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Post subject: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sat Dec 17, 2016 11:21 pm
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Hello Fellow Fender Fans - I just became the proud owner of a Fender Super Reverb (silverface) with non-original celestial 10" speakers. The caps were leaking and bulging. I changed them out and fired the amp up after retubing and setting the bias to 30MA. Played fine for 25 minutes.

Then it red plated VERY QUICLY, and took out the mains fuse. i put my Tube Dr. Bias tester and its running away to like 100MA just before i have to shut it down.

I noticed the 80uf 75v capacitor coming off the Bias pot was dead, so i changed them "BOTH" out (the one close to the pilot lamp.......bias still running away FASSSTTTT!

Can any of you guys that surf the board here point me in the right direction? By the way the TONE WAS CRAZY BEAUTIFUL for those 25 minutes :D

Thanks!


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sat Dec 17, 2016 11:38 pm
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Did you inadvertently install the bias-supply cap backwards?

It needs to be wired with the polarity reversed -- that is, with the "+" side connected to chassis ground.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sat Dec 17, 2016 11:47 pm
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Retroverbial - It is installed as you described, bias was "running away" before i swapped those 2 80uf/75V caps out.


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 12:51 am
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IS the bias ok, now? With the new bias caps? If not... check the 6L6GC sockets. May need cleaning and re-tightening.

The bias supply should read -44 VDC on one side of the first bias supply filter cap. Pot centered.

http://www.thevintagesound.com/ffg/sche ... _schem.gif


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 2:32 am
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Ryan41018 wrote:
Retroverbial - It is installed as you described, bias was "running away" before i swapped those 2 80uf/75V caps out.


Possible catastrophic diode failure?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 5:18 am
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Remove 6L6 tubes and fix bias circuit before putting them back.

Read bias voltage -30 to -40 dc ( BMW2002Ti ) at pin 5 on 6L6's socket.
You can power ON the amp without 6L6 to fix bias and check.

If bias volts are there, you may have a bad contact between socket and tube.
(BMW2002Ti )
The bias voltge are present at socket but not at tube


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 8:01 am
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BMW2002Ti wrote:
IS the bias ok, now? With the new bias caps? If not... check the 6L6GC sockets. May need cleaning and re-tightening.

The bias supply should read -44 VDC on one side of the first bias supply filter cap. Pot centered.

http://www.thevintagesound.com/ffg/sche ... _schem.gif


I will check the voltage there but at the cap, Sockets? They were fine for 25 minutes after the initial re-cap, re-tube and bias, but I will check them as well.


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 8:04 am
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Retroverbial wrote:
Ryan41018 wrote:
Retroverbial - It is installed as you described, bias was "running away" before i swapped those 2 80uf/75V caps out.


Possible catastrophic diode failure?

Arjay


Arjay do you refer to the diode nearest the pilot lamp socket? It does test good.

Thanks


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 9:34 am
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Yeah, the flyback diode on the small bias-supply board. If that tests good, then perhaps you need to retrace your work when the new filter caps were installed. Look for any soldering anomalies or wiring errors. One other possibility might be a bad bias cap. I know you reported it as new but I have seen brand-new parts that failed right out of the box. It's worth checking out.

Arjay

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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 10:45 am
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When you say you retubed it, did one of your new tubes crap out?
Its not unusual.
If only one tube red plated, try the old pair again.
If I encounter a failure in an old Fender amp, I look closely at the tubes first because old Fenders don't just crap out like that.
Even with bulging caps. If they're not exploded, they keep working.


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 11:13 am
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TimsAudio wrote:
When you say you retubed it, did one of your new tubes crap out?
Its not unusual.
If only one tube red plated, try the old pair again.
If I encounter a failure in an old Fender amp, I look closely at the tubes first because old Fenders don't just crap out like that.
Even with bulging caps. If they're not exploded, they keep working.


At this point this bias jumps and exceeds 150 that it doesn't have time to REDPLATE, it takes out the fuse after 10 -16 seconds.


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 2:31 pm
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Try another pair of 6L6GC, that are known good and you don' t mind if one or both blow out. If the amp measures -44VDC at the bias cap & has tight socket contact (esp pin 5) --- and you still have a 6L6GC going ballistic --- it's prolly the tube(s), which are shorted out.

Good luck! Post results.


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 9:56 pm
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BMW2002Ti wrote:
Try another pair of 6L6GC, that are known good and you don' t mind if one or both blow out. If the amp measures -44VDC at the bias cap & has tight socket contact (esp pin 5) --- and you still have a 6L6GC going ballistic --- it's prolly the tube(s), which are shorted out.

Good luck! Post results.



UPDATE - I discovered that one tube socket is creating the problem (i Pulled one tube) and the amp played (albeit as if has cold bias) so theres something going on with that one. The resistor the socket look good but I've got to put my DMM on it.

I tried to read voltage on pin 5 but the raised number on the bottom (inside of the chassis) are difficult to read. I've got to google which direction to count to pin 5.


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 10:35 pm
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Pin 5 is easy to find on most 6L6GC Fender amps. It has a 1500 ohm resistor tied to it, from pin 1. This is the grid stopper. Diagram shows layout looking at the bottom of the amp. looking up:

Image


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Sun Dec 18, 2016 11:18 pm
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Ryan41018 wrote:
The resistor the socket look good but I've got to put my DMM on it.


It's possible that the porcelain (or bakelite) has fractured with age. Static measurements may show nominal readings but with the HV applied there is arcing within. If that's in fact what's happening, replace BOTH power tube sockets simultaneously.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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