Two '77 Fender Twin issues

[Minty]

Hi folks! I just picked up two 1977 silverface Fender Twin Reverbs...they belonged to the band 'Yes' and have certainly seen years both on stage and in a damp garage. They're in a fairly miserable state and seem to have been modified in the past, but the price for both was too good to ignore!

This first one partially fires up...the preamp tubes and the 'on' bulb glow, but there doesn't seem to be any power getting to the power tubes as they aren't glowing or heating up at all. There seems to have been two wires cut from the transformer, (white & black with grey stripe), and I suspect this might be the problem. My question is, where do these wires go? And why on earth would they have been cut like that?



The other amp fires up fine and sounds great, apart from if you turn the reverb control up, a huge buzzy hum comes with it. If I disconnect the reverb cable from the back, it goes silent. I've cleaned the contacts for all the reverb tank cables and still no change... I feel like it's a grounding issue. What should I try next to fix it?

Thank you for reading!

[pdf64]

See http://bmamps.com/Schematics/fender/..._100_schem.pdf or more likely http://bmamps.com/Schematics/fender/..._135_schem.pdf
I think those wires are from the PT primary.

Is there a footswitch plugged into the TR with buzzy reverb? If so, try unplugging it.

[Minty]

See http://bmamps.com/Schematics/fender/..._100_schem.pdf or more likely http://bmamps.com/Schematics/fender/..._135_schem.pdf
I think those wires are from the PT primary.

Is there a footswitch plugged into the TR with buzzy reverb? If so, try unplugging it.

Thank you for the schematic! Am I right in thinking that the two wires should be going to the voltage selector? There are two unoccupied lugs on the pot...

Neither head came with a reverb pedal sadly!

[Steve A.]

If you are located in the UK the cut and taped off wires might be for 115VAC primary windings. Just a shot in the dark...

I often suggest that people pay an amp tech for a thorough diagnosis of an amp that doesn't work after replacing tubes. Sometimes that costs them less than replacing parts which turned out not to be bad.

Good luck!

Steve Ahola

[Minty]

If you are located in the UK the cut and taped off wires might be for 115VAC primary windings. Just a shot in the dark...

I often suggest that people pay an amp tech for a thorough diagnosis of an amp that doesn't work after replacing tubes. Sometimes that costs them less than replacing parts which turned out not to be bad.

Good luck!

Steve Ahola

Thanks for the reply Steve! Yeah I am based in the U.K, sorry I should have mentioned that.

[52 Bill]

The lead dress of the power transformer wires looks like the transformer has been replaced at some earlier time. As bad as they were in the '70s, even Fender didn't leave the leads full length inside the chassis. So the leads that were left unconnected may have been for voltages that the amp would never be connected to. The lazy/buzy tech saved some time.

If you look at the schematic, it lists the color codes of the multiple taps for the many international voltage windings. I would eventually go back in and clean up all of those wires, but fix the problem first as long as it is powering up.

The wiring to the heaters all look original and normal. Have you measured the voltage at the power tube sockets with a meter? Some 6L6 filaments are harder to see glowing as they are deeper into the main structure.

Oh yeah, Welcome to the place.

[Minty]

The lead dress of the power transformer wires looks like the transformer has been replaced at some earlier time. As bad as they were in the '70s, even Fender didn't leave the leads full length inside the chassis. So the leads that were left unconnected may have been for voltages that the amp would never be connected to. The lazy/buzy tech saved some time.

If you look at the schematic, it lists the color codes of the multiple taps for the many international voltage windings. I would eventually go back in and clean up all of those wires, but fix the problem first as long as it is powering up.

The wiring to the heaters all look original and normal. Have you measured the voltage at the power tube sockets with a meter? Some 6L6 filaments are harder to see glowing as they are deeper into the main structure.

Oh yeah, Welcome to the place.

Ahh that's a great reply! Thank you! I'll get right on measuring the voltage at the power tube sockets... Apologies for the upcoming novice question, but how do I use my multimeter correctly? Which setting should it be on? And which pins am I measuring between?

Thank you!

[Randall]

With all due respect Minty, if you aren't sure how to use a multimeter correctly, then maybe you shouldn't be poking around inside an amplifier that can kill you.

[Minty]

With all due respect Minty, if you aren't sure how to use a multimeter correctly, then maybe you shouldn't be poking around inside an amplifier that can kill you.

To be fair I am a novice when it comes to amps, but I've been building pedals etc for a while so I know how to wire things up pretty good, and I'm aware of the potentially immense danger that comes with working on an amp. I've just never had to use a multimeter before!

[52 Bill]

To be fair I am a novice when it comes to amps, but I've been building pedals etc for a while so I know how to wire things up pretty good, and I'm aware of the potentially immense danger that comes with working on an amp. I've just never had to use a multimeter before!

Well let's try a different approach for now.

Take the power tubes from the working head and install them in this chassis. Anything happen?

[Minty]

Well let's try a different approach for now.

Take the power tubes from the working head and install them in this chassis. Anything happen?

The power tubes in the working head actually originally came with the faulty head, so I don't think they're the problem. I have since retensioned the tube sockets so I'll give it another go!

[Steve A.]

I have since retensioned the tube sockets so I'll give it another go!

Yikes! I remember one of the now-famous amp techs back at the old AMPAGE forum here telling us how he was knocked across the room retensioning tube sockets without first draining the filter caps in an amp.

As even small shocks can cause you to flinch and cause other damage after draining the filter caps I will leave a jumper cable clipped from the positive terminal of the first filter cap to the chassis. (I've seen the sparks from voltage that built back up after discharging a cap and don't want that voltage going through ME.)

Steve

[Minty]

Yikes! I remember one of the now-famous amp techs back at the old AMPAGE forum here telling us how he was knocked across the room retensioning tube sockets without first draining the filter caps in an amp.

As even small shocks can cause you to flinch and cause other damage after draining the filter caps I will leave a jumper cable clipped from the positive terminal of the first filter cap to the chassis. (I've seen the sparks from voltage that built back up after discharging a cap and don't want that voltage going through ME.)

Steve

I only did it when I was absolutely sure it was fine. The amp hadn't actually been turned on for a few days! I have read a lot and watched a lot on the subject, so I am now extremely cautious when it comes to these things!

[Justin Thomas]

Not being turned on for a few days is no guarantee that caps are drained. I'll work on an amp that was on five minutes ago, if I drained the caps myself and put in my own clip lead, and checked with my own meter, before I'll poke around in one that someone brought me that's been "off for a few days." I don't trust my life to a $.10 bleeder resistor.

I'm not saying you don't know what you're doing. I'm saying that until you assure us that you know how to use a meter and do this without killing yourself, you will continue to receive such messages. I <HAVE> had my arm knocked behind my back for being, not careless, but not as careful as I should have been. It @$&#ing SUCKS!

Stay safe, and maybe get some hands-on live help from your local crusty old tube tech. It may save your life, and your amps. Yes, we all need to learn somehow. Most of us would NOT recommend learning the same way we did!

Justin

[The Dude]

Agree with the above. I would rather trust a meter than a clock when it comes to B+. Always verify caps are drained with a meter before you stick your paws in there.