Adding an extra cooling fan to a RA-100 head?

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Molly
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Adding an extra cooling fan to a RA-100 head?

Post by Molly » Fri Jan 13, 2017 12:01 am

Was looking at this image from a recent post and wondered if it might be prudent to add an extra cooling fan to my RA-100 head to help ensure this never happens to my amp (story goes that the owner's Mrs knocked the amp on whilst cleaning and it was left on for some time - still, shouldn't do this you'd think).

Could I add a fan by taking a feed from the existing one? Would it need to be of a certain spec in order to avoid noise?

Thanks in advance for any advice you're able to give.

Image

rbraga
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Re: Adding an extra cooling fan to a RA-100 head?

Post by rbraga » Thu Jan 26, 2017 10:14 am

Molly wrote:Was looking at this image from a recent post and wondered if it might be prudent to add an extra cooling fan to my RA-100 head to help ensure this never happens to my amp (story goes that the owner's Mrs knocked the amp on whilst cleaning and it was left on for some time - still, shouldn't do this you'd think).

Could I add a fan by taking a feed from the existing one? Would it need to be of a certain spec in order to avoid noise?

Thanks in advance for any advice you're able to give.

Image
Molly, that's my story hahaha.

As the technician said to me, one power tube failed causing this accident letting the board toasted. :shock:
But, I don't believe that it was caused by a overheat. I always check the temperature after I play and the RA-100 is always the coldest one (I also have a Bogner Shiva and an Engl Screamer).

Let's wait for more replies. I'm curious as well.

bandit2013
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Re: Adding an extra cooling fan to a RA-100 head?

Post by bandit2013 » Fri Jan 27, 2017 7:30 am

I have seen many fried boards in my time ( full time job as Electrical Engineer), not on amps though. Damage shown in image had to be cause by a short circuit since copper foil on the PCB substrate usually does not vaporized and burn the PCB. The short should have caused the fuse to blow, but if the current does not cause a rise in the AC power, it is possible to sustain an arc long enough at lower voltages that will cause this issue. Temperature of the arc is extremely hot (same as a plasma cutter) and if it is sustained long enough causing cracks in the copper foil, it will continue until the trace is gone and current stops flowing. As part of my job, destructive testing is always a fun thing to watch but not when it happens to your prized guitar amp.

If you have a cleaning service, or if you spouse or you are a clean freak, never use any cleaning agents (especially the spray type) on the amp. The RA100 Head and combo (combo being the most susceptible) to liquid chemical invasion due to the top vent. Any fluids that enter may eventually make contact with circuit boards. Cleaning agents that may contain ammonia will eat at the copper (even though the green solder mask as it is porous). Back in the day I designed products for hospitality industry and one test conducted was use of ammonia type cleaners (windex, glass plus and 409 (most active)) onto a live circuit to simulate the cleaning practices and to replicate what has happened to some claims. Best practice is to spray onto a cleaning towel or cloth before using on the amp. I generally use a nano-wax that is safe for vinyl like turtle wax liquid ice, on a micro fiber sponge. Does a great job keeping the tolex looking like new. I even use the same on my guitars (non-nitro finish).

bandit2013
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Re: Adding an extra cooling fan to a RA-100 head?

Post by bandit2013 » Sun Feb 05, 2017 11:31 am

One of my issues in relating to problems other have had with their amps in general (any tube amp) is that I assume all reportable issues are contained within the USA, The filament transformer (power transformer) can be sensitive to line voltage and frequency. That will change everything especially if a different power transformer was required due to line voltage of the country you are in (generally speaking).

I have had my RA100 head for 4 years now and have left the amp on for extended periods of time without having issues such as the pictures shown that one person experienced. With the head version, the tubes are facing up and heat is dissipated though the top grill and back of the shell. I normally keep the cooling fan set to run at high speed even if I am not using the power soak. The EL-34s can get very hot relative to a 6L6 tube but have not had thermal issues with the amp (tube cage and back panel installed helps air flow to reach more tubes from the fan). If your chassis is getting very hot due to overheating power transformer, chances are that the power tubes are not a good match for the amp. I have had this issue with my Mark V and Mesa (red) 6L6GC STR440. It would not take long for the center power tubes to fail (red plate) even with variac power at 45W (?, no clue why).

If the screen resistors are drawing that much current to cause the circuit traces on the power board to burn off, that is a result of a tube failure. I wish not to complain or flame a particular manufacture of tubes, but it was a certain brand of 6CA7 that caused the screen resistors to split and fail due to a hard short on my Mark V (yeah, I know, should have used variac power but too late and was a few years ago and lesson learned). Perhaps there is a good reason why Mesa used carbon comp resistors as they would fail first before the copper traces would vaporize.

I would highly doubt that any sort of damage would result from leaving the amp in standby for an extended period of time. Heater voltage is around 6 volts AC on the power tubes. Note that the high voltage is removed from the power supply so this could not have contributed to the failure unless it happened when the amp was taken out of standby, if the tube arcs or has internal short there is no telling what will happen if the fuses in the amp are not properly sized based on recommended values.

Please note: An additional cooling fan would not prevent the damage from happening in the picture as that was caused by a hard short in the power tube. If the resistor had failed due to overheating, the resistor would have failed open circuit stopping the flow of current as that is the common form of failure mode of a resistor. Could this have been caused by attempt to operate amp (taken out of standby) with no speaker load? The transient voltage as a result of an open coil on the high side of the OT will definitely breach the clearances or circuit spacing on the PCB if the voltage potential doubles or triples the supply voltage. An arc cause by such transient will definitely continue to flow current due to plasma discharge (sometimes the carbon will act as a conductor until the arc has reached a given distance that it cannot sustain current flow.

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