Speaker-driven reverb in Ampeg

[Packerswin14]

I've got an Ampeg Super Echo Twin ET-2-B.  Very cool amp, sounds great, but the reverb doesn't work (and apparently that's the best feature).  It's 'speaker-driven' reverb, has no separate transformer.  I guess I don't exactly understand the theory behind this method of obtaining reverb.  I've installed a new tank that I strongly believe is the correct one, the old tank springs were laying in the bottom, broken.  I've checked the tubes on my tester, and swapped them around just in case the tester was somehow wrong.  I've replaced all the electrolytics.  I've gone through and checked all the resistors in that part of the circuit, and replaced the caps in that section.  I pulled apart the footswitch and checked it and all the wiring to and from with the continuity tester, and no dice.  The footswitch works fine for the tremolo.

I've found some vague mention that has me intrigued.  This is a true stereo amp, apparently you can run a stereo signal into it, and also this has some influence on how the reverb sounds.  It's basically two reverbrockets in one box, plus some added features.  Anyone have any ideas?  Here's a link to the schematic:  http://www.kbapps.com/audio/schematics/tubeamps/ampeg/et2.html

[PRR]

Do both speakers make sound?

If you shake the tank, does it sproooooing?

[Packerswin14]

Yes, both speakers are working.  When I shake the tank, there is no amplified sound.  I know what you mean, I've worked on amps before where you get no reverb, but you do get amplified spring noise when you shake it.  I've got nothing with this one.

[sluckey]

Here's a more eye friendly schematic...
http://www.el34world.com/charts/Schematics/files/ampeg/et2b_superechotwn.pdf

No crash when you bang it? The tank is probably an 8Ω input and hi Z output. After verifying the tank I'd sub V6A, check voltages on V6A, then check all components in this pic.

[Packerswin14]

Hey, Sluckey,
I think the tube you mean is V5, not V6-- the hand lettered schematic isn't real clear so it's blurred together to look like a 6, but the circuit matches with v5.  I've tested all the resistors pictured and replaced one that was dead, but still no reverb.  I replaced the Elec. cap pictured, as well as the .005 film cap.  I checked the dimension pot, and while it might not range the full 100k anymore, it still sweeps across a wide range of resistances as I turn the knob.  For voltage, I got pin 1 at 0v (but was able to then receive the local AM country station), 2 reads 180vdc, 3 reads 7vdc, 4 was back to the radio station, 5 is 0v, 6 is 2.2v, 7 and 8 are 3.18vdc (these must be the filaments, since they add up to 6.3v).  That info help any?  Any nuggets of brilliance?

[PRR]

> 5 is 0v, 6 is 2.2v

If looking at Sluckey's plan, pin 5 is a plate and should be like a hundred volts, not zero. Pin 6 is its cathode, and should be lower than plate voltage, not higher.

Follow pin 5 to a B+ supply. Open resistor? Stray wire shorting to ground or pin 6?

Cathode pin 6 voltage should change with switch if switch is really wired like that (breaking cathode DC path).

It is hard to know how grid pin 4 gives radio if plate pin 5 is stuck at zero.

If it gets radio, it "ought" to take tank vibration.

[Packerswin14]

Sluckey's plan is correct, pin 5 is plate (It's a 6SL7 tube-- that's what's used all the way across for preamp and effects).  I remeasured, this time plugged into 120vac instead of my variac, which I had at 90% yesterday.  Must have just mis-measured, despite measuring three times because now I get 292 volts on pin 5, 11 volts on pin 6, which seems more reasonable, but still no reverb.  Guess maybe I could have 'fixed' a solder joint when I unsoldered a resistor to test it, but reverb situation remains unchanged.  Footswitch does not affect pin 6 voltage reading.  I did a continuity check on the footswitch and cables, and the switch does close and open properly, even when measured from the circuit board.  Any other ideas, anyone?

[sluckey]

Footswitch does not affect pin 6 voltage reading.

It should! Check resistance (power off) directly from pin 6 to chassis ground. Should be 2.2k with footswitch closed and open circuit with footswitch open. What do you really get?

[Packerswin14]

Finally got back to work on my amp.  Thanks guys, for all the ideas.  Sluckey led me to a fix for one problem, but still no results.  I verified that I bought the correct replacement tank.  I tried a new tube, even though the tube had passed both my testers.  I double-checked the footswitch where the wires connect to the board, and the footswitch is functioning correctly. Found the bad connection from pin 6 to the footswitch thanks to Sluckey, and replaced the 2.2k resistor just because I had it desoldered.  Still no reverb.  BUT I do get 'SPROING' when I tap the tank, so I guess that's progress. 

Voltages on 2 and 5 are about 178, pin 3 and 6 now down to about 2v, still getting radio on 1 and 4.

Any help would me much appreciated, I'm hoping for an Easter reverb resurrection.

[octal]

OK, so you've got your reverb recovery working properly now if you're hearing the sproing. So, there must be a problem on the drive side.

I'll assume you're plugging in to the correct side of the amp (that would be the OPPOSITE channel from the one you're hearing the "SPROING" out of.) 

So, you've got the 330 ohm resistor, the 25 uf bipolar cap (or two polarized caps of 50uf wired with their negatives connected together) and the 27 ohm resistor going to the tank. You can just jumper each of these parts one by one (assuming you're playing the amp quietly) and see if you get reverb- then you'll have found the faulty part. They mostly function to limit the drive to the tank to prevent damaging or saturating the drive coil.


