Triode Project - The Good, The Bad and The Ugly
 

On Sun, 4 Jul 2004 18:13:43 +0100, "Keith G"
wrote:


"Don Pearce" wrote



Mmmmm... that hum is 100Hz, not 50, so it pretty certainly comes from
the power supply rails, not stuff like ground loops. Do you have a
spare fat electrolytic you could hang across power supply in parallel
with the one already there? See if that kills, or at least reduces,
the level.


OK, that's interesting and would be good if I *don't* have any ground loops.
No I ain't got any spare electrolytics, but I can damn soon rectify*** that!
(I actually managed to use up all the components - I've come a long way from
me motorcycle-fettling days when I always ended up with a jamjar full of
'leftovers' for each bike..... :-)

Interesting thing is that, now that I've got it on the computer, I 'scoped
it**:

http://www.apah69.dsl.pipex.com/keith_g/show/humpic.jpg

The different traces swap places exactly when I switch the leads over, so
summat's up. (Buggered if I know what tho' - I'm not much cop at scopes
either!)

Which reminds me I haven't had an answer yet to an earlier question - that
top trace will slowly slide down the screen over a period of time. Anybody
got any idea what causes that?


**You sods are turning me into a bloody techie!!

***techie pun?

:-)


OK, if you have no major bits left over, you *definitely* built it
wrong - that's the law.

Good scope traces, though - but what are they of? I have the circuit
diagram of the amp (sadly not the power supply) in front of me.

d
Pearce Consulting
http://www.pearce.uk.com

Triode Project - The Good, The Bad and The Ugly
 

On Sun, 04 Jul 2004 17:22:21 +0100, Don Pearce
wrote:

On Sun, 4 Jul 2004 16:58:57 +0100, "Keith G"
wrote:


"Don Pearce" wrote


Keith - where's the MP3 of the hum? Maybe we can speed up the
tracking-down process.


OK, here's a clip from the record I've got on atm - Branford Marsalis
'Scenes In The City':

http://www.apah69.dsl.pipex.com/keith_g/show/hum002.mp3 2.5 Mb/1:49

The hum is before, during and after this 'end of track/beginning of next
track' snatch of music - which is to give you a sound level reference.

Recorded with an open mic, so it's a bit 'boggy' - no 'Line Out' luxuries
here, I'm afraid!! :-)

(Recording's another thing I've got no clue about! - Also not me best
speakers....!! ;-)

Mmmmm... that hum is 100Hz, not 50, so it pretty certainly comes from
the power supply rails, not stuff like ground loops. Do you have a
spare fat electrolytic you could hang across power supply in parallel
with the one already there? See if that kills, or at least reduces,
the level.

Actually, one of the nice things - the *only* nice thing? :-) - about
valve amps is that the HT current drain is low enough that you can use
a classic CLC 'pi' filter to virtually eliminate 100Hz problems on the
supply rail. I'd give it a go - if you find some spare space on the
chassis for more iron bits!

--

Stewart Pinkerton | Music is Art - Audio is Engineering

Triode Project - The Good, The Bad and The Ugly
 

Keith G wrote:

It's looking like the hum is 100Hz, but my scope telling me that it's
left-channel weighted. (If the scope can be trusted.....???)

Do you still get this if you swap the probe leads around? If the disparity
of amplitude shown on the scope doesn't swap around, your scope set up is
wrong.


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Triode Project - The Good, The Bad and The Ugly
 

In message , Keith G
writes

"Chris Morriss" wrote


Do you think the hum is coming from the directly-heated filaments of the
300Bs, or from the HT line? (Heater hum is likely to be 50Hz, HT hum is
more likely to be 100Hz)


It's looking like the hum is 100Hz, but my scope telling me that it's
left-channel weighted. (If the scope can be trusted.....???)


If it's the filament, then you could try using a resistive centre-tap on
the winding and earthing the CT, (or taking it to the bias circuitry or
whatever).

You may have a CT filament winding already of course....


