AC30/6 treble low output and thin sound. (Problem solved!)

[Dingleberry]

Hi.

My friend brought me his AC30/6 treble model and told me it lost about half of its volume and its not sounding right. It sounds thin and overly bright with no bass at all.
It had been serviced troughout (new electrolytics, new pots, some resistors and caps replaced etc.) couple years ago and it had worked flawlessly since until now.

I tried the amp and that's what it does. And it does it with every channel:
Vib/trem, normal and brilliant.
So the problem has to lie either in the P.I-stage or in the power amp?

Tried it with three different EL84 quartets and the problem still occurs.
Tried different P.I tube and no goal.

Replaced P.I input and output caps with new ones for good measure but no difference.
P.I resistor values tested ok.
I even tested it with another OT I happened to have on hand and still the same upper midrangey/trebly sound was all I got out of it.
Tried it with different speakers and still the same.

I have resoldered just about every solder joint in that amp so it's probably not a cold joint.

Now I'm kind of lost where to look? I'm out of ideas.

What could cause this kind of behaviour?

All voltages seem to be in acceptable range.
Plates vary between 320,8V - 321,1V. Should be good.
Screens vary between 313,9V - 314,7V. Should be ok too.
(390R/5W screen resistors at the moment)
1k5 input resistors are also new and tested ok likewise the 220k grid bias resistors too.

62R shared cathode resistor for all four power tubes gives 9,75V on EL84 cathodes so tubes are now biased about 12,6W each. Should be plenty enough but not too much.

Actually now at the same time one question popped into my mind.
Could bad 220uf cap parallel the bias resistor cause that kind of behaviour?

All help welcome... i think I'm losing my mind trying to find the problem which causes the amp sound like that.

-T

[Diablo1]

Just unsolder one leg of the bypass cap on the 62 ohm cathode bias resistor and see if it improves the tone. If no change, then it's not a bad bypass cap.

[Dingleberry]

Well it wasn't bad bypass cap...
This is getting really weird.

-T

[Diablo1]

Are your screen resistors good? Measure their value.

[Dingleberry]

Screen resistors are fine.
I see some weird oscillation on scope after P.I caps and all the way to the speaker terminal.
Could it be parasitic oscillation?

Got to scope more...

-T

[Masco]

Well, if you have an o-scope, inject a sine wave and see where the signal drops.

Also, check the 47k tail resistor read at PI?

[Dingleberry]

P.I resistors are ok. Checked and measured them. Tried changing them also... No goal. It's kind of hard to describe this situation and because my native language is not English, it's even harder. The signal seems ok all the way to the P.I but then after the P.I it looks a little "furry". In the speaker terminal it looks really "hazy". I have replaced every component in P.I. so looks like it's not a component problem. Tube is ok too. Tried it with four different tubes too. I have to try rewire the socket because a bad lead dress could probably cause that kind of behaviour. I've spent something like three full days chasing this problem and it is starting to drive me nuts... Sometimes problems aren't that obvious as you would hope.
-T

[LeftyStrat]

If you're going to try to rewire the PI socket, I would also replace it at the same time. It could be a bad socket or some kind of conduction on the socket.

[Dingleberry]

I should have some spare sockets on my workshop. Better replace it at the same time so I can eliminate one possible source. Bad sockets can sometimes cause really weird problems.
-T

[Dingleberry]

Oscillations are gone when changing the P.I. socket and
rerouting wires a bit.

BUT!

It probably wasn't the cause for thin sound and loss of volume...

When asked my friend about the amp and service history it turned out that the other speaker had been reconed lately by some amp tech. (and it had really cost a lot)

Well guess what.
The guy who reconed it had wired the speakers OUT OF PHASE!!!

Goddamit!

Didn't that "recone-guy" even bother to test the amp before he gave it back to my friend?

And didn't my friend instantly hear the differences when he got it?

"Probably not" seems to be the answer for both questions.

What did we all learn?

Sometimes the solution is not in the amp... and never trust any other amp tech;)

I have to update my "things to check before you even get inside the amp" -list. CHECK SPEAKER POLARITY!

Beer anyone?

-T

[jelle]

Good one! But in your defence, you did improve the amp by getting rid of that oscillation.

[Reeltarded]

L O Hell

Good deal! Nice job on not losing your mind!

(if you see mine anywhere, please PM me.. thanks!)

[alvarezh]

Here is another speaker one:

Some amps are build with shunted to ground main speaker jacks, countless times users connect the speaker plug on the aux speaker jack and the amp is dead.

Easy fix for sure, problem is one hardly ever thinks on the possibility of something being wrongly reinstalled.

Congrats on your finding. Certainly another thing for the "look for" list, thanks for sharing.

All the best.

[LeftyStrat]

Doh! Glad you found it. Two things to take away from this:

- Look for stupid shit first.
- Ask the owner about any recent changes.

Like you I probably wouldn't have thought to check speaker polarity. One to add to the list.

[Diablo1]

Now wait a minute pardner....
In your first post you said you tested it with different speakers.