SF Twin Reverb Distortion - Filter Caps???

[Krwkka]

Funky Resistors?

Mike: I don't know if you've fixed your friend's problem yet, but I thought I should share this.

I recently bought a 135 watt TR. I completely disassembled it to check all components (I know, too much time on my hands ). At any rate, I found several components that were funky. Perhaps relevant to your problem would be the big power resistors in that amp which were both way off spec. I don't remember what the values were, but they were way, way off. I'm no tech, but evidently these resistors are vulnerable and you might want to check them. You might want to check the 20 watt, 2.7k resistor on the rail. It's the big one mounted on the terminal strip between the main board and the rectifier board.

[earthmud]

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.

This problem is all over the net. I've had the same issue for over four months with my 72 SFvibroluxand still can't track down the source. Some people say it's filter caps, some say it's a blown speaker or OT. Like Bruce says it's a different fix every time but the issue is the same.

[earthmud]

http://www.thegearpage.net/board/sho...d.php?t=475093

Here's a clip from a guy that doesn't seem to know that his amp shouldn't sound like this. The noise rides under the note. It's hard to hear but if you turn it up loud it's there on all three clips when ever he lets a note or chord ring out. The issue seems to come on whenever gain or clipping is reached whether it's by a pedal or tube breakup.

[Krwkka]

I think that's the sound of his overdrive pedal.

[Lee_ranaldo]

if it's a SF 135 Twin, it's a master volume / ultra linear version.

I would check the Output transformer. I read somewhere that "internal arcs" in the windings of an Output transformer can produce some weird "popping" noises in the speakers.

My theory is..

Low notes amplified have more energy than higher ones, so your problem could be

- Mechanical (Loose componentes) (you already fix that i guess)
- Some weird behaviour in the componentes on the power supply, like rectifier diodes.
- Some resistors out of spec due to component aging.

I have almost a few experiences repairing tube amps, some of the gurus will spot the fault very quickly but i hope you repair your amp soon!

[mikeboone]

Problem Solved

The issue with this amp was an arc'd tube socket. The tube sockets are old black bakelite or phenolic type material and I didn;t immediately see where it had arc'd

Probelm solved

[Wes]

I'm having this same popping problem with a 135UL here....

Just like mikeboone I've done so much to this amp to try to eliminate
the big popping sound and I see that replacing the sockets resolved the
issue on mike's....

I'll post back here whether or not it works on mine....

[Krwkka]

That's great! I'll be waiting for your post.

[Wes]

I replaced all of the sockets with ceramics....no dice, still pops...

Ok, so far I've replaced:

-all plate resistors

-all coupling caps

-bias diode

-rectifier diodes

-grid resistors

-screen resistors

Someone worked on this amp somewhat recently and replaced
the filter/supply caps so those are most likely outta the question.
This tech also put in new tubes. I've swapped them around as
well as the preamp tubes and have gotten no relief.

At this point I think I may take the PI coupling caps down from .1 to
a lower value......that or rebuild the PI! gah!


Aside from that what would you suggest? Is it possible that it's in the OT?

Also, if I juice the master to ten and bring the pre up at intervals it pops
sooner than if I juice the pre to ten and bring the master up at intervals.
It seems definite that it's in/after the PI.

[Wes]

I swapped the .1 PI coupling caps to .022 and that reduced the pop. Still
a problem plus the amp doesn't sound as good and full with the smaller caps.

Off to study blocking distortion......

[Krwkka]

I wouldn't think blocking distortion would be a problem on a stock TR 135.

This might sound crazy, but I once had a TR that "Popped" and it drove me nuts trying to find it. One day I hooked up to a different speaker cabinet just to listen to the speakers and ... no pops?? It turned out that the cable running from my amp output to the speakers had something weird going on at the connector (where it splits out to both speakers). I disassembled, sucked out the old solder and resoldered and, voila! Problem gone!

Try using a different cable and see what happens.

[Wes]

grrrrr this is pissing me off.....


Good idea-

I'll try different speakers......gotta go get a cable....

[Wes]

Alright, I got a cable and plugged the amp into a 4X12 and the popping
was not present at all.

The speakers that are in this amp's cabinet are Pyle Drivers. I've never
seen these used in an amp and it seems that these are pretty crappy.

Welp... saved me a PI rebuild.