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Old 23rd November 2005, 11:00 AM   #1
Katapum is offline Katapum  Europe
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Default A BIG SCRATCH in my speakers? Help me please.

Hi all,

Some time ago I build a tube amplifier. Since then amplifier has a “little” problem.

ONLY when I turn power ON I listen a BIG SCRATCH in my speakers (95dB).

Any comments or suggestions are very welcome (and sorry my English).

Tube amp details:
1 x 274B
2 x 300B (2 x 60mA)
2 x 5842 (2 x 10mA)

itot=140mA+/-
B+=430Vdc+/-

PS: I only have one 274B tube. I listen nothing when I replace it by a GZ37.

Katapum
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Old 23rd November 2005, 11:07 AM   #2
ilimzn is offline ilimzn  Croatia
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I think you should check your schematic rather carefully, especially the connection of the diodes to the transformer (all of them!) - the way they are drawn in the diagram makes no sense and in fact the vacuum diode makes a short circuit every other half period of the AC.
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Old 23rd November 2005, 11:23 AM   #3
dhaen is offline dhaen  Europe
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The 274B is a directly heated rectifier, and will warm up quickly. The GZ35 is indirectly heated, and will warm up slowly. (Your schematic show the symbol for indirctly heated diode, but on the GZ34, the heater and cathode are joined at pin 8. I think this is just a symbol inaccuracy).

So, just to confirm what I understand:

With 274B there is noise,
With GZ34 there is no noise..?

If the power supply is the cause:
I think the "scratch" noise may be because of the ringing caused by the 2 * 10H chokes and the capacitor combination 1u + 270n + 100n + 33n.

If this is the case, try adding 220K 2W resistor across each 10H choke.

Of course there could be many other reasons, especially some instability in the amplifier as it warms up. This is just my "gut feeling".
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Old 23rd November 2005, 11:37 AM   #4
timpert is offline timpert  Netherlands
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The scratch you're hearing is probably from the rectifier arcing over internally at start-up, causing interference in the circuit via the 2 10H chokes. The 274B is a delicate tube, and quite prone to arc. Look at bright white sparkling spots on the filament during startup, this is usually a first sign of arcing, and a warning sign for more trouble. If you see this happening, quit using the tube in this application immediately, as the rectifier may fail in such a way that it takes other components with it.


I had the same problem with a 5U4G in a choke-input application (which should be easy on the rectifier during operation). The Russian version lasted longest, but eventually also started to sparkle and then arc. Directly heated rectifiers heat up so fast that the voltage rise caused currents to flow which saturated the input choke, which in turn caused the current to increase even more. With a GZ34 or GZ37 in the same position, measured current increases gently towards its nominal value as it heats up, and it comes nowhere near input choke saturation current (which, in my case, is about 500 mA) . As you have an input capacitor of only 1uF, your problem might be similar to mine, pushing the 2*10H chokes close to their saturation point.

There are two solutions: either a soft-start, that switches on full load after all tubes have warmed up properly, or a slow heating rectifier such as a member of the GZ family. The 274B reportedly sounds very nice, but it's too fragile to use without precautions.
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Old 23rd November 2005, 11:44 AM   #5
dhaen is offline dhaen  Europe
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Good point.
Is the problem new or old?
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Old 23rd November 2005, 03:03 PM   #6
Katapum is offline Katapum  Europe
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Hi,

Quote:
….capacitor combination 1u + 270n + 100n + 33n

???

Quote:
With 274B there is noise,
Yes. (274B Full Music)

Quote:
With GZ34 there is no noise..?
GZ34 isn´t a good replacement (B+ drop to much). I replaced with a GZ37

Quote:
Is the problem new or old?
It´s a very old problem.


Katapum
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Old 23rd November 2005, 03:35 PM   #7
dhaen is offline dhaen  Europe
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Quote:
….capacitor combination 1u + 270n + 100n + 33n


???
These are components on your schematic..

Quote:
GZ34 isn´t a good replacement (B+ drop to much). I replaced with a GZ37
My mistake, I meant GZ37.

Quote:
quote:
With 274B there is noise,

Yes. (274B Full Music)
I mean is there scratch noise?
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Old 23rd November 2005, 04:18 PM   #8
Katapum is offline Katapum  Europe
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Hi,

Quote:
These are components on your schematic..
Sorry dhaen.

Quote:
With 274B there is noise,
Yes noise is ONLY with 274B tube.


Where can I buy a new/good/cheap GZ37 tube?

Katapum
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Old 23rd November 2005, 11:05 PM   #9
Katapum is offline Katapum  Europe
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Hi all,

Quote:
If the power supply is the cause:I think the "scratch" noise may be because of the ringing caused by the 2 * 10H chokes and the capacitor combination 1u + 270n + 100n + 33n.
I simulated all PSU with PSU2. PSU2 shows graphic without ringing problems.

Quote:
Look at bright white sparkling spots on the filament during startup, this is usually a first sign of arcing, and a warning sign for more trouble. If you see this happening, quit using the tube in this application immediately, as the rectifier may fail in such a way that it takes other components with it.
274B looks ok at start up.


When I replaced tubes I didn’t change socket connections. Is GZ37 a DIRECT replacement to a 274B? If not how could I connect a 274B with two transformers (main and heater) without centred taps?

I fixed PSU schematic. Is it ok now?

Katapum
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Old 23rd November 2005, 11:22 PM   #10
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There is one correction still. The usual rectifier pin-out is pins 4 and 6 each to an anode, not anode and cathode as drawn.
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