Preamp Deal of the Century

HI Guys,

Don't know if anyone is keeping up with the thread anymore, but I have a problem with my Cortese and I was wondering if anyone might be able to help.

I am getting a spitting static noise through one channel. I have fliped all the tubes and the noise still comes through the same channel. I have a friend looking at it now. He thinks it is one of the adjustable resistors in the line stage. He replaced them, but there is still noise in the same channel. He is going to check connections in the power supply next. He has already checked connections in the preamp and they seem to be ok.

Does anyone have any ideas?

Thanks,

Tom

IMHO,

When the usual hit or miss tube swapping troubleshooting technique fails, you have to have a freq generator and a scope to solve problems like this. This is not just a matter of what "looks" broken anymore.

Your tech friend, how did he come up with the decision of replacing the resistor first?

Tell him to isolate the PSU circuit to the actual amplifier circuit. By feeeding very low freq signal at the input, you can tell if the power supply is the one with a problem looking at the modulation, if any, of the B+ signal. At mid to higher frequencies, you could test the real amp section stages by stages, assuming that the usual downstream B+ voltages checks out as it decrease when being decoupled from the stages, and see where the noise is coming from.

Please ask your tech about those. If he does not understand what you are talking about, find a different tech. It would be helpful also to plot the schematic first before poking around with the scope so that you can easily follow which point checks out in relation to the diagram.

Hope it helps......

ADDENDUM

My post above assumes that you already tried swapping amp, speakers etc. and the problem is in the linestage. If phono, and using a 6106 rectifier, I would take a look at the FET for that particular channel. The 6106 have a voltage drop of 60V, and a maximum dc current output of 125 ma. The 5AR4 on the otherhand is 17 and 250 mA. As you can see, the preamp is being starve not only with the voltage but also of the current when using a 6106. Since transistors are current driven, I would take a look at that first if it was me but using a 5AR4 rectifier.

I seem to remember that Mick was frustrated about units that was shipped back damage from using 6106 tube in the PSU before. IMO, it is possible that what i described above could be the reason.

I could be wrong of course ......

You may want to contact Michael Samra at Audio Asylum. He did some mods on a Supratek, and seems familiar with it.
It may just be a loose connection. I also did these mods to mine and my preamp sounds much better. Better than anything I have owned. Hope that helps.

Here's the mod thread and his name to email him.
http://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=148308

for what it's worth, when i first received the malbecs, one of them had an annoying popping, spitty sound which turned out to be the bias resistor on one of the KT88s. if your tech friend couldn't find any issue with any of the resistors, then i guess you could just email mick or kevin covi for assistance with trouble shooting.