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bibster 3rd April 2006 08:12 AM

5687 Linestage behaving odd...

My first post in this forum, which I've been following for quite a while now: What a superb source of info this is! And amazing how some guru's'round here manage to actually help others, without even being able to see or touch or measure the stuff, because they're not living on the same continent or so!

For far for the good thing, now for the bad...

I've decided some time ago to build my very first tube amp. It being my first, I've settled for a linestage, as my integrated (ss) amp has some problems with the source selector (some don't work anymore, rattle, etc..)
I've searched around a bit, and found this linestage.

Looked like something for me, and fun to, Tube rectified: nice :)

Found a local iron winder in Toulouse, ordered the tranny and the self, found some tubes (I've replaced the rectifier by a 5UG4), bought some 'off the shelf' C's and R's, some wood etc. etc. etc.
(Ow yeah: I've chosen to use 2 22K in // for the plate, instead of 7k)
Note that I've used both 'sides' of each tube (and not both tubes!!!) in //... the plates and cathodes are both on the same B+ (The plates share their R, the cathodes are on the same batteries, one pair of NiCd's per channel: The + on the cath, the - on the Gnd)

Yesterday I finished the top of the chassis (0.5mm Aluminium, glued UNDER 6 mm. MDF) drilled the holes, put the iron and sockets in, and started laying out, soldering, some more soldering, food, soldering, and at about 17h00 I decided that 'it was done'...

Fired up the beast, no explosions whatsoever, so I'start measureing:
Cathode: +2.7V
B+: 291v (hmm... bit to high)
Voltagedrop over Plate R (11K): 210V for channel 1, 184V for the second
(that's a plate at 81V and 107V) a bit far apart, not ?
(Tubes are not matched, I *think* they're not even the same brand.....:( )

okay, let's go for it: Plugged my airport Express in, (Not audio gear in my garage... this is a verrrry handy thing for such situation, this Airtunes thinggy!), and on the out, I plugged some 'line level accepting wireless headphones'... (It's for testing, not for enjoying!!!)

Hey! It worked! I'heard some music, wow!

But then: It sounds *real* odd.... I mean: Very odd, rather like faint or so.... but then again very colse for some intruments...

itunes said: Shuffle, so it shuffled, and I ended up hearing the album 'check your head' by 'the beastie boys', and I was stunned, like really stunned: The 'music' was again far away, but: And now we're getting somewhere, the SAMPLES and the DJ's scratching, the recorded voices etc, WERE VERY AUDIBLE, like 5 times as loud or so...
Knowing that the album was recorded on LoFi 8 tracks at home, I thought: To save tracks, I'd put this kind of 'additions' to the music on a single (thus mono) track... hmmm.....

So when the 2 signals (L+R) are EXACLTY THE SAME (As in: mono signal played over L+R) the amp works 'okay', but when they're NOT, the channel kind a like cancel each other... :confused:

Has ANYONE ever experienced such behaviour? And where shoud I start searching the cause? I just can't think of anything for the moment. I've tried:
useing 1 'side' of a tube,
just one tube
invert the phase of ONE input channel
but it didn't work out....

Maybe there's some feedback through the PS?

Anyhow, if someone's got sone hint or tracks to investigate: I'd be rather thankfull!!!!!

Merci, thank you, danke, bedankt, gracias, tige dank,


karma 3rd April 2006 08:50 AM

Tubes are not matched, I *think* they're not even the same brand

time to order a matched set;)

bibster 3rd April 2006 09:48 AM


Originally posted by karma
time to order a matched set;)
I agree, I agree....
(Anyone a good source for this in southern France?)

But still: Might that be the cause of my problem? I don't quite think so...

karma 3rd April 2006 10:21 AM

these guys have good service. not sure if thay ship that far e-mail them ask;)

get a picture of the underside of the unit so we can see the setup


bibster 3rd April 2006 11:08 AM


Well..... I'm a work now, so up-skirt picture will follow in 4 hours...

Anyhow: Here's the schematics for the PS and for the linestage.
Maybe this helps a bit.... ;)

Cheers, Paul

bibster 3rd April 2006 05:15 PM


Here the (verrry hi-res) pictures:
the linestage, and
the close up of one 5687.

Hope someone can help me now...

Just put both channels on the SAME signal (// cable) and than: No sound AT ALL.
So my IN phase thinggy in the 1st post should read 180 deg. OUT OF PHASE or so...

And: When I turn the power off, near the end, when there's allmost no power left, the 'effect' disappears...
Removing the B+ from one of the tubes, makes the sound okay as well: But then again I hear it in BOTH channels.....
Some feedback from output (out of phase!) to input, via the PS??

Cheers, Paul

schiller 3rd April 2006 05:49 PM

Hi Paul, before you buy new tubes, try the following:

Seperate the 2 cathodes. Use 2 220-470 ohm potis for each,1/2watt is enough, and 2 2200uF caps. Try to establish 10 mA per tube. It will not be easy, because of the common plate resistor. Every time you adjust the one, you will need to
re-adjust the other too. After some iterations it must be work - if they are not that different. Use multi-turn pot's e.g. the small blue trimmers von burns (about 1,5 euro each).

This works perfectly for me, but i have a choke load, which doesn't affect much the plate voltage, as i change the current.

If you don't manage to balance them, seperate the plates too, use 22k per tube and 2 output caps, which are tied together at the output jack. Again 10mA per tube, this time it will be very easy to achive.

The typical 5687 will end up with 80v at the plate and about 3v at the cathode for 10 mA .
Don't get crazy if the cathodes are not at the same potential, every value between 2-4 volt is acceptable.

From this setup you can expect about (Uk*0,7)*15 Vrms output, where Uk ist the smaller cathode voltage of the 2.

If, for examble, yor measure 2v at the cathode, expect 20vrms at the onset of klipping and a very clean output under 10vrms. This is more than enough for every amp.

It will sound "big" and rich, not the last word in refinement (at least with the tung-sol and raytheon tubes that i use), but very pleasant und much better than every sub 1500 $ commercial preamp that i have heard - and i have heard many.


bibster 3rd April 2006 09:05 PM


Well... heated my soldering rod, got my DMM out...
Fiddled a lot everywhere, measured again a lot of things... still nothing!

Got fed up: there was a lot of hum as well (sounded Like grounding problem)
Hmmm... made a new ground to chassis thinggy, measured resistance between some grounds, and then:
Input connector's ground to ground: a-lot-of-ohms.. Other input: Dito, outputs: Same

Odd.... well soldered (For my (lack off) skill :) ), so what can it be?

Well, these grounds are in fact rings around the connector, and they just don't make good contact! :smash: took one of these red plasticy things out between the nuts, and: BINGO!

Did all four of them, and we're having some good sounding sound here!
(Saw, router and glue tomorrow to finish the rest of the chassis)

Bedtime now, so I'll listen tomorrow!
(First idea: a lack of bass, but detailed)

(Happy) Paul

kevinkr 4th April 2006 02:27 AM

I noticed several of the resistors in your project are severely overheating and need to be replaced with ones rated at higher wattage.

Also I think you need to get some standoffs and mount your components to those instead. Flywiring as you have done is ok for prototyping but won't result in good long term reliability, and can also be quite hazardous if something shorts to your chassis or to each other..

Burnedfingers 4th April 2006 10:39 AM

I don't know if this is considered as fly wiring or not but your tack soldering job needs to be totally redone.

Don't just tack solder a wire to another component. Wrap the wire completely around and then solder it.

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