Fifty/Fifty Stereo - Weird, Mystery Noise

Hey...

The Fifty/Fifty Power Amp in my system has developed a strange noise and I'd like to hear if anyone might recognize and be able to help me ID the problem. Here are the charactistics.

For this description the amp has nothing in the input jacks - totally isolated from all components.

The volume and presence controls (which are the only controls, one L&R each) are completely off. No signal passing through the amp.

The sound is a shredding sort of noise, almost like a radio tuner signal that crackles and fluctuates but can't quite tune in. Like intermittent bursts of static.

It only comes out of the left channel.

It goes away when the Standby switch is turned off but is otherwise completely independent of the volume controls. Can't be made louder or quieter.

The level isn't obnoxiously loud and is over-ridden when I'm leaning into heavier work, but is annoying when I'm playing clean, and (of course) when the amp should be quiet.

The sound comes and goes - will be clean (gone) for several minutes then reappear with the occasional big burst, like a local electrical storm effecting radio reception.

I've recently outfitted the amp with a new round of tubes.


Does this sound familiar to anyone?

Thanks.

Cayce

Like intermittent bursts of static.

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That seems to me like a tube failure. It always reminds me of rodents running through the walls.

Even though you put in a new set, I'd advise trying to replace one 12AX7 at a time, starting with the main input tube (closest to your guitar "in" jack). There's likely only 3 12AX7 tubes: the input, then two phase-inverters (one each, close to their respective pairs of power tubes). Try the phase inverter (on the side you hear the noise) tube next, if the input wasn't the one.

If this doesn't bring you success, I'd still suspect the 12AX7s. Try cleaning the tube pins, and re-seating each tube in it's socket.

I think you might be confusing the 50/50 with the TriAxis (my pre-amp). The tubes I'm dealing with are 6L6s, but the advice is still good. This raises another question though: are there other internal tubes on the 50/50?

Cayce said:

are there other internal tubes on the 50/50?

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Those are the tubes TW is refering to. You need to open the amp to gain access to the three 12AX7's inside the 50/50.

t0aj15 said:

Cayce said:

are there other internal tubes on the 50/50?

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Those are the tubes TW is refering to. You need to open the amp to gain access to the three 12AX7's inside the 50/50.

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'zactly!

AHA! Why that sneaky little... I intend to fire off a harshly worded letter to Mr. Randall Smith and give him a piece of my mind. What kind of guy goes around hidin' tubes from people?!

Uhhh... on the other hand...

I guess a little investigation on my end – or on my 50/50 – would have paid off. Thanks Timbre Wolf... apologies for questioning your help. And thanks to you tOaj15 for settin' me all straight.

Now to pop the hood...

Cayce