Bibster's 5687 Linestage, the return

[bibster]

Its not entirely clear from your description, but I believe you need to connect a lead from your input/output phonos to your earth bar.

All input+output sockets are in the wooden back of the case.
All their GND's are linked together.
From these input/output-ground-rail, ONE wire goes to the U shaped ground-rail inside.

Kathodyne,

You're darn right about the not-quite-so-logical layout. It's true that I do indeed need to RE-arrange that, but... for the moment, I can't.
This is just part of the whole thing: LEARNING... I'd never put the power switch there where it's now (As you say these long feedings), and I'll put the 5U4 nearer to the chose (A lot of noise coming from there). The inputs are gonna be way further from the inputs/outputs in the next reincarnation of this linestage (don't you just LOVE diy? )

So I guess I'll just have to deal with this hum until a: I get the new tubes, and/or b: when I'll reincarnate this amp.

i'm not entirely sure....but doesn't the transformer shield need the same ground level as the circuit ground??

It's seperated by a 22 Ohm R, together with the powerline Earth, to avoid their noise.

why no grid leak?...(yeah the pot...but now it is dependant)

Haven't got a clue why.... Any idea for a value? I might try later on today....

Cheers, Paul

 

[Shoog]

Sorry, I missed that.

"Haven't got a clue why.... Any idea for a value? I might try later on today...."

The pot should do the job, but in case the pot goes open circuit, the tube will die. You don't want to decrease your input impedance much so think of 470K or even 1M.

Looking at your layout, rearranging things will probably sort the hum out., though your earthing seems logical enough. Does one channel hum more than the other. Your lucky that your hum problem is only slight on your first preamp project.

Shoog

 

[bibster]

PSU - HUM?

Hi,

Might the PSU be the source of my (constant) HUM?
I've 'psud2'-ed this PS with a 60mA (too much, I know, but let's overload is bit) Current Sink, and found a 9mV ripple (top-top)

I just haven't got a clue if that is
a: Achived in my PS (Could be more, could be less), and
b: Too much

I have however simulated the diyparadise simple5687 PSU, and found something like 1mV (TT again) ripple only.
Adding another 8H choke and 100uF LC filter after my LC would (Thank you psud) reduce my ripple to something like 0.9 mV(TT).
(But it'll cost me another 50€ to have a similar choke winded, alltough an el-cheapo would work as well for the moment, but the shoppe here in Toulouse doesn't sell any selfs as far as I know)

So: Who thinks this 9mV is (way) too much?
And: Should I increase my before-last C (47uF now) to something like 220? 400? 440?

(That last one suits my purse best I could pop in to the shop after work, and thus not see my beloved ones tonight )

Thanks, Paul

 

[Bas Horneman]

Might the PSU be the source of my (constant) HUM?

Hi Paul,

It seldomly is the PSU! Go see your loved ones tonight. Do not buy a cap...do not buy a choke. Go straight to your loved ones.

It is a pity your links to the schematic and picture of the amp don't work anymore. Otherwise we might be able to give you tips on the wiring.

Your problem is most likely a wiring problem..i.e. earth loop.

PS. Have you referenced your heater to earth yet via a virtual CT? Use something like 50-100R for that.

 

[bibster]

Hoi Bas,

It's not only a pity the links don't work anymore, it's also very strange!!! I'll see what I can do about that tonight at home, cosy beside my loved ones

I did make a virtual CT: 220K+22K (the latter bypassed with 100uF) between the B+ (266v) and the Ground-rail, which is in it's turn connected via a 22Ohm to the chassis+Screen+Power Earth.

Paul

 

[Shoog]

"PS. Have you referenced your heater to earth yet via a virtual CT? Use something like 50-100R for that."

The heater is reference to about +30V which helped with the hum. The only thing about the heaters is that they are still AC. I am a strong advocate of keeping the heaters AC whenever possible - but for me the 5687 is the exception.

Still I think its more than likely layout. The fact that the power line runs directly past the input/output phonos is a great source of worry for me. I would consider moving the power line, or if this is not possible can you use a screened cable to do the same run. If so tie the screen to the socket earth.

Shoog

 

[bibster]

Hi Shoog,

The fact that the power line runs directly past the input/output phonos is a great source of worry for me.

