Weird thing is happening

5150loveeddie · Post by **5150loveeddie** » Thu Feb 15, 2007 9:20 am

Guys, we are playing a 50w 1987 from 1969 MArshall head, basicly stock, and what is happening is after 3-4hrs of playing fully dimed it seems that one leg of the first screen resistor gets unsoldered (pin4 of V4)

I've reinstall that resistor twice, change it with a new one, doing that weird thing a few times...

Tubes in there are a mullard xf2, could the base of that tube get hot enough to melt a solder point????

I know I don't solder the best but damn I'm positive I did it ok??

robert · Post by **robert** » Thu Feb 15, 2007 9:26 am

I argue that the tube in this socket is the problem (obviously it pulls much too much screen grid current). You simply should swap the power tubes to test this.

Regards

Robert

5150loveeddie · Post by **5150loveeddie** » Thu Feb 15, 2007 6:41 pm

robert wrote:I argue that the tube in this socket is the problem (obviously it pulls much too much screen grid current). You simply should swap the power tubes to test this.

Regards

Robert

Got ya thanks...

Bias on the tubes are in range, plate voltage is low even 385/390vdc, bias at 33/33ma........I don't understand how this his happennig....

If it does it again, could this mean the tube in question is about to blow?

The amp sound really great...

Any other ideas Larry....??? anyone??

novosibir · Post by **novosibir** » Fri Feb 16, 2007 3:11 am

It's as robert already said, the screen grid resistor becomes too hot and then does unsolder itself. Now it's up to you, to figure out the reason.

Possibilities:

- Screen grid resistors with a too low wattage, therefore it becomes very hot.

- A tube with a very high screen current, therefore the resistor becomes very hot.

- You've used leadless solder - because the mixture with the usual solder gets a much lower melting temperature than leadless or usual solder ONLY.

Larry

5150loveeddie · Post by **5150loveeddie** » Fri Feb 16, 2007 8:52 am

Thanks guys...

It's a Xicon 5w cement resistor (that's enough...?) , what solder should I put in there instead?

High current pulling tube (xf2 here) is "normal" for the itseft or is it an indication of soemthing going wrong in it??

Thanks again!

SDM · Post by **SDM** » Fri Feb 16, 2007 10:57 pm

5w cement should be fine. What solder did you use? Have you measured the screens at idle? Try measuring the DC voltage on pin 4 of both output tubes at idle. More than a volt or so difference there, you may have a bum tube. Can go further there and measure pin 6 and pin 4 of a tube, then the actual resistance of your 1K screen resistors to figure the sceen current each tube is really drawing at idle. Just use Ohms law I = V/R. V is (pin 6 voltage-pin 4 voltage) here, R= actual measured 1K resistance, I= screen current draw at idle.

novosibir · Post by **novosibir** » Sat Feb 17, 2007 8:44 am

5150loveeddie wrote:... what solder should I put in there instead?

Unsolder both screen resistors and clean pin 4 and pin 6 with a solder puller pump, then use usual 60/40 solder (60% tin, 40% lead) or a Sn60Pb38Cu2, which contends 2% copper.

And as Steve already said, measure the voltage drop across the screen resistors at idle! With a 1K screen resistor your voltage reading equals the screen current in mA - with EL34's a 'heatlty' current is somewhat between 3mA and 6mA

Check also for a faulty tube, which first after switching from standby reads let's say 5mA - but within a few minutes is climbing up more and more in screen current.

Larry

5150loveeddie · Post by **5150loveeddie** » Wed Feb 21, 2007 12:25 am

Thanks both of you.......