Hey guys, trying to troubleshoot my friend's later 70's Super Reverb. This is the model with a Master Volume and higher wattage. The circuit most similar, if not exact, is this one: http://www.studiosoundelectronics.com/schematics/twin_reverb_sf_135_schem.pdf
He said the amp fell over a while ago... Yup.
What I can tell so far, is that the PT is giving all the correct voltages when in standby. When taken off standby, the 2K7 10W resistor right off the standby switch (bottom right corner of schematic) that eventually feeds the Tremelo and Reverb circuit gets really hot and eventually burns out followed by bad news. There is also a 30k 20W power resistor that goes to ground on that same side which is suspect because my friend said he "glued it back together" and the amp was working fine! I did some reading and found these discussions regarding these two resistors:
"Thinking about it, I can't see that the 30k 20W bleeder resistor is doing anything useful. The only benefit I can think of that it's providing is at switch on, before the tubes have warmed enough to conduct, it's reducing the surge voltage on the caps (500V rated) for the preamp nodes; but the standby switch is already mitigating for that. Plus such caps generally have a surge rating to accommodate this. Why not just increase the value of the dropper resistor from 2k7 10W to ~12k or 15k, which should provide a similar node voltage (along with commeasurate increase in power rating)? The benefits are about 6 watts less heat generated, and one less hot resistor waiting to fail. What may the design rationale for including those 30k bleeder resistors have been, which were often in new design Fenders from the late 70s through 90s?"
So on the suggestion of this person and several others, I disconnected the questionable 30k 20W resistor and upped the burnt out 2k7 resistor to several 3k Ohmite 5W resistors in series to get around 12k with plenty of current handling. But still the resistor gets too hot which lead me to this:
"the resistor that burned in this amp was the 2.7K 10W that normally only
dissipates about 2W. If the following 1K 1W isn't also burnt, about the only likely culprit would be a leaky or shorted 20uf 500V cap at that point."
From what I can gather, something on the other end of this 2k7 resistor is pulling a lot of current, which means a short, correct? It could be that 20uF capacitor or something further downstream in the Tremelo or Reverb circuit. Could the reverb transformer that is also connected to one end (Letter "A" box) of the 2K7 be the culprit? How in the heck would I troubleshoot that? Should I first go ahead and recap all the filter caps in the doghouse to clear up any issues there?
Ok, quick thought, while troubleshooting this amp, I've removed all the preamp/power tubes of course. So that being the case, there must be some component to ground that is shorted since it's not faulty preamp tubes in the Tremelo/Reverb sections. Does that help our process of elimination?
I could use any help I can get. Been teaching myself this stuff and it is getting easier but I'm not quite seasoned enough to rule out things very quickley. Thanks for taking a look!
He said the amp fell over a while ago... Yup.
What I can tell so far, is that the PT is giving all the correct voltages when in standby. When taken off standby, the 2K7 10W resistor right off the standby switch (bottom right corner of schematic) that eventually feeds the Tremelo and Reverb circuit gets really hot and eventually burns out followed by bad news. There is also a 30k 20W power resistor that goes to ground on that same side which is suspect because my friend said he "glued it back together" and the amp was working fine! I did some reading and found these discussions regarding these two resistors:
"Thinking about it, I can't see that the 30k 20W bleeder resistor is doing anything useful. The only benefit I can think of that it's providing is at switch on, before the tubes have warmed enough to conduct, it's reducing the surge voltage on the caps (500V rated) for the preamp nodes; but the standby switch is already mitigating for that. Plus such caps generally have a surge rating to accommodate this. Why not just increase the value of the dropper resistor from 2k7 10W to ~12k or 15k, which should provide a similar node voltage (along with commeasurate increase in power rating)? The benefits are about 6 watts less heat generated, and one less hot resistor waiting to fail. What may the design rationale for including those 30k bleeder resistors have been, which were often in new design Fenders from the late 70s through 90s?"
So on the suggestion of this person and several others, I disconnected the questionable 30k 20W resistor and upped the burnt out 2k7 resistor to several 3k Ohmite 5W resistors in series to get around 12k with plenty of current handling. But still the resistor gets too hot which lead me to this:
"the resistor that burned in this amp was the 2.7K 10W that normally only
dissipates about 2W. If the following 1K 1W isn't also burnt, about the only likely culprit would be a leaky or shorted 20uf 500V cap at that point."
From what I can gather, something on the other end of this 2k7 resistor is pulling a lot of current, which means a short, correct? It could be that 20uF capacitor or something further downstream in the Tremelo or Reverb circuit. Could the reverb transformer that is also connected to one end (Letter "A" box) of the 2K7 be the culprit? How in the heck would I troubleshoot that? Should I first go ahead and recap all the filter caps in the doghouse to clear up any issues there?
Ok, quick thought, while troubleshooting this amp, I've removed all the preamp/power tubes of course. So that being the case, there must be some component to ground that is shorted since it's not faulty preamp tubes in the Tremelo/Reverb sections. Does that help our process of elimination?
I could use any help I can get. Been teaching myself this stuff and it is getting easier but I'm not quite seasoned enough to rule out things very quickley. Thanks for taking a look!
Moved to the appropriate forum which is Instruments And Amps for guitar amplifiers. Please see forum sub-headers for guidance on where to post.
Thanks, kevinkr!
That's what I was thinking, Frank Berry. There's only so many causes of this symptom in that specific place, right? I went ahead and ordered all new electrolytics for this amp. We'll see what happens. If an elec is shorted, a multimeter would read it as some low resistance both ways? If it was a functioning capacitor, would it read infinite resistance one way?
That's what I was thinking, Frank Berry. There's only so many causes of this symptom in that specific place, right? I went ahead and ordered all new electrolytics for this amp. We'll see what happens. If an elec is shorted, a multimeter would read it as some low resistance both ways? If it was a functioning capacitor, would it read infinite resistance one way?
I know diodes do that for sure but also thought electros were bipolar because they blocked DC current one way. Thanks guys for the super quick replies. I'll post back when I get the caps and replace them!
*Edited my sentence after thinking about it
*Edited my sentence after thinking about it
Last edited:
Like it or not, those power resistors serve a purpose. That 20 watt to ground is the lower half of a voltage divider. It is a brute force method of lowering the B+ voltage for the earlier stages.
So you don't approve of the idea about increasing that resistor from 2k7 to 13k when removing that 30k one? The people who suggested that seemed to feel it didn't effect the amp poorly when they did it. I can certainly replace those resistors to stock values.
Ok, I finally found the cause. I replaced both power resistors to stock values and replaced all the filter caps. No more smoking! The power tubes were shot (as expected after a fall like that). That amp sounds great now. Thank you so much to everyone who contributed to the cause. It feels good to gain some experience like this. My friend is very happy now!
- Status
- Not open for further replies.