Super Twin

[casey]

Ok, im trying to fix a fender twin reverb.
everything works fine except that i get a
very grainey sound coming through it.
almost like a separate note is being played.
it's very noticeable on the low end when you
do solo work and what not.  whats gives?
i dont notice any loose connections, but of
course we're talking point to point here.
i do have a tube checker, but i havent tested the
tubes.  I looked at the filter caps, they look peachey,
but i have a feeling that they are not the culprit because i
dont have any severe hum going on....

[Mike Nichting]

It sounds like the tubes are under biased. Either check the bias of the tubes or have it checked and make sure the tubes are good first.

Good luck
Mike N.

[Mike Burgundy]

Mike's right - this is almost surely a bias problem.
If you're not comfortable with poking about in (LETHAL!)high-voltage circuits, get a qualified tech.
If the amp already has 1 or 10Ohm cathode reisitors installed, use those to measure the idle curretc through the tubes (use a DMM to measure the voltage over the 1 Ohm resistor: 30mV means there's 30mA running through it: Ohm's law!)
If they're not there, consider installing them. 1/2W 1 Ohm should do fine, although 1W is nicely rugged (though you're NEVER going to run 1 A through a tube - ouch)
You can measure the bias voltage - this might indicate the problem also. It should be somewhere around 30V, probably. Don't set bias by this voltage though! The resulting current varies a LOT from tube to tube.

By altering the bias voltage you can control the idle current, thus shifting the operating points of the tubes and making them run "hotter" (meaning more idle current) or "colder". The correct setting depends on the tubes (6L6) and their ratings, plate voltage (in a Twin some 400V if memory serves) and personal preference. Some people like really, really hot running tubes but this will shorten their life as well. I prefer not-too-hot, and often tweak them a bit by ear making sure I never exceed maximum ratings and that the tubes don't show any bad signs such as orange glow on the plates.
Assuming 400V(maybe a little higher) is correct, the ABSOLUTE max to bias the tubes at is approx. 50mA. DON"T DO THIS. It's likely to run the tubes red hot during prolongued playing.
I'd set the lot to 30-35mA and up until it sounds good - check every time you make a small adjustment! I reckon I'd prefer to stay below 45mA to extend tube life.

reading material:
http://www.geofex.com/ampdbug/ampdebug.htm

http://www.aikenamps.com/
(technical section)
hih

[casey]

thanks for the help guys....

i have one more question,

the "hum balance" on the back of the amp, this is where i change the
bias on this circuit right ?

[Ammscray]

thanks for the help guys....

i have one more question,

the "hum balance" on the back of the amp, this is where i change the
bias on this circuit right ?

A couple of things right away: the super twin circuit was a hot-rodded twin (obviously) but it was very underfiltered, and the voltages were higher than in the regular twin, so I would change those caps anyways, being that they are the originals...filter caps DON'T last for 35 years, especially when they're in that higher voltage circuit...and it doesn't matter if they "look" good...change 'em

You don't have to hear hum when caps are bad...ghost notes, or a slight lower octave on the bass strings means filter caps...

I'm not saying that you don't need tubes too, you probably do if those are the originals...if it hasn't been serviced in a while it probably needs a complete tune-up...

  The hum balance doesn't adjust the bias, it's a real crummy circuit that fender SF amps used and I would advise changing over to the normal BF bias pot circuit if you want to adjust it correctly...otherwise you need to change one or sometimes two different resistors on the inside...

If you just pop tubes in an amp with the stupid "hum balance" and adjust it, you'll only be getting about 15-20 Ma's on each tube if you're lucky...way too low! I like 30-35 in twin-type circuits, as was already stated...

FWIW the super twin really isn't one of fender's best amps...pretty unstable and sterile sounding...changing over to some of the earlier BF values, and also disconnesting that horrible push/pull master volume will make it a better amp...

[Peter Snowberg]

Fender used a variety of different configurations with bias.

In most (other) amps, the hum balance pot is usually just a low resistance pot (100-200 ohms typical) that centers the heater voltage around 0 volts. The control allows you to adjust for the non-perfect placement of the center tap of the filament winding.

In later silver Fenders, they used a fixed resistor to set the bias, and then gave you a control to bias balance the OP tubes (thus killing the hum). Ideally, you want two pots, one for bias level and one for balance. The second best alternative is to switch the bias balance control for a bias level control (ala black face) instead. Proper level is much more important than balance, but then you should use tubes that are fairly matched. The Fender hum balance allows for tube mismatches to be compensated for so that BOTH tubes will be at the wrong bias point, but at least you don't need matched tube to get there.  :?

