Fender Dual Showman Reverb: Verb and Vib problems

[ValveBytes]

OK

I finally got my hands on a dual showman reverb with push-pull master volume, sounds great stock so I'm not going to mess with it, best AMP I have heard! Silverface amps shouldn't get such a hard time

Anyway the reverb in it is very quiet and thin, if I tap the springs I get loud crashes seems good on the output side and when I put a scope on the input side of the tank the signal seems fairly strong and will drive an 8ohm speaker a little, but when I connect the scope to the output side of the tank I have to put it down to 5mv to see a blurry signal it is there but pretty weak, then again I don't know what it should look like as I don't have anything to compare it to?

the only other clue is that the 680ohm cathode resistor on V3 12AT7 driving the reverb was burnt out and the voltage reading from the cathode with new 700ohm resistor is 3v instead of 6v as in the schem, could this be the problem? have tried other valves/cables.

also the vibrato seems very weak the intensity hardly effects it, does this mean the bulb in the roach is dying? Speed seems to work fine, all related resistors and caps check out ok as far as I can test them. anything else I should look at?

thanks

[ValveBytes]

the cathode voltage is back up to 6v now I think I had a 12AX in there instead of a 12AT7 so maybe that explains it, I guess I'll have to assume to tank is weak and and transferring the signal to the other end, are there any other possibilities

[52 Bill]

Anyway the reverb in it is very quiet and thin, if I tap the springs I get loud crashes seems good on the output side and when I put a scope on the input side of the tank the signal seems fairly strong and will drive an 8ohm speaker a little, but when I connect the scope to the output side of the tank I have to put it down to 5mv to see a blurry signal it is there but pretty weak, then again I don't know what it should look like as I don't have anything to compare it to?

Did you open up the tank and visually inspect the springs, transducers, etc.? Is it the original tank or could it have been replaced with the wrongtype? The correct tank will measure about 1-2 ohms on the input transducer and about 200 ohms on the output transducer.

also the vibrato seems very weak the intensity hardly effects it, does this mean the bulb in the roach is dying? Speed seems to work fine, all related resistors and caps check out ok as far as I can test them. anything else I should look at?

The photocell in the vibrato "roach" is the weak link here, either cut open the heatshrink and replace it with a new one, or replace the entire opto-isolator unit.

[ValveBytes]

Hi Bill,

I just randomly decided to spray some contact cleaner in to the roach to see what would happen and the intensity improved a little! strange eh!? anyway it is more usable now.

The reverb tank is an accutronics as far as I know it is original made in USA springs are ok I already had to resolder the green wire on the output side to get that to work. One end measures about 145ohms and the other about 1ohm, what is strange is that if I use other tanks then it sounds pretty much the same, I have a couple of other ones that I haven't properly tested on another amp so they may also have the same fault? one is half the size of the accutronic one and the other is smaller two both from old organs.

[52 Bill]

I just randomly decided to spray some contact cleaner in to the roach to see what would happen and the intensity improved a little! strange eh!? anyway it is more usable now.

Good idea, maybe I'll try that the next time I'm working on one. I usually find that the new photocell makes such a dramatic difference in the depth of the effect, I just suggest that as the best fix.

The reverb tank is an accutronics as far as I know it is original made in USA springs are ok I already had to resolder the green wire on the output side to get that to work. One end measures about 145ohms and the other about 1ohm, what is strange is that if I use other tanks then it sounds pretty much the same, I have a couple of other ones that I haven't properly tested on another amp so they may also have the same fault? one is half the size of the accutronic one and the other is smaller two both from old organs.

The tank sounds like it is the correct one, make sure that you've got it plugged in correctly with the drive to the input of the tank and the output to the return. The other typical problem with this circuit is the reverb driver transformer. They will often fail, reducing the amount of drive to the tank.

Hope this helps.

[ValveBytes]

ok nailed the problems!

tried a slightly smaller transformer from an old hammond organ and now the verb is a lot fuller not quite is large as some twin reverbs I have heard but close... I wonder if that little guy will melt out? so now I need a new verb transformer and a roach! I guess I just assumed the transformer would be pretty reliable, maybe a bad 12AT7 tube caused the cathode resistor to burn out and also burnt out the transformer on the way!?

Anyway thanks a lot for your great help.

