1974 Super Reverb - Reverb Stopped Working

[Daradaramike3]

Hi All,


I am a long time lurker, first time poster (regarding an actual issue.  I think I may have asked about one of Tubenit's circuits once...but anyway).  I tried searching for older threads regarding my issue but still have some questions.


I was playing my 74 Super Reverb today and noticed that the reverb had stopped working.  About 5 minutes into playing and fiddling I noticed a bit of smoke and turned the amp off immediately.  The reverb has always been a little weak, but I haven't had the amp for too long so I can't say how far back the problem goes.


After the smoke, the reverb transformer was hot to the touch.  I tried to test the recovery circuit by knocking the amp and listening for the crash - the crash was loud and clear.  My test was less than ideal for sure.


I think that (maybe) narrows the problem to the reverb output - I tested the plate voltage on the 12AT7 and got 437vdc.  Higher than the schematic but not sure if I need to worry about it.  I then tried to test the continuity of the reverb transformer secondary and got a reading of 229 ohms.  That seems high and I'm wondering if the secondary is shot.


I also noticed the tube socket for the 12AT7 was darker and appeared more heat-damaged than the surrounding sockets.  Is this also something to worry about?


Is there anything else I may be missing?  Are there other, more simple tests I should do before I assume its the reverb transformer?  Any advice from people far more knowledgeable than myself would be greatly appreciated.


One other point - the amp was "blackfaced" including the bias supply and PI plate resistors.


Thanks in advance,
-Mike

[sluckey]

What is the voltage on pin 3 of V3? Should be about 8V. What is the measured resistance from V3 pin 3 to chassis? Should be 2200Ω/

[Daradaramike3]

Hi Sluckey,


The resistance came in around 695 to 700 ohms.  I couldn't seem to get a voltage reading on pin 3.


For the sake of learning, what am I trying to determine?


Thanks,
-Mike

[sluckey]

We're trying to determine if V3 is conducting. No voltage on pin 3 says it is not. 700Ω says it should be, unless that 700Ω is really 700KΩ. Replace the tube first since that's easy. Then replace that resistor with a 2200Ω 2W. Also replace the 25µF cap that's parallel to that 2200Ω. That 2200Ω resistor may be what smoked.

[PRR]

> knocking the amp and listening for the crash - the crash was loud and clear.  My test was less than ideal for sure.

Absolutely valid. We want mechanical vibration of springs to come out of the amp. Normally the Drive stage puts the vibrations in. If it may have failed, a knee is a handy diagnostic tool for proofing the Recovery stage (including tank recovery winding).

Be sure the tank In and Out wires are right at the tank and the chassis. The "in" coil will make "some" boiiing when used as the out, enuff to boiiing your knee, but not near enough to boiiing normal signal.

> trying to determine if V3 is conducting.

But--

> socket for the 12AT7 was darker and appeared more heat-damaged

That tube evidently _has_ conducted most of 4 decades. It's the hottest small tube in these amps, Fender didn't buy totally heat-proof sockets, so discoloration is expected.

Of course a tube can work 39 years and fail on its 40th birthday. Which is what Sluckey wants you to check.

> reverb transformer secondary and got a reading of 229 ohms

On nearly any Fender, this should be far below 10 Ohms (load, tank drive coil, is 4-10 Ohms, and we want RT winding to be lower).

Some DMMs on some audio coils read funny. You could repeat this test Power OFF and a jumper across the Primary to swamp the RT's inductance. If the secondary reads ~~1 Ohm, your DMM was just confused.

However 229 Ohms is a very odd value. We expect very low (OK) or nearly infinite (broken).

[sluckey]

I then tried to test the continuity of the reverb transformer secondary and got a reading of 229 ohms.

Somehow when I read that my mind saw primary instead. If the secondary is really 229Ω then that RT is shot. The stuff I mentioned about the cathode voltage and resistance still applies as well.

[eleventeen]

Sometimes that rev trans can get a bit "warm" because it gets some heat from the VERY hot 12AT7 rev driver and it DOES handle some power...unlike a preamp tube. If it's "hot" and you're getting those goofy voltages, I'll second the motion that the transformer is likely shot. Especially if you get burnt tranny smell. For sure, take note of the voltages on the 12AT7 reverb driver. Replace the 12AT7 with another one, yes, because it's easy. As soon as the amp is warmed up (1-2 minutes) if you get the same/almost the same voltages on the swap-in 12AT7, your tranny is probably shot.


That tranny gets practically full B+, near 450 volts, and if the internals are dried out, it could easily start with a partial short then work its way to a more serious one. Very plausible.

[Daradaramike3]

Hi All,


First of all, thank you very much for all of the help.  I was really happy to see so many responses after I came home from work today.


