Super Reverb AB763

[lowell]

It's tough trying to tell a customer that they're probably wrong. Ya know? ha ha!!!

[Tom Phillips]

...mines 10watts less of clean power. Doubt anyone could HEAR that difference though...

Concur. There are lots of other things affect the perceived sound of an amp. In addition the max clean power operating point is a subjective call if it's determined just by viewing the scope waveform. Both our measurements are in the same ballpark.

Did you date the amp you are working on? I'm curious for the record.

[g1]

Just to verify, when you were measuring the 50V p-p it was a distorted waveform, but now when you reduce the volume to clean it up you get 64W when measuring AC volts with your meter?
Have you double checked the bias, and if you bias it cooler, does the B+ come up?
Have you tried new power tubes? When someone says it breaks up earlier than before, weak power tubes are a likely cause. Someone real used to a particular amp may hear a difference we would think is only slight when measuring power with meter or scope.
Check those screen resistors!

[lowell]

My first thought when he brought it in was screen resistors. They ended up being fine. There are new power tubes in the amp. When I pulled the power tubes the B+ only came up to 425v... so I don't think cooling the bias will have much affect. The 50v p-p was clean right before clipping. Something must be wrong w/ my scope. I did however clip the lead onto the "calibrate" eyelet which says it should be 2v. It is indeed 2v. ??

[Chuck H]

Contrair monfrair... Cooling the bias will allow the tubes more OFF time so they can generate power during their ON time. As long as you don't have crossover distortion issues. Smetimes a cooler bias equates to MORE power. The drive voltage from the PI is the dictating factor.

[J M Fahey]

Scopes are great, but not that much for *measuring* .
You can never compare visually estimating a waveform height in a small graticule to the hard solid numbers you see on a multimeter´s display.
Hey, even an analog multimeter with its thin long needle against a large, finely graduated scale is more precise !!
Anyway, even your original measurement was not that far away.

[lowell]

Yall have a recommendation for a PT with 350-0-350 HT or thereabouts?

[lowell]

I ordered a PT from Mercury. It's a drop-in replacement with 335-0-335 HT taps. We'll see how this improves clean headroom.

[lowell]

The new PT seems to have increased clean headroom. The Av is now 460vDC. Seems good to me.

[Gman]

Secondary voltage on the AB763 Twin

Now things make more sense.

I have seen more than one circa 1969 SF Twin Reverb that had the exact same problem you describe.
The 022756 PT is stock correct and although it is possible that your secondary voltage is below average for a 022756 PT, yours is not the only one with this problem. Read on…

For comparison, I made the following measurements on a 1969 Twin Reverb with the same problem. The measurements were taken after a tune up which included new power tubes.

All transformers were original Fender stock units.
With 120V line power
PT HV secondary = 315-0-315V
B+ rail = 403VDC
Clean power output = 72W into 4 Ohms

I think that the amp you have pretty much left the Fender factory in the current condition.
There have been other postings discussing the same issue. For example the one at
AMPAGE Archive: 69 twin reverb power transformer question

I have no explanation for the customer's report that it used to sound better but that could be due to many other things besides the low voltage PT.

Cheers,
Tom

I just joined this site because of this thread. I went a little nutty with this very issue. Here's my story.
I recently modified my 1970 TR to blackface specs. I, too, had the low secondary voltage (315vac) issue and I think Mr Phillips is right when he said the amp probably left the factory that way. My PT p/n is F022756. I think this PT is supposed to put out 315vac. The amps coming out of Fender around this time period IMHO were not totally true to one particular spec. I suspect you have , as I have, a combination of the AB763 and AA270 spec., even though the tube tag inside my amp cabinet said AB763. The AA270 schematic shows 315vac coming off the secondary. Welcome to the world of early 70's SF Fender amps. I left the resistors in the filter cap pan original (AA270 spec) and it sounds great. It starts to break up around 6 or 7 on the volume. My power tubes are 6L6's not 6L6GC's.
I hope this helps.
G

[Tom Phillips]

Welcom to the forum Gman and thanks for your input.

My power tubes are 6L6's not 6L6GC's.

Who's brand is on those tubes? They are really old or some modern company took liberties with their marking.
Regards,
Tom

[Gman]

Oops, sorry, they ARE 6L6GC's. They are made by Groove Tubes and I went by the more predominant orange lettering that said "GT-6L6" so I thought they were just 6L6's and the GT stood for Grove Tubes. After inspecting the tube more closely, I found the real marking of 6L6GC. Sorry for the screw up. I had them installed in the amp back in 1982 in New Orleans but never used it much because I didn't like the tone. New filters caps and orange drop caps fixed all that (got rid of the brown turds).
I'm sure though that the PT is stock. As I went through the SF to BF process, I found a lot of instances where the components went back and forth between the AB763 and the AA270 spec.

I'm working on one now from circa 1972. It has a MV but no push/pull and Paktron caps. Does anyone know anything about these Paktron caps? Were they considered good tone wise? I'm going to replace one section at a time and play the amp each time to see where I get the greatest improvement.
All input is gratefully accepted.
G

[J M Fahey]

Does anyone know anything about these Paktron caps? Were they considered good tone wise?

Did "anybody famous" use them?
Doubt so.
Does any Internet Guru swear by them?
Doubt so.
Do they have Mojo?
Doubt so.
There's your answer, according to the terms of the question.

That they are among the best in the World, built specifically for Military, Aerospace, Medical and Industrial Process Control machinery hardly matters, compared to the first 3 parameters.
Oh well.
Paktron Multilayer Polymer Capacitors
Company Profile and Illinois Tool Works Inc. | Paktron Multilayer Polymer Capacitors
Tech Articles | Paktron Multilayer Polymer Capacitors

No, there are not articles touting their "tone" in guitar amps.
Maybe they are hiding something.

PS: being among the *real* "best in the World", I´m sure they still are (much) cheaper than Famous Mojo capacitors.

[Gman]

[QUOTE=J M Fahey;246684]Did "anybody famous" use them?
Doubt so.
Does any Internet Guru swear by them?
Doubt so.
Do they have Mojo?
Doubt so.
There's your answer, according to the terms of the question.

Do you need another hit on your crack pipe?
Doubt so.

[J M Fahey]

Did you at least Google them?
I did, and was amazed.
*GOOD* stuff