Calling capnjuan, I s'pose. ;)

[whoisalhedges]

Hi all, first post for me. I've got a '68 Thunderbird version 2, and she sings like a bird. For the most part. Only trouble is the reverb stopped working (I've got backup tubes, it's not that) and the second channel is dead - just noise - when I plug a guitar into it. Channel 1 is fine, and I can jumper the channels without noise; but when a guitar is just plugged into 2, I get loud static.

So... any advice?

 

[killdeer43]

I can welcome you to the forum, but the good capn is the man to respond to your query.

Welcome, :wink:
Joe

 

[capnjuan]

Hi all, first post for me. I've got a '68 Thunderbird version 2, and she sings like a bird. For the most part. Only trouble is the reverb stopped working (I've got backup tubes, it's not that) and the second channel is dead - just noise - when I plug a guitar into it. Channel 1 is fine, and I can jumper the channels without noise; but when a guitar is just plugged into 2, I get loud static. So... any advice?

Hi Al(?) (if you aren't Al, no, I don't know who he is ). I have to go out later this morning but send me a PM with your email address in it. I'll send you the v.1 and v.2 schematics ... as the expression goes, we'll then be on the same page of the hymnal.

Also, we have a member matsickma here who knows his beans about Guild amps and (to confuse things a little further) he says there's a v.1.5 Thunderbird; twin 7591-driven but in the later black tolex, two-tone grill cloth - looks something like this but has 7591s. Can you confirm that yours is a 6L6 model?

Something a little more thought out later today. J

 

[whoisalhedges]

Thanks, guys. Actually, I have a schematic - and yes, it's the version that looks like the 6L6 model, but has 7591 finals. No speaker-driven 'verb here.

I'm not an electrician by any stretch, I can pretty much swap speakers and tubes. I'm assuming the 12ax7 in the power amp section is the reverb recovery tube? Maybe that's my problem - still not sure why I can't plug straight into channel 2, though.

 

[capnjuan]

I don't think I'm going to be much help ... you may be looking at a trip to the tech.

Reverb

If you have a "... 12ax7 in the power amp section", no ... it's not the reverb recovery tube. The v.1 and v.2 models both use a 7247 (a/k/a 12DW7) as the final gain section / phase inverter either nearest the power tubes (v.1) or on the power chassis (v.2) - no reason to think they didn't do the same in the v.1.5 like yours. If you have a 12AX7 on the power chassis, it's because somebody subbed one in for the specified 7247 which has the same pinout but in any event, it's not the reverb recovery tube.

I don't have a v.1.5 schematic ... the only way to know for sure whether you've tested a known-good in that tube position is to sub in a known-good at V1 and work your way through the preamp; try it, works okay, try the next socket. FWIW; the reverb recovery only uses 1/2 of a 12AX7 but testing tube-for-tube will find a bad one if you've got one.

Reverb tray: that amp has a reverb 'tray' in the upper chassis; the springs could be broken or disconnected, the in/out wiring could be loose ... open ... or one/the other/both of the little transducers that connect the signal-in to the springs and springs to the signal-out could be shot.

6GW8: the model uses both halves of the 6GW8 to ramp the signal up and drive the reverb transformer. Substituting a known-good for what you have doesn't do any good if:
- there's no voltage; a dead filter cap in power supply or a burned-open plate resistor,
- one/the other/both the electrolytic cathode bypass caps on each half of the 6GW8 are shot; they don't last forever, or
- there are problems with the reverb tray or, occasionally, the reverb transformer.


Noise in Ch 2

If you've subbed in a known-good 12AX7 in the Ch 2 preamp and get the same thing, I'd look at the shorting arm on the input jack on Ch 1. Sometimes, the arm gets electrical junk on it and doesn't make good contact. When the jack is unused, it's supposed to be shorted to ground ... maybe it isn't.

Sorry; I wish I could be more helpful.

 

[whoisalhedges]

Actually, that's very helpful. I imagine you're probably right on about ch. 2 being a grounding issue, since there's no noise when it's not plugged in or when it's jumpered (speaking of, are the channels in phase? I don't hear any excess noise when they're jumpered).

I'll need to take her to a tech anyway, as I seriously doubt anyone's ever changed the electrolytics.

 

[capnjuan]

Okay; don't forget about the cathode bypass electrolytics in the preamp; might not be no good now but if you push the amp much, it's just a matter of time before they will be.

Good luck with it.