Fender '94 Twin Amp Noise...

[acorkos]

When I flip the standby switch, there's a static noise coming from the speakers. All front panel control pots are set at zero. And noisy with/without a guitar cord plugged in.

The noise persists when preamp tubes V1-V6 are removed. The noise is gone when V7 (effects loop) or V8 (PI) are removed.

It's not the tubes. It's not the plate resistors. It's not the power supply filter caps. I know because I replaced all those things. I've cleaned and retensioned all the tube sockets, too.

I replaced the .022uF PI/power tube coupling caps C32/C33. I also replaced coupling caps C29, C23 and C26 which run between V6 and the PI via the effects loop tube V7. Noise persists.

I also replaced power supply resistors 5W/1K R91/R154 and 2W/4.7K R103...noise persists.

Any ideas as to what to check next to determine where the noise is coming from? Thanks for your help!

The schematic is on page 2 of the link....

http://www.fender.com/support/amp_sc..._Schematic.pdf

[oc disorder]

Hi acorkos
I would have thought it would be a plate resister R146 even R 145 but you say you have replaced these.
If you insert a jack plug in the "power amp in" dissconecting the first half of V7 is the noise still there ?
That may help to narrow it down further.
Maybe explore where the negative feed back comes in and components around the presence control.

[acorkos]

If you insert a jack plug in the "power amp in" dissconecting the first half of V7 is the noise still there ?

when i insert a plug into the power amp in jack, the noise (and other normal sounds) disappears

[melvin]

It seems you've narrowed it down to V7b.

any resistor connected to it is suspect. could one of the cathode resistors inject hiss in a follower?

bad solder joint?

[acorkos]

R140 off V7B (100K) was the problem

[Stoffel]

Help wanted

R140 off V7B (100K) was the problem

Hey, my '94 twin amp (rev E) has the same noise problem, only in another part of the amp. The amp always had this problem, but my dealer told me that this was normal and at that time I believed him and took the amp home, bought a vox (quiet as a mouse) and forgot about the twin. 18 Years later I thougt it was time to fix that damn twin...!

I have already changed all caps and a bunch of resistors. The noise stops when I remove V6.

I isolated the preamp, the mixing part and the reverb part (by removing R54, R55, R63 and C25. Then I connected R65 (at the preamp side) to the ground. Noise persists. When I connect pin 7 of V6B to the ground the noise is gone.

According to these steps I suspected the resistors R65, R64, R66 and R67. Changed them all but the noise is still there. Don't now further what to do.

Any tips are welcome!!

[g1]

Then I connected R65 (at the preamp side) to the ground. Noise persists. When I connect pin 7 of V6B to the ground the noise is gone.

According to these steps I suspected the resistors R65, R64, R66 and R67. Changed them all but the noise is still there.

If grounding pin7 of V6B gets rid of the noise, then the problem is before that point. So R64,R66, and R67 are not suspects.
You have disconnected everything before V6 pin7, so the only other option seems to be V6 or it's socket. Have you resoldered it?
You did say elsewhere that Fender dealer told you it has the normal amount of noise? Have you done an A/B comparison with another amp of the same model and found yours to be noisier?

[Stoffel]

I retubed my amp with brand new JJ's and the problem still persists.
I haven't had the chance to do a comparison withe another twin amp, but it is simply too noisy. My vox is quit as a mouse, even with the volumeknob turned to 10 it is much quiter then the twin with all settings at 0.
I think my dealer didn't even send the amp back to the main fender dealer. I can not believe that this is normal...

Okay, I resoldered the socket. Even further: I changed sockets between V1 and V6. Same problem.

Now; pulled out V1, V2, V3, V4 and V5; With R63 removed and R65 only disconnected at the preamp side the following happens: When connecting R65 at the preamp side to ground: noise. When connecting groud to tube side of R65: noise gone, hum instead (cable between R65 and groud is very short; ground taken from screw of tube socket).

