Triode Project - The Good, The Bad and The Ugly
 

"Don Pearce" wrote


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.


It ain't a kit, other than a 'kit of parts' - see:

http://www.apah69.dsl.pipex.com/keit...de_project.htm

Triode Project - The Good, The Bad and The Ugly
 

Keith G wrote:

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 scope's probes will be very high impedance - you can just bung it on a
bit of circuit, and you'll still be able to hear what's going on. I'd
suggest chucking out everything other than the iffy amp (with a load) and
the scope, and trace the circuit through - compare the trace at the same
point on left and right channels.


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

If the hum is definitely in one channel, then I think the PSU will be okay.


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

Triode Project - The Good, The Bad and The Ugly
 

On Sun, 4 Jul 2004 19:45:55 +0100, "Keith G"
wrote:


"Don Pearce" wrote


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.


It ain't a kit, other than a 'kit of parts' - see:

http://www.apah69.dsl.pipex.com/keit...de_project.htm





Ah! I think Chris had a good idea, though - pull out the input
pentodes and see if the hum goes away. If it does, then pretty
certainly you need to improve the smoothing on that stage. The
Blumlein pair of triodes following should have a bit of power supply
immunity, so they shouldn't be anything like as much a source of
bother.

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

Triode Project - The Good, The Bad and The Ugly
 

Hi,

In message , Keith G
writes

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

You *can't* be a proper techie. If you haven't got any spare bits left
over at the end of a kit build, either you haven't build it properly, or
you got it from IKEA and only half of it arrived in the first place :-)

- 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..... :-)

Now that's more like it. My local garage do that, then offer to sell
them back to you as spares.

The records are great, btw. Lionel is doing his thing at the moment. Ta
very much!
--
Regards,
Glenn Booth

Triode Project - The Good, The Bad and The Ugly
 

"Chris Morriss" wrote


Here's an idea.

I guess your input stage valves are capacitor-coupled to the grid of the
300B.


Nope, the whole point of the amp is that the input valves are *direct*
coupled to the 2A3 triodes.


You could therefore run the amp with only the output 300B valves
in and see if the hum is still present.


I don't think that we a very good idea...


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.


The money's still on an earthing problem atm, but the 100Hz frequency is a
bit mysterious....

Triode Project - The Good, The Bad and The Ugly
 

"Don Pearce" wrote

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



Yes and the waveforms are the same height in SoundForge, so I'm suspecting
the scope and/or leads atm. (No big deal, the scopes only a learning tool
atm.)

Triode Project - The Good, The Bad and The Ugly
 

Hi,

In message , Keith G
writes

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.....???)

A spectrum analysis in Cooledit reckons that the channel difference is
tiny - within 0.5dB. I'd say it's happening to both channels.

--
Regards,
Glenn Booth

Triode Project - The Good, The Bad and The Ugly
 

Keith G wrote:

You could therefore run the amp with only the output 300B valves
in and see if the hum is still present.

I don't think that we a very good idea...

Should be okay, I'd have thought.


The money's still on an earthing problem atm, but the 100Hz frequency
is a bit mysterious....

100Hz is post-rectification - I'd have a look at the ripple on the supply.


--
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


Here's an idea.

I guess your input stage valves are capacitor-coupled to the grid of the
300B.


Nope, the whole point of the amp is that the input valves are *direct*
coupled to the 2A3 triodes.


You could therefore run the amp with only the output 300B valves
in and see if the hum is still present.


I don't think that we a very good idea...


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.


The money's still on an earthing problem atm, but the 100Hz frequency is a
bit mysterious....





Oh!, Didn't realise they were 2A3s. Sorry.
--
Chris Morriss

Triode Project - The Good, The Bad and The Ugly
 

In message , Keith G
writes

"Don Pearce" wrote


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.


It ain't a kit, other than a 'kit of parts' - see:

http://www.apah69.dsl.pipex.com/keit...de_project.htm






I've just had a look at one of the links to the original schematic from
your site.

I see that the original used a 460R resistor from the input ground to
the B-. Does your design do the same? I'm a bit concerned about having
a common B- for both L and R amps, and also a common input 'cold' return
line.

--
Chris Morriss