Strange overtones on certain low notes.

[Jazz P Bass]

The signal generator suplants the guitar as an input.
I have mine wired to a 1/4" plug.
I generally set the SigGen output at 100mvAC.
Run the amp into an 8 ohm dummy load (25 watt on a heat sink) & monitor the output with a scope.

[jmaf]

If you use the signal generator alligator clips to hook it up in the amp, remember to insert a dummy 1/4" plug in order to open the jack's ground switch.

[deci belle]

Thanks Jass and jmaf!! There's something I don't get yet— Jazz said:

Put the amp on a signal generator with a dummy load.
Monitor the output with a scope.
If the problem occurs at the output then you go sniffing.
Break the circuit in half.
Is it there.
Yes.
Break it in half again, towards the input.
No.
Break it in half towards the output.

I just don't know where to start: At this point I don't even care about fixing the amp!

I've never used the probes. I have x1, x10, x100.

I don't know whether or why to use the x or y (it's a dual trace as well) connections.

Just tell me where to put whichever connections and probe(s) somewhere logical and appropriate and then I think I'll be good to go for ridin' this scope for a while beyond that point. I need to get a feel for this as I'm not sure what probe is appropriate for certain circuits/voltages, ya know! It's goona be a hot amp and I've never used a scope before (I actually think this is kinda funny)!

Imagine that you have never actually used a scope before— yeah, that's me! haha!

[Jazz P Bass]

[QUOTE=deci belle;211889]Thanks Jass and jmaf!! There's something I don't get yet— Jazz said:


I just don't know where to start:

QUOTE]

[Jazz P Bass]

Start at the output.
The scope is "basically" a tool to visually see the electron signal trace.
Cool!
If you know how to properly use a DVM then you are 99% there.
Can you Google the manual for your scope?

[deci belle]

Hi Jazz! Thank you for helping— I hope my earnest approach doesn't seem disrespectful in any way. This is critical information. I've got the manuals. I've got 7 books. I've seen 30 youtube "tutorials". It's such an easy and common procedure, nobody can say how! It's so easy to bleed filter caps that it only took me three tries in 2 years to do it right. I was lucky. But now I know.

The scope (Tenma 72-320), signal generator (Heathkit 5218), dummy load (Weber Tru-load) I have are self explanatory. But just because I have a a free-range chicken on the loose, a knife, a couple of pots and pans, a bunch of ingredients and a commercial kitchen… know what I mean?. There's a lot more at stake here than a burnt bird. It is possible to screw up equipment not to mention readings that don't mean anything. As and Fs on my little string have a ghost "sheen": that's my bird.

So what is the recipe for "put the amp on a signal generator with a dummy load and monitor the output with a scope"?

If you would, please venture to describe exactly how you would ordinarily proceed to set up and monitor one single thing of your choice on an open and hot ab763 40~80w chassis from start to finish on your bench using your test equipment. I wouldn't fault you if you didn't go through the trouble— it does make me chuckle just thinking about it because it isn't easy to do! There are so many things that you'd normally do without taking those actions into account on such a simple procedure.

Everyone has their own way. I just want to know what one way is. Then maybe I'll know one way. Thanks Jazz!

[52 Bill]

I know that you didn't mean to... but why not start a new thread for this test set up discussion. I think that a lot of readers here would find it very helpful, but will never see it buried in an amp repair thread.

[protojim]

Valve head-

did you check your plate voltage to the tubes? and the current to the tubes individually?
I know in most fenders you measure across a 1 ohm resistor using a pot to set the idle current for the pair of tubes.
Or you can use a bias tester to check tubes individually.
do the math for total power dissipation of a tube- how many watts are your tubes dissipating?

It really sounds like you're running those tubes cold!

Deci belle - you need to start your own thread ok- you are hijacking his

[deci belle]

I know that you didn't mean to... but why not start a new thread for this test set up discussion. I think that a lot of readers here would find it very helpful, but will never see it buried in an amp repair thread.

Absolutely right Bill! It was relative, I have the same circuit— where's valvehead! It's not going to get answered here or anywhere else. I'll bet you could start a thread too~ but you won't.

You guys are right though so I apologize! I'm not in a hurry, just ready to learn— but Thanks!

[vavlehead]

Hey Guys,
thanks for all your replies.

The low G echo thing was the tubes... I got a bum set of Winged C 6L6s. I had a quad so I used the other 2 to swap. What are the chances that 2 tubes in a quad would produce the same kind of uglyness??? I threw in a brand new set of TungSol Reissues and all good.
I also re-soldered all of the points on the filter caps.

Played it for hours and all is fine. I dont know if re-soldering helped??

BTW I have 460VDC on the plates and I'm running 35ma on each tube. Those TungSols are super matched. I always use the OT shunt method and calculate the resistance of the meter ( I have two good DMMs ) along with the resistance of each side of the OT.
IMHO this is the most accurate way to measure idle current.

I also found that my Celestion Classic lead 80 had cone cry... or it added like "moaning" to the notes. It was very low but still a pain!! This was separate from the two issues above. I replaced it with a Eminence Commonwealth ... man its heavy!!! But sounds good.. it has High end sizzle right now but I suspect it will mellow out as I break it in. Its gonna take a while with 40watts!!