AB763 Bandmaster Intrigue

[TheAmpNerd]

Hi,

I have a Vintage Bandmaster that is really strange.
It is an AB763.

Originally posted by doctorx
URL=http://www.cnjradio.net/fenderamps/bandmaster_ab763_layout.gif]Here is the layout of the Bandmaster[/URL]

Interestingly, it has a high pitched squeal when turned
on.

This is not the result of the out of phase Brn/Blu lead on output tube socket
being on wrong socket (tried both ways, still squeal). This sounded like
a pre amp squeal.

I did find the following, and according to the owner
he took it in for service about five years ago.

I imagine a tube went south and took out the
CC resistors. there is still some carbon scarring
And some heater wire insulation was burnt and
Peeled back and separated (no insulation for about
And inch.

Output tube socket resistors replaced with
wire wounds and metal oxides (grid).

Mains filters replaced--however they were mis-wired.
I found them in with in series with a common ground,
and amplifier wiring leads to the caps were:
red to pos and black to the other positive. (on the second cap)

Because of the age of the amp, and the mis-wire all the electrolytics
have been replaced: mains, bypass, and bias.

Result amp still squeals.

TUBEs, have been installed with a known good test set.
They make no difference the amp squeals.

Stand by voltage = 450V.
Operating voltage = 413V.
Bias voltage to the tubes = -48.2
The other voltages are all close to spec.

The amp is stable and no squeal with out a PI tube.
With a PI tube installed it squeals.
With a PI tube and the .001 cap removed from the circuit
There is not squeal.

However, when my Fluke is measures the voltages
On the PI resistors….1M, 470, 1M.
On any of those it squeals again.

Every CC Resistor in the amp has drifted out of Spec.

That is it in a nut shell.

Any Ideas? I have the tools, and will post results and
Voltages accordingly.

 

[John Phillips]

Check that the OT output is not out of phase - not on the wrong socket, actually both wires on the wrong connections. I've worked on a mid-70s Bassman 100 that was wired up like that, absolutely stock. Strange but true. A real Friday-afternoon job... they'd "cured" it by grounding out the feedback loop at the PI end.

You can find out easily - temporarily disconnect the feedback loop, which is the 'other' wire that comes from the speaker jacks and goes back to the board. If disconnecting that wire stops the squeal, it may well be that.

 

[TheAmpNerd]

Originally posted by John Phillips
Check that the OT output is not out of phase - not on the wrong socket, actually both wires on the wrong connections. I've worked on a mid-70s Bassman 100 that was wired up like that, absolutely stock. Strange but true. A real Friday-afternoon job... they'd "cured" it by grounding out the feedback loop at the PI end.

You can find out easily - temporarily disconnect the feedback loop, which is the 'other' wire that comes from the speaker jacks and goes back to the board. If disconnecting that wire stops the squeal, it may well be that.

Hi John,

Been busy and haven't had the time
to check this amp again.

Besides new tubes (for testing)

spliced in:

New Output tranny
New Choke
New Power Tranny

Still the same problem.

So there must be something else going on.

AND

Lifting the feedback loop?

Appears to solve the problem...None of the squeel.

However

It is possible the amp is just out of phase with itself?

 

[John Phillips]

Originally posted by TheAmpNerd
Lifting the feedback loop?

Appears to solve the problem...None of the squeel.

However

It is possible the amp is just out of phase with itself?

Yes, that's the problem. Or technically, the problem is that it's in phase with itself . Someone must have hooked it up wrong at the factory.

I didn't believe it at first either when I came across the same fault on that Bassman... but it was so. OT secondary leads reversed, definitely stock.

That should be all you have to do to fix it - just swap the OT wires on the speaker jacks.

 

[TheAmpNerd]

Originally posted by John Phillips
Yes, that's the problem. Or technically, the problem is that it's in phase with itself . Someone must have hooked it up wrong at the factory.

I didn't believe it at first either when I came across the same fault on that Bassman... but it was so. OT secondary leads reversed, definitely stock.

That should be all you have to do to fix it - just swap the OT wires on the speaker jacks.

John,

But this doesn't explain when I use a different output transformer
that it also squeals.

Thinking, if the amp is out (in) of phase, I also tested the feedback
wire...when i place it on the other OT wire, it doesn't squeal.

Trying to think; which is the BEST solution?

Also, at some point your Bassman, and my Bandmaster started
this...I can't belive it came this way from the factory.

Strange, indeed.

