Reverb Tank for marshall JCM800 ?

[walkman]

The reverb on my Marshall JCM 800 sounds quite weak.

It is a 4210 and the tank is an accutronics and is listed on the schematic as 4FB2A1C. I've taken the tank out and cannot make out the stamp on it however have measured the DC resistance. The input is 32 ohms and the output is 200 ohms. From the specs online something is wrong as both the input and output DC resistance should be 200 ohms.

I have another reverb tank that I got from an old organ (solid state from memory). From what I've been able to identify it is equivalent to an accutronics 4FB2B2D.

From Need Reverb Tank For Marshall 2205 - AMPAGE Archive Enzo gives a very helpful description of what the code for the reverb tank means. However while he points out that an A type tank and a C type tank can be interchangeable what about a B type tank. Also does the 2 between the B and the D indicate there is a lock leaver? What does that mean? And the D for the mounting position?. I've measured the input and output DC resistance and it is 170-180 for each. I did notice that the input of the tank is grounded to the chassis of the tank and the output of this tank is not grounded to the chassis of the tank, whereas the tank that's in the amp now does have its output (and input) grounded to the tank.

I have tested this reverb tank in a peavey studio pro 112 and it sounded great much better than the stock tank in that amp.If I understand the specs correctly it should have the right impedance / same impedance as the 4FB2A1C.The Marshall reverb is tube / transformer driven and the peavey is solid state with no transformer.

Would this tank work for the Marshall? I think I could hook it up and see and would not damage the amp but would appreciate some understanding before doing so.

[g1]

You should be able to hook it up and see if it works. The impedances seem to be FB type, so better than the BB you have.
Does it have a locking lever? Only you can tell us that! The 2 between the B and D does designate that. It is a method of locking the springs in place during transport. Perhaps you removed it when you took the tank from the organ?
The output jack not being grounded may or may not be a problem. It could affect hum level in the reverb, or it could cause squealing depending on the circuit. If necessary you should be able to ground the output jack to the case somehow.
The last D designates it is meant for vertical mount with the jacks pointing down. This should not be a big deal.
Try it out and see how it sounds. If you get any bad hum or squeal, disconnect it and ground the output jack.

[walkman]

Tried it and though the reverb sounds better it is much weaker so that I have to crank the reverb right up to hear any reverb. Never mind I will get the correct tank for the amp

[nevetslab]

Looking at the code listing for the Accutronics 4FB2A1C, it calls it out as:

4: Reverb Type 4 Long Pan, 2 spring
F: Input Impedance 1.475k ohms
B: Output Impedance 2.250k ohms
2: Decay Time Medium
A: Connectors Input Grounded, Output Grounded
1: Locking Devices No Lock
C: Mounting Plane Vertical, Connectors Up (meaning mounted to inside of front grille, connectors face up, NOT on ceiling, open side up

I just got thru servicing one of our JCM 800's, a 2205 Preamp/50W, and it doesn't have the original Accutronics tank in it. Someone has installed a Model P-RMOD 4FB3A1B in it. It's a Long Delay, and it's mounting plane is Horizontal, Open side Down. However, in this instance, the tank has been mounted on the ceiling of the cabinet.

Some of the tanks (may be all of them?) are very sensitive to their mounting configuration. Taking a tank intended to be mounted vertical, connectors down (onto the inside of the grille panel) and instead mounting it on the ceiling, it may not work, or work very poorly. On this tank that's installed on our JCM 800 2205, and the response of the tank is VERY weak. In responding to this thread, I'm going to pull the tank out and see if it responds better open side down, or if it can work mounted vertically, connectors down on the back side of the grille (not in accordance to the specified mounting configuration).

I never have gotten to the bottom of why there is this issue with the tanks NOT responding correctly if mounted other than that specified (and assuming the suspension springs aren't compromised by the alternate orientation). Anyone out there know why?

Looking at this MOD tank in our Marshall JCM 800, the DCR on the input is 200 ohm, and the DCR on the output is 225 ohm.

In your case, as there is a coupling transformer to drive the tank, though driven thru an ECC83/12AX7, it needs a higher input impedance, then one finds on Fender Twin Reverb's, who uses both stages of a 12AT7 to drive their coupling transformer, and it's tank (4AB3C1B) has a very low input impedance.of 8 ohms.

You can find the breakdown of the reverb tanks by it's model number here https://www.amplifiedparts.com/tech_...specifications

Added note....after pulling the tank out of our Marshall amp, some dummy left the foam packing in the tank, so the springs were VERY HEAVILY damped! I also noticed that while this tank is intended to mount horizontally, open face down, the 4 springs suspending the tank assembly (in the pan) were positioned so when mounted that way, the weight of the tank properly suspends it. As it was, it's bottoming out into the pan. But, you can re-position those springs to make it suspended for open face UP, which is what I'm trying now. That took care of it, now properly suspended for open face UP.

[52 Bill]

I never have gotten to the bottom of why there is this issue with the tanks NOT responding correctly if mounted other than that specified (and assuming the suspension springs aren't compromised by the alternate orientation). Anyone out there know why?

The only issue becomes one of alignment of the ferrite bead/magnets in the transducer/transformer gap when the springs are loaded by gravity.

I've never really noticed that much of a difference, but then again I have never really tried different tanks in different situations.

[J M Fahey]

Yes, there is some influence of position, similar to having guitar strings misaligned with polepieces/screws/alnico cylinders .

It will still work, but ..... not best results.

As of the reverb lock, it's a quite useful piece of hardware: a piece of steel wire shaped in such a way that you can turn one end which is long enough to protrude through the back panel and acts as a lever, and the reverb tray gets pressed against a piece of foam, locking the springs for safe travel in the back of a pickup over bad roads, etc.

Many factories stopped using them because it limits reverb tank mounting to positions which allow said lever to be accessible to user, pity because it really did what it promised.

Of course most of today's bands do not go on the road weekly and the few who do use foam padded Anvils, but in the old times, for locally known bands who played all nearby towns, it solved a real issue.

Guess it can still be ordered OEM .