Non-Traditional 15 watt Liverpool with PCB

[vincenz]

After some tests:
-All the connections to ground are ok (less than 1 ohm)
-I if remove V1, no more hum, nor hiss.
-Only the GAIN potentiometer does affect the hum.
-See my Voltage chart below, the V3 voltages seem to be wrong ?!??

[img:116:165]http://img4.hostingpics.net/thumbs/mini_801900Voltages.jpg[/img]

Here are some better pictures.

[img:165:123]http://img4.hostingpics.net/thumbs/mini_641498livp1.jpg[/img]

The big 1k/25w resistor is connectect by under the board (yellow line)
[img:165:123]http://img4.hostingpics.net/thumbs/mini_269195livp2.jpg[/img]

[surfsup]

I don't think your v1b should be lower than your v1a

Are those in the correct order? or is v1b used as your input?

[martin manning]

See my Voltage chart below, the V3 voltages seem to be wrong ?!??

Yes, your PI seems to be drawing more current than expected. Check the resistor values and connections. The solder joint in the center of the snippet below looks questionable to me. V1 and V2 voltages look fine to me. Have you tried different tubes in V1? You might have to try several before you find a quiet one.

[vincenz]

well,
I double checked all the resistor values, I also redid these solder points, just to be sure...

I tried others preamp tubes, and it seems to work better now. Actually, all the tubes I had in this packages was bad !!
So, now with good preamp tubes (I tried 2 or 3 differtents in each position),
only a little hum remains but still a lot of hiss, I guess it is because of the topology of this PCB variation...

This amp sounds not too bad, but is very noisy compared to my typical Express build... At this point it will not be eligible for the live, nor the studio...

[M Fowler]

That is odd the PCB Express amps I've built were more stable then traditional builds?

[martin manning]

So, now with good preamp tubes (I tried 2 or 3 differtents in each position),
only a little hum remains but still a lot of hiss, I guess it is because of the topology of this PCB variation...

Not likely. I would suspect a cold solder joint or a defective component such as a noisy resistor or leaky capacitor. Is it still the case that the hiss stops when you remove V1? if so then the issue is likely to be somewhere around there. Which controls affect the level of hiss?

[vincenz]

Is it still the case that the hiss stops when you remove V1?

yes it is

Which controls affect the level of hiss?

only the GAIN. Nothing changes with the bright switch, nor the 3 EQ pots. Of course it increases with the master volume ...



EDIT:
In order to exclude any cold solder joint, I re-soldered all points.
Regarding the signal caps, I already replaced all of them with fresh good Malory.
Therefore, I guess now I should replace all resistors

[martin manning]

I would try replacing the plate resistors on V1 with MF type first, but what values are the grid stoppers on the first two stages?

[vincenz]

I would try replacing the plate resistors on V1 with MF type first, but what values are the grid stoppers on the first two stages?

Grid stoppers are both 47k on V1. On V2 I have a 22k

[martin manning]

47k should be more than enough. You can try replacing the plate resistors, but I find that the biggest driver is the tube itself.

[vincenz]

47k should be more than enough. You can try replacing the plate resistors, but I find that the biggest driver is the tube itself.

Ok I will try replacing the plate resistors...
Regarding the tubes, I tried 5 differents (good NOS that I know they are quiet, some modern ones, and 12AT7 as well). More than less the hiss does not change...

[martin manning]

If that doesn't improve things, perhaps you can look for oscillation. Do you have access to a 'scope?

Here's something else, not likely to cause hiss, but this is a ground loop isn't it? As I understand it the PCB trace to which this wire is attached is grounded at the mounting screw near the input jack.

[vincenz]

The last mods:
-replaced all resistors around the V1, as well as the two 22uf. (Parts are from the leftovers of a Ceriatone kit)
-changed the presence layout for the same as on RJ's build
-removed the 2 solder points on the potentiometers (ground loops ?)
-modified the bright switch: I kept only the 100pf on a 2 way switch

I think this baby is getting a little better, thanks to you Martin !
More quiet now. Still a good amount of hiss, so it is not perfect, but it will be acceptable...

[IMG:165:97]http://img15.hostingpics.net/thumbs/mini_461395finalbuild.jpg[/img]

[Colossal]

Still a good amount of hiss, so it is not perfect, but it will be acceptable...

Hiss at idle is normal. It's a high gain amp! if you are happy, now try different preamp tubes to find the best sounding and lowest noise.

[vincenz]

I changed once again the V1 and V2.
actually... thanks to the last mods, now the amp is very quiet, quietest then my Express
And, by the way, it sounds amazing !!!


Here's the pair of winners:
This PCB-Liverpool is the upper one, into a custom-made cabinet. The old Ford Monterey nameplate is only for groove purpose

[img1794]http://img11.hostingpics.net/pics/916584pair1600.jpg[/img]