Noise issue with 70 SR

[Mars-Hall]

I have a strange noise issue with this amp.  The noise seems to be frequency related to an A note. Also is touch sensitive, in that picking lightly, the noise does not occur.  My first thought was bad speaker, when tested with another cab, noise persisted.  It's underlying noise, heard in the back ground.  The touch sensitivity led me to believe it was power section related.  I replaced all electrolytic caps, installed new power tubes and swapped the rectifier.  I tried a 5U4GB which diminished the noise to some degree.  The amp has been converted to bias adjust and removed the large resisters from pin 5 of the power tube sockets.  Chop stick test shows no sign of cold solder joints, I re-flowed all areas disturbed to be on the safe side. Last thing I replaced all preamp tubes with known good, to no avail.

[sluckey]

and removed the large resisters from pin 5 of the power tube sockets.

What large resistors are you talking about?

[Mars-Hall]

and removed the large resisters from pin 5 of the power tube sockets.

What large resistors are you talking about?

My apologies, it's a 2000pf cap to ground.  I'm gonna change the plate resistors next.  Hopefully that will cure it.

[Colas LeGrippa]

the power tubes have plate resistors ? 50 ohm or so ?

[shooter]

don't see any on this AA270 flavor

https://el34world.com/charts/Schematics/files/Fender/Fender_bandmasterreverb_aa270_schem.pdf

[Mars-Hall]

the power tubes have plate resistors ? 50 ohm or so ?

I was referring to the 100k resistors in the preamp section.

[Mars-Hall]

Today I lifted the board to see if there was conductivity, noise persists.  Upon closer inspection, I found these plate resistors for V2 looking a little dark.  However, both read within tolerance.

[shooter]

noise persists

looks or tolerance don't have much to do with solving noise.  Unless this is a restoration and not repair, put Metal film 1W in the early gain stages.  If it's a resto, use a scope, find bad R, order NOS, lots 

[Mars-Hall]

I replaced all plate resistors and still have the noise issue.  Even though the pots don't make noise when rotated, I discovered a small amount of DC voltage at channel 1, treble and bass  pots. Also at channel 2, volume and treble pots.  Could these leaky caps cause this noise, as described? 

[shooter]

a small amount of DC voltage

less than 1 or greater than 2?
sometimes meters are deceptive at real low volts

[shooter]

Could these leaky caps cause this noise, as described? 

sorry ment to say, yes, leaky caps can cause weird stuff since the "right hand" side DC usually goes to next stage grid and messes with it

[Mars-Hall]

a small amount of DC voltage

less than 1 or greater than 2?
sometimes meters are deceptive at real low volts

I'm reading between 3-4 mv.

[shooter]

That low and I'm looking for another noise source, 1vdc and up, I'm swapping caps

The noise seems to be frequency related to an A note

this still the noise?

[Mars-Hall]

That low and I'm looking for another noise source, 1vdc and up, I'm swapping caps

The noise seems to be frequency related to an A note

this still the noise?

Now the noise is mostly lowest 2 octaves only.   I tried to upload a short video but the file is too big.  I even tried editing it down.

[Mars-Hall]

Here is the noise example.


https://youtu.be/Hr00DG5JquY

[shooter]

sounds like a guitar payer noise way more than a tech noise.

sounded like when you don't play, it's quiet?
for fun roll a known good tube in V1, check
IF no effect, move to .......until the "last tube" before PI

[Mars-Hall]

sounds like a guitar payer noise way more than a tech noise.

sounded like when you don't play, it's quiet?
for fun roll a known good tube in V1, check
IF no effect, move to .......until the "last tube" before PI

Already went down the tube road, more than once.  The noise is volume and touch sensitive, in that at lower the volume or lighter playing, no noise.   Not sure what you mean by, "guitar player noise more than tech noise".  Is that another way of saying, "loose nut behind the steering wheel". I've tried the amp through another SR cabinet. Brand new tubes in the power section, rectifier included, and swapped out each pre, one at a time.  Re-tensioned all tube sockets.  Also installed new 470 ohm resistors on the power tube sockets and all the plate resistors.  Replaced all electrolytic caps.  Been reading in the tube amp library, that's what brought me to the coupling caps. It seems the DC voltage at the pots is negligible. Also tried unscrewing and lifting the board, with a chop stick. 


The only other oddball thing about this amp, has a very thin coated wire, only seen on a handful of amps, from this era.  Perhaps there's a bare spot somewhere touching to ground, that I haven't noticed. 

[shooter]

sharpen up your meter probes to a "pin point", jab them into the board at various places and look for conductance (ohms, low values).  old fender boards are like fenders on old Chevy's :)

or
put a distortion pedal up from and learn to love the Metal sound 

[66Strat]

Those 2,000 pf shunt capacitors were implemented by Fender to prevent oscillation in the power amp. Try putting the shunt capacitors back in place.

[Mars-Hall]

Those 2,000 pf shunt capacitors were implemented by Fender to prevent oscillation in the power amp. Try putting the shunt capacitors back in place.

Due to the sloppy wiring?   This amp has excessively long runs in places. Looks like a rats nest. However, those shunts were removed years ago, this noise is relatively new.

[66Strat]

Those 2,000 pf shunt capacitors were implemented by Fender to prevent oscillation in the power amp. Try putting the shunt capacitors back in place.

Due to the sloppy wiring?   This amp has excessively long runs in places. Looks like a rats nest. However, those shunts were removed years ago, this noise is relatively new.

Partly sloppy wiring. The other piece to the puzzle, that doesn't show in the blackface schematics, is the capacitance formed by the cloth backed wiring lying flat against the chassis. Listening to the clip that you posted, to me, it sounds like there is some oscillation going on somewhere in the circuit. The shunt capacitors, to me, seem like a likely cure as that was Fender's solution. Alternatively, you might try a 100 pf to 200 pf capacitor capacitor between the plates of the phase inverter. Oscillation in this area was a concern even with the blackface and tweed circuits.

[dude]

Raise your action and try, if too low action or a high fret, you'll get that slight buzz when you first strike a note.

[Mars-Hall]

Ok, what are the big rectangular resistors, in this photo, intended purpose?  These are what I remember removing.  It was a long time ago...

[sluckey]

Those are cathode resistors. They add a bit of cathode bias to the amp, also a convenient place to monitor when adjusting the bias.

[Mars-Hall]

Those are cathode resistors. They add a bit of cathode bias to the amp, also a convenient place to monitor when adjusting the bias.

That makes sense, I have the 1 ohm 1 watt resistors there now.

[66Strat]

IMO, no harm, no foul in replacing the 150 ohm cathode resistors with the 1 ohm resistors. It will simplify the biasing process.

[66Strat]

I don't know why Fender added the cathode bias in addition to the fixed bias supply, but in addition to biasing the tubes, the resistors also introduced some degenerative feedback.

[shooter]

introduced some degenerative feedback.

hmm, and caps makes a more stable PA 

fwiw;
I looked back at my repair notes for a '65 SR
came in biased at 23ma (-57) plate 470vdc for ~11W
I bumped it to 47ma (-47) plate ate 450 for ~ 18W per tube
This unit had neither the 150 ohm or 2000pF and rattled my teeth at 7