Dyna MKIII blowing fuse at 90volts

[petertub]

Thanks to everyone for their input. I now have several things to take care of. I'm in Prescott, AZ and get my supplies from "Tubes and More" in Mesa Arizona. I'm now thinking of scratch building a 12AX7 preamp to go with the Dyna. Does anybody have a favorite design for one with volume, bass and treble controls? I haven't been able to find a complete schematic online. I've got a 5MK9 power supply I'm thinking of using. Thanks again to all. I'll update everyone when I get my Dyna running.

Dyna PAS would do fine here. Boards could be bought from erhard-audio

 

[lzugner]

Does anybody have any known issues with the 5MK9 rectifier?

 

[vinylkid58]

Does anybody have any known issues with the 5MK9 rectifier?

An obscure Japanese half wave 7-pin rectifier? Na. 🙂 https://frank.pocnet.net/sheets/133/5/5M-K9.pdf

jeff

 

[lzugner]

jeff: No issues or, I wouldn't use it?

 

[rayma]

Pretty sure he means Na, wouldn't use it.

 

[lzugner]

Well, I had all the tubes tested and they were all good, I reset the bias which was about 1 volt too high, brought it up slow and no fuse blow. I'm running it at 110VAC for now to make sure it is healthy. I still intend to replace the selenium rectifier and make the "yellow sheet" mods suggested by Rayna. But for now, I'm back in business. Thanks to all for your input.

 

[rayma]

Do those one at a time, just in case.
You'll have to rebias after replacing the diode.
You may want to check the value of the bias resistor, sometimes they get off.

 

[lzugner]

Thanks, will do. I was also meaning to ask.....I wanted to install a VU meter. Where would that go? I saw a vid on that once, but now I can't find it. Between the preamp output and the Dyna input, maybe?

 

[rayma]

Yes, but use an op amp buffer with it, so you don't add distortion.
That will also let you adjust the VU level independent of the volume.