I just got me a 1973 super reverb. I'll have to fix the reverb first and then set it up for surf.

When I bought the amp the reverb was operating but minimal (on 10 you got a faint reverb sound as if you were on 2). When I got it home, (carried upside down on the back seat of my car for 2 hrs) the reverb was gone. There is no reverb in the guitar sound, the knob does nothing but if the footswitch is on and the tank shaken, there is the huge splash sound as it should be. It's as if the reverb is there and operating but the guitar's signal doesn't go through it. Any ideas?

The tank it has now is a 4AB381B (I guess the '8' is actually a 'B' or maybe an 'E'?). V3 and V4 tubes have to do with the reverb as well. Has anyone here experimented with the tubes and tank parameters?

https://zakandthekrakens.bandcamp.com/
https://www.dirtyfuse.com

RaistMagus wrote:

There is no reverb in the guitar sound, the knob does nothing but if the footswitch is on and the tank shaken, there is the huge splash sound as it should be.

Obviously the pan's output transducer and the reverb recovery stage are working fine.

Next basic steps would be:
1) checking the pan's send and return cables and the pan's input transducer for DC continuity with your R-meter.
2) swapping the 12AT7 reverb driver tube for a fresh tube (and check the 12AT7 socket pin tension - with the power off and the caps discharged).

If those things don't give you the answer, it could be a blown reverb transformer (or pan input transducer), or some other part of the reverb send circuit (like a leaky coupling cap going to the 12AT7 grid).

The tank it has now is a 4AB381B (I guess the '8' is actually a 'B' or maybe an 'E'?). V3 and V4 tubes have to do with the reverb as well. Has anyone here experimented with the tubes and tank parameters?

I think you mean 4AB3B1B - a standard pan for BF and SF fender amps - parameters are listed in the Accutronics site. http://www.accutronicsreverb.com/

He who dies with the most tubes... wins

Surf Daddies

double post (I hit reply instead of edit)

https://zakandthekrakens.bandcamp.com/
https://www.dirtyfuse.com

Last edited: Apr 23, 2012 05:50:00

The most important thing is #2. With a transformer-coupled reverb driver, your pan needs the correct input impedance to provide the correct reflected load through the reverb transformer to the reverb driver tube. In virtually all fender reverbs, this is 8R input impedance. ("A" type). Now if you had a transistor- (or even direct tube-) driven input transducer, you would go for a higher input impedance.

The next most important thing (from a purely academic point of view) is #3. Ideally you need an output impedance which will minimise losses to the recovery stage's input impedance, otherwise you will lose precious reverb pan signal. Output:input impedance ratios above about 1:5 are considered sufficient for maintaining a good impedance bridge. However, seeing as how the input impedance is 220k on most fender tube recovery stages, and the maximum output impedance available on commercial reverb pans is 10k, then any type A, B or C for #2 will work with a tube stage, with lower output impedance making more 'noise' in the recovery-side signal.

Item #5 is also fairly critical, and you want B or C type connectors with most fender reverbs, because you usually want one (but neither both or none) of the cable connector sleeves to be attached to the pan casing in order to shield the pan from EM interference. (If you have both sleeves grounded to the pan case, then you only want one sleeve grounded on the amp chassis, otherwise you will have a nasty hummy ground loop)

The other stuff is all up for grabs, but if the mounting plane (#7) is suboptimal, you won't get as-good-a-boing from the springs

He who dies with the most tubes... wins

Surf Daddies

Last edited: Apr 23, 2012 11:26:05

Thanks for the awesome info Tubeswell!

I'm back with news.

The extra tank was very similar after all and I tried it. I got the same results. Reverb on/off switch worked, reverb knob worked, I had a splash sound when I shook the tank but no reverb when I played.

I also swapped V3 with a 12AX7 and nothing got fixed.