I also tried something else: I was thinking: if the noise comes from before the V6B maybe I can lower the volume of this noise by lowering the amplificationfactor of V6B. I did this by lowering the 100K (R67) to 50K. (Factor divided by 2). Guess what: noise still as loud as it was before the change... Maybe I have to look between V6B and V7B? I don't know anymore...

[Stoffel]

Ok guys, the quest goes on!

Today I find that R139 (pin 8 of Tube V7B) was a 100K resistor! It should be 1,5K! Changed it and the noise was remarkable less then before!
I think it's a good progress, but the amp is still too noisy for me.

When I connect pin 7 of V6B to the ground the amp is very quite (as it should be). No noise or hum at all.

When I remove R54 and R55 and ground R65 at the preamp side the noise persists, so it seems that the noise is coming from the reverb section...

[g1]

Today I find that R139 (pin 8 of Tube V7B) was a 100K resistor! It should be 1,5K! Changed it

R139 is in series with R140, which is 100K. So you may want to double check those 2 locations are labelled correctly on the board, and that R140 is 100K.

so it seems that the noise is coming from the reverb section...

I have attached the complete service manual. Near the back there is a service bulletin about noisy reverb. It looks like it may be a fair bit of work, but if it solves your problem then I guess it's worth it.

[Stoffel]

Both R139 and R140 were 100K resistors. I doublechecked this before I changed it into a 1,5K resistor.

Thx for the service manual, but I already have the 2nd mosfet on the print. There is an E revision board in my amp.

If I turn all pots to 0 the amp very quit. Turning the presence up makes it noisier again. Turning the reverb up makes it noisier too.
I wonder if this noise is normal for the amp.

Does anyone owns a '94 twin? And is it quiet? Or is it noisy too? I would like to know.
I compare it with my Vox H1TV. With this one I have to look at the light to know if it is on or not. Love it!

[GT11RR]

Both R139 and R140 were 100K resistors. I doublechecked this before I changed it into a 1,5K resistor.

Thx for the service manual, but I already have the 2nd mosfet on the print. There is an E revision board in my amp.

If I turn all pots to 0 the amp very quit. Turning the presence up makes it noisier again. Turning the reverb up makes it noisier too.
I wonder if this noise is normal for the amp.

Does anyone owns a '94 twin? And is it quiet? Or is it noisy too? I would like to know.
I compare it with my Vox H1TV. With this one I have to look at the light to know if it is on or not. Love it!

HI,

know this is old as he'll but I have one of these amp and being the 100w beasts they are, yes, when on and idle they are a bit on the noisy side. Another thing too: there are 12 vacuum tubes in there (!) and in are technological wireless age they are prone to interference from all kinds of modern home devices (eg. cell phones, WiFi, noisy ac coming out the wall, lights, tvs, etc. etc.) the thing is though

if you're playing it at anywhere near the volume it was designed to operate at the noise is totally acceptable.

if you're playing it in your bedroom at 2am (hahaha, we've all been there!) yes it's on the noisy side.

[Stevie B]

Looking at this old thread cuz I have a noisy 94 Twin Amp.
I actually have 2 of them - one is noisy and the other is quiet.
So I was considering replacing the filter caps since the amp is over 20 years old.
But when you add up all the filter caps - there's 13 of them!
And the 4 for the bias supply are little radial lead caps.

I was wondering if the 16 volt power supply filter caps (4 of them - which adds up to 17 total) would create some hum in the signal path?

[g1]

I think the IC's running off LV supplies are only in the switching circuit, not the signal path.
I assume you've tried swapping in the tubes from your other amp?
How about pre-out into the other amps power amp in, or vice versa? Does any way eliminate the hum?

[Stevie B]

Yes, I realize the 16v is for the switching stuff - but I was wondering if it could affect the smoothness of the power for the bias supply (the 16v is coming from the same PT secondary as the bias supply).

I've swapped around the tubes (I am always searching for the quietest 12AX7).
But I haven't tried the pre to power of the opposite amp - I should've already thought about that - thanks.