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In Reply to: RE: Eico HF 30 rebuild, hums badly... posted by gkargreen on December 22, 2015 at 10:28:10
Randy
You aren't grounded properly or one of the EZ81s are bad or you could have a leaky coupling cap feeding the EL84s.I have seen new caps leak at times..Just lift the side going into the output tube grids and see if you are leaking DC..This is most likely improper grounding tho.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Follow Ups:
thanks, Michael, tested the tubes and they are good, as noted, the output tube plates and screens have a 1.5 v p-p on them, it is a sawtooth waveform, so I am guessing there may be some issue with the ground in the power supply caps...
there was a suggestion about the grounding of the filaments which are normally grounded via the outboard preamps which I missed, once the filaments were grounded the majority of the hum was removed!
Michael, connecting the cap ground only to the center tap of the power tranny work great, no hum, but also NO SOUND! Without the caps grounded to the chassis, there is no output, it ain't working! Grounding the - caps/power tranny center tap to the chassis, we are working fine, but still have that damn sawtooth buzz in the sound, arrgh!
Lets put the right amount of capacitance in and go from there.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Thanks, Michael, your suggestion of hooking the power supply caps directly to the center tap worked great!
It's great having him here.
ET
The red/yellow wire on the transformer secondary that goes to ground,try grounding the filter caps to that as opposed to chassis ground and see what happens..In other words,use the red/yel as your ground without going to chassis and see what you get.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
What Mike said!
It sounds like the ground side of the cap(s) is floating independently of the center tap on the secondary.
Randy
Did you change the coupling caps? If you left the original wax paper caps in,you are asking for trouble.It's still grounding most likely.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
You did check the wiring on the heaters of the output tubes? So that AC from heater line is cancelled by push-pull effect.
8^)
Hi, Fender, I just rechecked the filament wiring, it is wired to the schematic which was also how the original owner wired it, i.e., all pin 4 wired to the same wire and all the pin 5 wired together with pins 3 of the 6AV6 and 6C4 wired to pins 4 of the output tubes and pin 4 wired to pin 5 of the output tubes, just like the schematic. I just moved the ground of the power supply caps to a separate ground rather than the star ground, no difference except now the in-rush current is now about 4 amps rather than 1.5 amps from before, this damn amp gets more confusing every time I do something to it, thanks
Just a swag.. did you replace the original 2 prong AC cord with a 3 prong and attach the safety ground wire to the chassis?
The bitterness of poor quality remains long after the sweetness of low price is forgotten
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