Yaqin MC-10L buzz

[Jo3r1]

Hello

A couple of days ago I switched on my Yaqin MC10-L, but within seconds the V1 powertube gave a bright lightflash and took the main fuse of the amp out. (bought the amp second hand, and the tubes were fairly old I guess.)

I replaced the four powertubes and the fuse, but now the amp has an annoying hum/buzz on both channels. Before, the amp was dead silent. All tubes are biased (0.35) and I also tried switching them around a bit to see if it makes any difference.

I did all the checks as described in this PDF (http://homepage.ntlworld.com/lez/YAQIN MC10/A LOOK AT THE YAQIN MC10L.pdf) on page 26-40.
By doing this I found out that the 450V and negative 35V are off a bit. Around 505V and -42V. all other measurements were within spec.

Can these voltages be the cause of the buzz? What can cause them to be so high and what can I do about it to fix them?

Thanks a lot!

 

[tubeworld]

B+ looks higher than usual. Have you checked all the PSU capacitors and see if they're still OK? Do you have a scope? If you have, you can check if the PSU ripple is too high to cause the hum/buzz.

tubeworld.

 

[Hi_Q]

If the hum/buzz is on both channels one would assume a problem with one of the power supplies as they are common to both. Is the amp an older model without voltage equalisation resistors across the two smoother strings? Without these resistors there is a chance that one of the smoothing capacitors, due to different internal leakage currents, gets more voltage than it should but this problem can usually be easily checked. Look for 'domed' tops on the four top mounted capacitors, if they are flat topped then the chances are they should be OK. If your amp does not have the visible top loaded capacitors then you can relax in the knowledge that voltage sharing resistors are mounted inside. So what else could be causing this problem? Bridge rectifier failure - not nice in the MC10L as it is not easily replaced and requires almost total strip down. But then again, rectifier failure would either blow the power fuse again or give a low or even zero B+. The only other components that could give this problem (grasping straws here without having the amp on my bench) is the hum canceling resistors, each 100 Ohms, and go from one side of each Heater rail to chassis Ground. A fault could blow a resistor and cause unbalance and make hum be heard. A long shot I know but the worst of it is the fact that these are on the same board as the rectifier! You should be able to check for something around 50 Ohms to Ground on any valve heater pin and chassis ground. It will be 50 Ohms because the 100 Ohm resistors are being placed in parallel by the valves heaters being connected.
Does the hum/buzz disappear if you pull the two innermost small tubes? This effectively isolates the pre-amp section from the output section and if the hum persists then the problem is indeed in the output stages and has to be B+ or Bias supply problem. If the Hum/Buzz disappears then it would indicate a problem common to both pre-amp channels, mmm back to those heater resistors again or a bad B+ supply. Not much to go on but hope it helps, Les