Deluxe Reverb Buzz

[Ted]

I've got a 68 Deluxe Reverb here.
Sounds pretty good except when you play between bottom G and A# on the E string of a guitar it's got a soft underlying buzz. I note that this is around 100Hz. (Mains freq here is 50Hz)

Its present on both channels. It doesn't seem to be vibration related as its present when the amp is physically separated from the speaker.

I can't seem to recreate it with scope and sig gen.

Things I've tried which haven't worked:

i) Tube swapping - this slight varied the tone and intensity of the buzz. (With PI, 6V6s and rectifier)

ii) different speakers.

iii) The amp has had the output stage snubbers removed at some point in its life so I tried replacing them with 1200pF.

iv) Clipping in known good PS electrolytics (it looks like its had them replaced recently anyway)

v) Lead dress around PI and output stage.

vi) Replaced all resistors which were more than 10% out of tolerance. (All signal electrolytics look to have been recently replaced.

vii) A different guitar.


ANy suggestions?

[Jazz P Bass]

Fender DR

Is this a test of a clean sine wave.
If not, you "may" be hearing the onset of clipping.

[Steve Conner]

First of all, if it's a fixed bias amp (I think the '68 might be) did you replace the bias supply filter cap? That is, the 80uF, 75v cap near the bottom right of this schematic. If this is dried up, you don't hear more hum when the amp is idling, but everything you play gets modulated at the mains frequency. (Think a 50Hz vibrato.)

http://www.ampwares.com/schematics/d...verb_boost.pdf

Many old amps hum at twice the mains frequency because of undersized filter caps. Some people think this is part of the vintage mojo. The Internet jargon word for it is "ghost noting", or something.

The UK's mains frequency is 16% lower, so it'll be 16% worse than in the States.

If you drive the amp into clipping with your sig gen, you should see a kind of squishing of the waveform peaks that travels along at 100Hz. Try a sig gen frequency near 100Hz, 200, 300, 400 etc, and you should hear a beat tone.

[Ted]

Its not even close to clipping.

I've tried a square wave to see if I can make it ring.

The buzzing is only present just after the note is struck - as the note dies away theirs a point where it stops.

Bias electrolytic is OK.

Looking with a scope it appears to be visible on the output of the PI (both sides) - that is there's a certain fuzziness to the signal while the buzz is present.

[Steve Conner]

Oh, well I'm not too sure then... If it starts and stops suddenly, maybe it's some kind of parasitic oscillation? Does it change if you poke the chassis wiring around with a chopstick?

Or maybe you have a leaky capacitor somewhere that's causing the DC voltages on one stage to be wrong, and it's clipping prematurely. Do the voltages agree with the values on the schematic?

[Jazz P Bass]

Fender DR

Can you look at both sides of the PI coupling caps with your scope.
You may have to work your way back & find the source.
PI's are tricky cause you have voltage on both sides of the coupling cap.
You could try disconnecting the feedback resistor (820 ohm) & see if it gets worse.

[pdf64]

Does the problem occur when the amp chassis is seperated from the speaker cab, with V2, V3, V4 & V5 removed?
Even after tube subs?
If yes, then replace all resistors that carry dc, even though they're still within tolerance. I view those carbon comps as a consumable, like electrolytics.
If the problem is still there then, re-tension the octal tube sockets, re-flow solder joints, give all component leads and wires a waggle to check for fractures. May even be a bad link wire under the board.

Re schematic, if it's got the snubbers, it'll be an AB868.
http://www.schematicheaven.com/fende...b868_schem.pdf

[Ted]

Does the problem occur when the amp chassis is seperated from the speaker cab, with V2, V3, V4 & V5 removed?
Even after tube subs?

It does. It also does it with V1,V3 & V5 removed through the reverb channel.

I forgot about shared pre-amp cathode resistors - its not just the PI and power stage which share components - those 820Rs are looking mighty suspicious...

Re schematic, if it's got the snubbers, it'll be an AB868.

The valve chart inside the amp says AB763 (I never trust those things anyway) but it has definitely had snubbers fitted..

[Ted]

What a nightmare this amp turned out to be.

Replacing all the dc current carrying resistors almost fixed it, but it there was still a slight rattle there (not frequency related). On looking closer I saw that the baffle was cracked from speaker hole to edge, top and bottom. Took out the Altec - cone had been inexpertly repaired with rubber cement and was a little bit crunchy.. Made a new baffle from 15mm birch ply. Put it all back together and sounds lovely - crisp and clear, breaking into a wail as you wind it up. That Altec will need reconing one day. But I think thats enough for now.