Fender Twin Screeching

[TD_Madden]

Probably V3 (12AT7). Does it have a tube-chart?

[Rob Mercure]

YGM,

First off, as you can tell we're recommending, don't get hung up on conincidence - it can totally turn you around and in the meantime drive you crazy (ooops, did I dribble again?). And as I recommended, have you pulled the reverb connectors and cleaned them at both ends? It only takes one dirty end to make problems.

As mentioned preamp tubes last a lifetime often so definitely hang on to your old tubes. Any recommendation to replace an entire set of tubes comes from someone who makes a profit selling you tubes - my '67 Vibrolux reverb has only had the rectifier and PI tube replaced - 31 years later, after years on stage, I'm still riding on the original output tubes (but if this amp doesn't have individual output tube bias I recommend that you get a good tech to install it).

Have you taken a pencil and tapped each tube in the amp with the volume at a meduim level? Most often when a tube's responsible for screeching it's microphonic (has a 6SL7 once that I could actually sing into!). But to save money I'd probably only buy a 12AX7 and a 12AT7 and replace suspect tubes one by one cuz it's unlikely that you've got more than one bad tube - assuming that this is you problem.

Rob

[pdf64]

I recently fixed a Mesa Boogie that was doing a similar intermittant screeching thing, new power tubes cured it. Never known that failure mode before!
Don't expect new production tubes to last as long as Rob's 60s vintage tubes. The power tubes in my buddy's pro tube twin amp were giving a flabby and tired after a year's hard use.
If you have the effects loop engaged in that amp, does turning down the send or return levels have any effect on the screech? Does the tremelo modulate the screech?
Peter

[yesguitarman]

I recently fixed a Mesa Boogie that was doing a similar intermittant screeching thing, new power tubes cured it. Never known that failure mode before!
Don't expect new production tubes to last as long as Rob's 60s vintage tubes. The power tubes in my buddy's pro tube twin amp were giving a flabby and tired after a year's hard use.
If you have the effects loop engaged in that amp, does turning down the send or return levels have any effect on the screech? Does the tremelo modulate the screech?
Peter

I'm not using the effects loop. I haven't tried to see if the tremelo affects the screech. The screech is so loud the first reaction is to flip the standby on. Volume controls have had no effect on stopping the screech when it starts. If I flip the standby off after just a few seconds the screech is right there again, even with nothing plugged into the input jack.

[MWJB]

"Volume controls have had no effect on stopping the screech when it starts. If I flip the standby off after just a few seconds the screech is right there again, even with nothing plugged into the input jack." We have known this for a few days now, what exactly have you done to isolate/identify where the screech is coming from?

Is there any screech with all the preamp tubes removed?

[yesguitarman]

"Volume controls have had no effect on stopping the screech when it starts. If I flip the standby off after just a few seconds the screech is right there again, even with nothing plugged into the input jack." We have known this for a few days now, what exactly have you done to isolate/identify where the screech is coming from?

Is there any screech with all the preamp tubes removed?

Ok, I'm a true neophyte here so please be patient with me. Is it safe to pull the tubes with the amp powered? Aren't they hot, do I need gloves? Do I leave the standby switch in the normal play position when pulling the tubes? Do I start at V1 an pull the tubes in ascending numerical order?

Thanks

[Rob Mercure]

Yes you can pull the tubes with the amp powered but turn down the volume controls as you pull each one cuz it will cause much pop and boom. And I'd recommend a cloth work glove.

As to which tube to pull that probably depends on personal preference. I would first pull the 12AT7 nearest the output tubes and see what happens. This is the phase inverter (PI) and with it out the only tubes connected to the speaker (via the OT) are the output tubes - if the noise remains it is in the output circuit. If that changes nothing then I'd pull the 4th tube which is the reverb recovery and also channel mixer - 12AX7 - if the symptoms diminish I'd suspect the reverb recover section but it could be the mixer side. If this doesn't change anything then systematically start from tube 1 and remove the tubes. But, just so you can determine which actual stage is at fault be sure to restore a tube to it's original position once you've found that it doesn't make any difference. It would be easy to pull all of the tubes, have the problem fixed, and wind up with a spare set of tube with a hidden "suspect" in the spares waiting to jump out and bite you.

As you remove and restore the tubes make sure to gently rock the tubes in their sockets a bit - this will often either make the syptom worse, pointing to a bad tube or dirty socket - or improve the syptom pointing to a dirty socket. Sockets are best cleaned with a product called DeOxit (Caig corp) followed by 99% isopropyl alcohol (to remove any of the lubricant in the DeOxit which can attract conductive dust and cause carbon tracks on the tube socket which then cause all sorts of gremlins). I think you know aleady not to operate the amplifier without a speaker to prevent damage to your output tranny and output tubes.

Rob

[MWJB]

Is this a 65 BF Twin or a pro tube twin? I seem to remember a reference to an effects loop (not a feature on a 65 Twin). If pro tube, V4 is the PI - go to Fender.com/support/amplifier schematics & print off schem & layout, you will see what number tube does what (or ask here, making sure you have a reference & can understand the answer).

It's good practice to power down when pulling out tubes, preamp tubes don't typically getr very hot BUT the 12AT7 reverb driver will most likely be too hot to be comfortable! Pull them ALL out one by one, write on them with a sharpie where they came from.

Replace one by one, starting with PI (V4 if pro tube, V6 if a 65), then 12AX7 reverb recovery, then 12AT7 reverb driver, then the preamp tubes one by one. Power up between each installation, volumes & tone controls set to full, and flick the end of the preamp tube - you are trying to prompt a noise. Preamp tubes V1 & V2 may give a "tink" sound when tapped/flicked...this is normal, any whistling or extended ringing sounds, screetching, frying eggs, or loud "baaaarrrrgh" sounds are not normal, replace whatever tube is dioing it.

[yesguitarman]

OK folks, it's definitely a reverb issue. If I play the amp with a guitar plugged straight into the input jack, with no reverb, no screech. As soon as I turn the reverb control past 1, the screech returns. Which tube or tubes (V number) should I replace to address the issue. The amp is in pristine condition. There is absolutely not a spot of corrosion on the reverb spring tank RCA jacks.

Thanks for all your help,

Tom

[MWJB]

If it's a pro tube twin, replace V5 12AX7 & V7 12AT7.

If it's a 65 Twin replace V3 12AT7 & V4 12AX7.

[yesguitarman]

If it's a pro tube twin, replace V5 12AX7 & V7 12AT7.

If it's a 65 Twin replace V3 12AT7 & V4 12AX7.

Thanks guys, I think that did it. I replaced both V5 and V7 last night and the amp sounded great! I also replaced V1 with a new 12AX7R2 tube. The amp sound sooo much better with this tube than the standard 12AX7. Thanks for the tech at The Amp Shop in West Palm Beach for suggesting that preamp tube.

Thanks again,

Tom

[yesguitarman]

Guys, this issue has returned several times. Each time I have replaced V5 and V7 and the screeching from the reverb section goes away. The amp plays fine as long as I keep the reverb at 0. What other components in the reverb section is causing these tubes to go bad. For clarity, the amp is a 2000 Fender Pro Twin (the one with a single input jack).

[Enzo]

I would ask if you removed those tubes, then tried re-installing the exact same pair of tubes?

My first impression is feedback in either the pan or the cabling.

[Jazz P Bass]

Has anyone messed with the reverb pan?

There is a drive end and a pickup end.

The pickup end is marked OUTPUT.
That end should be mounted as far as possible from the power transformer .