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    Is my amplifier blown?

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    gabis8

    Posts : 3
    Join date : 2012-11-28

    Is my amplifier blown?

    Post by gabis8 on Wed Nov 28, 2012 10:19 pm

    Hello,

    I'm a real novice with tube amplification!

    I have just acquired a ST-70 from Ebay. Unfortunately I didn't have the chance to enjoy it for very long. Today I received a new preamplifier, so I decided to try it with my amp. I connected the amp to the speaker and preamp to amp, turned the power on, and started to play music. But no sound was coming. So I rotated the selector switch, still no sound. What I made is a bad idea. I disconnected the output of the preamp and heard a huge thump through the speakers. I realized it's not a good thing to play with the preamp output while the amp is still powered, so I turned off the amp, and connected it to another preamp output. I then turned on the amplifier and a small flash occurred at the rectifier tube, and the amplifier went dead. Nothing is working anymore.

    Does anyone have a clue what happened? I hope I did not damage my dynaco too much!
    Thanks very much for any help.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: Is my amplifier blown?

    Post by anbitet66 on Wed Nov 28, 2012 10:45 pm

    gabis8,

    If you turned off your amplifier for just a few seconds to make the switch, then powered it back up, that was the cause of the rectifier to flash. The rectifier is now bad and if the amplifier has the correct fuse, it also failed. You need to replace the 5AR4 rectifier and your 3A fuse.

    The next question on my mind is: Was this an all original amplifier, or was the amplifier gone over? That is were the capacitors replaced, new tubes, replaced resistors as needed? New quad capacitor in the power supply? Otherwise the amp will not have the reliability it should.

    And most important, the rectifier mod should be done to prevent this from happening again. There are old posts on this forum about it. It involves installing 2 silicon diodes to pre-rectify the voltage from the power transformer, saving the 5AR4 from future problems.


    gabis8

    Posts : 3
    Join date : 2012-11-28

    Re: Is my amplifier blown?

    Post by gabis8 on Wed Nov 28, 2012 10:58 pm

    Hello anbitet66, thank you for your response.

    The amplifier have had all the parts on the board renewed, but I'm not sure about the quad capacitor. The seller did not respond to this question. I've heard before about the rectifier mod, that's something I plan to do in the near future. Thank you for your advises. I'll correct the 5AR4 and the fuse and see what happen.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Is my amplifier blown?

    Post by Bob Latino on Thu Nov 29, 2012 6:40 am

    Anbitet66 has given good advice. The problem with turning a Dynaco ST-70 amp OFF and the ON again quickly (short cycling) is that the voltage in the rectifier drops almost immediately BUT the voltage stays high in the quad cap (DC storage) for a longer period of time. If you turn the amp ON again maybe within 10 or so seconds after you turned it OFF, the (higher) voltage from the quad cap can flow back into the rectifier tube causing a brief "flash" which CAN take out the rectifier. Now and then someone will Email me that > "All the tubes light up but there is no sound and also no bias voltage". This is a classic "bad rectifier" symptom. Keep in mind that just because a tube lights up does not mean that is is good.

    There are two solutions you could use ...

    1. Do the diode mod as explained on the forum here > ST-70 diode mod The diode mod prevents the backward flow of current into the rectifier.

    2. Use the VTA TDR (Time Delay Relay). The TDR gives a 17 to 20 second time delay before high voltage is applied and if you turn the amp OFF the delay resets and no high voltage is applied for another 17 to 20 seconds. The TDR, although not absolutely needed with a tube rectifier if you just remember not to short cycle your amp, is a good idea if you you live in an area which has quick ON/OFF/ON power outages.

    The sad thing is that the older USA, British and German made GZ34/5AR4 rectifiers of the 1950's and 1960's were made of better materials, had better quality control and lasted much longer than any rectifiers presently made. (China, Russia etc.) The older rectifiers could take a little short cycling abuse and not die. Apparently the newer crop of rectifiers made today are not as "robust" as the older rectifiers.

    Bob

    gabis8

    Posts : 3
    Join date : 2012-11-28

    Re: Is my amplifier blown?

    Post by gabis8 on Thu Nov 29, 2012 9:36 pm

    Thanks you for this explanation, Bob.

    If I understand correctly, placing diodes prevent the rectifier tube from receiving back voltage from the quad cap.

    Is there some danger doing the diode mod? I guess I should discharge the quad cap before attempting to do it.

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