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Old 14th December 2007, 07:04 AM   #1
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Default I Damaged my Amp?

Hi Guys
This is my first post so take it easy on me.
I was the proud owner of a great sounding Chinese Yaqin MC-100B tube amp.........Twin Power supplies,2 x 12 AX7 for preamp,4 x 6SN7 and 4 x KT88 output valves........until the other day.
It was a bad day.......the laser in my CD player stopped.So I took the cover off it to have a look,but could do nothing to revive it.
The power was still connected to the CD player and an interconnect lead(which was connected at the time to the amplifier) fell into the CD player with the shield of the interconnect hitting live power.The live power travelled up the interconnect and into the amp for a very short period of time....I have earth leakage breakers which trip quickly.
I connected my laptop to the amp to see if I could still play music.
The music sounded ok except for very weak vocal.
This amp has alternative inputs on the front which bypass the preamp stage(which is the 2 x 12AX7's i'd assume) so I connected to them and it sounded quite acceptable albeit less output.
I disconnected the amp from power etc ...removed all the valves and removed the bottom cover to see any obvious damage....which there was.
The input selector switch has a small PC Bd connected to it(which houses some cable connections)....this is where the live power has found an easy path to earth...black mark on case which sits very close to the PC Bd.There was also a small amount of track burnt on the PC Bd.
I'm trying not to bore you guys too much!!!
I repaired the PC Bd and replaced the 2 x 12 AX7's.
Connected everything back up and it sounded FAT(with the new 12AX7B's) for about 5 minutes and then the sound deteriorated back to the thin crappy sound after the accident.
I must still have a problem and I feel Ive probably taken out the new 12AX7B's.I need HELP and I don't seem to be able to find a circuit diagram anywhere on the Net.
I'm not a Dummy....I'm actually an Electronics Tech by trade but have had little experience with Valves.....although I do understand (to a point) how amps work.
I just hope I haven't taken out one or both power transformers!!
Cheers
Bloke669
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Old 14th December 2007, 11:38 AM   #2
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Unlikely that you've cooked the transformers.

Step 1: Find a schematic or at least trace out the input stage.

Step 2: CAREFULLY measure the DC voltages at the cathodes, grid, and anodes of the input tubes. Indicate them on the schematic. These are much higher voltages that you're used to, so be VERY careful of both your hands and the voltmeter probe.

Step 3: If the likely cause doesn't jump out at you, post your schematic and voltages- I'll give you a 90% chance that someone here will figure it out within a few hours of that post.
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Old 14th December 2007, 05:15 PM   #3
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Step 4: post a few pictures of the insides.
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Old 16th December 2007, 07:55 AM   #4
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Thanks for the quick response guys!
I have listed below every voltage for all tubes just in case it may help.
I will supply some pictures if needed....just not sure on the regulations for this forum on uploading pictures.

LEFT CHANNEL RIGHT CHANNEL
12AX7B 12AX7B
Pin 1(Plate T2)....314V DC........................321V DC
Pin 2(Grid T2)......167V DC........................168V DC
Pin 3(Cathode T2)....169V DC...................170V DC
Pin 4(Heater T2)....3.5V AC.......................3.5V AC
Pin 5(Heater T1)....3.5V AC........................3.5V AC
Pin 6(Plate T1).....166V DC.........................169V DC
Pin 7(Grid T1).......0V DC.............................0V DC
Pin 8(Cathode T1)..0V DC...........................0V DC
Pin 9(Heater CT)....3.5V AC.........................3.5V AC

LEFT CHANNEL Tube 1 Tube 2
6SN7GT
Pin 1(Grid T2).........................290V DC.......................154V DC
Pin 2(Plate T2).......................428V DC.......................289V DC
Pin 3(Cathode T2)..................295V DC......................175V DC
Pin 4(Grid T1).........................296V DC.......................170V DC
Pin 5(Plate T1).......................0V DC............................296V DC
Pin 6(Cathode T1).................303V DC.........................177V DC
Pin 7(Heater).........................3.5V AC...........................3.5V AC
Pin 8(Heater).........................3.5V AC...........................3.5V AC

RIGHT CHANNEL Tube 3 Tube 4
6SN7
Pin 1.......................................156V DC..........................293V DC
Pin 2.......................................295V DC..........................429V DC
Pin 3.......................................178V DC..........................300V DC
Pin 4.......................................171V DC..........................294V DC
Pin 5.......................................293V DC...........................0V DC
Pin 6.......................................179V DC...........................302V DC
Pin 7........................................3.5V AC............................3.5V AC
Pin 8........................................3.5V AC.............................3.5V AC

