Noise in SFDR

[David Vinson]

Any troubleshooting suggestions?

In my late-seventies Deluxe Reverb, there's a background "pink noise" hiss in the Vibrato channel. It occurs when I turn the volume knob up to 3 or higher on this channel, whether or not there is anything plugged into the amp.

The noise does not occur in the Normal channel.

Here's what I've done so far to clean up other issues:

-The ticking from the vibrato circuit was cleared by disconnecting
the foot switch (I never use it anyway).

-replaced all the tubes except the 5U4
(JJ's for the 6V6; Mullards for the 12AT7, and TungSol for the 12AX7s)

-Rebiased (with 445V, the plate current is 22)

-Removed the 1200pF caps shunting from the 6V6 grids to ground

None of the filter caps appear damaged; but they don't hold a charge very long after the amp is off (they discharge to <8VDC within 10 seconds)

I was thinking about blackfacing the PI, but want to clear up the noise problems first so I have a good starting baseline of what the amp *should* sound like to start with.

Thanks VERY MUCH for any ideas!

[mykey]

very common problems with this type of preamp:
cracked resistors, plate and cathode resistors for preamp tubes
shorted or leaky electrolytic capacitors at preamp cathodes
to change all of those plate and cathode resistors on this side
of the preamp, and to change electrolytic caps on cathodes
of preamp tubes costs less than $10.
I suggest you try that, I like to use metal film resistors instead
of carbon.
the carbon composition resistors used in the production
of these amplifiers have glued on leads. over time they tend
to fail, and I have seen this type of failure in many older
Fender amps. the newer metal film resistors have welded on
leads and lower thermal noise.
the electrolytic caps used to bypass cathode resistors go bad
with age, as do nearly all electrolytic capacitors.
many times this simple replacement will cure the noises. hope
this helps...

[David Vinson]

Where to begin...

Thanks for reply, mykey.

More questions, more questions...

a) Since the sound isn't in the PI (it would be in both channels if so); then I should start in the first stage of the Vibrato channel & work towards the power stage, yes?

b) Is there a location in the Vibrato channel that tends to fail first? (e.g., the components for Reverb driver V3, Vibrato driver V5, etc).

Or, since the Vibrato components (around V5) need to be redressed anyway (to get rid of that stupid ticking), mebbe I should start there?

c) Any suggestions on component suppliers - best selection (and price)?

d) While I'm in there, should I do some wiring redress - If so, which are the crucial changes to make, and might there be *really clear* online pics to reference a blackface wiring dress? I don't have access to the real deal.

TIA,

David

[mykey]

a) Since the sound isn't in the PI (it would be in both channels if so); then I should start in the first stage of the Vibrato channel & work towards the power stage, yes?

yes

b) Is there a location in the Vibrato channel that tends to fail first? (e.g., the components for Reverb driver V3, Vibrato driver V5, etc).

no, but it's usually a plate or cathode resistor, the resistors that heat up the most

Or, since the Vibrato components (around V5) need to be redressed anyway (to get rid of that stupid ticking), mebbe I should start there?

Im not sure that you will get rid of all the ticking, maybe reduce it. old capacitors that are leaky would cause to much DC bleeding and not enough DC blocking, this could conceivably cause ticking sounds.

c) Any suggestions on component suppliers - best selection (and price)?

Michael Percy is one source of high quality low noise parts,
http://www.percyaudio.com/Catalog.pdf
but they cost more. I think better parts are worth it.
give me a day or two and I might have some recommendations

d) While I'm in there, should I do some wiring redress - If so, which are the crucial changes to make, and might there be *really clear* online pics to reference a blackface wiring dress? I don't have access to the real deal.

Ill try to find some, I know these layouts are around...These used to be in the Fender service manuals. but most of all I would change any ceramic disk capacitors (bright switch, treble control) to silver mica, this reduces the 60 cycle hum drastically.

get back to you soon, good luck with it.

[mykey]

wait a sec, look on the back of the chassis and tell me what the
model number is cause there was more than one layout diagram.

[David Vinson]

Model

Thanks, mykey.

The chassis is stamped "A76050". It is a late 70's, I think: It has the pull pot for "Boost"; but there is no serial# on the tube chart in the back of the amp.

