Fender Dual Showman Reverb Problem

[RiccardoSG]

Hi, I'm Riccardo and I'm new in this forum, I hope that someone can help me.
That is my problem: I have a Fender Dual showman Reverb silverface with push-pull master volume that for the second time broke the fuse ( 3A slow). The first time I opened the Amp and I sow a tube that seems to be like broke, I repleced it, the other component seems in good conditlin, I try to adjust the balance of the tube with oscilloscope but weren't possible to do it fine because the tube were not matched ( 2 were matched but 2 were unmatchen). Afte a month (yesterday) I had the same problem but this time I haven't see a tube that seems broke.

I have only an oscilloscope and not a tube tester.

What can I do?




P.S. Sorry for my bad English!!!!

[Jerry]

You should probably replace all of your output tubes. If you have a volt/ohm meter, measure the resistors that are mounted on the tube sockets. There should be a 470 and a 1500 ohm resistor on each tube socket. If you don't have a volt/ohm meter, you'll need one if you are going to service your amp. If you need a schematic for your amp, look on the schematic heaven website.
This might be your amp, I don't know, you'll find others there.

http://schematicheaven.com/fenderamp..._100_schem.pdf

[Don Moose]

R.G.Keen's excellent Tube Amp Debug Page is your friend.

[RiccardoSG]

Hey guys, thanks for the council, I will do that things and than I will let you know!

[RiccardoSG]

I Tested the 470 and a 1500 ohm resistor on each tube socket and they are OK.

In the tube amp debug page I see "Remove the power tubes, replace the fuse, and see if the fuse still blows when powered up with the power tubes out. If it still blows, the tubes are not your problem". It is not dangerous for the amp? Probably it is a stupid question but I don't know...

[MWJB]

No it's not dangerous for the amp. Fit the power tubes one at a time, powering down between each installation, and see if any particular tube/socket causes your fuse to blow.

If the fuse blows with no power tubes installed, suspect the rectifier. If the rectifier is OK disconnect & insulate all power transformer secondaries and see if the fuse still blows - if so, the PT is shot (as your amp powers up & only fails intermittently we shall assume that your PT is OK).

If you had a cathode biased amp (Dual Showman is not cathode biased), you would need to install all the power tubes at the same time to ensure that they did not draw too much current.

Whilst you have the amp apart, solder a 1ohm (matched & exactly 1ohm) resistor (2W) between each power tube cathode & ground. Read the voltage at pin 8 of the tube socket (<200mV) to establish how much plate current each tube is drawing (<33mV would be good - mV converts to mA). Neither a tube tester nor an oscilloscope will be much use at this point.

[RiccardoSG]

But the fuse don't blow instantaneously, I play the amp for a few minutes (10-15) and than the fuse blow. I let the amp powered up without power tubes for 15 minutes?

[MWJB]

Then something is drawing a lot of current, I'd suspect one of the power tubes/sockets in the first instance.

Play the amp with one tube (one that doesn't blow the fuses), then 2 tubes - one either end of the OT, say in sockets 7 & 10, then try the same 2 tubes in sockets 8 & 9 - measure plate, screen & grid voltages at pins 3, 4 & 5 (now you should have eliminated the sockets as a cause). Then add a third tube...and so on. Any red plating in the tubes when you play the amp is a bad sign (too much current).

In fact you'd be better off buying a matched quad (you need them anyway), install the 1ohm resistors at the cathodes and measure the current draw for each tube. Rebias to 30mA per tube (set negative voltage at pin 5 to -50 to -60vdc with no tubes installed before putting the new tubes in & fine tuning bias) and see what happens.

[RiccardoSG]

Thanks to your councils I found the problem: the tube number 7 is broke.
I had not install the 1ohm resistors at the cathodes and measure the current draw for each tube but I measured the negative voltage at pin 5 without the tube and that are the value:
Tube 7: -48,7V
Tube 8: -48,9V
Tube 9: -49,1V
Tube 10:-49,0V

But now my question is: Why I broke for two times the tube in the same position (tube number 7)? It is a case? It is because the tube 7 and 8 were unmatched between them and between 9 and 10 ( 9 & 10 are matched)?

What can I do?

Naw I ordered 6L6GC matched quad but before to install I want to be shure that the 7 don't blow again!!!! Must I change also the driver tube ( tube 6)?

[MWJB]

Yes, it's a good idea to change the driver tube.

What are the plate & screen voltages at pins 3 & 5 of the power tube sockets (you'll need the tubes installed). The negative voltage on it's own doesn't tell us anything really, other than all the grids are getting bias voltage, they can still all have voltage and the amp be underbiased (too much current). That's why you need to ascertain plate current.

[RiccardoSG]

With two tubes plugged in I read this values:

Soket 7=> Pin3= 484V
Pin4= 483V
Pin5= -50V

Soket10=> Pin3= 494V
Pin4= 490V
Pin5= -49V

Than with the same two tubes I read:

Soket 8=> Pin3= 493V
Pin4= 490V
Pin5= -50V

Soket 9=> Pin3= 493V
Pin4= 489V
Pin5= -50V

How can I interpret this values? Which is the next step?

[MWJB]

If the same tube in socket 7 draws more current and therefore less voltage, when compared to what happens to thst tube in a different socket, it looks like the problem is socket related. Change the socket for V7.

Whilst you are in there, install the 1ohm 2 reasistors between each power tube socket pin 8 and ground. Make sure the resistors are really 1ohm, by puting your ohmeter leads together (zero resistance) then noting the default reading on the meter, maybe 0.4 ohms? In this example a 1ohm resitor will actually read 1.4ohms on your meter.

[RiccardoSG]

I replaced soket 7 and its own 470 and 1k5 resistor, I installed a 1ohm 2W 1% accross pin 8 and ground on each soket.
I puted on two tubes and I mesured the mV at pin 8:

V7 Pin8 = 46mV
V10 Pin8 = 33mV

Whith same two tubeon the other soket:

V8 Pin8= 48mV
V9 Pin8= 33mV

There are a lot of mV on V7&V8!!!!
I haven't tryed to exchange the tube between them, is it possible that a tube is short-circuit?

[MWJB]

If the negative voltage at each grid is pretty much the same, I'd suggest that you have 2 pairs of tubes, one pair is not matched to the other.

Swap the tubes about and see if the high current follows the tube or the socket to confirm?

[RiccardoSG]

I have a matched quartet of 6L6 and a lot of single 6L6. Before to install the matched quad I want to be shure that the amp is OK. I tryed different tubes and them drow different current, some drow 33mV (mA) other drow 48mV!! Should I use the balance pot to set the bias? Is that its function?