LEFT CHANNEL TUBE 1 TUBE 2
KT88-98
Pin 1(Shield)...............................0V DC..............................0V DC
Pin 2(Heater)..............................3.5VAC..... .......................3.5V AC
Pin 3(Plate).................................522V DC..........................521V DC
Pin 4(Screen Grid).......................521V DC..........................521V DC
Pin 5(Control Grid)....................... -62V DC........................ -60V DC
Pin 6(No Pin)................................. OV DC........................... 0V DC
Pin 7(Heater)................................3.5V AC..........................3.5V AC
Pin 8(Cathode)..............................OV DC..............................0V DC

RIGHT CHANNEL TUBE 3 TUBE 4
KT88-98
Pin 1............................................0V DC..............................0V DC
Pin 2............................................3.5V AC............................3.5V AC
Pin 3.............................................521V DC........................521V DC
Pin 4.............................................520V DC.........................521V DC
Pin 5............................................. -60V DC........................ -58V DC
Pin 6..............................................0V DC.............................0V DC
Pin 7..............................................3.5 V AC..........................3.5V AC
Pin 8...............................................0V DC............................0V DC

Some of the voltages fluctuated by + or - 1V DC.
To the eyes of a novice it looks to me like the Grid and Cathode(T1) 0V DC readings on the 12AX7B's is where the problem lies.
Just not sure on the OV DC readings on the Cathodes of the KT88-98's is Cool!
Cheers
Bloke669
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Old 16th December 2007, 11:16 AM   #5
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OK, from those voltage readings, it would appear that the input stage is either an SRPP or a cascode. The voltages on the "bottom" tube look suspicious- the cathode should be a couple volts positive of the grid. Check the cathode bias resistors, grid leaks, and (if any) grid stopper resistors for the pin 6,7,8 sections of the 12AX7s. I'll bet that the grids are flopping around loose.
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Old 16th December 2007, 11:48 AM   #6
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Valve amps are really simple things, and pretty well 'bomb proof', there's not much you could have damaged, and the valves are VERY unlikely to have been damaged at all.

But like anything else, it would make life easier if you had a circuit diagram, just the front end is probably all you need?.
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Old 16th December 2007, 04:28 PM   #7
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Being a novice and having all components on a PC Bd doesn't help.
I'm sure a hard wired Point to Point design would be much easier to trace and fault find.....and I know would sound better!
I've emailed the manufacturer in China to see if I can get a schematic.....I won't hold my breath on this one.
If any of you guys could direct me to a schematic that could be "similar" to my setup (1 x 12AX7....2 x 6SN7....2 x KT88 per channel) that I could use as a guide....I'm sure that could help.
I realise there is various configurations that valves can be connected in and what I'm asking for is a bit silly.
In the meantime I'll try and make a sketch of the input cct.
Cheers
Bloke669
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Old 16th December 2007, 04:33 PM   #8
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Just as a quick check, could you measure the resistance between the 12AX7 grids and ground?
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Old 17th December 2007, 02:49 AM   #9
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Hi SY
Left Channel 12AX7B
Pin2(Grid T2)........o/c to grd.
Pin7(Grid T1)........53 ohms to grd

Right Channel 12AX7B
Pin2.........o/c to grd
Pin 7........17 ohms to grd

Cheers
Bloke669
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Old 17th December 2007, 02:49 AM   #10
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Hi SY
I just tried something else that I think may help.
I disconnected the two leads (one for each channel) from the volume control to the input circuit on the PC Bd.
The resistances from the grids of the 12AX7B's to ground are as follows

Left Channel
Pin 2........o/c to grd
Pin 7.........0.509 MOhm to grd

Right Channel
Pin 2.........o/c to grd
Pin 7.........0.511 MOhm to grd

When the shield of the interconnect touched active power the surge travelled through the input RCA's and found an easy path to earth at the input selector switch.....lifting a track on the small PCBd connected to the input selector switch. I repaired this.
Next in the path is a toggle switch which allows you to use the rear RCA's for direct connection to the pre amp input or the front RCA's to bypass the input cct and go directly to the 6SN7's I'd assume.
Next in the path is the volume control which connects directly to the input cct.
I can only assume that the fault is in the volume control or one of the other two switches.
If this proves to be the case I'd be very tempted to eliminate all this.........I only ever have 1 input connected to the amp...so I could use one set of RCA's(the front set closest to the volume control would be the pick) and direct wire to the new volume pot if that needs replacing.
This would eliminate the long run of cable from the rear RCA's to the volume pot...the input selector switch and the pre amp bypass toggle switch and extra wiring.
It would simplify things and make it a bit tidier.
Cheers
Bloke669
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