Silverface Fender Twin has nasty bass distortion

[Whymeworry]

Hi all,

Recently, I took to replacing my Silverface Fender's old filter caps with some new Sprague Atom filter caps.
After doing so, I noticed that when playing an instrument, if the Bass nob was set past 1, I would hear slight out of tune bass distortion getting worse the higher the Bass nob was set. It will also get progressively worse the higher I set the channel volume, but is not affected by the position of the master volume. It is also much worse in the Vibrato channel than in the Normal Channel.
Seeing as the Master Volume has no effect on diminishing or worsening this distortion, I assume the problem lies somewhere before the power tube section of the amp, but honestly have no idea.

Thank you all, any advice helps,
Nick

[Chuck H]

What are you plugging into it? Please include details. Example: 7 string guitar, EQ @ +10dB@ all frequencies, clean boost set at +6dB, amp input.

Fresh preamp tubes may help. But the BF and SF Twin circuits are really all about loud and clean. The EQ is scooped and bass heavy. Most guys find that early design Fender amps don't overdrive well with the bass above one or two. But this would apply to overdriving the entire amp, power tubes and all. I think it's possible your just over boosting the input and expecting too much. BUT...

Since you did a cap job it's possible that something is miswired or incorrectly installed. A lack of filtering in the preamp might cause what you describe. I know it's a PITA, but re-check all your work to be certain everything is connected to the right places and installed in correct polarity.

[Whymeworry]

I play a 6 string guitar and don't use overdrive or anything, my pickups are also pretty low output. And before the cap job this problem was not present. To clarify, the problem is low frequency out of tune bass distortion only, not natural tube overdrive. I also tryed changing preamp tubes, which didn't help. Everything appears to be wired correctly, If it would help I could post a picture of the doghouse. Thanks for the reply

[Chuck H]

Well, to analyze the evidence presented...

Simple rig, didn't do it before changes were made. = The changes caused the problem. There is a slim possibility of coincidence and something else may have failed at the same time that the the filters were changed. But that doesn't seem likely since the problems symptoms could be indicative of a lack of preamp filtering. The most likely cause of the problem is incorrect wiring, polarity or a ground fault. If there is no mistake in the wiring, polarity or ground connections then the next most likely cause of the problem is that you installed bad parts (rare, but it happens). If the parts are not bad then the problem is something new that occured at the exact moment that the filters were replaced (even more rare, but it happens). And your indication that the amp worked fine before means that it's not misuse. Your indication that there is a master volume in use means it's probably not the power amp. And I can't think of anything else that would cause your problem other than a ground fault or a lack of filtering in the preamp. So... I think there is a wiring error. But you really shouldn't take it that hard. None of us on the forum would be here if we did. I've checked and re checked things five times only to have someone here point out a clue that finally leads me to my wiring error. Good luck.

[Whymeworry]

I know the caps aren't bad, as I changed back to the original caps after the problem happened to see, then to another set of caps, but the problem persisted regardless of the caps. Here are some pictures of the doghouse

I seem to have melted a bit of the plastic covering on two of the wires, could that have caused the problem?

[Chuck H]

I'm as clumsy with an iron as anyone (one reason I haven't posten build pics here ) unless any exposed core is touching something it shouldn't there is no problem. Honestly, pictures under the doghouse don't mean much to me since I don't have a memorized image of what it should look like under there. That board is just a tether point. I can't trace the circuit just looking at that board. But you do seem to have your polarities correct. Of course, there are wires from the doghouse that go under the board and into the amp. The possibility that something has gone awry on the under side of the board should be examined. A solder blob touching where it shouldn't or a lead that poped out of sight when the solder was melted, etc. The board should be lifted. The actual circuit paths should be examined to make sure all the components are connected correctly. It's really not enough to simple work slow and try to keep everything connected where it was. It's emminently better to know circuits if you are to catch errors. I know you probably disconnected and reconnected one cap at a time and are sure that everything is where it out to be by virtue of it being just how it was. But mistakes happen. And, as mentioned, there may be a fault or a short under the board.

[Whymeworry]

So, basically what you're suggesting is I should unscrew the doghouse board and look under it?

[52 Bill]

The photos are not real clear, but the dropping resistors have been changed. The second one appears to be a 100K, probably not the correct value.

[olddawg]

Some of those connections look pretty dodgy (resistive) to me. Especially the 20uf caps. I would pull it completely apart and start over. Use solder wick and flux to clean up all the eyelets.. Use a temperature controlled iron and don't overheat stuff. Take a picture before you start. (Hopefully you took a picture before you started in the first place, it's always good practice.) Make sure nothing could touch or arc to the chassis or the dog house. Make sure there are no solder splashes, etc.

[Whymeworry]

I've at least redone the job 4 times now, so I am honestly a little hesitant to redo it just because it looks a little finicky. I do use a temperature controlled Iron, and I do use flux, but what temperature would you suggest? I usually keep it around 350c.

[Whymeworry]

These are the schematics for the amp: http://www.webphix.com/schematic%20h..._100_schem.pdf

Looks like 100k is correct for that position.

[Chuck H]

The photos are not real clear, but the dropping resistors have been changed. The second one appears to be a 100K, probably not the correct value.

Good catch. And I think your right. That resistor is clearly brown/black/yellow = 100k. And is certainly the wrong value. The other resistor looks to be brown, black, orange =10k. The correct values for EVERY SF Twin I looked at indicate the values should be 2.2k (where the 10k is now) and 10k (where the 100k is now). I think I would start here if "I" were fixing this amp.

[Whymeworry]

For whatever reason my response to 52 Bill kept saying it had to be looked over by moderators first. Maybe this time it will work. These are the schematics for my amp: http://www.webphix.com/schematic%20h..._100_schem.pdf
and it looks like all the resistors are in the right place according to this. I have a 100 watt 4 speaker version of the SF Twin called the Quad, so that may be why there is some confusion on this point. The resistors are causing no problems, I assure you.

[Chuck H]

And that schematic indicates that the resistor values should be 2.2k and 10k.

What are your preamp tube plate voltages?

[olddawg]

For whatever reason my response to 52 Bill kept saying it had to be looked over by moderators first. Maybe this time it will work. These are the schematics for my amp: http://www.webphix.com/schematic%20h..._100_schem.pdf
and it looks like all the resistors are in the right place according to this. I have a 100 watt 4 speaker version of the SF Twin called the Quad, so that may be why there is some confusion on this point. The resistors are causing no problems, I assure you.

I've seen a few of those. They weigh a ton, lol. Have you tried a different speaker cab just to make sure you don't have a rubbing voice coil or something?