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PostPosted: Mon Dec 26, 2016 12:37 am 
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Bottle Rocket

Joined: Mon Sep 05, 2016 3:53 am
Posts: 12
Hi guys.

When I've been playing my RA-100 for an hour or so I start to get that 'fluttering' / 'crackling' noise. It's at a consistent volume and doesn't change / disappear when I change channels or switch the amp down to 50w. It seems independent of anything I do which is making narrowing down the cause quite difficult.

Any ideas?

Thanks.


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PostPosted: Wed Dec 28, 2016 9:59 am 
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Mark II

Joined: Sat Jun 02, 2007 8:30 am
Posts: 109
If it's happening on both channels try replacing V3, V6 or V7 as these are the preamp tubes that are in the circuit no matter which channel you're on.


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PostPosted: Sat Dec 31, 2016 10:48 pm 
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Bottle Rocket

Joined: Mon Sep 05, 2016 3:53 am
Posts: 12
Thanks for the reply and advice.

Discovered it was the reverb tube. When I toggled the reverb on and off the crackling went away.

Would like to add more cooling to this amp if possible. Was concerned to see that post where the guy's RA-100 had all but destroyed itself just because his Mrs inadvertently left it on stand-by.

Cheers.

Molly.


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PostPosted: Sun Feb 05, 2017 11:09 am 
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Dual Recto

Joined: Fri Feb 22, 2013 7:31 am
Posts: 2244
Location: North Carolina
Yep, if you remove V1 and V2 only the clean channel will work as those tubes are used on Hi/lo only. If you remove v4 and v5 only the hi/lo channel will work (assuming you put back V1 and V2).

V3 can be a major culprit for noise (12AT7) as it is always present in the signal chain (FX send and return). If you remove it, you will get no sound.
V6 reverb circuit. this one can be switched off on the back panel with the bypass switch (or use the hidden jack next to the reverb send/return wires).
V7 is the last tube in the chain, PI tube.

I had to track down a noisy tube with my first RA so I started pulling preamp tubes (also to get a better idea how the circuits are mapped). I bought my RA's used so no manual came with them. I do have the downloaded copy for reference.

The RA100 has two fuses. One on the mains, the other on the output/attenuator load. I would check the secondary fuse for correct type as this is not a slow blow fuse. Also there is a light that indicates power tube failure or fault on the back of the amp. Fuse is next to the fan seed selector switch labeled as H.T. fuse. I have had the light come on before due to a faulty tube.

From the Manual on page 13.

H.T. FUSE: This fuse and indication LED provide additional protection to the circuit should a power tube arc or fail. In an amplifier that has the ability to reduce the listening volume of extreme output settings, this level of protection is warranted and welcome. Should a power tube fail, it will first blow this High Tension fuse and reduce the likelihood of circuit /component damage. If you ever hear a strange hum or “shorting” noises, hit STANDBY right away, then look here to see if the Red LED is lit indicating a power tube failure and the H.T. Fuse going open. If it is lit, shut the amplifier off and replace this fuse with a 1 amp “Fast Blo” type fuse and the faulty power tube (you should see it glowing red hot in the center). Sometimes it may be necessary to replace the adjacent power tube as well, as it may have been damaged when its nearest neighbor shorted.

_________________
Current amps:
TC-50, JP-2C, MK V, Roadster, RA100
Old friends I sometimes miss:
Mk III (blue stripe), Mark IVb-WB


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