the remaining two tubes so a bias reset is in order. This could also mask a low B+ issue as well. You really need to pull the chassis and check the power supply, both electronically and with your eyes. It is not out of the realm of possibility that you have an electrolytic cap leaking right now or other issue with the B+. Additionally, what is the service history, ? this is an older amp and may very well be in need of maintenance, the amp is talking to you. With a Fender chassis this can be done rather quickly, under an hour to check the PS. Personally at this point, my opinion is that this is not gig worthy even at low volume without checking and confirming PS voltages.

Many techs at this point, just for the shear age of the amp, would replace all PS caps ( 5 ) as well as the bias cap when pulling the chassis thus giving the amp a new reference point. If possible also change the screen resistors on the power tube sockets, clean and re-tension the sockets as well. This would all be considered preventive maintenance. Once the chassis is out ..well..you get the point...


Don't pull the motor just to look at the bearings....

good luck

t

 

[BobbyZ]

I didn't list which tubes it'll get by without.

Normal channel will work with V-1 and V-6 and every thing missing in between.

Verb Channel V-2, V-4 and V-6.

So a two channel Fender reverb has a lot of "spare" tubes built in. Sure you might not have one channel or trem and reverb but if a tube does go bad you can still make alot of noise.

With a TR you can run on a pair of tubes. Yeah it'll raise B+ and you should check bias bias. But IME it's been close enough. If you just need to do a gig and the tubes aren't glowing red on the plates you'll be fine.

TPrior is right about the electrolytic caps. I just change every single one in an old amp.
That's electrolytic caps the rest I only change when they're bad. So far an black and silverfaced Fenders that's been never.

Another spot on these (depending on the year) is that resister that sits sideways on the board. Sometimes as the board warps that one gets loose. Good as place as any to look at first if it ain't tubes. Two seconds with a soldering iron will fix it.

 

[Coop47]

Lots of great info in this thread!

Any advice on what to replace these stock like but completely dissimilar looking
speakers would be welcome - all the 12" speakers I see have really unfamiliar
names (i.e. not VR stuff...)

Regards and I get to say MC until NYE...
p

I didn't like them for my sound, but for warm smooth tones for Jazz gigs, you might like Jensen C12Ks. For a clearer brighter sound and some big time weight savings, the Eminence Lil Texas is a great speaker. I'm currently running my 72 TR with an Eminence Swamp Thang and a Lil Texas. If I run across another Lil Texas for cheap, I think I might switch out the Thang. Hope this helps.