So, judging from the schematic, the problem could lie in:

a) the 'Reverb send' jack on the amp

b) cold/broken solder joints of the socket of V3

c) bad reverb transformer

How can I test each of the above and find out what's wrong? I got a multimeter and I'm not afraid to use it but I'l appreciate some advice on how to use it, what to measure and where, amp on or off etc.

https://zakandthekrakens.bandcamp.com/
https://www.dirtyfuse.com

Thanks! I'll check what I can from the above and I'll be back with the news.

https://zakandthekrakens.bandcamp.com/
https://www.dirtyfuse.com

RaistMagus wrote:

When going into a 70s SF amp to replace capacitors or other parts that failed (jacks, pots...) is there any special treatment I should give to the solder joints after removing the old parts? Will soldering paste be enough for removing impurities form the old parts to be soldered?

(Anyway, I intend to use a compressed air spray to remove dust or other dirt that's possibly in the amp and some electric contact cleaner on the speaker plugs and similar spots)

Use desoldering braid to take out the old solder first then heat up the part and remove it. Use rosin-core ~60/40 solder to resolder it. Wipe your hot iron tip on a damp/wet rag in between soldering to keep the tip clean. This will keep the joint clean. Dirty joints can create unwanted noise problems.

Make sure you don't end up creating a short between the board and the chassis - otherwise you may blow fuses, etc

He who dies with the most tubes... wins

Surf Daddies

Last edited: May 10, 2012 15:02:38

Old tubes start pulling more current and the voltages (and the gain) drop. So when that happens, yes its a good idea to re-tube.

Re; your voltages with the new tubes, in tube terms, there's really not much difference between 250 and 270, and the difference can be accounted for by changes in wall voltage, different batches/brands/age of tubes, or even differences in tolerance in the value of resistors (esp if they're carbon comps) etc, so its not worth worrying about. The voltages written on those vintage Fender schematics should only be taken as ballpark anyway. I seriously doubt if all the amps made came out of the factory with exactly the same voltages. CC resistor values are known to have manufacturing tolerance of 5% or even 10% (get a bunch of 4k7s or 10ks or 100k etc and measure them and you will see). Having said that, a potential source of voltage difference can be drifting values in the supply resistors, especially if they're old carbon comps that get decrepit with heat over time. But as I noted, if you're getting 250 plate volts, I wouldn't worry about it.

Re: filter caps, the time to replace them on a vintage amp is when the amp starts humming. Its usually a good idea to replace the electrolyic caps if they've been in there for a longish time (like 10-20 years) and haven't been used regularly, because that's the circumstances when the dilectric compound dries up. But if the amps working fine without any extraneous hum issues, they should be okay for now.

Re: your meter, you can buy reliable cheap-as meters new these days, so why not go and get a new one?. I usually have 3 or 4 meters on the go at any one time - its also handy taking simultaneous voltage readings, especially when biasing output tubes.

The plate of the LTP on my copy of the AB568 schematic looks like 340, which is what you'd expect for a 12AT7 with an HT of 450 and a 22k tail resistor and a 47k plate resistor.

However none of these things would be making the reverb not work. Looking at your voltages, V3 looks to be in the ballpark. So have you checked the pan transducers and cables and RT primary and secondary for DC continuity with your R-meter setting?

He who dies with the most tubes... wins

Surf Daddies

Last edited: May 14, 2012 04:59:19

RaistMagus wrote:

Pan transducers and cables
I tried a good tank and good cable and I still can't get the guitar signal reverberated. Here's what happens with the orginal tank when I jolt the amp with Reverb at 5 and only the return-from-pan cable plugged. In the vid I give it two jolts far apart, it sounds as if I'm constantly shaking it...
The send-to-pan rca jack is well soldered and I get continuity readings.

Maybe the table has slightly wobbly legs. But the noise means that the recovery side of everything is working. So somehow a signal isn't getting into the send side. Seng as how you changed the pan and cable, then that leaves the RT and the driver tube.

The voltages on V3 look okay, but the 500pF coupling cap supplying the 12AT7 grid might be leaky. Pull the tube and measure the DC voltage on pin 2 (or 7) of the socket to ground. It should read 0 VDC if the coupling cap is okay. It the cap is leaky there will be DC there when you measure it, and high VDC can throw off the bias of the following stage, even as far as causing loss of volume.

Otherwise its the RT (see below)

Supply Resistors
I'll check if they have waded.

