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-   -   ready to give up (http://www.diyaudio.com/forums/tubes-valves/80209-ready-give-up.html)

tokyofist 25th May 2006 07:57 AM

ready to give up
 
i'm coming up on my third month of trying to get the hum in my first tube kit down to a listenable level. the amp in question is an ella from diyhifisupply. i had a thread on their AA board, which can be found here.

i finally broke down and rebuilt the thing from the ground up, to no avail. i still have hum. confoundingly, it measures only 3.6mv and 4.8 mv in the L/R channels, respectively. i know that's pretty low, but i can still hear it from the listening position 2+ meters away. my speakers are only 90dB efficient.

i feel as if i've run out of things to try here. i have yet to swap out tubes, because this is my only tube amp and i have no spares lying around - i would have to order some. plus, i have hum in both channels, and i imagine the chances of having two bad tubes are pretty slim.

at this point, i'd appreciate any ideas/advice/offers to buy the damned thing :bawling:

-dave

EC8010 25th May 2006 08:06 AM

Take a photograph of the underside and post it here...

Welcome to the forum, but please use the "shift" key.

bibster 25th May 2006 08:33 AM

Sorry I can't help you Tokyofist,

But I'll say: Welcome to the COTWCGROTH ('club-of-those-who-can't-get-rid-of-their-hum').
Folow my thread a bit, it might have some usefull info for you, I'll follow yours!

Cheers, Paul

martinab2 25th May 2006 08:56 AM

Hi there,

I'm just guessing here as I'm not familiar with your amp but...

If I'm reading your post on audioasylum correctly, the hum got worse after you 'redid some of the grounding points'. Did you add to or change any of the grounding points while you were doing this? e.g. additional connections to the chassis, extra wiring etc. If so, you could be looking at a ground loop. Most valve amps need to be wired in a star ground arrangement - all the grounds return to a single point. If this has been altered you might get some noise problems as a result.

In a few of my own projects I had really bad hum because the heater supply was floating an not referenced to a similar voltage to the cathode (usually ground). This affects pre and driver stages the most and can vary with the tubes you use as well - very confusing
Check that the heater supply is referenced to something - this is normally done either by connecting one side to ground, a resistor network, or a capacitor depending on the design so check the schematic.

testlab 25th May 2006 11:52 AM

Are you running dc heaters? If not, try running a 100 ohm resistor from each side of the filament supply to ground. DC heaters would probably be better though.

SRMcGee 25th May 2006 12:07 PM

Tokyofist:

Voltsecond, a frequent contributor to the Bottlehead forum, maintains a website where he provides a number of suggestions for managing hum on the Bottlehead Foreplay preamp. You might try checking his website out -- perhaps there's something that could easily be implemented in your situation that would reduce your Ella's hum.

Regards,
Scott

SY 25th May 2006 12:21 PM

Second EC. A clear photo of the wiring will give our bloodhounds a high probability of spotting the grounding problem.

ray_moth 25th May 2006 12:28 PM

Is the hum 60Hz or 120Hz? In other words, is it coming from the power tranny (primary or secondaries, including heater circuit) or from the B+ filtering? I know nothing about the Ella (except that it's got a silly name ;) ). However, the cures for 60Hz hum are not the same as those for 120Hz hum.

If it's 60Hz, check for unshielded signal-bearing components (e.g. decoupling caps, grid resistors, long grid wires) in the vicinity of unshielded or untwisted AC bearing leads. Are your OP tranny and power tranny magnetic fields at 90 degrees to one another, as they should be?

Is your heater circuit grounded at its centre tap? This can be achieved either by a centre tap on the 6.3 v winding, if there is one, or by using a pair of, say, 100 ohm resistors bridging the 6.3 v line with their junction grounded. A pot could be substituted for the 100 ohm resistors, with its slider grounded; this enables you to choose the position of least hum (in theory, anyway).

Have you tried connecting the heater centre tap to a voltage divider from B+ to ground, so that the heaters are at about +20v, with the centre tap grouded also via a 10uF capacitor? This prevents the tube heaters from emitting electrons and interfering with the operation of other electrodes.

Another problem that can cause 60Hz hum is if one side of your rectifier bridge or full-wave rectifier is not functioning.

If it's 120Hz, this suggests that your B+ filtering is deficient. You could try changing, or increasing the size of, the smoothing/filtering capacitors and/or using chokes in place of resistors in the filtering line.

A very common cause of hum, which can be difficult to diagnose, is a poor grounding scheme. Hum can be picked up easily by the sensitive early stages of an amp if there are ground loops or bad grounding connections. Using the chassis as ground at more than one point is a very bad idea. Multi-point grounding is the best approach, whereby all ground connections for each individual stage are to the same point; the individual stage grounding points are then connected together. The idea of this is to ensure that ground currents do not flow between different stages. Someone else can probably explain it better!

Good luck.

SHiFTY 25th May 2006 12:59 PM

You need a triple star ground for max hum reduction-

Star 1: all signal grounds- input jacks, volume control, output tube grid resistors

Star 2: all cathode bypasses, output grounds & other non-power supply grounds

Star 3: all power supply grounds & PS capacitor negatives

These 3 stars should then be connected together at one point, and connected to the chassis. Also you could try shielded cable in the input section, with the shield attached to the signal star ground. The filament power supply should be grounded at one end or with resistors as someone else said.

Also have you tried swapping the preamp tubes for some other ones? Even swapping sides- you could see whether the hum folloes the valves- they might be noisy...

I also suggest connecting a battery powered source like an ipod or similar to make sure you don't have a simple earth loop. Also try moving it away from other mains powered gear.

Don't give up! You will surely solve it with a little detective work :)

tokyofist 25th May 2006 04:14 PM

Wow, a lot of great advice. Thanks!

EC and SY: I'll get a hi-rez shot of my amp up tonight. Here is a link to a picture of someone else's if people are need to get an idea of the layout. Mine is laid out in almost identical fashion.

martinab2: The initial problem posted on the AA thread was a real lawnmower hum caused by a loose ground connection - it has since been resolved. The issue presently at hand measures small, but is still too loud to be acceptable IMO.

testlab: Heater voltages on the stock ELLA are 6.3 VAC. If I were to run 100 ohm resistors from the filaments to ground, would that be for all valves or just the output section? driver section?

ray: I have yet to acquire an o-scope, so I can't say for sure what frequency I'm dealing with. There is plenty of noise coming from my tweeter as well as the woofer. Could 60/120hz hum find its way up there?

shifty: The ELLA uses a ground bus down the center of the chassis (see attached pic). When I've been testing it, the amp hasn't been connected near any other gear, so I feel it's safe to say the problem is within the amp itself.

Thanks again fellas, I'll post a picture later tonight.


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