Sputtering, spitting, and bad breakup past volume 6

[Chuck H]

The first filter cap would be the two series wired 70uf caps. The 20uf after the choke is the second. You shouldn't need additional ufs over the stock values. I think it's more likely that there is an error in the wiring and the addional ufs you added helps with the current condition. Also, just because a cap "tests" ok doesn't mean it is. Your meter is testing at voltages significantly lower than the amps operating voltages. Though the cap is likely alright.

The amp would operate, pretty much, with the filter string somewhat cattywhompus. It's the "pretty much" part that causes foolies here. Just because the amp is working doesn't mean it's not wired wrong. Since the stock filter values don't cause motor boating in tens of thousands of stock amps and clones I don't think the values should be suspect. Since you did work on this circuit in the amp and it's easy to get lost between the doghouse and the inside of the chassis I think the wiring IS suspect. You may have nodes wired to the wrong places or nodes that are accidentally coupled with no filtering on some circuits, etc. Just go over it carefully and be sure everything in the power supply rail is wired correctly, grounded correctly and going to the right places.

[Contech]

OK, I'll go over the wiring carefully and try to see if there is any difference from the schematic.

By the way, when I tacked on the filter cap, it was with the 20uF cap on the the right side. See picture. Regardless. I think I will just replace that one 20uF cap and see if that fixes the problem. But I will definitely chase the wiring through the chassis.

Thanks for your advice Chuck. This has been an excellent troubleshooting exersize for me.

[Contech]

Hi Chuck, well I just finished tracing the wiring for all the HT lines. I was pretty careful and I even isolated a few wire runs and check continuity (since I can't see under the board). Everything appears to be wired correctly. I'm going to re-heat the ground connections on the filter board as well as where it connects to the brass plate inside the chassis. Perhaps there is a cold solder joint or something not making a solid ground connection. I'm also going to replace the 20uF caps one at a time to see if one of those are bad.

Thanks for you help. It's gotta be somewhere...

[Chuck H]

Well if you checked continuity and you think that cap you bypassed is supposed to go to the choke then it confirms there's errors in how the filter caps are wired. Because that cap is supposed to go to the first two preamp tubes. Perhaps be certain you know how it's supposed to be wired and check everything again.

[Chuck H]

...

[g1]

That's not the one after the choke. That is the "D" node shown on the schematic. Some would call it the "last" filter, it's for the input stages.
That particular position cap will often cause motorboating when bad, so that may be all it is.
Disconnect one leg of the original and try it with your sub tacked in there.

(edit: I'm assuming it's wired correctly but agree with Chuck that it needs to be verified)

[Enzo]

Yes, instead of assuming it is just more cap that solved it, it might be a different cap was the key, so disconnect the existing cap and tack the new one in its place. That is the test. This circuit has been working fine with the existing value caps in it since, well, 1963, so we shouldn't need to enlarge the cap.

We determined we have the AB763 revision, and we have the schematic, so I suggest we refer to the "B-node" or "D-node" rather than "First" or "Last" or "The one by the Buick".