65' Fender Twin - Help Needed Troubleshooting

[TheeDoctor_G]

Hello,

I've recently become the proud owner of the Fender 65' Twin I bought it second hand and I am very pleased with it, it's in perfect condition for 20 year old amp - visibly anyway.

Switching the amp on into "ready to play" mode, there is an audible hiss present. This is present when all dials are set to 0 on both channels and is then amplified when volumes, eq controls are turned up. I have just replaced the output valves and few of the 12ax7's and the 12at7 phase inverter, this has not repaired the problem

Further diagnosis:
Hiss disappears when either V6 (PI) or V4 (half reverb recovery and half gain stage for vibra... I mean, tremolo channel) are pulled.
Also, the hiss is louder when the tremolo (vibrato) channel is turned up than when the normal channel is.
The hiss sounds like "moving air", if that makes sense, and is certainly not a hum!

Further, minor problem:
The LFO is leaking when the trem circuit is switched on.


Thanks very much for all your help in advance!

[Steve Conner]

Many tube amps have an audible hiss. The more powerful the amp, the louder the hiss will be. So in the case of a Twin, it could be normal.

Then again, maybe some of the plate load resistors have gone noisy with age.

[R.G.]

1. What does pulling V1 do to the hiss?
2. What does pulling V2 do to the hiss?
3. What does shorting across R26 do to the hiss?
4. What does shorting Ring of J7 to ground do to the hiss?

[TheeDoctor_G]

1. What does pulling V1 do to the hiss?
2. What does pulling V2 do to the hiss?
3. What does shorting across R26 do to the hiss?
4. What does shorting Ring of J7 to ground do to the hiss?

Thanks for your replies. None of the above kills the hissing - obviously it kills the amplification of the hiss, no valve = no channel haha

[Enzo]

That is the point of pulling the tubes. If we pull a tube and it no longer hisses - meaning we broke the circuit poath for the hiss to be amplified - then that tube is after the source of the hiss. Or is itself the source. Likewise, if we pull a tube and the hiss remains unchanged, then that tube is before the hiss source.

These tests help localize the problem within the amp.

[soundguruman]

It's normal to have hiss, and there is more hiss in the tremolo channel, that is normal.
LFO is normal with the trem circuit ON.
All is normal.
In old Fender amps, the resistors and capacitors can get noisy. If they get too noisy, people overhaul them.
While it is true that the newer components have less noise, people would rather have an amp that has never been worked on.
When a twin has hiss like that in the trem channel, it usually sounds killer cranked up really loud.
So, I recommend that you play it on "10" and your neighbors will love it.

Seriously, there could actually be a bad resistor or capacitor, in that section of the circuit, that is causing excessive noise.
Being this old, it is not that unusual, if you think this really is a 65' twin, there could be several noisy parts, and it's not the first time
an old twin needed some overhauling.

[Enzo]

SGM, for the love of god... '65 Twin Reverb is a model name, not the date of manufacture. It is a reissue model from the 1990s.

[Steve Conner]

What?! I assumed it was the 65ft oversized model.

[Enzo]

No, the oversized model is my wife...

[Jazz P Bass]

Wenzo, he's picking on you.

[soundguruman]

SGM, for the love of god... '65 Twin Reverb is a model name, not the date of manufacture. It is a reissue model from the 1990s.

"for the love of God"...
So which is it a 65 twin or a 65 twin reissue?
Maybe somebody should have said "reissue"
But none the less, the same problems.
I am not a member of the psychic friends network, maybe Enzo is. We should just "know" from our psychic abilities.

But anyhow, there were a bunch of noisy carbon resistors used in some of these reissue amps, they were really junky.
Sometimes the best thing is to just change all the resistors in the signal path. You could actually have noisy parts.

[52 Bill]

"for the love of God"...
So which is it a 65 twin or a 65 twin reissue?
Maybe somebody should have said "reissue"
But none the less, the same problems.
I am not a member of the psychic friends network, maybe Enzo is. We should just "know" from our psychic abilities.

In the first paragraph the OP calls it a 20 year old amp in perfect condition.

[g1]

In the first paragraph the OP calls it a 20 year old amp in perfect condition.

Oh you cynical psychic hoax busters, I was liking the idea of Enzo's psychic amp healing powers .
Aside from the 20yr. old tip off, '65 Fender Twin is an official Fender model name. No psychic abilities required.
Many more examples of such "reissue" model names here: FenderŪ Support

[52 Bill]

Enzo's powers go way beyond psychic healing!

[Enzo]

You don't have to be psychic to read and pay attention to the question.