rushing with crackle sound coming from my st-35

Hello all, i have a st-35 with a complete rebuild(component removal and replace). upon plug in the transformer cut out and then needed replacement also, done deal. since then the left channel has had some rushing and crackle sound until the unit warms up and then it almost stops on a metaphorical switch of "on and off" and then stays quiet until the next time you power it back up or it cools down. the noise only lasts about 10 minutes or so but has me concerned. i have tried to replace the 7247'2 and the el 84's but still proves to be somewhere else. the unit did get pretty hot and is there the possibility of some overheated components? the resister's are all 1 watt carbon film, and pio caps. the only thing i did not replace are the output transformers. i need some professional opinion's. thanks guys and gals.

If you already have replaced the tubes for the left channel then I'd say the problem is either a leaky cap, or a weak resistor in the preamp section of that channel. Try powering up with the pre amp tube(s) removed for that channel, leaving only the EL84's in to see if the noise stops. This will tell you if the output tubes and associated resistors, caps, and output transformers are the culprits should the noise continue. If the noise does stop, check the resistors and caps associated with the 7247. Again, I'd suspect a leaky cap first, but a weak or drifting resistor can cause the rushing noise you hear.

Tony

Thanks tony i really appreciate the advice, i will do some diagnostics and get back to you.

Did you replace the tube sockets? And are the tube pins clean?

If you swap tubes from the good channel with the tubes from the noisy channel does the noise move?

Hi,
I suspect capacitor (C-4) but if all of these smaller caps are original...I would
replace them all.

Kevin

The latest,............. I checked all resistors and caps for integrity and they all checked in fine. Jim's advice about the pins being clean was the answer, so far so good. I also reheated a few of the solder joints that could have looked questionable. Thank you all, the little amp that can is sounding wonderful!!!

Yea, that sounds familliar. Bad contact in the tube socket can cause many weird problems. I should know that. Except I tend to forget. My PAS3x had that problem when I got it from a seller on evilbay a couple of years ago. The seller was honest to say the preamp had a hum in one channel that he couldn't isolate. When it arrived, he even removed the tubes and wrapped them so none would break. Wouldn't you know when I put them back in and powered it up, the hum was there.

As soon as I moved the tubes in the socket, one caused the hum to disappear... completely! I cleaned the pins of each tube, checked for cold solder joints and have been using it noise free since. Go figure. At least you got to the bottom of it, and it didn't cost much to find the solution.

Tony

hagmo wrote: The latest,............. I checked all resistors and caps for integrity and they all checked in fine. Jim's advice about the pins being clean was the answer, so far so good. I also reheated a few of the solder joints that could have looked questionable. Thank you all, the little amp that can is sounding wonderful!!!

Keep in mind that the connector (pins & socket) used on a 9 pin tube is really pretty primitive. It's not gas-tight, the already quite small pin diameters vary some from pin to pin, the socket contacts vary in diameter... a tube socket and tube pins have to be in excellent condition to work properly.

Clean the sockets and pins at least yearly and you'll be much better off. If the socket doesn't grip the pins tightly re-tension or replace it.

I clean tube pins with a medium bristle brass brush (purchased from Ace Hardware if you are in the USA) and DeOxit D-5. Practice using the brush on a couple junk tubes until you get the hang of it, it works well. Brush then flush the pins with D-5 and wait a minute, then wipe the pins with a clean paper towel. Voila! Clean the small 9 pin socket contacts with the tiny dental brushes drug stores sell for cleaning between your teeth (and D-5).

FYI - the majority of the tubes I get returned as "defective" are fine - they just suffered from poor pin to socket contact. And sometimes just the act of pulling out one tube and reinstalling it or installing a different tube is enough to temporarily create adequate contact. That's enough to fool you into thinking the first tube was bad - it may not be.

I'm glad I could be of some help!!