Don't discount the filter caps just yet. If they're original, change 'em. It's only a matter of time before they die anyway. Electrolytics are not very linear caps to begin with and that only gets worse with age.

Another possibility is that your amp has lousy coupling caps. Fender did some really strange component purchases. Well, not really since they were just mass marketed consumer products.

I have a (silver) twin and the coupling caps were a combination of film and ceramic. Most of the caps (including the larger tone caps) were plastic encapsulated blue film caps, but at the end of the preamp there was a big ugly dark reddish-brown ceramic cap. This cap was quite microphonic (read as: nasty overtone pickup). I changed out that ceramic and a couple others including the treble cap in the tone stack. Suddenly the amp sounded a LOT better, but it lost a lot of the classic "grain" from the tone.

The improvement in tone was sure worth it and I've done this upgrade on a few amps.

If you want to keep some of that original grain but still clean up the signal, consider swapping all the ceramic caps with high quality ceramics like NP0 types (a.k.a. C0G). You can only get them so big, so you may have to parallel a couple caps to get your value.

For larger caps I have been very satisfied with SBE (Sprague Orange Drops) 418 series. They have a bit more character than the 715 series. Lots of boutique people use Mallory 150 series. I've come across them and they sounded nice in the circuit, but I've never recapped an amp with them.

Good luck,

-Peter

[casey]

thanks for all of the awesome information.  this amp is a black face
that has had the transformer replaced with an absolutely huge one.
almost all of the wires in the amp are plastic covered except for the huge transformer which has cloth covered wires.  there is also a small transformer that has cloth covered wires.  its in great shape and
has a wierd switchable eq section in it and a distortion circuit as well....
ive worked on bassmans, super reverbs, and others and i must say
that this is a wierd animal.  i am not for sure what year it is.
the filter caps are grey mallory's.   are those stock?
the amp has been serviced before, that much
i can tell...  it's is definitely post c.b.s.
messy wire job too....

sorry for all of the questions, but this stuff is very interesting and it
seems as if yall know what your talking about.

[Ammscray]

thanks for all of the awesome information.  this amp is a black face
that has had the transformer replaced with an absolutely huge one.
almost all of the wires in the amp are plastic covered except for the huge transformer which has cloth covered wires.  there is also a small transformer that has cloth covered wires.  its in great shape and
has a wierd switchable eq section in it and a distortion circuit as well....
ive worked on bassmans, super reverbs, and others and i must say
that this is a wierd animal.  i am not for sure what year it is.
the filter caps are grey mallory's.   are those stock?
the amp has been serviced before, that much
i can tell...  it's is definitely post c.b.s.
messy wire job too....

sorry for all of the questions, but this stuff is very interesting and it
seems as if yall know what your talking about.

No such thing as a blackface super twin...your amp is a later SF circuit but with a "black" front panel...they made a few in the 70's...NO "real" blackface amp ever came with the hum balance circuit, or with plastic insulated wiring...

Man, by what you said, I'd say all the mods and swapped tranny (you didn't mention which one, PT or OT) and such are the culprits, but who knows what they did or where the tranny came from??

Grey mallorys are NOT stock, so they're probably OK, but check your voltages!

Just curious, what is the plate voltage?

I've seen some super twins up over 550V! Ouch! But it's no wonder that Ted Nugent was blowing them up every other day on the road...

[casey]

if your looking at the back of the amp, the big tranny is to the right.  This
thing is absolutely huge.  anyway, ill let you guys know how it's going, and if i get it working right...

thanks for all the good info.

[Ammscray]

if your looking at the back of the amp, the big tranny is to the right.  This
thing is absolutely huge.  anyway, ill let you guys know how it's going, and if i get it working right...

thanks for all the good info.

PT

[Mike Nichting]

I forgot to add, please be very careful in there~!!  If you are not comfortable with any part of fixing it please seek the help of a tech as we would like to keep you around a while longer :-)

  If you don't know what you're doing, and I'm not saying you don't, you could easily be killed or suffer a severe shock~!!!

If it has all of those mods I would look up a tech in your area that is good with Fenders and have him/her do the work for you. If they are a good tech they will work with you to get the tone you want.

 Best of luck bro~!!

Mike N.

[casey]

thanks dude.  here is a pic of one like it...would yall happen to know
what year this one was in production?

http://pawntrader.com/pics/sep9/fender.jpg

[Ammscray]

thanks dude.  here is a pic of one like it...would yall happen to know
what year this one was in production?

http://pawntrader.com/pics/sep9/fender.jpg

77-80

[casey]

cool