wow was I surprised by this! Rebuilt this amp carefully (at least the rebuild was done by looking at the amp and reproducing all the wiring according to the original build and following along the schematic...) but it is humming like crazy, there is about 1.4 v p-p on the screens and plate of the output tubes and at the input cap ( a new film cap of 35 ufd/500 v rating), the hum on the second cap is almost 0 v (a 10 ufd film cap) so it is all on the output tubes and 1st pole of the filter. Check the wiring against the schematic repeatedly and no errors I can see, there is some funky cross connections on the rectifiers using NC pins but that should not be an issue, also, the power tranny "bucks" (makes initial noise) on turn on even though there is a surge of only about 2 amps, it also "bucks" again after a warm up as the power comes up, afterwards it is as quite as can be. Anyone got an idea of what is going on? thanks!
Eico HF-30 rebuild, hums like crazy
- Thread starter gkargreen
- Start date
ferninando
Lunatic Member
just guessing:
cold solder joint
miswire
wrong part
gnd problem
defective part/tube
cold solder joint
miswire
wrong part
gnd problem
defective part/tube
plate and screen off first node 40 ufd, second node is 20 ufd... Ferninando, using was rebuilt due to really lousy owner build and general dirty condition as found, all wiring is new and rechecked multiple times for accuracy, tubes used are new/tested, this is one damn bugger of an amp!
Probably not. I suspect the connection for the preamp would have made the connection so they could have a common point between both devices. I've heard amps hum like mad when this connection was missed. I can't/won't say it is the problem here, but I wouldn't go looking for something else without trying it first.
Take a look here. Look at both "artificial center tap" and "humdinger".
http://www.valvewizard.co.uk/heater.html
You can experiment to see if the heaters are the problem. Check the heater circuit to ground with an ohmmeter. If no connection, then ground one side of the heater circuit with a clip lead. If it does make an improvement it won't be the best case scenario but it should give you an idea if it is the source of your problem. If it works, then you can add the resistors.
All the best,
John
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whoaru99
Epic Member
I understand what it's supposed to do.
But, my point remains, I think there is something "broken" per se, since I doubt in original form it had a hum problem without the virtual ground. What is causing the hum should be identified and fixed, not implement further modification confusions to try to mitigate what's actually broken.
But, my point remains, I think there is something "broken" per se, since I doubt in original form it had a hum problem without the virtual ground. What is causing the hum should be identified and fixed, not implement further modification confusions to try to mitigate what's actually broken.
John, read the article and then did as you suggested, BINGO! Yes, grounding the filament thru a pair of resistors (I used 47 ohm/side, an EICO preamp schematic noted the use of a hum balance pot of 100 ohm from which I got my value) and the hum just disappeared! The difference is wonderful! So now, could I use a 100 ohm pot and do it that way and what wattage would I need? thanks so much! BTW, the article was great, thanks for the link!
BTW, John, I just noticed you have Lyn Olson ME2, are those the floorstanders that Lynn wrote about years ago in Positive Feedback? I think they were called the Aerius or something...
I don't believe I ever said it did NOT have a hum problem before, it has ALWAYS had a hum problem when I rebuilt it. The original build was so poor in workmanship and the overall condition from being left in someone's bard for 40 years that there was no way I was going to try and turn it on before a rebuild had been done. And as been said, the grounding scheme for the filaments may have been done in the preamp connection for these amps, the Eico preamp schematics I saw DID SHOW a hum balance pot across the filaments so the suggestion, which I implemented, worked in the same fashion that I bet it was originally intended...
No accusations intended, I did note in this thread why the rebuild was done before testing, as far as the lack of a filament ground, it's not in the schematic nor was in the original build which is what threw me, but as noted by nerdorama it is somewhere in the circuit needed, and was probably done at the plug-in preamp section, I didn't get preamps with this pair..
Glad you got the Eico working quietly. A 100ohm pot would work fine. 6.3 volts on 100ohms would be 0.4 watts so a 1W or larger rate pot should be ok. I've not had much experience where using a pot worked measurably better than a pair of resistors. I've always used a pair of 100-220 ohm resistors. I've never heard, or even seen, the HF-30's. I'll bet that much EL-84 power would sound good.
I built both of Lynn's speaker versions, the Aerial floorstanders for a good friend and the ME2 satellites for myself. I still have the hard copy of Positive Feedback (when it was a printed mag) with the first version of the plans that I ever saw. The Aerial is a difficult plan to build but they sound very good, especially with a good tube amp.
John
