Rectifiertrouble

[OddvarR]

Hi. My technician tells me that my ampfuse blows often because the standbyknob is connected directly to the rectifiet and therefore sends to much volts into the tubes. Is there any other way to connect this? It's a Plexi 50w.

[neikeel]

Hi. My technician tells me that my ampfuse blows often because the standbyknob is connected directly to the rectifiet and therefore sends to much volts into the tubes. Is there any other way to connect this? It's a Plexi 50w.

Hmm tell us more!

Year/model/spec etc.

[OddvarR]

Hi again. It doesn't seem to be an alternative to upload pics here, so I have put up to links to my Facebookpage.
My tech said the wire from the standbyknob isn't going through the biassystem and therefore sends to much voltage to the tubes the first milliseconds the amp is turned on, which may burn the rectifier. He has now put the third new one in, and it's possibly strong enough-

https://www.facebook.com/photo.php?fbid ... =1&theater
https://www.facebook.com/photo.php?fbid ... =1&theater

[danman]

The older Marshall 50 watters did have a wiring error and many rectifiers and power transformers were damaged over the years. The bias feed was wired to the cold side of the standby switch which meant that the tubes didn't receive any bias voltage until after the amp was taken off of standby. It is a common fix nowadays to move the bias feed wire to the other side of the standby switch (the side that is always hot) so that the bias circuit starts receiving power as soon as the amp is turned on. This way is much easier on the power tubes, rectifier and PT.

[OddvarR]

Thank you Danman, informative. Since I am not an electrician, could you kindly point out the way to do it? On my amp it seems like the stanbycables are going to the bridge rectifier and to one of the filtercans?

[bill bokey]

The bias feed on your amp is at the right place but the standby switch is after the rectifier, maybe that's the problem as it's switching DC and not AC.

[neikeel]

That is why I asked for make model and iseally some pics, can't open facebook at work!

[OddvarR]

Hi' I tried to upload pics, but got error messages.
It is a Metro 50w plexi amp put together as the spec says, as far as I can see.

[neikeel]

Hi' I tried to upload pics, but got error messages.
It is a Metro 50w plexi amp put together as the spec says, as far as I can see.

In which case if the original metro wiring was followed it is unlikely to be the bias feed issue that came about in the econd half of 69 and was reversed a couple of years later

Looking at your picture the bias feed is correct, it comes off the red wire (ac) directly from the PT. If you use standby for 30 secs or so before switching on the HT your tubes will have seen bias, 60 secs even better. So I do not understand what your tech means.

The build has used some nice parts but, to be honest, the quality of wiring is pretty average

If it were mine I would tear it all down and start again.

[OddvarR]

Thank you again. The wiring wasn't done by me luckily. I bought the amp this summer, but the fuse have a habit of blowing the minute I turn it on. My tech has installed a stronger rectifier.
(You mean it should be cleaned up and twisted right?)

[OddvarR]

And it sounds great, if not fantastic...