Well, as a matter of fact: I've taken the powerswitch out of the powerline, and connected the poewrsocket directly to the PT.
This did NOT, I repeat, NOT have any noticable effect on the hum. Pity... (Shows however how much hum there is!)
I can't (for the moment) go back to DC heating, cause the output voltage drops too much, thus underheating the tubes (By the way, my philips tubes arrived yesterday!)

I'll scratch my head tonight over a possible earth-loop. (Why doesn't this darn DMM have a 'auto grouploop detection mode' ?)

Paul

 

[Shoog]

I like to run my heaters at about 6V rather than the usual 6.3V. I have heard plenty of warnings about this but no hard evidence that it will harm the valves. Infact the only hard evidence I have seen is that you can expect a near doubling of valve life with a 10-20% drop in heater voltage.
If you had access to a 12.6Vac winding, then rectifying that and putting it through a 7812 would give you a nice 12VDC supply. That is the arrangement I am using on my 5687 which I am listening to now. Great !!!

Unfortunately having lowered the plate resistor to improve your operating point is likely to have worsened any power supply induced hum. Was there any increase when you did that?

Shoog

 

[bibster]

Unfortunately having lowered the plate resistor to improve your operating point is likely to have worsened any power supply induced hum. Was there any increase when you did that?

You make a good point there.
Unfortunatly (Or rather fortunatly?) this did not affect the hum. At least not as I recall, and I would have recalled it if it did. (I'd have jumped of my chair, opened the champane etc.. )
Therefore, I suspect the hum not being originated in the PS. (Am I right in this, guys?)

Is there some guideline to follow, finding the groupndloop that most certainly causes my hum? I've found some info, but that mainly covers loops between different pieces of gear.

Could there be some magnetic source of my hum, like the PT blazing 50hz in some signal wire? (I suppose some wire AFTER the vol. pot, as the hum is (95%) volume independant, the last 5% is becomeing audible when you move the pot past 75%)
And how could I easily check on this?
My chassis is 6mm MDF with 0.5mm aluminium glued underneat, cause I thought Au is good a electric conductor, but not magnetic, hence my choice.
Should I fold some little aluminium box around (at least) the undersided of the tranny to shield it, thus to keep 'da flux' inside?

Regards, Paul

BTW, shoog: my heaters are at about 6.0V AC now... these new JAN 1985 phillips 5687 draw a hint more current maybe.

 

[Shoog]

Good idea to include the aluminium plate (though I would have used copper), but heres a thought - have you put an earth strap onto it. If not you could be spreading hum about.

Shoog

 

[bibster]

Hi Shoog,

Thanks, for your positive remark I've used one of the screws that fix the 5U4's socket to fix some sort of strip-thinggy.
The PT's screen is connected to it, as well as (via the small R) the groundrail.
Screen goes to powersocket's Earth.

Paul

 

[Shoog]

Try cutting that strap and replacing it with a number of straps that go strait to the mains socket. Especially think about straps from next to yout PT and Choke. It probably wont help, but its worth a try.
Its amazing how much potential difference can develope between various points on a metal case - think in the range of a couple of volts !!


Shoog

 

[bibster]

Hi,

Some visual info 'bout the hm I'm talking about....
(RMAA Noise image)

 

[bibster]

Hi,

Is that typical rectifier hum? The 100Hz makes me think so. Could it be my 5U4 radiates it into my circuit? (The signal wires allmost cross it underneath, at about 3" below)

I could sod my present case, keep just the PS in there for the moment) , and make a new case, just for the amp. Should I move the last PS RC filter into the amp case than (As they are seperate for each channel)?

Cheerio, Paul

 

[Shoog]

Shouldn't really be necessary to scrap the case. Re-running some wires and a second look at the grounding should sort it out. Yes it does look like rectified hum (ie 120hz).

Shoog

 

[bibster]

(ie 120hz)

Ici la france, mon chèr....
50Hz, PAL (and cheese and wine) and such
No 60Hz, Never The Same Colour (and Coke and Hamburgers) stuff...
(Ignore this, my american friends!)

However, how should I run my wires (without scapping the case) when I want to have'em far from the rectifier?
Other thing, to check if it's the 5U4, could I (Just to check) replace it by 2 1N4007's?

Paul

 

[bibster]

The pot should do the job, but in case the pot goes open circuit, the tube will die.

Could you explain why? Open circuit == pot at full blast? In the original schematics they're not there, nor in the diyparadise.com simple 5687 preamp.

You're lucky that your hum problem is only slight on your first preamp project.

