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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!

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!
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.

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.

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.

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.

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

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