1987 marshall combo el34 valves overheating

jwamp · 10-18-2009, 11:09 AM

Hi all you amp heads!! I have a problem with one of my marshall amps the output valves are overheating, having checked the heater voltage is below 6.3 volts A/C. But the amp doesn't have any life , its just flat, no brightness to the sound.

jwamp · 10-18-2009, 11:10 AM

Thanks Great sounding People with a marshall blast

T-Bird · 10-18-2009, 11:13 AM

Hi.

Welcome to the forum.

Less than 6.3V? By how much? Measured how?

In any case this IMHO falls into the category: "If You Have To Ask, You Probably Can't Fix It Yourself".

A trip to a tech might be the easiest and cheapest solution.

Regards
Sam

jwamp · 10-18-2009, 12:40 PM

Quote:

Originally Posted by T-Bird

Hi.

Welcome to the forum.

Less than 6.3V? By how much? Measured how?

In any case this IMHO falls into the category: "If You Have To Ask, You Probably Can't Fix It Yourself".

A trip to a tech might be the easiest and cheapest solution.

Regards
Sam

Thanks for your quote the 6.3 volt was measured by a multi meter, and I am a Tech but not trainned in valves, as you can see I am learning, so if you have a solution it would be greatfuly received.
jwamp

Wilder Amplification · 10-18-2009, 02:29 PM

Define overheating. Are the plates "cherrying out" (i.e. red plating, plates turning red, etc etc)? IF this is the case, it could be as simple as rebiasing the amp, or those tubes are going south. Worst case scenario, you have a problem in the bias supply. Check and make sure you have a negative voltage on pin 5 of all the power tubes.

This could be why you have less than 6.3 volts on the heaters since the secondaries are all wound on the same transformer core, power valves going overcurrent (i.e. drawing too much current) can be stealing it away from everything else, causing the voltage drop on the heaters.

jwamp · 10-18-2009, 03:04 PM

Quote:

Originally Posted by Wilder Amplification

Define overheating. Are the plates "cherrying out" (i.e. red plating, plates turning red, etc etc)? IF this is the case, it could be as simple as rebiasing the amp, or those tubes are going south. Worst case scenario, you have a problem in the bias supply. Check and make sure you have a negative voltage on pin 5 of all the power tubes.

This could be why you have less than 6.3 volts on the heaters since the secondaries are all wound on the same transformer core, power valves going overcurrent (i.e. drawing too much current) can be stealing it away from everything else, causing the voltage drop on the heaters.

Thanks I have checked the bias, it had been slightly out, perhaps the tubes are going faulty, the heater supply is low about 5.9 volt A/C, there is also some A/C on the DC power rail it is coming from the valves. Have replaced rectifier Diodes to be on the safe side.
Many thanks jwamp

Wilder Amplification · 10-18-2009, 03:14 PM

Quote:

Originally Posted by jwamp

Thanks I have checked the bias, it had been slightly out, perhaps the tubes are going faulty, the heater supply is low about 5.9 volt A/C, there is also some A/C on the DC power rail it is coming from the valves. Have replaced rectifier Diodes to be on the safe side.
Many thanks jwamp

5.9 volts isn't a bad thing and could be attributed to the accuracy of your meter. Now if it were 5.5 that'd be a definite problem.

What's your plate voltage (voltage on pin 3) and how much idle current did you bias it to?

Also, you mention AC on the DC side...how and where did you measure that? If it's excessive "ripple" that it is you're seeing you may be due for a filter cap replacement.

10-18-2009, 11:09 AM	#1 (permalink)
jwamp Junior Member Join Date: Oct 2009 Posts: 4	1987 marshall combo el34 valves overheating Hi all you amp heads!! I have a problem with one of my marshall amps the output valves are overheating, having checked the heater voltage is below 6.3 volts A/C. But the amp doesn't have any life , its just flat, no brightness to the sound.

10-18-2009, 11:10 AM	#2 (permalink)
jwamp Junior Member Join Date: Oct 2009 Posts: 4	Re: 1987 marshall combo el34 valves overheating Thanks Great sounding People with a marshall blast

10-18-2009, 11:13 AM	#3 (permalink)
T-Bird Senior Member Join Date: Jul 2009 Location: Finland (Northern Europe) Posts: 151	Re: 1987 marshall combo el34 valves overheating Hi. Welcome to the forum. Less than 6.3V? By how much? Measured how? In any case this IMHO falls into the category: "If You Have To Ask, You Probably Can't Fix It Yourself". A trip to a tech might be the easiest and cheapest solution. Regards Sam

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