Well.... seen that noise graph I post earlier on? It is getting on my wick this hum... bl**dy annoying! (And the fact I can't seem to figure where it's coming from!)

It is a pity your links to the schematic and picture of the amp don't work anymore.

Well, they seem to be working again! (Hint hint.. )

Your problem is most likely a wiring problem..i.e. earth loop.

Well, I've redone some earth rewireing (Sorry no pics yet)
I'll describe what I've done:

Cut the ground rail as to start at the first tube and end at the other one (ie. just one straght line between the tubes)
The (common) ground of all but the last PSU caps goes to the middle (at about...) of this rail, as well as:
The 0 off of the tranny
the (common) ground of the in/out phono plugs
The ground end of the V divider for the virt. CT for the heaters
The gnd. off of the pot.

Regarding the shielding of the signal wires (Do I really need to do that??):
Shields connected at input.
NOT connected at 'source selector switch'
Shields connected to POT (for the part between switch and pot)
Shields connected to POT (for the part between pot and Tube)
Those last two (At the pot) are connected to eachother ('in' and 'out' of the pot AND L+R) AND to the ground rail.
Shields NOT connected to ground at the tube
At the output:
Shields connected to GND at the tube, NOT at the phono plug

also, I've:
Add a 1000uF cap right after the choke
Add a mute switch (dpst) which grounds the outputs
Add a 10K (That was all I had, I'll fetch some 470K or 1M later) grid leaks (Near the tube, is that okay?)
Replaced the ground to chassis resistor to a very low 0.22R one. This one is simply placed of the PT, between the Shield and the 0. Shield is still connected to power-earth & chassis.

And ofcourse this didn't help at all.... (X-ept for the 1000uF, this reduced the hum A BIT)
(Well at least, when I use the mute switch, the hum is gone, just like when I tumble the power switch)

Now, WHAT should I look for? Any good ground-loop detection/determination techniques that I am unaware of?
Could it be something else? If so, then what?

regards, (a rather desperate) Paul

 

[Shoog]

"Could you explain why? Open circuit == pot at full blast? In the original schematics they're not there, nor in the diyparadise.com simple 5687 preamp."

The grid needs a ground reference, if that is absent there is excessive plate current and the valve melts.

"Those last two (At the pot) are connected to eachother ('in' and 'out' of the pot AND L+R) AND to the ground rail."

Take that to one of the valve GND's, ie small signal ground. This also applies to the phono sockets earth.

Is the power switch still on the front ?

Did you try an earth strap from the chassis, next to the choke, to the power socket ?

Otherwise you have done all you can do.

I would point out that I never got my 5687 hum free until I implemented DC heaters. Most Valves are very happy with AC on the heaters, some are not. Remember the 5687 was originally designed as a computer logic gate where small hum was never really an issue.

Shoog

 

[arnoldc]

I sympathize with your frustration bibster...

You know what? The worst (in terms of hum) preamp I ever made was a two-chassis 76-6SN7 from Sound Practices (Berman).

I used a bus arrangement, and all the parts neatly laid out and I can't get rid of the bloody hum. And like yours, has nothing to do with the volume position.

In the end I threw it away (i'm not discouraging you, it's just me) and went back to how I wired my first pre-amp, a Foreplay. Star ground, no hum!

All my preamps are wired this way I think i'm still "lucky" as far as hum goes.

Nothing to help you really, just a story to tell

 

[bibster]

The grid needs a ground reference, if that is absent there is excessive plate current and the valve melts.

I'll never stop learning...

"Those last two (At the pot) are connected to eachother ('in' and 'out' of the pot AND L+R) AND to the ground rail."

Take that to one of the valve GND's, ie small signal ground. This also applies to the phono sockets earth.

Err... take them of, and put them where? This rail is the 'small signal' ground as well...
Should I make a different seperation of grounds? What should I keep together then?
For the 'small signal' I suppose:

• Cathodes' RC
• Input (off of the pot)'s shield
• Output's shield
• 1M output resistor
• Grid leak resistor

The PSU's grounds should be with the tranny's 0
The chassis should be with the Poer earth
They should come together in some one point somewhere (How?)

Is the power switch still on the front ?

Yep, but if I take it out of the circuit, there's no change in hum.

Did you try an earth strap from the chassis, next to the choke, to the power socket ?

I'll try tonight! And I'll put a switch in the heaters to see if it's them that cause the hum.

Thanks, Paul