ll I want for Christmas is no more hum!

[words453]

I am working on an Ampeg Reverb Rocket that has an annoying hum. I have changed the output tubes, phase inverter, electrolytics, OT, any out of spec resisitors and coupling caps and bypass caps. The hum goes away only if I ground pin2 of v4 or pull v4. I lifted the lead to pin2 and injected a signal from an MP3 player into pin2 and had no hum from the output stage. Any suggestions would be appreciated.

[words453]

FWIW, I also lifted the lead from pin6 of v3 to the 0.1 coupling cap and the hum was still there.

[Mick Bailey]

You've eliminated the PI and output tubes as the source, but have you swapped the preamp tubes for known good examples? Tubes are my first suspect.

[words453]

V1 and v2 are odd tubes that are not made anymore and only available used. But pulling them makes no difference in the hum. Only pulling v4 or grounding pin2 of v4, stops the hum.

[Chuck H]

If pulling any preamp tubes makes no difference then the hum is probably due to a grounding or power supply fault.

[g1]

If grounding the PI grid makes the hum disappear, but disconnecting the coupling cap to PI at V3 end does not, then I would guess that wire is picking it up somewhere.

[Jon Snell]

You should be looking at V4 only.
Re connect the feed to pin 2 again.
Connect a link lead exactly as the green link shows.
Does the hum stop?
Yes = Is the 470k grid leak resistor grounded properly?
No = dig further in towards the valve pin 2

[words453]

You should be looking at V4 only.
Re connect the feed to pin 2 again.
Connect a link lead exactly as the green link shows.
Does the hum stop?
Yes = Is the 470k grid leak resistor grounded properly?
No = dig further in towards the valve pin 2

Hum stops, grid resistor good and grounded.

[Jon Snell]

Look just before V4; V3 pin six is the mixed output to the Phase Splitter.
The Phase Splitter we have discovered is not the issue, so move back to the previous section;
Link V3 pin 7 to ground (Green link) and test.; see where where we are going with this ...

Place a link as shown between pin 7 and ground on V3 (Green link). Does the hum stop, if it does, remove the link and go to the next part.
Link V1 pin 10 to ground Red link. Power off first!
If the hum doesn't stop; remove the Red link and Link V1 pin 9 to ground (Blue link).
Move back towards the input until the hum stops. The last part you linked out, is where the issue is.

V1 pin 3 is acting as a diode clamp for the tremelo, you cannot link out pin 3 to ground as it will upset your mixer/driver stage bias.

I know it seems good to take a stab in the dark but that is not good fault finding.

[words453]

Grounded pin7 v3, hum is still there

[Jon Snell]

Grounded pin7 v3, hum is still there

Go further forward and find where it stops.
Is this the same post as Haase posted?

[words453]

Fixed!! It was the .001 capacitor from pin6 v3 to ground. Even though it measures good, it must have been microphonic. Thanks everyone for the advice and Merry Christmas!

[words453]

I celebrated too soon, the hum is back

[Chuck H]

I celebrated too soon, the hum is back

Okay. Was it actually gone for a moment or could there be dirt in the diagnosis?

[words453]

I tacked in a new .001 capacitor in and the hum was gone. Then I trimmed the leads and soldered everything properly and the hum was back. I guess it’s possible that one of the components wasn’t making good contact at the junction. If I lift the 0.1 capacitor from the junction with v3 pin6, the.001 cap and the 47k plate resistor, the hum goes away. If I leave the cap in place and lift any other component from that junction, I get hum. I have replaced both the 0.1 and .001 caps and the 47k resistor. Any suggestions?