Mcintosh Tube Amplifier Protection Circuit

[marcutan]

Hi,

I have a Mcintosh amplifier that failed to switch on after going into "protection mode" because of a bad tube. Could anyone explain how a simple "protection" circuit is like? Or point me to web resources on such circuits?

Also, what's the best way to identify a bad tube?

Thanks.

Cheers,
Tan

[kevinkr]

Originally posted by marcutan
Hi,

I have a Mcintosh amplifier that failed to switch on after going into "protection mode" because of a bad tube. Could anyone explain how a simple "protection" circuit is like? Or point me to web resources on such circuits?

Also, what's the best way to identify a bad tube?

Thanks.

Cheers,
Tan

Not enough information here to formulate any sort of answer. What model mac, when was it made? Tube amps unless equipped with a sophisticated mpu generally have little more than fusible resistors and fuses for protection.

Bad tubes can be identified by inspection, one with a white getter has air in it and is bad, beyond that harder to know. If the filament is not glowing it could be a bad tube or a circuit problem.

Substitution for small signal types is usually low risk, and is the only avenue open to the average non tube savvy audiophile. Power tube substitution can be very high risk because you have no idea what caused the previous one to fail and whether or not there is also damage that may cause the new one to expire quickly as well.

McIntosh is still in business and has a reputation for really caring about its customers and its product. The first thing to do would be to just contact them.

[marcutan]

Thanks for the reply, Kevin.

I was working in California and recently returned to my country, where I was able to investigate the mcintosh amplifier failure.

I have a Mcintosh MC2000. I replaced 2 output tubes and found a fuse that was blown within the mcintosh. the fuse is a small 50mA, 250v fuse just before a standby transformer, which resides close to the mains supply wiring. I look at the circuit and it appears that the protection circuit is based on a simple standby transformer with a 5v electronic switch. There are fuses in almost all the PCBs (power supply, amp main board, meters etc) of the amplifier.

The amplifier was working fine for 30mins with the tube and fuse replacement. After 30mins, the same fuse blew.

I am wondering if it is due to power supply surges? Or I should bring the amplifier for servicing?

Thanks for the help.

[kevinkr]

Originally posted by marcutan
Thanks for the reply, Kevin.

I was working in California and recently returned to my country, where I was able to investigate the mcintosh amplifier failure.

I have a Mcintosh MC2000. I replaced 2 output tubes and found a fuse that was blown within the mcintosh. the fuse is a small 50mA, 250v fuse just before a standby transformer, which resides close to the mains supply wiring. I look at the circuit and it appears that the protection circuit is based on a simple standby transformer with a 5v electronic switch. There are fuses in almost all the PCBs (power supply, amp main board, meters etc) of the amplifier.

The amplifier was working fine for 30mins with the tube and fuse replacement. After 30mins, the same fuse blew.

I am wondering if it is due to power supply surges? Or I should bring the amplifier for servicing?

Thanks for the help.

I'd contact McIntosh for advice on what to do next. Service seems advisable, find a recommended service center in your country if possible. I don't know enough about this amplifier to be able to give better advice. It seems unlikely that power surges would blow just this particular fuse, more likely that there is a marginal component in the circuit it is meant to protect, or potentially a minor design issue.

[rsheldon]

I think the problem may be in the cathode resistor(s). I replaced all the 220 ohm cathode resistors in my MC2000 as the original ones were failing for no obvious reason. I think that perhaps McIntosh may of had some defective parts.

I was finding that after one of the resistors failed (open - not burned) the other tube in the pair would overheat and finally fail. When this happened the amp appeared to be working (biased ok) but the sound was very hard sounding.

Good Luck,

-Ron