AB763 Deluxe reverb clone - Ugly break up

[MWJB]

I'm not surprised, it's not a big jump. 40-50uf at the first filter would be more typical, the early BF Deluxes were underfiltered.

Any joy with those chassis pics yet?

[GregS]

All voltages are very close to the ones shown at the schematic, with all tubes installed

B+ = 412
POWER TUBES:
pin 3: -34
pin 4: 409
pin 5: 410

From the information you've provided here it seems you have the connections for the plates and grids reversed on the power tubes. Pin 3 is the plate and 410V sounds about right for the plate. Pin 5 is the grid which should have the negative voltage.

Perhaps this is simply a typo though as I would think the results would be worse than simply bad distortion if wired this way?

[el_fela]

yes sorry it was a tiping error... anyway, it's a good observation. I will upload some pics tonight. Thanks!

[olddawg]

Let's start with the obvious. Have you tried using a different speaker cabinet to see if it isnt the speakers?

[el_fela]

I'd like to add up that the amp I built before this one had the same problem, and I'm using the same power transformer... everything else is new. I cant afford another one right now, but I wonder if its possible that a transformer failure os bad design could be the cause. I must say Its a pretty big iron.

[txstrat]

I don't think it's the power transformer. The PT just provides the current the circuit needs. No more no less. Lets say the PT can provide 500mA. If the circuit draws only 180mA the PT would be happy but won't change the sound to the bad.

Bad design, maybe, as long as you changed something which is essential for the sound.

Have you added anything to the circuit - be it ever so little?

I'd check the lead dress and the grounds of the circuit again.

[el_fela]

I tried different speakers before.
I did some scope tests, I listened to the sound signal at different points of the circuit, with PT tubes on and without them. Again, with the PT tubes on there is some distortion present.

The circuit is exactly as fender's, but with mid controls.
There are no wiring or layout mistakes, I checked more than a hundred times.

Here are some low res pics:

[MWJB]

"The circuit is exactly as fender's, but with mid controls." I'm sorry to say that it isn't, it really isn't. You really need to check out some pics of an AB763 Deluxe chassis, then compare to yours. Even if you used exctly the same parts, the way you physically wire up the components affects the sound.

Your wiring needs to be a lot neater, it doesn't have to be a work of art to look at, but it needs to be neat enough to function properly. Is it really that costly to buy a circuit board? I would want to see some grommets on those chassis holes where wires pass through too.

Is that chassis really strong enough to support that power transformer?

Gut it, buy a proven circuit board & rewire it up, pay attention to keeping grid wires short & avoid unnecessarily long runs.

[Bruce / Mission Amps]

I cant tell the difference in tone

There is none... it's another NET driven Red Herring... 44uF...32uF.... not really quite
enough difference.
Also, what is the value of the cap feeding the phase inverter... it looks big for a what should be a 1000pF-.001uF cap.

[J M Fahey]

*Any* amp distorts from about volume 6 up, some much earlier.
If you set tone controls to an "ugly" sound, that will be much worse.
To listen to something more "normal" set treble to 10, bass to 10 if that´s what you like, and play some music, full chords , starting with volume on 1 , then raise it step by step, so we hear how distortion sets in.
I suspect your amp works reasonably well.
*If* it sounds like that on 1 or 2, then you have a problem.
What speaker are you using?
How are you micing it?
Be realistic, that combo is not a wall shaking metal monster, but an *excellent* Blues/Rock/Country/Funky machine.
You have one zillion You Tubes showing them or other AB763 variations, all with basically the same excellent sound.
Or listen to Belly up for Blues or 1.FM (blues) Internet radios, they provide tons of tasty sounding guitar amps, many of them quite close to what you have.
Testing it with a Strat or Tele or any single coils will be also very interesting, that´s what Leo made them for (not that they don´t sound very good with humbuckers also)
Good luck.

[el_fela]

Thank you all for your replies.

I'm going to make some recordings with several amp settings, so maybe it can help someone else.
For the recording before I used an Oktava mk-219 into a mackie VLZ-pro and then into an M-audio Delta 66.

