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kingoftheblues8 15th November 2012 06:53 PM

1966 Super reverb problem child
Okay guys,I need some serious help. Heres what we have

1966 Fender Super Reverb. Someone has monkeyed with this one. Theyve Soldered in SS Diodes in the Recitifier tube. Added 1Ohm Resistors to gorund across Pin 8 of Outpiut tubes. Replaced several caps, mostly all coupling caps and electrolytic filter caps.

Heres the issue. One of the Output tubes is having bad thermal issues. When switche don it glows bright orange, and the other normally. After taking off standby, the amp has a low hum. The Left output tube becomes very very hot. Both are reading close to each other at 468 v off, but heres the main issue ive found: measured at pin 8, the hot tube has a reading 42.3 mV, the other? 0 mV!!! whats going on, ive trie tracing the layout as close as possible (with whats original in this amp) and everything looks fine. Ive replaced both output tiube sockets with new 1.5k reistors and 470 grid resistors to spec. PROBLEM still exists.... any ideas? please help

snaggletooth 15th November 2012 07:44 PM

I would be suspicious of the quality of the connection of that 1ohm resistor to ground. Did you measure the resistance between pin 8 and ground?

kingoftheblues8 15th November 2012 08:12 PM

I have not yet, I will measure in about 15 minutes, and update... Cant these resistors be eliminated? Whats the purpose?

Enzo 15th November 2012 09:43 PM

1 ohm resistors between pin 8 (the cathode) and ground are added so you can measure the voltage across them, and by Ohm's Law determine the current through the tube. 1 ohm is not going to upset anything in the circuit. Many amps come with them already there.

What do you mean they soldered diodes "in the rectifier tube"? Did they add them between the transformer wires and the rectifier tube? Or did they solder them across the socket in PLACE OF the rectifier tube?

Putting a couple diodes in series with the AC coming into the rectifier just protects the amp from a shorted rectifier tube. When we are talking 450-500 volts, the half a volt or so the diode drops is irrrelevant.

Putting diodes in place of a rectifier tube results in DC just as with the tube, but the voltage will be somewhat higher.

First, verify the 1 ohm resistors are OK. There is no purpose in removing them, unless they are defective. They are useful to have.

Next, remove the power tubes and power up the amp. Measure for B+ voltage (your 468v) at BOTH pin 3 and pin 4 of every power tube socket. If it is missing at pin 4, that might explain the zero volts on pin 8. Then also measure at pin 5, you should have something near -50v.

It is normal for the tubes to get hot. 42ma at 468v is about 19-20 watts idle dissipation for the tube. That should be fine, the zero one is the side with the problem. There wilol be the glowing oorange tube heater in the center of each tube, but when the tube overheats, we then start to see the metal structure inside starting to get red hot. We call that "red plating" and it means the tube is coinducting too much current.

And never forget, the most likely failure in any tube amp is the tubes.

Your two power tubes are in a push pull arrangement, and that naturally cancels power tube stage hum. If one tube is not working, the natural balance is gone, and the result is usually hum. Consistent with your findings.

GOYA 25th November 2012 05:33 AM

First thing I'd do is switch the power tubes and see if the problem follows the tube or stays in the socket.

cyclecamper 26th November 2012 06:05 PM

Absolutely! Suspect bad tube first!

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