PA-774 transformer buzzing

for the DIY ST35, the Dynakit and every other PP EL84

Re: PA-774 transformer buzzing

Postby corndog71 » Sun Jun 02, 2013 11:52 am

separks wrote:
corndog71 wrote:What purpose do the 100R resistors serve? Could I run one heater secondary to 2 EL84s and one of the driver tubes and then hook the other heater secondary to the other pair of EL84s and driver tube and then connect both of them to the EL84 cathodes like in the stock circuit?


It serves to provide a voltage divider for phase cancellation (think hum pot). Indeed, you can separate the filaments.

Re-reading the thread, is your amp working and it just has a noisy power transformer? Search the forums - it seems this topic comes up a lot. I'd be tempted to cook in an oven to "reseal" the varnish. I'm sure it's been done before, but mind any odors! (sick)

Shannon


Ok, cool. I think I'll try separating the heater secondaries. And yes, the amp has been working all this time but with a noisy PA774. In fact it's my best sounding amp! (y)
I'm almost positive it's due to a noisy power line in my apartment. I've even tried replacing the PT and got the same buzzing. Plus, other devices hum when powered on. (Ceiling fans)

My brother is hooking me up with an oscilloscope sometime soon and then I'll be able to see what's going on with the AC. I hope to come up with some kind of filter to kill the buzzing.
some people call me Rob
corndog71
 
Posts: 105
Joined: Thu Feb 03, 2011 10:45 am
Location: midwest

Re: PA-774 transformer buzzing

Postby TerrySmith » Mon Jun 03, 2013 8:28 am

In my honest opinion the PA-774 is just too small for it's intended purpose. Every one I have had hums, buzzes, vibrates etc, just the nature of the beast.

Something I have done to quiet them down is to fill the end bell with RTV silicone, just enough to fill in the empty portion and cover that with a thin piece of plastic or paper so it doesn't get on the windings. The RTV I use is Permatex Ultra black, it's low odor and non corrosive. This seems to quiet down a lot of noise- even in a Hammond 270-HX!

Another trick I use for E-I power transformers is isolate them with rubber mounts. I take some 3/16 fuel hose, cut four pieces 1/4" and four 1/8". Get longer mounting screws and nylon locknuts, place the 1/4 pieces between the trans and chassis, put the longer screw in, place the 1/8 piece under the chassis then the locknut and tighten until it barely makes contact. I've tried faucet washers, but are too hard. Vacuum hose won't work either, not reinforced.
T. Smith
User avatar
TerrySmith
KT88
 
Posts: 973
Joined: Sat Jan 31, 2004 12:51 pm
Location: Maryville TN

Re: PA-774 transformer buzzing

Postby corndog71 » Mon Aug 12, 2013 1:30 pm

Solved!

So I rebuilt my ST35 in a new box with a new layout and decided I wanted to try hooking up the filaments as per the original ST35. It took me a day before I realized the PA-774 wasn't buzzing anymore. It was dead quiet! To be clear I used green & green/white for V1,3,4 and brown & brown/white for V2,5,6. I then connected pins 4&5 of V1,2 to the common cathode test point of the EFB.
some people call me Rob
corndog71
 
Posts: 105
Joined: Thu Feb 03, 2011 10:45 am
Location: midwest

Re: PA-774 transformer buzzing

Postby corndog71 » Wed Aug 14, 2013 8:03 am

I guess the excitement of getting my amp working and looking all sexy got the best of me. I was listening last night and when I went to turn the amp off I heard the buzzing again. It doesn't seem as loud as before but I haven't confirmed that. I should also mention I moved to another apartment on the other side of the building. I'm pretty sure it gets a different electrical feed but it's still noisy as even the ceiling fans in the apartment have a bit of hum to them.
some people call me Rob
corndog71
 
Posts: 105
Joined: Thu Feb 03, 2011 10:45 am
Location: midwest

Re: PA-774 transformer buzzing

Postby EWBrown » Wed Aug 14, 2013 12:52 pm

The problem could be that your AC power line is noisy or "dirty" from the effects of numerous CFL bulbs, light dimmers, switching power supplies, distorted 60 Hz waveform, and possibly some unwanted DC voltage riding on top of the 120VAC.

/ed B
Real Radios Glow in the Dark
User avatar
EWBrown
Insulator & Iron Magnate
 
Posts: 6389
Joined: Wed Mar 19, 2003 6:03 am
Location: Now located in Clay County, NC !

Re: PA-774 transformer buzzing

Postby danf » Fri Aug 23, 2013 2:29 pm

Sometimes very modest changes in the feet/support of the chassis make a difference to this kind of buzzing. Try lifting up/pressing down on each corner of your chassis to see if that affects the buzz level. Another possibility: the worst buzz I dealt with came from sympathetic vibration of a steel bottom cover on an ST-70. I replaced it with a (non-magnetic) SS cover and all was silent. You are obviously a perfectionist - I know the signs!

Best,
Dan
danf
 
Posts: 49
Joined: Fri May 29, 2009 5:52 am

Previous

Return to stereo 35

Who is online

Users browsing this forum: No registered users and 4 guests

cron