fender 65 twin reverb reissue making siren noise after replacing all preamp tubes

[funkylimb]

Hi,
I was long overdue for a tube change so I replaced all the preamp tubes in my fender twin reverb reissue with new ones. Now when I turn the amp off of standby there is a siren noise -- i hear it with the volume turned all the way down. It does get louder when I increase the volume on the vibrato channel. I was originally troubleshooting a noisy reverb/vibrato channel and was hoping the tube swap would help -- but it made it worse :-)
Any ideas? They would be much appreciated!
Thanks,
-James

I uploaded an audio file of the problem to dropbox:
https://www.dropbox.com/s/h4nv6h4pxptzpl0/Squeak.m4a

[Justin Thomas]

Suggest replacing the "New" tubes one by one with the old ones. Betcha one of your new preamp tubes is a noise-monster. Preamp tubes can last quite some time, so you prolly shouldn't replace them without a sure diagnosis, and then, one at a time. If the reissue is the same as the vintage, it should only be V3,4,5 involved in reverb & trem.

Justin

[funkylimb]

Suggest replacing the "New" tubes one by one with the old ones. Betcha one of your new preamp tubes is a noise-monster. Preamp tubes can last quite some time, so you prolly shouldn't replace them without a sure diagnosis, and then, one at a time. If the reissue is the same as the vintage, it should only be V3,4,5 involved in reverb & trem.

Justin

Thanks Justin,
I removed V6 12AT7 and the amp was quiet as can be. I tried two other 12AT7s in that socket and had the same problem. I then took out the V4 12AX7 and the amp was quiet as well. Replacing that with other 12AX7s didn't fix the problem either -- so I imagine there is an issue in that circuit (2nd gain stage for channel 2 and reverb return amplifier)?
I downloaded the amp schematic and was checking voltages at some of the test points in the circuit. I wasn't getting any voltage reading off of two resistor checkpoints. One was crazy high -- reading 400 volts when it should have been 110mV. Is that a sign of bad resistors or some other component?
Thanks again for taking the time to help me out!
-J

[Enzo]

Please tell us what readings you got and where you got them. It is hard to help when we hear a reading was crazy high at 400 volts...somewhere in the amp. V6 pin 6 or top of the B channel volume control, or the left end of C123, and so on. Be specific.

[funkylimb]

Please tell us what readings you got and where you got them. It is hard to help when we hear a reading was crazy high at 400 volts...somewhere in the amp. V6 pin 6 or top of the B channel volume control, or the left end of C123, and so on. Be specific.

Hi Enzo,
Sorry about that and thanks for the help. Also of note -- the siren sound subsided and now there is just a lot of loud fuzz noise. I just retested and here are the readings.

TP 13 - should be 110mv
R28
400V left 286V on right

TP10 - should be 1.1V
CP30
no voltage reading

TP11 should be 1.9mv
R26
no voltage reading on either side

R30 and R31
Loud buzz when i touch with the probe on either side - also no voltage reading

[Justin Thomas]

Okay, looks like the Test Point voltages in OVALS are AC, and the Test Point voltages in RECTANGLES are DC. Is that maybe causing some of the confusion? Do you have the schematic, and are the controls set where they should be for the tests? I'm on a tablet, but if you can post the schem or a link so we can all be on the same page, that'd be cool.

Most likely I won't be much more help here, so I'll duck out. Good luck!

Justin

[funkylimb]

Ah -- i did not know that. Thanks Justin. I was testing in DC -- when the kids go to bed I will take another peak. Here is a link to the service manual which has the schematic:

http://ampwares.com/schematics/65_tw..._@20manual.pdf

[funkylimb]

I looked at the schematic and everything I checked was supposed to be DC -- so the readings I previously posted are accurate.

[Richard]

Readings in ellipses are AC, in rectangles DC. That should be noted on the drawing but it isn't on the schematic I have. The B+ is what you're measuring if you are measuring DC, 400VDC and 286VDC at R28 are about what they should be. Are the tubes in? The schematic should list the proper control settings and input level that the voltages were measured at. TP10, 11, and 13 are the reverb send, return, and recovery respectively. R30 and 31 should buzz when you touch a probe, it's effectively the same as touching the tip of the guitar cable.

The fact that you hear the noise with the volume turned down suggests that the initial gain stage V1a and V2a are not the problem. Does the reverb level affect the sound? Pull tubes one by one starting at V1 and working your way up. When the noise stops you are closing in on the problem.

V6 is the phase inverter, pulling V6 tells you it's not the power amp. If you remove V4 (reverb recovery and blend) and V1 (normal channel) and you still get the noise the the problem is probably around the phase inverter. V3 is the reverb send. V5 is the vibrato.

By the way, don't think of it as your amp squealing, think of it as horror movie sound effects.

[EDIT: slow typer, left window open, did not see other posts]

[Richard]

You won't get the proper AC readings unless you have a 37mv 1kHz input.

[funkylimb]

Richard -- thank you. You've made this novice's head spin :-)
I'm not sure how to achieve that input . . . a signal generator?

But I did some more troubleshooting and made some progress. Part of the problem was a bad tube in the V2 location -- that took care of the crazy horror movie sound! I'm not sure why my initial tests didn't find that.
But there is still substantial 60Hz hum so i starting pulling the other tubes and the culprit seems to be V4. I tried swapping that tube but am still getting crazy amounts of hum. I tried turning the knobs to see if that changed it but the only one that affected it was the volume control.
I know tube amps have some noise but it seems excessive.

[Enzo]

If you pull V4 and the hum goes away, that does not mean V4 was causing the hum. it MIGHT be, but all it really tells us is the hum is coming from V4 or anything before it. Everything in the vibrato channel runs through V4 to mix in with the other channel and on to the powr amp. SO if say V2 were making noise, pulling V4 would make it silent.

[Richard]

Yeah, I start typing and remember other things. I could have been clearer. I don't get much hum but some hiss with my SRRI, I expect your Twin would be similar Sometimes a spring reverb can pick up a little buzz. On my amp I wouldn't consider it substantial in any way. Does turning down the reverb knob have an effect? Or unplugging the return? On my amp the reverb has a slight hum whereas the rest of the amp has hiss. Perhaps the tank is not grounded well so it is picking up more hum than normal. Have you tried a different outlet? Your mains may be a source of hum. I know my guitar makes a lot less noise at other places besides my house.

I don't know if ovals for AC and boxes for DC is a convention or if the drafter left the note off the plans. On other schematics from Fender the notes specifically say VDC or VAC.

[funkylimb]

Thank you all for the replies -- they have helped out tremendously. I ended up biasing the amp today which is something i should have done the last time I replaced the power tubes. The tubes were being run fairly close to their max capacity. After doing this the hum was quieter so I tried to adjust the hum pot and low and behold, my amp is pretty darn quiet :-)
I'll take it.