The filaments have their own transformers where the CT (if it is one - grey
and blue secondaries twisted togetther??) is taken up to a 200V rail which
it shares with the cathode resistors and bypass capacitor as in:

http://www.apah69.dsl.pipex.com/keit.../ampkit124.jpg

The red and yellow secondaries supply 2.5V @ 3A directly to the filament
(measured about 2.69V, IIRC)





Here's an idea.
I guess your input stage valves are capacitor-coupled to the grid of the
300B. You could therefore run the amp with only the output 300B valves
in and see if the hum is still present. If it is you know that the +ve
feed to the anode of one of the 300Bs is wrong in some way. If the hum
goes away, but comes back with the input valves in, then you might have
faulty decoupling on these in some way.

I see the URL you gave shows the PSU schematic poking out from under the
chassis. The 40u + choke +100u should give adequate smoothing for the
output triodes.
--
Chris Morriss

Triode Project - The Good, The Bad and The Ugly
 

On Sun, 4 Jul 2004 17:50:52 +0000 (UTC), Stewart Pinkerton
wrote:

Mmmmm... that hum is 100Hz, not 50, so it pretty certainly comes from
the power supply rails, not stuff like ground loops. Do you have a
spare fat electrolytic you could hang across power supply in parallel
with the one already there? See if that kills, or at least reduces,
the level.

Actually, one of the nice things - the *only* nice thing? :-) - about
valve amps is that the HT current drain is low enough that you can use
a classic CLC 'pi' filter to virtually eliminate 100Hz problems on the
supply rail. I'd give it a go - if you find some spare space on the
chassis for more iron bits!

--

Stewart Pinkerton | Music is Art - Audio is Engineering

True, but what bothers me is that this is a kit, which presumably
should work OK without going to those lengths. I'm wondering if the HT
for the first stage has perhaps been taken from the wrong spot on the
RC chain.

Have a look at http://www.worldaudiodesign.com/pdf/sup5688upg.pdf It
is interesting that they are using KT88s, but haven't bothered with
ultralinear tapping of the output transformer.

d
Pearce Consulting
http://www.pearce.uk.com

Triode Project - The Good, The Bad and The Ugly
 

On Sun, 4 Jul 2004 18:27:01 +0100, "Keith G"
wrote:


"Chris Morriss" wrote


Do you think the hum is coming from the directly-heated filaments of the
300Bs, or from the HT line? (Heater hum is likely to be 50Hz, HT hum is
more likely to be 100Hz)


It's looking like the hum is 100Hz, but my scope telling me that it's
left-channel weighted. (If the scope can be trusted.....???)


If it's the filament, then you could try using a resistive centre-tap on
the winding and earthing the CT, (or taking it to the bias circuitry or
whatever).

You may have a CT filament winding already of course....


The filaments have their own transformers where the CT (if it is one - grey
and blue secondaries twisted togetther??) is taken up to a 200V rail which
it shares with the cathode resistors and bypass capacitor as in:

http://www.apah69.dsl.pipex.com/keit.../ampkit124.jpg

The red and yellow secondaries supply 2.5V @ 3A directly to the filament
(measured about 2.69V, IIRC)



The MP3 has virtually identical hum on both channels - it certainly
doesn't have anything like the disparity that the scope picture shows.

http://www.donepearce.plus.com/odds/keith_screen.jpg

d
Pearce Consulting
http://www.pearce.uk.com

Triode Project - The Good, The Bad and The Ugly
 

"Don Pearce"

OK, if you have no major bits left over, you *definitely* built it
wrong - that's the law.


Yes, that *is* a bit worrying! :-)


Good scope traces, though - but what are they of?

OK, they are the 'hum' from the Branford Marsalis clip played from the
computer to a little Technics amp which is connected to the scope - one pair
of speaker outlets (A) drive a pair of speakers, the other speaker outlets
(B) are connected to the scope. Enables me to hear tones at the same time as
I can see them**. (Crosstalk between the speaker outlets notwithstanding)

The thing is that a) the better of the two traces (left channel) showed the
'fuzziness' of hum and appeared to confirm your 100 Hz (I didn't do the
sums - I just saw 2cm waves @ 5 ms/cm) and b) the traces swapped over
exactly when I reversed the leads from the speaker outlets.....


