SF Twin Reverb Distortion - Filter Caps???

[mikeboone]

I have a Twin (mid 70's) I am looking at for a friend that is distorting really bad when a hard note is hit on open E (6th) string.

Its farting out in a disturbing fashion, it sounds like a tuba at times. It happens on higher volumes (4+) with master volume above 6.

I have checked all the tubes in another twin and the speakers are fine.

I read a thread in here some time back that this could be a symptom of the filter caps being old. This amp has original filter caps.

I am going to replace them anyway, I just don't like shotgun troubleshooting.

Any wisdom would be appreciated.

Mike

[stokes]

Sounds like caps are a good bet.

[mikeboone]

I changed the filter caps and no joy. It is still doing the same thing. I notice it pops if I palm mute the open low E as if the speaker were bottoming out.

The bias is around 30ma per tube.

[Enzo]

Look up "blocking distortion." See if that fits your symptom.

[pdf64]

Or maybe a problem caused by vibration coupling at high volumes. What happens if you isolate the amp chassis from the cab, or plug into a different cab? Peter.

[jrfrond]

Exactly what pdf64 said. You need to isolate the chassis from the speaker sound source. Another cabinet is the best bet, because it isolates the chassis AND really troubleshoots your internal speakers at the same time by process of elimination.

If the problem disappears, what I would do next is to plug an audio oscillator or other sound source into the amp, turn it on and bring it up to listening level, then pull the chassis out but leave it hanging into the back of the cabinet a few inches (it's a bit of a balancing act) so that you can leave the speakers (and sound source) plugged in. Then poke at the wires and tag board with the back of a Sharpie, a chopstick, or any other non-conductive item. I've seen caps and wires break loose in those amps many times, and this is the best way to find them. It may or may not be the problem, but it's a good place to start.

[mikeboone]

Thanks guys... New info is very helpful

I have, by default, eliminated the vibration problem. The amp chassis is on my bench and it is connected to the cabinet via a 10' extension speaker cable that I made specifically to test combo amps with their own speakers.

I also poked around looking for fractured solder joints and microphonics with my trusty chopstick.

I will do the research into blocking distortion and see what comes of it.

Thanks again...
Mike

[Bruce / Mission Amps]

I've had this issue with so many vintage amps over the years that it is utterly amazing!
The one thing that I have found is that is almost NEVER the same part(s), tubes or the same solder joint problem creating the issue... but, it is much more often then not a cold or dry solder joint in the circuit... typically on the eyelet board.

[jrfrond]

And speaking of eyelet boards, there is also the phenomenon of comtaminated eyelet boards causing problems. Usually it's humidity, because the vulcanized fiber composition is hygroscopic (water-absorbing) and will conduct slightly when damp. My guess is that this is due to the addition of carbon black or leftover sulfuric acid residues from the vulcizing process. These compounds are inert until you add water to the mix, and then you have some conductivity. This problem can be vexing because there is no one smoking gun.

When dealing with the boards, I make use of a heat gun first to dry it out, then hit ALL of the solder joints with RMA flux and reflow them. It's easier and quicker to do this first rather than poking, probing and guessing. 99% of the time, you'll cure what ails the amp.

[mikeboone]

You guys are going to make an Amp Tech of me yet!!!

I really appreciate all the input and challeges you guys throw my way.

Mike

[Borg54]

Crank that baby up and pop test it through with a little noise. Sounds like a week resistor, really to much resistance some where . I would re-flow the board joints first, its usually does work wonders.

[mikeboone]

I systematically re-flowed every solder joint on the eyelet board. Yea I just shotgunned it - but no luck.

I checked for DC on the signal side of all the coupling caps and saw nothing more than about 100mV DC.

It's common to both channels. Hmmm?? PI? Reverb ckt?

I'll keep poking around, but in the meantime, if anyone has any suggestions.

[mikeboone]

Oh and by the way... I have discovered that this amp is some funky higher powered (135W) version of Twin that was made circa 1979.

Thought maybe that would help...

[mikeboone]

Recap on Progress

This is a synopsis of where I am on this... Any suggestions?

Amp has all new tubes. I checked them in my Super Reverb, all OK.

Speakers are good. Checked on Super Reverb.

I checked all the voltages against schematic and they are OK. The bias is OK around 41mA per tube.

I looked for DCV on the signal side of all the coupling caps.

I changed the filter caps.

I re-flowed, over time, every solder joint on the eyelet board and the tube sockets.

I tried lower gain preamp tubes (12AU and AY).

Enzo suggested possibly blocking distortion, but I am relatively new to this game and didn't really absorb what I was reading.

Relative to blocking distortion, I tried to lift the feedback loop as suggested in some reading - no change, just reduced clarity.

[Dave Curtis, dB AudioTech]

And speaking of eyelet boards, there is also the phenomenon of comtaminated eyelet boards causing problems. Usually it's humidity, because the vulcanized fiber composition is hygroscopic (water-absorbing) and will conduct slightly when damp. My guess is that this is due to the addition of carbon black or leftover sulfuric acid residues from the vulcizing process. These compounds are inert until you add water to the mix, and then you have some conductivity. This problem can be vexing because there is no one smoking gun.

When dealing with the boards, I make use of a heat gun first to dry it out, then hit ALL of the solder joints with RMA flux and reflow them. It's easier and quicker to do this first rather than poking, probing and guessing. 99% of the time, you'll cure what ails the amp.

That (tweed disease) was much more of a problem in the older Fender amps before they started treating the boards with the parrafin stuff (IDR what it's called). He has a mid '70s amp.