Little Help On Magnatone 440 Reverb

[jcollins]

I've got a Magnatone 440 on my bench. I've never worked on one of these before. The problem is the reverb isn't working. The problem seems to be
in the driver section. If I turn the reverb up and slap on the amp, I can hear
the tank crash.

The reverb is directly driven by 1/2 of a 6DR7 Tube. On this particular tube,
every single other tube was bad. I ordered a replacement tube and installed it and it still didn't work. I found that the plate load resistor, a 10 watt 4K job, was open. I couldn't find an exact replacement so I used a 5K 10 watter. I also replaced the 1 uF coupling cap between the plate and pan input. After all this I tried injecting signal at the grid of the 6DR7. Barley any signal gets through to the plate. The cathode resistor reads OK to ground as does the 470K feedback resistor.

If I run my signal generator right into the reverb tank I can hear signal getting through and I hear splashy sounds. I'm thinking the reverb tank is OK.

Ordered a second replacement tube, thinking that the one I purchased was maybe bad (they are cheap) and that didn't work either.

As I turn the reverb up, the volume of any signal input in the front of the amp
is decreased.

It appears to me that the reverb driver circuit isn't working right, but for the life of me I can't figure out why.

The schematic for this amp is here:

http://www.schematicheaven.com/barga...natone_440.pdf

although the one on my bench is slightly different. It has a 6eu7 as the first
stage of the preamp. I have swapped the 12ax7 that comes before the reverb circuit.

OK. As I was writing this I realized I never measured the voltages on the cathode nor the grid. The grid reads 0.04V and the cathode reads 220V!
Yikes somethings not right there.

Any ideas?

[Enzo]

The schematic file is taking forever, but looking at a 480 schematic, I see the same 6DR7, 4k plate load, and this one has 330 ohms in the cathode leg. What is yours?

In a circuit like the one I see, the only way to get 220v on the cathode is to either have 2/3 of an amp flowing through the tube, have a shorted tube, or the cathode resistor is either open or not making it to ground or your meter was gounded to a negative supply somehow.

WHat DC voltage is at the plate?

If the cathode measures 220vDC, measure then the voltage at each end of the cathode resistor. DOn't assume it must be the same because the cathode is wired to it. CHECK. By measuring the voltage at the "ground" end of the cathode resistor we find if there is indeed a ground connection or if there is more resistance or an open.

If the tube were conducting 666ma, the 330 ohm resistor would be dissipating almost 150 watts. Wouldn't do it for very long, I'd bet. Obviously if your circuit is different, the math changes, but still...

You now have more than one of these tubes. DO all the tubes you have yield the same or similar voltage readings?

[jcollins]

The schematic file is taking forever, but looking at a 480 schematic, I see the same 6DR7, 4k plate load, and this one has 330 ohms in the cathode leg. What is yours?

In a circuit like the one I see, the only way to get 220v on the cathode is to either have 2/3 of an amp flowing through the tube, have a shorted tube, or the cathode resistor is either open or not making it to ground or your meter was gounded to a negative supply somehow.

WHat DC voltage is at the plate?

If the cathode measures 220vDC, measure then the voltage at each end of the cathode resistor. DOn't assume it must be the same because the cathode is wired to it. CHECK. By measuring the voltage at the "ground" end of the cathode resistor we find if there is indeed a ground connection or if there is more resistance or an open.

If the tube were conducting 666ma, the 330 ohm resistor would be dissipating almost 150 watts. Wouldn't do it for very long, I'd bet. Obviously if your circuit is different, the math changes, but still...

You now have more than one of these tubes. DO all the tubes you have yield the same or similar voltage readings?

Thanks Enzo. Yes the cathode resistor is 330. I'll do the test you suggested tonight. DC voltage at the plate was around 300V as I remember.

[jcollins]

OK. I pushed this to the side for a while but now I'm back on it. I was mistaken about the cathode voltage on the 1/2 of the 6DR7 that drives the reverb tank. I remeasured several times. I'm actually getting 25V across the cathode resistor. It is grounded. I checked.

Signal is getting to the grid of that tube. Nothing is coming out the plate.

Another strange thing. There is no voltage drop across the plate resistor. It reads 270 on boths sides.

I swapped in the other tube I bought. Got the same reading.

The other half of the tube appears to be working. It's part of the recovery
circuit and I can hear the reverb splash if I shake the tank.

Any help is much appreaciated.

[jcollins]

El Bumpo for this problem.

[Steve Conner]

My guess is that the coupling capacitor failed short and partly melted the reverb tank driver coil before burning out the plate resistor. The tank driver coil now has a bunch of shorted turns, and the tube can't drive it, but your signal generator still can with its low output impedance.

Either that, or the cable and connectors to the reverb tank have a short, and you used a different cable, or moved the cable and the short went away, when testing with your generator.

Try disconnecting the tank and see if signal now appears at the plate.

The only explanation for volt-drop across the cathode resistor with none across the plate resistor, is that the cathode resistor is open or the plate is shorted to B+. Or maybe someone sold you a "6DR7" with the wrong pinout.

[Don Moose]

I wonder if the grid socket barrel is making good contact with the grid PIN on the tube?

[52 Bill]

"Or maybe someone sold you a "6DR7" with the wrong pinout."

Is the reverb drive using the front section of the tube, pins 1-2-9?
Are pins 2 and 3 tied together on the tube socket?

I've had this happen before, the RCA Manual says that the grid internally should be hooked to both pins 2 and 3. But if the tube was manufactured by someone else it may only be connected to either pin 2 or 3. Try tying pins 2 and 3 together on the tube socket and see what happens.

[jcollins]

An update on this amp. I actully finished a few weeks ago.

I'm an idiot. The big resistor that I thought was 5K was actually 5 ohms.

It had something like 5 ohms K on it.

Put in the right resistor and it works just fine.