Noisy output from 6eu7 phase splitters - related to AC filam

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dramadisease

Well-known member
Joined
Jun 17, 2004
Messages
110
Location
portland - or
so i decided to rebuild my stereo over the weekend (voice of music old tube model) and i put in mostly solen coupling caps and vitaminQ's to the output tubes, and the thing just sounds awesome BUT! i still get this same noise off the plates of the phase splitters! it has nothing to do with them coupling to the output tubes, as they generate this hum on their own. also its only in the second triode and not the first.
its 60/120hz and is affected by the hum balance, but balancing doesnt cure the problem. the 6eu7's are ac filament, can they be dc? it does this even with no input signal. maybe like 1.4Vrms AC of noise... really damn anoying... the thing is the power tubes run quiet as hell and the whole thing has been recapped....
ideas anyone??
canyou clean up an ac filament supply??
thanks!!
-bryan sours
 
..any tube can run with DC filament voltage..

In order to make sure that this is the origin of your hum, try heating up the amp, and then disconnect the heaters for the 6eu7's momentarily. If hum goes away instantly (but signal or background hiss continues for some seconds), then this is your hum source.

Try alternative filament wireing - larger diameter cable (well twisted), alternative grounding, alternative hum cancelling circuit?

And, oh, by the way - if you post a schematic, all this will be much easier..

Jakob E.
 
It didn't use DC heat when new, and it didn't humm enough to matter. Find the real problem. DC heat may not even help.

Check ALL the resistors. Especially check for balance around the phase splitter. I've seen a pair of "47K" resistors drift to 70K and 90K, unbalancing the splitter and not rejecting the power supply well.

I hope you replaced the power caps, not just the signal caps.

Could be a heater-cathode short. 6EU7 is not an expensive exotic tube. Replace it. Or if your soldering iron is hot, re-wire the socket for 12AX7: same tube, different pin-out. 6EU7 had better noise specs, but we know it was a premium 12AX7 with incompatable pinout so you could not stick a cheap low-spec 12AX7 in there. On today's market, clean new 12AX7s are readily available. (However an NOS 6EU7 may be a lot cheaper than the insane prices that NOS 12AX7s are fetching.)

How can you tell if there is hum when you disconnect the power tubes? The speaker should be dead quiet in that case. Are you probing a signal tracer in the inverter?
 
here are some schematic shots from a WEBCAM! sorry!!!!
http://Imagedeposit.com/ad_images/ImageDeposit_dot_com_FREE_Hosting_BIG_UserID_29511_Image_number_02450E5D00.JPG
http://Imagedeposit.com/ad_images/ImageDeposit_dot_com_FREE_Hosting_BIG_UserID_29511_Image_number_0245AF3100.JPG
http://Imagedeposit.com/ad_images/ImageDeposit_dot_com_FREE_Hosting_BIG_UserID_29511_Image_number_024615DA00.JPG

you'll have to cut and paste them into a window...the links dont work..


tried 2 new 6eu7's hum still there - ill go back and verify the resistors, up until this morning i had assumed it was in the driver stage and not splitter - i was wrong -
the hum is easily visable on my oscilliscope looks very close to an octave above heater frequency (60hz)
ill go double check those resistors!
ofcourse i recapped the psu!! it desperatly needed it too - the schematic is in 3 parts.
kinda hard to read i know.... theres the 'loudness' pot which is volume - which HAD this messed up loudness filter on certain taps on the pot call "tone-o-matic" i ripped that out - i bypassed the eq (uses the same type of encapslated resistor/capacitor network) and ran a 220k resistor off the balance to the last line amp stage.
the hum was present before i even started recapping - its been there a while!
thanks! ill go check the resistors
-bryan
 
wuuups!
found some drifted plate resistors - 335k instead of 220k.....
maybe i should have slowded and thought that concept through on my own rather than bugging you 2! hhaha
ill sub some replacements in and see what i get.
one question - how would drifted plate resistors result in less rejection from the psu?
thanks!
 
hi gang,

Here's a cool article from 1953 about hum. Engineers had a different view of things then; notice that converting to DC heaters was considered a last ditch solution after about 50 other options.

I'll have this article up on my office's server for a week or two. Please grab it sooner rather than later. Hope it helps, or at least entertains.

http://www.newcontrol.com/ndj/hum1953.pdf

-yours, robbing the Man of server space,
-neil
 
> hum was present before i even started recapping

Nevertheless, it didn't hum much when it was new on the showroom floor. It probably hummed more than we expect modern gear to hum, but those old designers knew how to keep it to a minimum.

> i bypassed the eq

That EQ has about 20dB loss. If you just bypassed it, then V2 (Volume-amp) noise is 20dB louder than the designer intended.

> ran a 220k resistor off the balance to the last line amp stage.

I would expect 220K to a grid to pick up every scrap of hum in the room. Try 220K from BAL to grid, 27K from grid to ground, to approximate the tone control loss and impedance.

> how would drifted plate resistors result in less rejection from the psu?

Push-pull stages tend to reject power supply garbage about 10 times better than single-ended stages, IF they are really balanced. 5% mismatch is no big deal, but my 30% mismatch caused problems.

If you really have 335K resistors marked 220K, then this thing has run so hot for so long that all the carbon-composition has baked to crumbles inside the resistor shell. And since a 220K shouldn't run very hot (my 47K ran HOT!), you may have a lot of over-baked resistors. Don't even trust the ones that read "OK": they can be OK when cool and open-up when the chassis gets hot. (My "47K" that read 90K one day read 110K the next day, and infinity after I gently cut it out of the chassis.)

> Here's a cool article from 1953 about hum.... up on my office's server for a week or two. Please grab it sooner....

Do you mind if I host it? Here? I can hold it there until I retire or am fired.
 
[quote author="PRR"]>
Do you mind if I host it? Here? I can hold it there until I retire or am fired.[/quote]

Host away; the more the merrier. Glad you think enough of it to host it.

-neil
 
ok so i replaced all the resistors in the second line amp stage and in the phase splitter stage, then removed the 220k resistor off the balance pot, threw in a 47k from grid to ground on the second line amp stage (all i had laying around that was close), and the hum significantly got better. i mean alot better probably about 8 db lower - it still hums, but its not nearly as annoying - so i guess at this point i should figure out how to get rid of the rest of the hum?
when the power amp tubes are in alone i get nothing, no hum coming off that stage. all the wiring through those 2 line amp stages that caries audio is shielded nicely. any thoughts as to where i should start checking? since all the tubes are operating how they should (proper resistors that aint burnt to a crisp inside), and balance affects the hum but volume doesnt really....i would assume it would be coming from those same 2 amp/splitter stages.
or is it the fact that the P2P wiring on this amp is a monsterous cludge...? (i didnt doit! ahhaa)
-bryan
 
> balance affects the hum but volume doesnt really

Then it is the stage between the Volume and Balance pots.
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