Rocket Hum

[cbass]

Ahh don't give up yet.Have a beer, burn one don't look at it for a day or two.Come back when you're not frustrated

pics from different angles can help those trying to help you.

A new component thats bad is possible but its a lot more likely to be a bad connection or error on your part.

[cbass]

Measure AC voltage at the B+ that should give an idea if your filter caps are filtering.

[fishy]

+1 on the cover

Seems as though you have checked the grounding a number of times.
Have you actually measured the resistance relative to the chassis and safety ground.

Have you cleaned up the area at the ground connections to ensure you are connecting fully to the ground point?

Just because the hardware its tight is not always a good indicator.

[mickephoto]

Measure AC voltage at the B+ that should give an idea if your filter caps are filtering.

I checked all voltages and they are very close to the Ceriatone chart.

[mickephoto]

+1 on the cover

When I've used covers in the past they may have helped a bit, but not for this kind of hum.

Seems as though you have checked the grounding a number of times.
Have you actually measured the resistance relative to the chassis and safety ground.

No, I haven't done this. I'll try it. What would be a "good" value?

Have you cleaned up the area at the ground connections to ensure you are connecting fully to the ground point? Just because the hardware its tight is not always a good indicator.

This is worth looking into as well. The ground lugs to chassis seem very solid though. There are 4 in the layout - see photo. One thing that's different compared to the layout found here at AG are the way the filter caps ground. Again, I've tried plenty options.

[mickephoto]

Also check to see if your bus bar connection to the back of the pots is getting a path to ground via the pots themselves. If so then you have a ground loop that you can easily fix.

I removed the pots from chassis an isolated temporarily. Only connection to ground was through the bus wire. I had some hopes doing this, but it didn't help.

[Cliff Schecht]

I had this problem with my Rocket and another amp at the same time. Turned out that both amps had loose hardware somewhere that was the source of a bad ground connection. It sounds like you are having a similar issue, either a ground is not connected securely or some hardware isn't tightened down. Go over every screw and bolt and make sure they are cranked down!

Don't know if this got ignored or what but did you check that your hardware is all down tight?

[fishy]

No, I haven't done this. I'll try it. What would be a "good" value?

They should be low resistance. I have usually measured o.1 to 0.3ohms. Make sure you see what the leads measure first.

[Roe]

try adding a big choke ala Brian May/Pete Cornish (7h 260ma) between gz34 and OT. this should improve the filtering quite a bit

[dorrisant]

Is it possible that it could have something to do with the PT, or bad (new) filter caps. I'm not sure what else to try, except...

Eliminate this possibility quick and easy... I made this tester in an old 1/4" jack... took longer to make it than to actually test my OT! Turn the standby switch off, plug the tester into the main output jack and lift the OT wires going to the power tubes. Touch the wires to a 9v battery and release... If the bulb flashes, your OT is fine... end of test!

www.premierguitar.com/Magazine/Issue/20 ... ester.aspx

Tony

[mickephoto]

I checked resistance on all grounds and they measure 0.4ohms or less.

The OT battery tester sounds like a good thing to try.

The filter caps are normally not oriented this way in a Rocket, but I had to due to space (chassis screws). Could this induce hum perhaps?

A few more photos...

[xtian]

The filter caps are normally not oriented this way in a Rocket, but I had to due to space (chassis screws). Could this induce hum perhaps?

I did my caps the same way in two Rockets built last month. No issues.

[mickephoto]

Measure AC voltage at the B+ that should give an idea if your filter caps are filtering.

I measured about 2.5 volts AC on the B+ on startup. After a while it went up to 4V. Power tube plates all have the same AC (DC is 340). Pre tubes have almost no AC.

[mickephoto]

The filter caps are normally not oriented this way in a Rocket, but I had to due to space (chassis screws). Could this induce hum perhaps?

I did my caps the same way in two Rockets built last month. No issues.

Good to know, thanks!

[Tillydog]

If you have the busbar soldered to the pots, the connection from the ground lug to the busbar is superfluous and a classic ground loop (see my highlights on your drawing below).

You also have a ground lead shared between the 3 x 22uF preamp filter caps and the 3 other HT main reservoir caps. These should have their own, separate lead to the ground lug per the layout (may well be your problem).

Happily, you can fix both of these by transferring the ground connection wire from the busbar to the -ve connection of the 3x22uF caps (& break the connection between these caps and the other 3 reservoir caps).

This may, or may not have anything to do with your problem, but it's bad practice in any case, so you should correct it. (I suspect this is what badtweed was alluding to earlier)

Also, do me a favour, and just check you have a 56k resistor in V2 cathode, not 5k6 - I can't make out the colours well enough in the photos.

0.02p

Andy