Yaqin mc10L

[Hi-Q]

Hi Gerry, I use the svetlana 6N1P available from Watford Valves.

http://www.watfordvalves.com/product_detail.asp?id=1479

For biasing the output stages you need a good multimeter to connect across each of the 10 Ohm resistors associated with each valve.
The original voltages I measured as being around 330mV (0.33V) but I set my outputs to 300mV and seems to be ok. Running a bit lower will extend the life of the valves. I recently checked my amp after it had been in use for 12 months and found each valve had dropped to 250mV so I adjusted each control to bring them all back to 300mV.
I think that when the day comes that you cannot get 300mV any more, then its time to change the valves. But before you put the new ones in make sure you back down the controls for minimum voltage with the old ones in circuit. Then let the new ones heat up for a good 15 minutes before setting each as required.
Les

[gasman]

When i checked the bias on mine 3 of the valves were set to 0.4v and the last one was 0.9v, have reduced them all to 0.3v,so it does go to show that you cant trust the factory settings on these units so checking does seem to be a must.

[rk-one]

Boys you are great (HI-Q u're my hero )
2 last questions because i'm really noob : in which position must be set multimeter and i don't undertansd if i'have to put the red pin on the upper side and the black pin in the lower side of the resistor or the black in other position.

thank YOU!!!!!

Gerry

ps is there any way to check plate voltage? or it's no necessary?

[Hi-Q]

Use a digital multimeter set to 2V DC. If you get the meter leads round the wrong way it will still give a correct reading except a minus sign will appear to the left of the readout. If my failing memory serves me right, the control pots are wired correctly such that turning the pots anti-clockwise reduces the valve current and hence the multimeter reading. The controls are fairly course so adjust very carefully. Wow! 0.9 that's high and I am not surprised the anodes were not glowing red.
I just heard today that a friends MC10L has blown its mains transformer! It's a pity that I cannot get access to it (he lives a long way away) as I am sure there must be a fault on the amp other than just blaming the higher UK voltage, which he does! Best to check the heater volts if you can, two yellow twisted wires around V2 on the pre-amp board at the top edge of board. This of course is AC volts and expected to be approximately 6.3V. Take care you do not short these wires out as it will cause loss of heaters on the left hand side and bring forth a burning smell. I know!, how do I know? Well of course I found out the hard way - damned crocodile clips shifted!
It burns out the PCB track on the main board and that is a bit more difficult to get to, but that's another story

[rk-one]

Job done!
original bias was not good 33.8 38.9 - 34.4 36.3
Now all at 32.6 thank You Hi-Q

Now i wanna try better tubes (sed or jj kt77)
and new 6n1p, please post your opinions and experiences about tubes.

Gerry

[gasman]

Hi All, Some great advice from Les, Tested the heater voltage on my MC10L. came out at 6.5v thats close enough for me but just one thing to note, on my amp which is very new, the heater voltage terminals are on the other side of the board next to V3, Cheers. ps, hope you mean 0.326v RK.

[rk-one]

Yes 0.326 v

[gasman]

Did you manage to get the heater voltages while you got the lid off Gerry, I did mine, came out at 6.5 v so only just over, near enough for me, Glad you got a result with your biasing, Do you like the sound of the MC10L, Cheers Brian.

[rk-one]

No sorry.When i'll open it again i'll check it.
I like mc-10l but i'd like to improve details and make sound better and firmer the bass. I heard the yaqin ms-300b and has better and more accurate highs and larger scene but not so great in the medium-low zone.
I wanna try new tubes but i've to choose which brand.


Gerry

[Hi-Q]

Well I am wondering if replacing the 100nF couplers with 220nF Audio grade may help give a bit more bass response. The other components I would like some help with are the NFB, namely 5k1 in parallel with 100pF. Are they worth tweaking with? I am certain the 100pF is only there to stop supersonic oscillation.
Note that the circuit shows the grid stoppers in the right place, at least on my amp, they are directly placed between grid and the 100k resistor at each valve.
Les