Good luck!

[octal]

Looking at that schematic again, it looks like it's 2X 25 uf connected back to back, so if installing a single bipolar, a 10uf would be close enough. I wouldn't be surprised if those cap(s) are dried out and limiting your reverb drive to next to nothing.

[jeff]

 
 Do you have an Accutronic 4Fxxxxxx tank? If so, I believe the tank is the problem.

The schematic calls for a 4F tank, but it's not an Accutronic 4Fxxxxx tank. See who made the old tank, it's probally not Accutronic. 4F to one company(I forgot the name) does not equal Accutronics 4Fxxxxx. If you're using an Accutronic tank you want one with 4A as the first numbers. To run off the speaker, the tank should be ~8 ohm. Accutronic's 4Axxxxx is 8 ohm, but their 4Fxxxxx is 1475 ohm input.

 You fixed the return. First check the transducers then, if possible, transplant the springs and try the old tank or try a 4Axxxxx tank.

 Maybe someone here can tell you what the best output impedance should be, but I've use 4ABxxxx & 9ABxxxx tanks sucessfully for speaker driven reverb.

[octal]

You could also measure the DC resistance of your new tank with an ohmmeter and compare it to Jeff's chart. Let us know what you find!

[jeff]

After checking to be sure you have an 8 ohm tank(accutronic 4Axxxxx) check to be sure both the RCA's sleeves are grounded. If your new tank has isolated input or output(for accutronic any letter other than A for the 5th digit) and both RCA's shields are not grounded, touch the two RCA's shield's together (hopefully the RCA's are not insulated and are exposed like the fender type).

Here's why that's important: To avoid ground loops you isolate one jack at the amp or at the tank.

Some tanks ground the input transducer with the input cable's shield and the output with the output's cables sheild and isolate the two jacks in the tank.

Other amps isolate either the send or return jack from ground. Both transducers ground's and both RCA's sheilds are connected to the tank and grounded by the other jack. If your amp is like this, and you use a tank with isolated jacks, one transducer won't be grounded. If this is the case get a 4ABxAxx tank or see if you can ground the ungrounded reverb send or return jack.

DIGIT #1 - REVERB TYPE
1 for Type 1 or 4 for Type 4

DIGIT #2 - INPUT IMPEDANCE
A = 8 Ohm
B = 150 Ohm
C = 200 Ohm
D = 250 Ohm
E = 600 Ohm
F = 1475 Ohm

DIGIT #3 - OUTPUT IMPEDANCE
A = 500 Ohm
B = 2250 Ohm
C = 10000 Ohm

DIGIT #4 - DECAY TIME
1 = Short (1.2 to 2.0 sec)
2 = Medium (1.75 to 3.0 sec)
3 = Long (2.75 to 4.0 sec)

DIGIT #5 - CONNECTORS
A = Input Grounded / Output Grounded
B = Input Grounded / Output Insulated
C = Input Insulated / Output Grounded
D = Input Insulated / Output Insulated
E = No Outer Channel

DIGIT #6 - LOCKING DEVICES
1 = No Lock

DIGIT #7 - MOUNTING PLANE
A = Horizontal Open Side Up
B = Horizontal Open Side Down
C = Vertical Connectors Up
D = Vertical Connectors Down
E = On End Input Up
F = On End Output Up


What's No Outer Channel(5th digit E)?

[terminalgs]

just to rule out simple stuff:

the signal from the 'left' speaker feeds the reverb which goes to the 'right' side amp.   (1)  make sure the volume is turned up on the right side amp,  and (2) make sure signal is being applied to the correct preamp circuits:  the stereo jack may be like the stereo jacks on magnatones, which can fool you (requires a TRS stereo cable so if you use a mono cable in the stereo jack, it shorts the 'ring' to ground and effectively turns off the 2nd channel).  these seem to be unlikely, but just to rule it out....

do you have a signal generator with a probe?  if so, or if you can rig one,   apply a signal to pin 4 on V7 and then on V6A to see if those stages are passing signals... (i guess you don't have an o-scope).

[Packerswin14]

Jeff Wins!  He nailed the problem.  Indeed, I did have the wrong tank, as regards to jack grounding.  I had gotten the tank number I needed from a couple of websites, but they obviously both had the wrong letter code for the grounding configuration.  I needed to ground the input jack of the tank to make it work.  This also verifies that the old tank is bad.  I did reattach the springs in the old tank, and checked the impedance, so there must be something else wrong with it.

Thanks to everyone for your help!  I learned a little more about reverb tanks and circuits.

[Willabe]

Great! Now how does this amp sound? Some guys realy like the reverb in Ampeg amps.


                Brad           

[jeff]

 Glad I could help. My suggestion would be(if its not a pain in the butt) instead of grounding the tank, ground the send jack. Only because I think insulated jacks on tanks are more common and it'll be less trouble if you ever want to try a different tank(8,9). Or if you ever sell the amp and the next guy has to/wants to replace the tank he'll see the numbers on the tank and replace it with the same one and have the same problem. And if he does get a grounded input/output tank, it'll still work.

Here's a thought: If it is already grounded maybe the original problem was you got a bad RCA cord?????? I never even thought of asking that until just now!