4.03.2003

 
Post from AudioAsylum re: my Lindell amp

Posted by Jim McShane

Re: old Lindell guitar amp - no info on web - tube id - ?'s
------------------------------
Let me see if I can help...

"The tubes light up but no sound, not even a buzz. One of the wax and paper
capacitors is blown apart, can't read the values, but there is another capacitor
that looks the same and it is marked ".01mfd +-20% 600 W.V.D.C. Dumont 157." It
doesn't have a line around one end or a + marking."

"- is this an electrolytic?"

No, it's a coupling cap. Replace both of them, if you haven't got proper caps
(or tubes) drop me a note privately.

"- does this cap being blown up point to a likely suspect?"

Hard to say, it's so old it may have just come undone on its own. The tubes are
definitely suspect, as are the filter caps.

"The rectifier tube is a 12AU6, then there are two other tubes, one is marked
50C5, the other is indecipherable, but looks to be similar in construction,
except it has a clear top.

Actually the 12AU6 is not the rectifier, it's a small pentode that's used as a
voltage amp. The 50C5 is your output tube, and the mystery tube is likely to be
the rectifier. It might be a 50DC4, but...

The amp probably does not have a power transformer, and the tube heaters are run
right off the AC line. So if you add 50 volts for the 50C5 and 12 volts for the
12AU6, then the rectifier needs to drop about 50 volts, that's why I mention the
50DC4 (50+50+12=112 volts-close enough to 115). Some units also used a resistor
in series with the heaters, and they often used a 35W4 rectifier. I'd bet you
find a large resistor of 100 ohms or so in there, and that rectifier is a 35W4.
The 35W4 was VERY common.

The B+ is developed with a 1/2 wave rectifier runnning straight off wall AC too.
This is a DANGER! Don't run this amp or test this amp while it's plugged in
unless you know EXACTLY how to handle it. The chassis may be "hot", so a nasty
shock would occur if you touch the chassis and an earth ground at the same time!

I STRONGLY recommend you use an isolation transformer to work on or run this
amp!!

Replace all the caps you can find, replace the tubes (they're cheap!), clean up
the controls and tube sockets, jacks, etc., GET AN ISOLATION TRANNY, and you'll
probably be good to go. You might find some resistors that act up or are
cracked/damaged, but the caps and tubes are a must, as is the ISO tranny. Be
sure the line cord insulation is 100% too!


Pics of the Lindell:


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