-jim

[Bruce / Mission Amps]

tried a slightly smaller transformer from an old hammond organ and now the verb is a lot fuller not quite is large as some twin reverbs I have heard but close... I wonder if that little guy will melt out? so now I need a new verb transformer and a roach! I guess I just assumed the transformer would be pretty reliable, maybe a bad 12AT7 tube caused the cathode resistor to burn out and also burnt out the transformer on the way!?

Anyway thanks a lot for your great help.

-jim

The output from the tank is less then 8mav-10mvac on a good operating reverb tank.
With out seeing or testing it I think yours is probably just fine.
It is VERY rare to see a bad reverb transformer.
I wouldn't replace it yet.
However, yes by all means do use the 12AT7 but pull your new 700 ohm resistor out and replace it with a 1500 ohm to 2200ohm resistor and bypass it with a new 10uF to 22uF cap.
I'd be curious to hear if that helps.

The "LDR bug" is notorious for getting weak.
The lube had nothing to do with the mechanics of it but in there, you most likely helped couple light to the cad cell or something like that.
Replace both of the cathode bypass caps on the vibrato 12AX7 circuit.
If that doesn't help replace the bug.
I helped WeberVST design and beta tested their cheap little vibrato bug and they are not over priced so you can swap it out for less then $10.00.

[ValveBytes]

I went back to the original reverb transformer with a 2.2k resistor and a 22uf cap on the 12AT7 and it sounds just as quiet as before, the cathode voltage to ground now measures 7.5v instead of 6.1v on the 12AT.

hmm

[52 Bill]

I've seen the reverb transformers fail in a few Fender amps. I always figured that it was caused by running the amp for extended periods with either with a open cable connection or an open input transducer in the tank, leaving the driver circuit with no load.

[ValveBytes]

I would agree that it looks to be the transformer.. took the old one to pieces just to have a look see.. the wax has pooled at one end but other than that it seems fine, measures 666ohms on the primary and 1ohm on the secondary the two sides don't seem to be shorted together, so should I assume the secondary is shorted somewhere against itself?

would it be possible to unwind the secondary and find short if there is one? it just seems to be ok though, as you do get a signal similar in magnitude on the secondary as you put in on the primary?

still not certain, but it does work with the little transformer from the hammond? maybe that transformers gives a stronger signal on the secondary so it makes it appear that the old one is broke, with the little one you get a decent reverb but it doesn't have the depth of other twins I have used, sounds like it is on 7 when it is one 10...

[52 Bill]

I remember seeing that before. Pinkish colored wax? Try R.G.'s transformer test circuit at the geofx site. Maybe you can determine if there are any shorted turns without dissection.

Some tanks are more efficient than others, have you substituted a known good tank? Are the transducer coils in the tank loose on the laminated core structures? I've never tested the theory, but some say that if you tighten the fit of the coils to the cores, there is a better transfer of energy.

[Bruce / Mission Amps]

I would agree that it looks to be the transformer.. took the old one to pieces just to have a look see.. the wax has pooled at one end but other than that it seems fine, measures 666ohms on the primary and 1ohm on the secondary the two sides don't seem to be shorted together, so should I assume the secondary is shorted somewhere against itself?

I just pulled a few reverb transformers down off the shelf and one off of a 1969 Fender with reverb and they all measured between 1600 and 1775 ohms on the primary.

[ValveBytes]

I put together that little circuit from geofx and got no flashing lights! so the transformer is dead also the goes with what Bruce is saying about the primary being too low..

Managed to fit the hammond one in to the shell of the old fender one sounds pretty good, but that one also measures around 600ohms on the primary and about 1ohm on the sec, it works with the neon bulb test though as do all the other tx's I have.

Reverb = nuked transformer and weak tank
Vibrato = tired old bug optoisolator

well the vib and verb are good enough to be useful effects now so I'll just wait until I have enough money to get new parts, needs new filter caps too, from the date codes on this thing it looks like it was made in 1982 but I thought they stopped making the Dual Showman Reverb in 1981 must have been one of the last made the build quality seems as good as the other silverface amps I have looked at.

I wonder if it uses the ultralinear transformer to get 135W clean? has loads of character in the sound, to my ear anyway not sterile at all.

thanks a lot for all the help, experience is so valuable in these situations.