I have the circuit
diagram of the amp (sadly not the power supply) in front of me.


OK, I've posted the ps circuit....


http://www.apah69.dsl.pipex.com/keit.../pscircuit.jpg


.......but I daren't post the whole thing as I don't know what the form is
with publishing people's circuits???




**not so dusty for a thick git, eh? ;-)

Triode Project - The Good, The Bad and The Ugly
 

In article , Keith G
writes

"Don Pearce"

OK, if you have no major bits left over, you *definitely* built it
wrong - that's the law.


Yes, that *is* a bit worrying! :-)


Good scope traces, though - but what are they of?

OK, they are the 'hum' from the Branford Marsalis clip played from the
computer to a little Technics amp which is connected to the scope - one pair
of speaker outlets (A) drive a pair of speakers, the other speaker outlets
(B) are connected to the scope. Enables me to hear tones at the same time as
I can see them**. (Crosstalk between the speaker outlets notwithstanding)

The thing is that a) the better of the two traces (left channel) showed the
'fuzziness' of hum and appeared to confirm your 100 Hz (I didn't do the
sums - I just saw 2cm waves @ 5 ms/cm) and b) the traces swapped over
exactly when I reversed the leads from the speaker outlets.....


I have the circuit
diagram of the amp (sadly not the power supply) in front of me.


OK, I've posted the ps circuit....


http://www.apah69.dsl.pipex.com/keit.../pscircuit.jpg


......but I daren't post the whole thing as I don't know what the form is
with publishing people's circuits???




**not so dusty for a thick git, eh? ;-)





They reversing the 5 volt heater windings?, hasn't the GZ34 got a
cathode????
--
Tony Sayer

Triode Project - The Good, The Bad and The Ugly
 

On Sun, 4 Jul 2004 19:08:29 +0100, "Keith G"
wrote:


"Don Pearce"

OK, if you have no major bits left over, you *definitely* built it
wrong - that's the law.


Yes, that *is* a bit worrying! :-)


Good scope traces, though - but what are they of?

OK, they are the 'hum' from the Branford Marsalis clip played from the
computer to a little Technics amp which is connected to the scope - one pair
of speaker outlets (A) drive a pair of speakers, the other speaker outlets
(B) are connected to the scope. Enables me to hear tones at the same time as
I can see them**. (Crosstalk between the speaker outlets notwithstanding)

The thing is that a) the better of the two traces (left channel) showed the
'fuzziness' of hum and appeared to confirm your 100 Hz (I didn't do the
sums - I just saw 2cm waves @ 5 ms/cm) and b) the traces swapped over
exactly when I reversed the leads from the speaker outlets.....


I have the circuit
diagram of the amp (sadly not the power supply) in front of me.


OK, I've posted the ps circuit....


http://www.apah69.dsl.pipex.com/keit.../pscircuit.jpg


......but I daren't post the whole thing as I don't know what the form is
with publishing people's circuits???




**not so dusty for a thick git, eh? ;-)




You'll be an engineer by the time you're done!


Well, see another post where I look at that file, and the hum on the
two channels is essentially identical. The PSU circuit doesn't seem to
quite stack up with the power amp one I have. There are connection
points A-D, presumably along a chain of resistors and capacitors, for
the various valves. Does the input valve anode resistor connect to
point B on the PSU?

d
Pearce Consulting
http://www.pearce.uk.com

Triode Project - The Good, The Bad and The Ugly
 

"Don Pearce" wrote


You'll be an engineer by the time you're done!


Oi! You watch yer language!!



Well, see another post where I look at that file, and the hum on the
two channels is essentially identical. The PSU circuit doesn't seem to
quite stack up with the power amp one I have. There are connection
points A-D, presumably along a chain of resistors and capacitors, for
the various valves. Does the input valve anode resistor connect to
point B on the PSU?


Don, I think we are at cross-purposes - see other reply.....