YAQIN MC100B.
Fizzed for a few seconds then dead.
I have followed the guidance from this link:
http://g4cnh.com/public/A_LOOK_AT_THE_YAQIN_MC100B.pdf
from Page 9 'when things go POP!
Mains input fuse was blown.
After replacement with identical type, all measures ok (as described in pdf) without valves fitted.
If a valve is faulty, is there any way of identifying which without a tube tester?
ATM, I cannot even identify which channel failed.
I could fit a fuse for each channel B+ as described in the pdf and identify faulty valve by elimination
Question 2.
I have been running for umpteen years without previous problems.
UK mains is 240V but amp is for 220V.
From measurements above, heaters are 6.8V & B+ is 530V.
Is it worth fitting a bucking transformer?
I will at some point move this amplifier on: I have been intending to build its SS Class D replacement since 2019.
Therefore I don't really want to replace a set of 4 valves.
Fizzed for a few seconds then dead.
I have followed the guidance from this link:
http://g4cnh.com/public/A_LOOK_AT_THE_YAQIN_MC100B.pdf
from Page 9 'when things go POP!
Mains input fuse was blown.
After replacement with identical type, all measures ok (as described in pdf) without valves fitted.
If a valve is faulty, is there any way of identifying which without a tube tester?
ATM, I cannot even identify which channel failed.
I could fit a fuse for each channel B+ as described in the pdf and identify faulty valve by elimination
Question 2.
I have been running for umpteen years without previous problems.
UK mains is 240V but amp is for 220V.
From measurements above, heaters are 6.8V & B+ is 530V.
Is it worth fitting a bucking transformer?
I will at some point move this amplifier on: I have been intending to build its SS Class D replacement since 2019.
Therefore I don't really want to replace a set of 4 valves.
That heater voltage is probably OK if the valves were not fitted. you expect 10% reduction when loaded.
Did the fuse blow on a hot start or a cold start? Is it a slow blow or fast?
Are you confident to take voltage measurements when the amp is on? If you can prepare a grid for every tube and valve pin before hand, ground one lead of a multimeter, then collect the DC voltages on every pin, using ONE hand only (other tucked away), and post that data here, then there is a good chance one of the experts here could identify an inconsistency.
If that data looks OK, and the fuse was slow blow, I have heard that they can degrade over time.
Did the fuse blow on a hot start or a cold start? Is it a slow blow or fast?
Are you confident to take voltage measurements when the amp is on? If you can prepare a grid for every tube and valve pin before hand, ground one lead of a multimeter, then collect the DC voltages on every pin, using ONE hand only (other tucked away), and post that data here, then there is a good chance one of the experts here could identify an inconsistency.
If that data looks OK, and the fuse was slow blow, I have heard that they can degrade over time.
Well, as first post, I did hear a fizz.
Music continued to play but not sure whether one or both channels as it then cut out pretty quick with the blown fuse.
It was from a cold start within a few seconds.
I have one pair of JJ KT88s. (£49/pair in 2009) virtually unused.
I am tempted to buy another pair (£100/pair 2025) so I can fit a full complement of matching valves.
From reading similar threads, a faulty valve can be an intermittent beast so identifying which one could be a fools errand.
Even when identified, replacing one alone would mean odd valves.
It is on the original set it was supplied with (bought 2009).
I have carried out the checks in the pdf I linked.
All high power resistors measure ok.
Coupling caps seem ok: approx 200K in circuit which is the path through the 68K (actually 100K) cathode resistors & the bias resistors.
All measure the same anyway.
This gives me the incentive to get on with its replacement and move it on.
Incidentally, I still have the original packaging in the loft and the outer box is red stamped '240 volt' so despite what is on the rear of the amplifier (220 +/- 10%) it may indeed be a 240V version for the UK market.
That would explain the not overly high off load heater voltage.
Now I am wondering what it might sell for.
New close to £1000 with tax & shipping.
Music continued to play but not sure whether one or both channels as it then cut out pretty quick with the blown fuse.
It was from a cold start within a few seconds.
I have one pair of JJ KT88s. (£49/pair in 2009) virtually unused.
I am tempted to buy another pair (£100/pair 2025) so I can fit a full complement of matching valves.
From reading similar threads, a faulty valve can be an intermittent beast so identifying which one could be a fools errand.
Even when identified, replacing one alone would mean odd valves.
It is on the original set it was supplied with (bought 2009).
I have carried out the checks in the pdf I linked.
All high power resistors measure ok.
Coupling caps seem ok: approx 200K in circuit which is the path through the 68K (actually 100K) cathode resistors & the bias resistors.
All measure the same anyway.
This gives me the incentive to get on with its replacement and move it on.
Incidentally, I still have the original packaging in the loft and the outer box is red stamped '240 volt' so despite what is on the rear of the amplifier (220 +/- 10%) it may indeed be a 240V version for the UK market.
That would explain the not overly high off load heater voltage.
Now I am wondering what it might sell for.
New close to £1000 with tax & shipping.
If you take the voltages when powered up, assuming the fuse does not blow again, then you can see if there is an imbalance or inconsistency anywhere. I have had a fuse blow when I switched off and switched on again within 20 seconds, say. I think you need to keep an open mind about the state of the tubes.
Output valves replaced with new JJ.
Works perfectly.
Now in 'for sale' thread.
As Holmes said, once you have eliminated the impossible, whatever remains, which every Valve Amplifier website says is most probable, must be the truth.
Faulty output valve: 15 year old, original valves.
Works perfectly.
Now in 'for sale' thread.
As Holmes said, once you have eliminated the impossible, whatever remains, which every Valve Amplifier website says is most probable, must be the truth.
Faulty output valve: 15 year old, original valves.