With the voltages you have, they probably aren't worth worrying about.

Hum
Hum is non-existant, even with volume on 10 (guitar volume shut off) there is absolutely no hum. These caps are 40 years old and don't hum!? I see three of them have been replaced.

Then the bad ones have all been taken out for now

Meter
The meter is new and I also tested it on an AAA battery, I get the 1.5V as I should. I must be doing something wrong with the pin#3.

You should get a lowish ~2V cathode-to-voltage on all the V1, V2 and V4 cathodes and on the LFO stage's cathode and about 8-9V on the V3 cathode. The LTP cathode will be highish. The CF stage on the tremolo tube will be about 17-19V or so.

Reverb tranny (I hate this word)
I wasn't able to follow your very specific instructions "The primary winding can be measured from Pin 1 (or 6) to the screen supply filter cap node (with the 12AT7 unplugged). The secondary winding can be measured from the pan send RCA jack tip to the jack's sleeve (with the cable unplugged)". I measure the red-black and blue-green cables resistance (12AT7 pulled) and get readings at about 1-2V (I presume the red-black is one winding and green-blue is the other). All 4 transformer cables have spots where they are soldered to other stuff so I use these spots to measure the coils' resistance. Is that ok? I'll try again this afternoon...

The DC continuity test with your R-meter is just a quick check to show whether the primary and secondary winding of the RT are intact and not shorted to each other or to the chassis.

The primary winding is connected between Pins 1 and 6 (which are wired in parallel) of the reverb driver socket (V3), and the filter cap node from whence the RT primary takes its power supply (point 'C' on the schematic - sorry I should've checked that properly before I tried to explain that earlier). Measure the DC resistance between these two points with your R-meter, with the tube unplugged and your amp off. It should be about 800R to 1k. If its less than that, the primary could be shorted (but you would need to do a AC voltage test on it (with a variac) to know for sure). What's more, if the primary and secondary are shorted to each other, you won't get a good DC resistance reading on the primary.

The RT secondary winding is between the RCA 'to-pan' jack and the ground return. Measure between those points with your R-meter (and the pan cable unplugged) it should be about 1-2R.

To AC-test the RT, you need to de-solder all the primary and secondary leads, and (carefully) hook up a low (~2VAC) source to the secondary leads and (carefully) measure the resulting VAC across the primary leads. With 2VAC on the secondary, you should get *95-111VAC across the primary (depending on the turns ratio of your particular RT - they can be between ~47:1 (for a 18k reflected load with an 8R transducer) to ~55:1 (for a 25k reflected load with an 8R transducer). I wouldn't put any more than say 2VAC onto the secondary, which is why you really need a variac (and two VAC meters) to do this. If the RT windings are shorted, you wont get the expected voltage ratio, and the RT should be discarded.

(* I had to think about the numbers here for a minute -its early in the morning when I wrote this)

Good to know it doesn't need a recap. I'll start reading about various tubes brands characteristics and get me some new ones.

Better off with NOS pre-amp tubes if you ask me. They may cost more (and buying any tube is a bit hit-and-miss) but they generally last longer/sound better.

I had the luck to listen to a 69 SR, same as mine, SF cirquit, Alnicos etc. On 8 you could play ACDC. It was heavenly, as if the amp was designed and built to play like this. No farting out, no mushy bass, no nothing. I want to eventually make my amp sing like this.

Farting out can be a variety of things. But first you need to check for blocking distortion at one or more of the grids.(which is a whole 'nother story). But BTW, how old are the output tubes? And how cold are they biased?

Tubeswell, I cant thank you enough for your assistance. Let me know your address and I'll send you our new CD, it's the least I can do.

The postage would kill you. I'm around the other side of the world.

He who dies with the most tubes... wins

Surf Daddies

Last edited: May 14, 2012 15:11:03

The only other thing I can't recall if you checked or not was for DC continuity through your pans input transducer (with your r-meter). I'd love a copy of your CD my friend. PM-ing you with my addy.

He who dies with the most tubes... wins

Surf Daddies

Last edited: May 17, 2012 01:11:04