The speaker I'm using is a weber 12f50a 50W (or something like that)... I also have a 4x10 cab and there is no difference with the noise.

Another weird thing that happens is that I get some low frecuency undertones when doing some third chords, using 4th and 3rd string. If I hit the strings hard enough I can liste a very low note. Seen it on the scope to. It happens only after the PI, not before. I didnt find much info about that.

About the break up, I found that it gets worse with less filtering, but it doesnt get better with more... Maybe there is some relation, but i wouldnt know.

Also, what is the value of the cap feeding the phase inverter... it looks big for a what should be a 1000pF-.001uF cap.

The value is correct, but i will try with a smaller cap, cause the bass is almost annoying.

After some reading about the PI ( The Long-Tail Pair ) I tried a larger value for the 470 ohm bias resistor, and it seemed to improve overall tone a bit.

I'll get back to you soon.

Felipe

[cminor9]

my vote is that you have an oscillation. Looking at your lead dress I agree with MWJB: you have some cleanup to do.

or, how new is your speaker? I have had a few new webers that, before they were broken in, produced some weird overtones like you describe. I had one in a 5f2 I built where every time I played an F# I'd hear a dissonant overtone, like my note was going flat. I decided I'd wait and just play with the amp a bit before I fixed it. Well, it went away. Same thing with a deluxe reverb I built, weird overtones. Both used new weber speakers, and both problems went away. I blamed it one speaker break in, which is pretty important with Webers.

[Greenmachine]

Shouldn't that output transformer be at a 90 degree angle to the power transformer?

[el_fela]

Shouldn't that output transformer be at a 90 degree angle to the power transformer?

Tried moving the transformers around... no noticeable difference.

my vote is that you have an oscillation. Looking at your lead dress I agree with MWJB: you have some cleanup to do.

Yes i know, im trying to improve my tidiness. About oscillation and lead dress, I think I have isolated the problem, or at least i think its not before the PI. But moving wire around didnt change anything, so I think it can be layout related, as MWJB said before.

¿What can be done to furhter isolate this?

I made a recording, but i was in a hurry so it has some digitally removed console preamp saturation (just a little) and my playing sucks.
Seems to soudn nicer with a cheap squier strat than with my hagstrom HIII.

Ab763 deluxe reverb by el_fela on SoundCloud - Create, record and share your sounds for free

Thank you all for your replies.

Felipe

[J M Fahey]

Hi Felipe.
Well, at last we could get to actually listen to your amp, instead of relying on verbal descriptions.
>>>Your amp is fine<<<
It sounds like any other AB763 played by you.
Up to half volume, and with sensible EQ settings, sounds clean snd Fendery.
From 5 up it starts to crunch, (like anybody else) and on 10 it clearly distorts .... the normal and to be expected behaviour.
On the last 10% of your recording, you set it on 10 but put bass on full and treble on 0 ... or so it sounds.
Since there are no "real, "musical" highs to mask it, you hear only the bass notes distortion, which passing through a bright "Fendery" speaker (Jensen/BlackPowder/Legend125/Weber12F150) will sound buzzy.
jm2c , based on what I hear. May be wrong, of course.

PS: I suggest you to listen to, among others, to Internet radios "Belly up for Blues" and "1.FM" (One Dot FM, their Blues channel)
There you will listen to tons of Fenders and Fender inspired amps, some way more distorted than yours , but, oh, man, what gorgeous sounds !
Basically, you should not play "against" your amp, but using what it can reasonably provide, even its weaknesses , if any, to your advantage.
Many of our loved and respected guitar sounds were created (out of sheer need) by those cool 50's Chicago Bluesmen with their undersized 40W Bassmans playing for thousands, with no PA available, or Rock'nRoll players (such as Elvis' own, or Chuck Berry, etc.), same conditions.
They all often played their amps on 10 (some even went up to 12, no kidding)
Dual Showmans with 2 15" JBLs were created specifically to provide clean high headroom ... but those who couldn´t afford those very expensive amps just pushed to the limit what was available.