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Post subject: blowing fuses
Posted: Thu Jul 05, 2018 1:30 pm
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I have a1970 dual showman reverb I have recapped it with a recap kit , new tubes when I powered it up I had a low ( very low ) hum then it threw the breaker on the power switch and the fuse , new fuse and it keeps blowing the fuse . any help will be appreciated
thamx
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Post subject: Re: blowing fuses
Posted: Sun Jul 08, 2018 7:29 pm
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Pull the power tubes and see if the problem went away. New tubes can be bad out of the box. Check over your work with a magnifier.


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Post subject: Re: blowing fuses
Posted: Sun Jul 08, 2018 9:49 pm
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TimsAudio wrote:
Pull the power tubes and see if the problem went away. New tubes can be bad out of the box. Check over your work with a magnifier.


+1

As well, inspect the installation of your new electrolytics, paying particular attention to their polarity -- especially the bias-supply cap which should be reversed ("+" lead to 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: blowing fuses
Posted: Mon Jul 09, 2018 1:51 am
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Use this to save your fuses;

https://www.youtube.com/watch?v=wRFRwOnLsZI

Or Google ; 100 watts bulb current limiter

ImageCapture d’écran 2018-07-09 à 04.49.55


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Post subject: Re: blowing fuses
Posted: Mon Jul 09, 2018 7:44 pm
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After following the previous advise of the repliers to your posted problem and you still have issues.

With a good 2.5 amp slo-blo fuse installed.
There is no need to take the amp off of standby.
All your initial measurements should be VAC.
Before turning on the amp, plug it in and verify wall outlet, both fuse holder connections and one side of the on/off switch are nearly same voltage's, the standby switch should not see this voltage, if it does you on/off switch is bad or your power cord is improperly wired.
Turn amp to on and verify your Power Supply (transformer) is working properly.
If working you will have power to the Lamp(6.3vac) and all tube heater connections(with or without tubes installed), AC voltage to the - bias voltage supply diode(between PT-Diode approx. +60vac), AC voltage to the AC to DC diode rectifier(between PT-1st diode in the chain x2 of approx. +315vac).

Then you can test for DC voltages, on the - bias voltage supply output side(after the diode, at the bias pot, and at each output tube's pin 5) should be approximately -52 volts. Then verify diode rectifier output DC voltage/B+should be approximately +410dc (measured at the junction of the two diode chains)

As long as the power supply is good you can safely rule it out as the problem.
If you have problems w/ any of the above voltages, correct them before proceeding further.
In other words break down each specific area whether AC or DC and each sub component area whether lamp&heaters, negative bias supply or diode rectifier.
Diagnose and replace faulty transformers, resistors, capacitors or diodes as needed.

If all is good you are ready to flip that standby switch to on.


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