Long story short, it looks like it was a mix of bad tube and an inadequate replacement for the cathode resistor for the 12AT7.  I guess I jumped to conclusions when I saw smoke and completely overlooked the tube.  Once I put in a new one, I had reverb again...and I should have known to try that first and foremost.


However, I also decided to figure out why I was getting such a low resistance to ground from pin 3.  It turns out that the cathode resistor had been replaced with something that was very small - looks like maybe 1/2 watt and it was cooked.  When I measured across it I was getting 695 Ohms, obviously way too low of a resistance according to the schematic.  Also, the cathode bypass cap was completely missing.


Since PRR said the measurement across my reverb transformer secondaries was strange, I figured I had better try to measure it again since I am new to this.  Last night I tried measuring across the output rca jack per a recommendation from Gerald Weber in a guitar player article and that was my mistake.  When I opened the amp up tonight I noticed the lead to ground was actually off of the reverb input jack so I measured again across those wires from inside the amp and got 1 ohm.  Whoops.


Was the tube getting adequate bias?  Could the low value of the resistor have lead to the failure of the tube, or was it the general stress this tube experiences, i.e. the high plate voltages, that killed it?  For reference it was a newer EHX 12AT7, though I'm not sure of the age.


It looks like there are also a number of wires that have been burnt by a soldering iron, some so bad that the pvc insulation is almost completely melted through.  Between the that and the underrated/missing parts, I think my best bet is to order some parts from Doug and clean it up.


Sorry for not testing the tube first before posting, but thank you for giving me a sense of what to look at and enabling me to uncover the issues I mentioned.


-Mike

[PRR]

> the lead to ground was actually off of the reverb input jack

That was gonna be our next suggestion (eyeball ALL wiring).

> Could the low value of the resistor have lead to the failure of the tube

Sure didn't help; but that tube is worked TO its limits, so failure happens.

> clean it up.

Has to be done every 40 years or so.

On TV, 'CCC' Wayne has a 1921 car, all original, under 10K miles on it, which was apparently taken down for decarboning in 1931 and never put back together. Sometimes the years get ahead of the mechanic.

[eleventeen]

Certainly makes sense that if the cathode resistor is too low, it will cause the tube to overcurrent. As PRR said, that tube is worked to its bloody limit in that application as reverb driver. (If you get the amp working and it sounds like you are on your way, if you grab that tube after some period of operation you will definitely notice it to be quite a bit hotter than any other 12A_7 in the amp. Take your skin off in some cases. So normally, it's really being worked. If the tube is "set up" (by the too-low cathode resistor) so that too much current flows through it, it will not be able to swing enough volts through the plate load of the reverb tranny and...surprise...it won't furnish quite enough drive to the reverb can thru the transformer. Makes 100% sense. Good for you that you don't have to replace the transformer, fortunately they are cheap, but they're more expensive than a tube and a heck of a lot more expensive than a resistor!

[Daradaramike3]

Okay, parts have been ordered from Doug!  I will replace the cathode resistor, bypass cap and melted wire and will report back here with the results


>On TV, 'CCC' Wayne has a 1921 car, all original, under 10K miles on it, which was apparently taken down for decarboning in 1931 and never put back together. Sometimes the years get ahead of the mechanic.


That sounds about like every pedal project I have started.  Not sure how many loaded pcbs I have for tubescreamer variants and pedals that are half way through a mod. 


>If the tube is "set up" (by the too-low cathode resistor) so that too much current flows through it, it will not be able to swing enough volts through the plate load of the reverb tranny and...surprise...it won't furnish quite enough drive to the reverb can thru the transformer.

I hadn't considered the trade-off between current and voltage and how it impacts the tubes ability to drive the reverb circuit, but now that you mention it I think it makes perfect sense.  As it stands now, I think the reverb is a bit weak compared to the average Fender.  Once I get everything back in place I'll see how it changes.


Thanks again for the advice!  Hopefully I will have this resolved shortly.

[sluckey]

As it stands now, I think the reverb is a bit weak compared to the average Fender.

When you put that missing cathode cap back in the reverb will get much stronger.

[Daradaramike3]

Hi all,


I am happy to report this issue is resolved!  I received the parts I needed from Doug yesterday after ordering them on Friday night - super fast shipping!


With a new cathode resistor and bypass cap the reverb is nice and strong.  Haven't had a chance to clean up the burnt wires yet, looks like I'll have to wait for the weekend.


I just wanted to say thanks again for the help!  I learned quite a bit in the process and I was able to get my amp running again in almost no time at all.

[Willabe]

Good job! 

[Daradaramike3]

Thanks Willabe!