I've been using a couple of drawings that I scanned out of my copy of Weber's "A Desktop Reference of Hip Vintage Guitar Amps". Although there are some discrepancies between the two, they have helped me get started:

- A schematic named "Deluxe Rev-Amp w/ Volume Boost".
... that's Fender's drawing #014641 rev C.

- A layout diagram numbered "AA763".

Is the latter as good as anything out there for lead dressing? Any other pointers to online references or must-have books would be awesome - Thanks for the input whenever you have time to look into this!

Great tip about the ceramic caps & hum. I'll come back to that later - I like to tackle one problem at a time, so that I can undo a change if I really hork it up.

DV

[Enzo]

Your ticking is an old and well known problem with certain Fender amps. If you disconnect the footswitch it turns off the trem oscillator so there will be no ticking. If you don't plan to use the trem, don't worry about it.

Your amp is a Deluxe reverb version AA763. You can find that schematic at Schematic Heaven. (ALong with many many others)

Turn the bright switch off and on. That will show you how much hum that cap is adding or not. As a rule, treble caps don't tend to add hum.

[David Vinson]

Parts Suppliers &amp; Schematics

Thx Enzo. I'll try that test for the cap. Are there other disc caps I should consider replacing?

Two other questions:

1) Are there some parts suppliers that folks in this forum like to use
(in addition to http://www.percyaudio.com/Catalog.pdf )

2) Are there other online pictures and/or drawings that layout the lead dress for the blackface. I'm not sure I have the AA763; the two schematics I have from SchematicHeaven.com show some differences:

The schematic "AA763" does not show the Boost pull switch, and has (I think) BF-style components (no 1200pF cap on 6V6 grids; and in the Phase Inverter the grid-to-cathode resistors are 1-meg and the coupling cap into the grid is .001 mF)

The schematic "Deluxe Rev-Amp w/ Volume Boost" shows the Boost pull switch, has the 1200pF "anti-oscillation" caps from 6V6 grid to ground; the PI has 330K resistors from Grid-to-cathode (vs 1Meg) and coupling caps are .01mF.

So, I'm thinking my amp is more like the second schematic. The layout drawing I have is for the AA763, not the Volume Boost version.

[Enzo]

I misunderstood. If you do not have the AA763, and your drawings don't match, Schematic Heaven has a ton of them, you should be able to dinf the one that matches completely and let us know which it is.

[Jag]

He's got the 77-79 pull boost verision. I was helping him with this amp a couple of weeks ago.

David, as I said in the other thread, your hiss is probably the plate resistors. You've got the right idea on where to start there.

It's probably a good idea to change the power supply caps as well. Regardless of how good the may look, they're 30ish years old and probably won't hold on much longer. This will likely do alot to get rid of that hum.

[mykey]

here are some of the favorite parts sources:
http://www.mouser.com/
http://www.alliedelec.com/
http://www.turnstyle.com/nsc/catalog.asp
http://www.digikey.com/
of course there are many more, but there's a
starting point for shopping. check stock before
ordering, and tell anyone you order from to:
"ship complete order only" this way you avoid
paying multiple shipping charges on the same order.

[David Vinson]

Great info

Thx mykey,

That's exactly what I was looking for!

Darn, now I have to go shopping :-D

[David Vinson]

Yup

Thx, Jag.

After I got the baseline operation going (thx for you rexcellent help!), it took awhile to figure out my next fix--the hiss. We covered so much ground in that thread that I forgot you had mentioned to check the plate resistors.

...then it took awhile for the info to sink in that the obvious place to look is the plate resistors of the Vibato channel's first gain stage. (heh, it only took three different people saying the same thing (you, mykey & Keen's tube amp debugging page)

http://www.geofex.com/ampdbug/ampdebug.htm

Sorry to make y'all repeat yourselves.

[David Vinson]

Fuzzy

Thx very much for the suggestions, Enzo. Based on everyone's input, I'll be replacing the plate resistors as first target, Power Supply caps, and maybe ceramic discs...This may be all I have to do (Always open to further ideas :-)

So at this point, I'll search for a layout drawing of the Pull Boost version; and report back on what happens as I swap out components.

Thx again guys,

DV

[mykey]

Thx mykey,

That's exactly what I was looking for!

Darn, now I have to go shopping :-D

shop wisely, grasshopper.