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MC275 output tube failure question

kmanusa

kmanusa

AK Subscriber
Subscriber
Well I guess it had to happen sooner or later but one of the Penta Labs KT88s blew in my MC275 IV yesterday. :sigh:

I took the bottom cover off to check and there were no obvious smoked resistors and the 2 fuses were still good. So, I pulled the remaining 3 Pentas and replaced them with a quartet of RAM Labs KT88s that I had originally bought for an RM-9 amp. I then played the amp for about a half an hour last night and it seemed to work ok.

Today I turned the amp on and immediately noticed one channel was distorting. I checked all the tubes (miniatures too) on my B&K 700 tester and one of the RAM KT88s was dead. :tears: I pulled the bottom cover again and began to probe the resistors and diodes on the board. The large 220 ohm and 100 ohm resistors nearest the KT88 sockets all measured within tolerance. But, diode D4 was shorted.

My questions are:

1) what type of diode is D4 (my schematic is of the old MC275 which does not appear to use any silicon diodes)?

2) Are there any "usual suspects" parts (resistors, diodes) to check when a tube blows?

I wonder if the diode blew when the Penta tube failed or when the RAM KT88 failed? Or maybe it blew and caused the tubes to fail? It is hard to tell the impact of a blown diode without a schematic.
 
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I believe the newer model schematics are available in the Service Manual from Mac. I'd suggest you call the parts department to confirm. I picked up the C2200 Service Manual this way. I always want to have schematics of all my gear where possible.

Cheers,

David
 
dshoaf said:
I believe the newer model schematics are available in the Service Manual from Mac. I'd suggest you call the parts department to confirm. I picked up the C2200 Service Manual this way. I always want to have schematics of all my gear where possible.

Cheers,

David
Click to expand...

Thanks, will do. I neglected to mention in my original post that in both instances of KT88 failure, the same tube (V10) blew.
 
kmanusa said:
I neglected to mention in my original post that in both instances of KT88 failure, the same tube (V10) blew.
Click to expand...
I was just going to ask you that! :yes: That is extremely suspicious (with respect to there being anything wrong with the tube(s). One thing I think you should check (besides resistors, caps and diodes ;-) is the other tube of that pair. If it's very weak (which it probably isn't, but . . . . .) it could cause a good tube in the other socket to be overworked to the point of self-destruction :D

You mentioned two fuses. I've never looked under my 275, and I do have a current schematic but I didn't notice a second fuse inside the chassis?
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nsgarch said:
I was just going to ask you that! :yes: That is extremely suspicious (with respect to there being anything wrong with the tube(s). One thing I think you should check (besides resistors, caps and diodes ;-) is the other tube of that pair. If it's very weak (which it probably isn't, but . . . . .) it could cause a good tube in the other socket to be overworked to the point of self-destruction :D

You mentioned two fuses. I've never looked under my 275, and I do have a current schematic but I didn't notice a second fuse inside the chassis?
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Click to expand...

Neil-- my 275 has 2 small slow blow fuses mounted on the circuit board. These are separate and apart from the chassis mounted AC line fuse.

I did check the RAM KT88s on the B&K before putting them in and they all checked out perfect. I will check the remaining 3 good tubes to make sure they are still "good". BTW, I am using 4 CV4024 Mullards in place of the 12AT7s and 3 JJ Tesla ECC83s in place of the 12AX7s.
 
A schematic that matches your model is key. Check all tube socket voltages with no signal at idle. Make sure you check the 220 ohm 5 watt sand block resistors whenever a tube fails. Make sure your small tubes are of a known reliable brand.

Thanks,
Ron-C
 
I just got off the phone with the Parts department. I ordered some diodes and 4 220 ohm 5 watt resistors. Even though they checked out ok cold, I did not like the appearance of these sand block resistors so I am replacing them.
 
The 220 ohm resistors are a fuse. They will cleanly open if damaged so yours are OK. DO NOT replace with higher values over 5 watts as this will allow transformer damage if a tube totally shorts out.
Did you order the service manual?

Thanks,
Ron
 
ron-c said:
The 220 ohm resistors are a fuse. They will cleanly open if damaged so yours are OK. DO NOT replace with higher values over 5 watts as this will allow transformer damage if a tube totally shorts out.
Did you order the service manual?

Thanks,
Ron
Click to expand...

Will do. I ordered the parts from the McIntosh parts department and they assured me the 220 ohm resistors are rated for 5 watts dissipation. These resistors have very small cracks in them and although there is no obvious sign of heat damage, I wonder if they have been a little stressed.
 
Well here is an update on the blown KT88 on the MC275IV.

I received the diodes and resistors along with the service manual from McIntosh. I replaced the blown diode and after taking another look at the 220 ohm screen grid resistors, I decided to take Ron's advice and leave them alone.

I checked all diodes and many resistors on the circuit board and all measured ok. I replaced the mini tubes and checked DC voltages on the KT88 sockets; all measured within tolerance and so I put the KT88s back in. So far, the amp seems to be working fine with no issues.

I can only surmise that the first KT88 failure took out the diode and that is what killed the second KT88. So, one lesson learned is to check diodes as well as the 220 ohm resistors when you lose an output tube.

There is a diode in the screen grid circuit of each output tube that ought to be checked in the event of tube failure. There are 4 of these diodes, marked D2 through D5 on the circuit board.

One note about working on this unit-- removing the circuit board appears easy because unsoldering of the connecting wires is not required. But, you need to be careful in removing the blade connectors and handling the wiring harnesses. Also, soldering to this board was fairly easy although this is a thick, high quality mil-spec type board. If you take the plunge, take your time and don't forget the flux remover after the soldering is done.
 
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kmanusa said:
Well here is an update on the blown KT88 on the MC275IV.

I received the diodes and resistors along with the service manual from McIntosh. I replaced the blown diode and after taking another look at the 220 ohm screen grid resistors, I decided to take Ron's advice and leave them alone.

I checked all diodes and many resistors on the circuit board and all measured ok. I replaced the mini tubes and checked DC voltages on the KT88 sockets; all measured within tolerance and so I put the KT88s back in. So far, the amp seems to be working fine with no issues.

I can only surmise that the first KT88 failure took out the diode and that is what killed the second KT88. So, one lesson learned is to check diodes as well as the 220 ohm resistors when you lose an output tube.

There is a diode in the screen grid circuit of each output tube that ought to be checked in the event of tube failure. There are 4 of these diodes, marked D2 through D5 on the circuit board.

One note about working on this unit-- removing the circuit board appears easy because unsoldering of the connecting wires is not required. But, you need to be careful in removing the blade connectors and handling the wiring harnesses. Also, soldering to this board was fairly easy although this is a thick, high quality mil-spec type board. If you take the plunge, take your time and don't forget the flux remover after the soldering is done.
Click to expand...

Did you happen to take pictures?
 
After reading through this thread I'm alittle concerned.Is this a "common" issue with the 275?It's one thing to have to go and buy a new tube if it fails, but to have to do a internal repair because of a tube failure doesn't make me feel good.I'm not a DIYer as far as electronics so then it would be off to the shop$$$!Tube failure are not uncommon, so doesn't the 275 have any protection circut to prevent internal damage?

Randy
 
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