Tube glow - can anyone help?

Kivitel

Kivitel

New member
Hi folks. I've been having some weird issues with my old amp here, which is basically a fender twin with a solid state preamp. I've been detaching it from the cabinet and I tried to connect it to an external cabinet today, and I noticed that the output was really low...like, turned up all the way (we're talking 80 or so tube watts) it wouldn't get louder a soft voice. It did, however, sound completely overdriven.

I was looking at the tubes and noticed that 3 of them are glowing blue when they get input (ie it fades with the input level). They are also glowing red at the filament - including the one that doesn't glow blue.

So: the problem is low output. Weird clue: one tube not lighting up. What's the solution? Do I just need to make sure it's pushed in? (It worked before I took the tubes out and replaced them, and I know they're in the right position because I tested it when I put them back in). Would a single tube reduce output this much if it isn't connected?
 
Re: Tube glow - can anyone help?

Update: tried pushing them around, making sure they're in. They're in, but the same one won't light up. I tried getting sound out but to do that the volume has to be way up, and now they're making a weird popping noise.
 
Re: Tube glow - can anyone help?

The blue glow is normal.....it just means there's some oxygen in the tube. The one that isn't glowing probably doesn't have any oxygen in it.

The red glow is the tube heater.

I'd double check all your cables and connections.
 
Re: Tube glow - can anyone help?

Sometimes on speaker extension jacks, there's an extra metal flap that breaks the circuit. If you're getting no sound other than the preamp buzz, it means the whole power section isn't working. It's either because one of the ext speaker jacks is an open circuit, the FX loop jacks are an open circuit, or a tube problem.

I'd look at all the jacks you've been using, and the amp most likely needs all new tubes and the bias set. By continuing to run it, you're stressing components, so try and diagnose it without running the amp speakerless.
 
Re: Tube glow - can anyone help?

yeah, I seem to remember the power-amp out jack didn't really snap on the cable. Would that explain low output / crackly poppy noises?
 
Re: Tube glow - can anyone help?

Oxygen ??? ...in vacuum tubes?...blue is normal.

Anyway; is this amp by any chance a SF 70's version of the Twin ? The tube sockets in mid to late 70's Fender stuff have a way of not making very good contact with the pins. Drain the caps and re-tension the pins...or the tube is dead/kaput/DOA.
 
Re: Tube glow - can anyone help?

Hey! Might the problem be that I am running the amp to one speaker now instead of two? I thought this wouldn't make a difference...80W into one speaker rather than two. But does this make a difference with tube amps?

To clarify: originally 80W into 2 speakers
Now running into one speaker
Is this an issue with tube amps?
 
Re: Tube glow - can anyone help?

No---that would be a mismatched load (8 ohms vs 4) but it wouldn't normally cause any trouble in a healthy Fender. From here, it's kinda hard to figure out what's going on, but it worked before you switched or pulled the tubes (clue #1). One new (?) tube is not working in a socket that worked before (clue #2).

-put the old (or known good) tubes back in and see if it works.
-No? You could have a bad socket(s)---see a tech.
-other issues may include bias settings, screen or grid resistors (just pulling the tubes may have cracked one that was marginal).
 
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