Hello there. I’m new to valve amplifiers and would like some advice please. I’ve just bought a K4040 which has been modified in several ways, including upgrading to KT88 output valves. It has also had the LED bias panel removed ‘for better sound quality’. Could somebody please advise me as to how I can check/set the bias without the LEDs?
Thanks,
Les G
Please take a close look at the board.
Each cathode resistor (close to each tube) has an indication ‘0.4V’
Carefully adjust the bias so that you measure 0.4VDC over this resistor.
Be very carefull: Voltages present in this amplifier can kill !
Please note that we do not encourage nor guarantee this modification.
Let’s just hope that it has been performed by skilled people.
Should you ever decide to return the amp for inspection/repair, keep in mind that we can only accept it in original, unmodified condition.
Thank you for the prompt reply. Is it possible to buy replacement parts for the LED circuit? If yes, could you please tell me where can I purchase them and how much would they be?
Do you think I would need to change any other circuitry or settings to compensate for the KT88s?
Regards
Les G.
Sorry, we do not sell direct.
The assembly manual mentions all necessary parts and values.
These are off-the-shelf parts, available from your Velleman distributor.
Assembly manual:
velleman.eu/support/downloads/?code=K4040
Normally, there is no need to change the circuit, when KT88 tubes are used.
Excellent. Thank very much.
Les G
[quote=“VEL417”]Please take a close look at the board.
Each cathode resistor (close to each tube) has an indication ‘0.4V’
Carefully adjust the bias so that you measure 0.4VDC over this resistor.
Be very carefull: Voltages present in this amplifier can kill !
Please note that we do not encourage nor guarantee this modification.
Let’s just hope that it has been performed by skilled people.
Should you ever decide to return the amp for inspection/repair, keep in mind that we can only accept it in original, unmodified condition.[/quote]
Is it not beter to regulate with a good Volt meter?
'O.4 V between the point and the chasis of over the resistor?
Yes, you need a multimeter set to ‘DC volts’.
I haven’t had a chance to look closely at the amp, but I am very pleased with the clarity of sound it produces compared to my Marantz PM 6010OSE, which I had thought to be very good until I compared the two!
How do I access the test points? Do I need to take the cover plate off the base, or work from the top by removing the chrome plate? Is there a diagram or photo that would show me where to connect the multimeter to take the readings and where the adjusters are?
Thanks,
Les
We have the impression that your experience with electronics is limited. That is fine, but we do wish to warn you that the voltages in the amp can kill ! It is absolutely necessary that you know what you are doing.
Sorry, there is no ‘how-to’-diagram available, as originally, the user did not have to perform this operation. Hereby a link to the assembly manual of the kit:
velleman.eu/support/downloads/?code=K4040
Yes, I am inexperienced as you say. Thank you for the warning; I am conscious of the high voltages in the amp and will be extremely careful.
I have a link which shows a picture of the K4000 bias being adjusted. Is the K4040 adjusted manually in the same way? adrian-kingston.com/K4000.htm
The amp was transported by courier with the valves removed and carefully packed seperately. Do you think the bias will need resetting after the journey? I’m not sure what happens if the bias is out?
Thanks for taking the time to advise me.
Yes, it is more or less similar: remove top plate, place multimeter over cathode resistor (we recommend insulated alligator clips) and adjust potmeter until you read 0.4V.
It is a good idea to check bias monthly and to perform adjustment after transport.
Make sure amp has heated a couple of minutes before adjusting.
Next, re-check and fine tune after e.g. 1 hour.
Thank you. I must say, I’m really pleased with the amp. It smooths out the harshness I found in some instrumentals (oboe, piano to name a few) that I found with the Marantz.
Thanks for your help.
Les
Just so people know , I have done some in-depth fixes on my Velleman K4040 including determining why some people might blow tubes. The guide is located at here https://skydrive.live.com/redir?resid=D25B243EF6E53584!195&authkey=!AHV1Mqi76n-YnQQ and hopefully people find it useful. Every build may be different but I did find some common problems