Tube Amps / Music Electronics
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|2/29/2000 1:28 AM|
||Burnout Resistor across 6L6 in Bandmaster Reverb?|
I'm a newbie to getting inside of amps. I got ahold of a SF Bandmaster Reverb to try the Vibroclone project, and have started to Blackface it using Gerald Weber's book as a guide.
With the chassis out, I noticed that the resistor across one of the 6L6s is cracked/burnt out. Is this a sign of something wrong elsewhere in the amp, or is it just something that happens with age? It also surprised me to find Mallory caps. I always read that Fender started using crap, and the stamp inside the chassis says December 22, 1971.
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|2/29/2000 2:05 AM|
Well I'm far from an expert but I would say you need to replace the resistor. Use a very high quality. Probably burnt due to a bad tube. I have seen a lot of those caps in SF Fenders of the era, white with blue print. Don't know if they are that bad a quality.
What's a "Vibroclone project"?
|2/29/2000 4:03 AM|
The resistor is either a screen grid resistor which would be a 470 ohm 1 watt or a 1.5K 1/2 watt grid stopper resistor.
The tube in the socket with the burned resistor is most likely shorted and that is why the resistor burned out. Replace the resistor and the tube and you should be good to go.
|2/29/2000 4:32 PM|
I've got a SF Bandmaster Reverb head. So what is the Vibroclone? Has anyone made one? Is there a link to a schem?
Inquiring minds want to know.
|2/29/2000 4:08 AM|
Dan, Having not seen your amp, but having seen enough old Fenders leads me to guess that the cracked/burnt resistor is across pins 4 and 6 of the power tube. This is your screen grid resistor, usually 470 ohm in Fenders. Definitely replace the resistor, and while you are at it replace the resistor across the same pins on the other power tube. I'd recommend 2 watt resistors. The only other resistor across the power tube sockets in your amp are the grid stopper resistors across pins 1 and 5, and these 1.5K resistors rarely ever fail. Mallory caps are no surprise in '70s vintage Fenders. Good luck. Bryan Prud'Homme
|2/29/2000 7:36 AM|
Bryan: have to differ on this one..seen many of
the little 1.5k break in half with a touch..
burnt and bad..some may even look good..no burn marks,and still be bad. But also the 470ohm too..
|2/29/2000 8:30 AM|
to Richie & Bryan:
"Can't we all just be friends?" (from Rodney King) Just kidding...
So what is your opinion on rewiring these tube sockets in case someone down the line decides to put in EL34's? Of course you'd check with the customer first, but if you are going to replace both resistors on the tube socket, why not wire it up to handle EL34's as well as 6L6's?
I've been putting 1k/5w resistors between pins 4 & 6, tying pins 1 & 8 together, and then putting the 1k5 grid stopper right on pin 5 (with heat shrink tubing to help support the resistor on the lead). Would you recommend different values here?
P.S. For an older amp I'd vote for changing all of the resistors on all of the tube sockets rather than just the one that is bad.
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