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Old 28th July 2009, 07:58 PM   #1
vince76 is offline vince76  Italy
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Default new build - abdellah SE KT88

Hy all,
I'm new here and this is my first build.
The circuit is the Abbellah KT88 as Gendrano has modified. except for the fact I substituted the 5 mH inductance with 250R resistors and omitted the two 10 uF condenser connected to the 47K res going on the plate of ecc88.

The amp works, so I'm happy (it is a good news for a first builder)
Anyway I have some answers for you experienced builders:

I left off the standby swich, should I include it for some reason like improving tube life?

I hear a little hum, I believe it is 50Hz related hum. what can I do? better B+ filtering or DC on filaments?

when the amp is on I can hear the rectifier rattling. is this normal?

When no source is connected to the amp (I included a source selector) I can hear a Prrrrrrr coming out from the speaker... what the hell is happening? Maybe I was wrong when connected the grounds of the two sources together? hope can understand me

hope someone can help me.

Vincenzo
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Old 28th July 2009, 10:08 PM   #2
vince76 is offline vince76  Italy
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Default about hum

just discovered that only one channel hums.
should I swap the output tube to see if the hum is related to the tube?
how to verify it doesn't depend on the ecc88?
thank you,
V.
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Old 29th July 2009, 05:32 AM   #3
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Vincenzo...
On the subject of the "rattling" sound your hearing..Quite probably it is your choke on the power supply. Of course that is assuming it is a choke input PS.
Chokes are notorious for buzzing badly...imparting the buzzing sound into the chassis. One needs to isolate a choke from the chassis with a soft pliant material to absorb vibrations.
Do you have a schematic for us to see??
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Old 29th July 2009, 02:51 PM   #4
vince76 is offline vince76  Italy
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Default schematic

Thank you Richard for your reply. I will try to put something between the choke and the chassis. The schem I followed can be found at:
http://diyaudioprojects.com/Tubes/KT88/

I found that only one channel hums and if no source is connected the hum stops. Maybe the double triode or a wire too close to heater ac wires?

Vincenzo.
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Old 29th July 2009, 09:13 PM   #5
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Hi Vincenzo-
I built this same amp, and it's dead quiet, so I'm not sure where the hum is coming from.
You say "I found that only one channel hums and if no source is connected the hum stops." This sounds like a ground loop with one of the channels. If you short the input jacks to ground, the center of the RCA connector to the outside, do you still hear the hum?

I'm also not sure where the ecc88 came into this build, I'm using the 6N1P called for on the schematic. Is it possible that the ecc88 has too much gain for this amp? Others more knowledgeable than myself can comment on this.

My KT88 build

Glenn
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Old 29th July 2009, 09:59 PM   #6
alexg is offline alexg  Philippines
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Default Re: new build - abdellah SE KT88

Quote:
Originally posted by vince76
Hy all,
I'm new here and this is my first build.
The circuit is the Abbellah KT88 as Gendrano has modified. except for the fact I substituted the 5 mH inductance with 250R resistors and omitted the two 10 uF condenser connected to the 47K res going on the plate of ecc88.

The amp works, so I'm happy (it is a good news for a first builder)
Anyway I have some answers for you experienced builders:

I left off the standby swich, should I include it for some reason like improving tube life?

I hear a little hum, I believe it is 50Hz related hum. what can I do? better B+ filtering or DC on filaments?

when the amp is on I can hear the rectifier rattling. is this normal?

When no source is connected to the amp (I included a source selector) I can hear a Prrrrrrr coming out from the speaker... what the hell is happening? Maybe I was wrong when connected the grounds of the two sources together? hope can understand me

hope someone can help me.

Vincenzo
What is the voltage rating for the capacitors that you used on the PSU? If it is 500V or 450V, then you need to use a standby switch as the PSU is capable of producing more than 500V when the tubes are cold.

If you are just hearing the hum on one channel, then please check your wiring and where you have routed the signal wires, it might be close to the heater wiring. BTW, did you twist the heater wires? It can also be a "cold solder" on the side that you hear the hum.

I have built a few of these amps, and all of them are very quiet.

Regards,

Alex
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Old 30th July 2009, 12:32 AM   #7
vince76 is offline vince76  Italy
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hy Glenn,
thank you for the input. very nice and clean buils.
I'm trying to move the ground point for the input source ground...
hoping that's the problem. It's night, tomorrow I will trying again
following your ground scheme if possible (mine is point to point)

V.
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Old 30th July 2009, 12:23 PM   #8
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Old 30th July 2009, 02:24 PM   #9
vince76 is offline vince76  Italy
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thank you Alex
I need a standby switch. I didn't put a switch in because I used for on off switch a recicled one from a computer PSU. I wanted to use two of them but realized one was under rated since it says 250V 5A.
The heater wires are well twisted. I will look for a cold solder, but I'm sure it isn't since as I turn off the amp, while the caps are discharging and I can listen to the music, the hum disappear. It have to be 50 Hz!?!?!

Vincenzo
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Old 30th July 2009, 07:14 PM   #10
piero7 is offline piero7  Italy
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controlla le masse vincè...
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