Thanks

 

[John Phillips]

Try swapping the leads on the new OT too. My guess is that the problem will go away. It may not be the output wires that are reversed, it could be the primary wires, either of the feeds to the output tubes, or the plate wires to the PI tube. If removing the feedback loop stops the oscillation, it's a phasing problem somewhere in the PI or output stage. Connecting the feedback wire to the other OT output wire will certainly stop it - that's a ground connection.

That Bassman left the factory like that, I'm quite certain. The OT output wires were the wrong way round (by color code), and the soldering looked absolutely original. Not only that, the problem had been 'cured' by bypassing the 100-ohm resistor at the PI ground (which kills the negative feedback loop), which also looked like a factory soldering job.

I also once worked on a Vibrolux where a wire had not been soldered into the eyelet - just poked in there and left. No solder had been applied to the wire end, ever. Totally original - which I know because the guy who owned it bought it new and it had never been worked on. The funny thing is that it had worked perfectly well for about ten years before it started to cause trouble.

Fender's QC was maybe not as high as you think, and mistakes like this definitely did occur. They were cranking those amps out by the tens of thousands.

They even made mistakes at the design stage... they built Champs with totally the wrong bias for about fifteen years, and no-one noticed.

 

[TheAmpNerd]

John,

Yeah, I came across a Deluxe Reverb that ahd the same thing.

Someone got in a hurry, took a break, or for what ever reason
left and eyelet unsoldered. I was saying to myself "holy ****,
this thing still works after all these years w/o solder!"

Who said "Solder isn't glue" ?

Anyway, thanks for the heads up on the feedback wire.

I'll double check the PI wiring,

I'll also post anything else I find.

Oh, swapping the primary leads on the OT was the first
thing I tried same squeal. It kept sounding like a
pre amp grounding squeal that you can get sometimes.

Ciao for now

 

[TheAmpNerd]

UPDATE:

After changing the OT Seconary leads,
the amp works fine w/o squeal.

Wiring the green to ground and black to tip
did the trick. Keeping the feedback wire where
it was.

I've also taken care of all the other problems
with the amp. All new filters,
New final coupling caps,
Fixed the Standard CC resistor blow up,
New heater wire between outputs,
New resistors on outputs
added gound sense (measure bias).

She works like a champ now,
er ah, I guess that should be...

Works like a Bandmaster now. : )

I can post some pics if any one is insterested.

 

[Boogie92801]

I have a bandmaster reverb. SInce you are on the topic. I was wonder what you would sugest to tame the mids and tighten the bass. I recently had it tuned up but the voicing is not what I wanted. It has all new caps and tubes. I am running into a custom 2x12 closed back cab with two Texas Heats.

 

[TheAmpNerd]

Originally posted by Boogie92801
I have a bandmaster reverb. SInce you are on the topic. I was wonder what you would sugest to tame the mids and tighten the bass. I recently had it tuned up but the voicing is not what I wanted. It has all new caps and tubes. I am running into a custom 2x12 closed back cab with two Texas Heats.

I should think with a 2 x 12 closed back your bass should
be plenty tight. However, I haven't tried the speakers
in question and don't know if they accentuate the mids.

If you are working with a local tech, then work through
him to see what he's done. With all new caps,
is that electrolytics (mains), bypass (inside), coupling?
He should also be able to assist with voicing
the amp to your liking.

Also, if Bandmaster Reverb, with the 100K slope
resistor, plenty of mids should be scooped. You
can bump that up to scoop them even more.

I also assume you are using the tone controls
and turning the Mids down?

You can also try a pair of .022uf coupling
caps instead of the .1uf .047uf combination.

That's a start any way.

Get back and let us know whats been done specifically.

 

[darkbluemurder]

Originally posted by TheAmpNerd

Also, if Bandmaster Reverb, with the 100K slope
resistor, plenty of mids should be scooped. You
can bump that up to scoop them even more.

I also assume you are using the tone controls
and turning the Mids down?

If I remember things correctly only the later Bandmasters have mid controls and only so on the vibrato channel. If you do not have a mid control you can try to turn up the bass and treble although I tend to keep the bass rather low to avoid mushiness and achieve better sound projection when playing with the band.

 

[TheAmpNerd]

Originally posted by darkbluemurder
If I remember things correctly only the later Bandmasters have mid controls and only so on the vibrato channel.

You are quite correct; that later amp is
the Bandmaster Reverb thinking AA1068 or 69?.