Mesa Subway Reverb

[tubesornothing]

Hey gang, I am stumped with a non-working Mesa Subway Reverb. It just does not work. I tried a bunch of things:

- different driver tube
- 2 different tanks
- 2 different cables
- even soldered in a new reverb transformer

Basically I am getting a signal going into the driver stage (about 100mV), and getting about 100mV going to the actual tank. But nothing coming from the tank at all. And this is with 3 different tanks?!?!

I measured the voltages:

B+ for the driver tube 330V
B+ at the plate 328V
Cathode 3V

Suggestions?

[tubesornothing]

One would figure if I had a signal going into the reverb tank and none coming out the reverb tank or cables would be the problem.  But I tried three different tanks and at least three different sets of cables.

So then one might think, well perhaps the input to the recovery stage is being grounded out or something.  But I injected a signal into the recovery stage and it worked fine.


*sigh*

[Willabe]

Basically I am getting a signal going into the driver stage (about 100mV), and getting about 100mV going to the actual tank.

I think that seems off?    

Isn't a guitar pup's output around 100mV going into the preamp? 

Only a 100mV going to the 12AT7? I don't think that's enough to drive the 12AT7. I have a book that says Fender BF verb require's around 5V of driver signal.

I'd look to see if you've got a bad coupling cap going to the verb.

                        
                                            Brad      

  

[tubesornothing]

Yeah a strat is around 100mV.  I wonder if your book is saying 5V at the grid of the driver stage, or 5V at the tank input?

Regaurdless I will check the coupling cap, and see if there is any DC on the grid.

[Willabe]

Yeah a strat is around 100mV.  I wonder if your book is saying 5V at the grid of the driver stage, or 5V at the tank input?

At the grid.

[Fresh_Start]

Does your reverb driver tube have a snubbing cap (.002?) connecting the plate & cathode?  First thing I'd try is lifting one end of that cap.  Also check value of grid return resistor.

Cheers,

Chip

[PRR]

> signal going into the driver stage (about 100mV), and getting about 100mV going to the actual tank

Seems right. Hard-loaded 12AT7 has gain near 30, but reverb transformer has step-down ratio near 30, so grid voltage and tank voltage may be similar.

You checked all the right stuff. Now it comes down to the stuff that "can't go wrong". Bad cables, bad plugs, confused which is in and out.

[Willabe]

Seems right.

Huh? 100mV into the grid of the 12AT7 verb tube driver is enough to drive it?

If a guitar pup (Strat) is about 100mV that goes into the input of a tube preamp, with standard Fender bias, we have what, a 1V or 2V gain(?), 10 to 20X of what came in (30db?) then it goes to tone stack/volume, so loss, but still more than came in, than another gain stage, now way more than 100mV, even after the FMV tone stack loss, then split off in a Y to verb 12AT7 driver grid, what the grid see's/recieves has got to be, still, at this point 10/20X's, or more, than 100mV.

OTOH, if the Fender (Hammond) type verb 12AT7 driver tube's grid only needed to see 100mV to drive the verb PT & tank that would mean we could inject our guitar signal directly into the 12AT7's grid and have full reverb. But that's not how it's set up, there's no droping R's to cut down the signal to the driver's grid.

PRR, I don't get it?     


                     Brad                   

[PRR]

> guitar pup's output around 100mV

Zero to 100mV (or more).

Soft sounds are less.

The reverb should reverberate the soft stuff too.

Yes, the 12AT7 stage _can_ take more signal. Half-volt, whole-volt, maybe more. (5V would be un-clean but that may not matter.)

But the OP says " nothing coming from the tank at all", and the observation of 100mV at grid, 100mV at tank. That, to to me, says the driver tube and transformer are working right, or at least not-wrong.