Fender Red Knob "The Twin" All Kinds of Problems

[The Kabash]

the insane buzz happens when the guitar is plugged in and turned up. unplugged guitars makes 'normal' amp hiss. I will check though. also, all the jacks cause this sound (4 inputs), would all the jacks be causing this if there is a problem in one?

[Chuck H]

I'll repeat what others have said... If the amp is quiet with nothing plugged in, and set the same as you would use it with a guitar plugged in, and the amp is quiet with the guitar plugged in but with the guitars volume control at zero, with the amp set up as you would use it when the guitar is turned up, but the amp is noisy when the guitar is turned up, with the amp set up the same as it has been, then the noise is getting into the amp through the guitar pickup. (that's a period, here's another). If you've determined the problem isn't your guitars by using them with other amps in another location then the problem is a noisy environment or a bad or improper cable between the guitar and amp.

Here's a thought... Did you take your guitar cord into the music store when you tested your guitars? Are you sure it's an instrument cable and not a speakers cable? If there's no problem with the cable then I'll bet six beer (German) that your guitars have single coil pickups. If yes then they are picking up some awful noise that isn't present at the music store but is at your house.

[The Kabash]

I took the amp to a different environment yeterday (stated above) and the guitars a few days ago. Instrument cables were taken to GC and I have a les paul studio and an eastwood airline town and country (humbuckers).

[ogeecheeman]

I have one of those #!&^ sh&$ bags on the bench right now.same deal.after canvassing the globe I managed to find suitable pot replacements. fried 1 ohm biasing resistor,open grid resistor,busted switch jacks everywhere,broken speaker jack, can't get reverb to go-new tank. as far as buzz my friend-look for a broken jumper wire from front panel control board to tube sockets or an open ground. anyone with reverb answers,new reverb pot,new tank,new tubes,new wires.

[TimmyP1955]

Sounds like time to strip it out and turn it into something useful.

[Xenrelic]

Try a different 12AT7, the 3rd preamp tube from the left, closest to the 6L6 tubes. The filter caps also might need replacing.

[Jquijano]

a) Disconnect the cable at the input of the reverb tank. Measure the AC voltage at the cable while you have signal running through the amp with reverb turned up. We know the return side of the reverb is working, so this will check the send side.
b) If turning the guitar volume off makes the amp quiet but it is noisy with nothing plugged in then there is some problem with the input jacks. There is a switch in the jack that is supposed to ground the input when nothing is plugged in. So with nothing plugged in it should sound the same as with the guitar plugged in with guitar volume at zero.
c) The fact that the amp is quiet with the guitar volume at zero implies that it is the guitar picking up the noise from the environment. You should try the amp somewhere else.
d)What is the condition of the channel select switch? It may be interfering with the footswitch.

Reverb used to work on both channels of my red knob twin ans now no reverb. I found one broken 12at7 tube, replaced it and still no reverb. Any clues of what could be wrong? It used to work 2 days ago.

[g1]

Reverb used to work on both channels of my red knob twin ans now no reverb.

Didn't you have trouble with the switch?

[Jquijano]

Didn't you have trouble with the switch?

I sprayed some deoxit d5 and it fixed it. Now no reverb. Very faint when max to 8 or 10 on the reverb pot. Could it be the tranny? Where should i check?

[Jquijano]

Input side of the reverb tank measured 1.2 ohms, output measured 214.5 ohms. Wires have continuity so it must be the reverb transformer. Any other consideration?

[g1]

How can you be sure the switch is good?
You can test the reverb transformer with a speaker. Connect the cable that goes to tank input to a speaker.
But that will only prove the problem is in the drive circuit, not necessarily the transformer. If you hear signal through the speaker, then the trouble is in the recovery side of the circuit.

[Jquijano]

How can you be sure the switch is good?
You can test the reverb transformer with a speaker. Connect the cable that goes to tank input to a speaker.
But that will only prove the problem is in the drive circuit, not necessarily the transformer. If you hear signal through the speaker, then the trouble is in the recovery side of the circuit.

Groove tubes 12at7 suck. One came out broken and i replace it with the same brand and still no reverb. I swap it with an original fender 12at7 the amp brought and reverb came back. I did tightened all the male and female reverb jacks.

[Chuck H]

It's not Groove Tubes fault a tube arrived broken. That's a shipping error. That said, Groove tubes sells anything they think is viable qualifying it with a description. And THAT said...

Fender reverb circuits do put a lot of stress on 12at7 tubes. I wouldn't be surprised if newer makes don't hold up like the old ones. But let's not be too quick to blame suppliers for the problem. The amps still exist. The least bad option for a tube has to be supplied if these amps are to continue operating. This has ALWAYS been Groove Tube's MO. They could probably supply a tube that works (provided the shipper didn't fracture it!) even if it weren't as good as what was once upon a time.

[Jquijano]

It's not Groove Tubes fault a tube arrived broken. That's a shipping error. That said, Groove tubes sells anything they think is viable qualifying it with a description. And THAT said...

Fender reverb circuits do put a lot of stress on 12at7 tubes. I wouldn't be surprised if newer makes don't hold up like the old ones. But let's not be too quick to blame suppliers for the problem. The amps still exist. The least bad option for a tube has to be supplied if these amps are to continue operating. This has ALWAYS been Groove Tube's MO. They could probably supply a tube that works (provided the shipper didn't fracture it!) even if it weren't as good as what was once upon a time.

Thanks Chuck for your great advice as always. Amp is sounding fine. Still thinking if it is really necesary to swap the original 470 ohms carb composite resistors and 1.5k 1/4 watt resistors on the Power tubes. Only one measured 465 ohms, the other ones measured 472, 471, 473 ohms.

[Jquijano]

How can you be sure the switch is good?
You can test the reverb transformer with a speaker. Connect the cable that goes to tank input to a speaker.
But that will only prove the problem is in the drive circuit, not necessarily the transformer. If you hear signal through the speaker, then the trouble is in the recovery side of the circuit.

As a matter of knowledge to test if the reverb transformer is working or not i just need to plug the one of rca terminals to one of the speakers.