Help restoring a beautiful Princeton AA964

[fossilshark]

Hello all! I've got this vintage Princeton AA964 (silverface, no reverb) in the shop today and I am looking for advice on restoring it. This is the oldest amp I've ever worked on and I am admittedly a little nervous, I want to do a really good job on it.

It was bought brand new in 1970 and seems to only have had 1 tech inside it previously.

Unit has developed a nasty hum and it seems the vibrato is always on even with the pots turned all the way down. What is the first things I should check?

Looks like the Mallory can was left installed but disconnected and replaced with some IC cans, as well as a bias supply rebuild and an up to date power chord.

-Should the original blue molded caps, ceramic discs, and carbon comp resistors be replaced as preventative maintenance?
-The rectifier tube looks original (Sylvania), should that be replaced as preventative maintenance?
-Any advice on dating this amp? The SN is A 09353, wich I read should be a blackface amp from 1966, however to me this looks like a silverface (on account of the silver control panel)

Attached is what I believe to be the correct schematic, please correct me if I am wrong.

The google drive link is a video showing how it sounds currently, and pics of the amp.

I welcome ANY advice on getting this beauty bulletproof again.

Video ---> https://drive.google.com/file/d/1Mvx9pyDlZAoBjU8PG8T5LfAYrtSQN60M/view?usp=sharing
Pics ---> https://drive.google.com/drive/folders/1gez3-3ZqFtMlfRK2xmrIKTZouysb0rLl?usp=sharing

ALSO if anyone has a better way of uploading hi-res pics please let me know.





 

[shooter]

what's the "end-game"??


as close to original as possible,
highly modified test bed


my belief is "make it work well"
play it for a week
decide, if anything needs "tweaking"


so no swapping out old stuff til you prove it's bad, new tubes are fine, but modern quality could add rabbit holes you don't need

[mresistor]

Do you have another speaker or cabinet you could plug into?  That speaker sounds terrible.  That is the correct schematic.   The chassis stamp ser # is for 66 but the date codes on the iron says a different story.  For instance the OT shows the 14th week of 1968.  Not to mention it has the drip edge moulding which is correct for a 1968. 


I would plug into a different known good speaker cab.  Also you're going to want to verify the bias is set correctly for 60% or so max dissipation for a 12 watt tube, although you have 14w 6V6s installed.

[TitaniumValhalla]

Agree with other posters- check the bias, check voltages, change out that dog turd speaker and see where you are.

[HotBluePlates]

-Should the original blue molded caps, ceramic discs, and carbon comp resistors be replaced as preventative maintenance?

NO!

Fix only broken things, especially on a vintage collectible amp.  Most of the dollar-value is "collector value."

Unit has developed a nasty hum ...
The google drive link is a video showing how it sounds currently, and pics of the amp.

The photos show someone has already been in the amp & replaced all the things you would probably replace.  All the gray metal oxide resistors are new, all the filter caps are swapped, and all the cathode bypass caps have been replaced.

Your video didn't demonstrate "hum" but did demonstrate a gated, ring-modulated sound.  Possibly all of the filter caps do not have a good ground connection, and/or the cathode bypass caps are short-circuited.  Check the former with resistance measurement (looking for <1Ω) not continuity (which can be as much as 200Ω on some meters).  Check the latter by unsoldering one side of the cathode bypass caps (to remove a short-circuit if a cap is shorted).

... it seems the vibrato is always on even with the pots turned all the way down.

That's normal for a Vibro Champ and a Princeton.  You have to use the footswitch to turn off the tremolo in those amps.

-Any advice on dating this amp? The SN is A 09353, wich I read should be a blackface amp from 1966, however to me this looks like a silverface (on account of the silver control panel)

This amp was made in 1968.

  - All transformers have 1968 date codes.
  - There is a 1968 stamp on the chassis by the new filter caps for when the chassis-assembly was completed.
  - You photographed a 2nd 1968 chassis stamp on the outside of the chassis.
  - The Mallory cap can probably has a 1968 date code, and the same for the pots (couldn't make them out in the photos).

The serial number is not a solid tool for dating, especially near the "edges" of a date-range.  That said, this amp's serial number is close to that which is documented for early silverface Princeton amps.

[stratomaster]

As stated above, someone has already been in here and changed a lot of parts -- I suspect unnecessarily.

Just a quick look through of the pictures reveals a solder starved chassis ground connection, cold solder joints at the HT dropping resistors, a squirrelly bias adjustment mod, and a potentially mis wired 3rd HT node. 

You'll need to be on your game with this one as nothing can be trusted.  Start from square 1 and get the input AC wiring rock solid, then get the voltages on the secondary of the power transformer within reasonable ranges at all nodes.  Then work from input to output of the signal path.  Use a scope if you've got it, or a listening amp if you don't.

Lots of changed parts like this indicates one of a few things--none of them good.
- an inexperienced tech whose work needs to be scrutinized heavily
- an unethical tech who was padding his bill at best/stealing valuable vintage components at worst
- a desperate tech chasing a problem by shotgunning parts -- the problem may not have been addressed fully in this effort.

Always check these old Fender amps for stray DC.  This can affect output stage bias and the cathode voltage/bias for the preamp tubes.

[mresistor]

I geatly enlarged the photo of the IL electrolytics and I don't see any cold solder joints.   However ..  there is almost no support for the ground lead ends of the IL electrolytics, i.e. the capacitors are just laying there and gravity is holding them down, in the pictue. That's a big no no in a chassis that will be upside down in the cabinet.  In this case there should be at least a terminal strip added for the capacitors to tie to.  The negative leads are then soldered to the point where the HT center tap and 6.3VAC center taps are grounded.
On the positive side there is a (grey-red-red?) 2 or three watt resistor hanging off the board and it is carelessly tied to the positive lead of one of the electrolytics, and then the capacitor lead is looped and soldered to the yellow wire.  What a mess.  No physical support, just laying there.


If if were my amp I'd take that whole mess out.  I'd test the original CE / Mallory capacitor. Are we even sure that there is a bad capacitor in the can? If so, then go here  https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-40402020-f and get a new one and replace the old one.

[AlNewman]

Some of the original carbon comps look like they've been burnt.  If there's drifted or open resistors, it could also contribute to the strange behavior of the amp.

Doesn't sound like a speaker to me, but it's easy enough to sub another one in to test.

[HotBluePlates]

Some of the original carbon comps look like they've been burnt. ...

Can you point them out?

"Brunt" carbon comp resistors literally burn (like this and this), which is why those metal oxide resistors are called "flame proof."

... the IL electrolytics ... are just laying there and gravity is holding them down, in the pictue. ... there should be at least a terminal strip added for the capacitors to tie to. ...

I believe there is a bare wire from those caps to the solder terminal used by the PT B+ and heater center-tap wires.  I believe we don't see it because it is physically behind the two cloth-insulated wires.

[Willabe]

I don't see any burnt or discolored R's from high heat.

[mresistor]

That could be HBP   maybe the OP would tell us if that is so.  That mo esistor on the other side is 8.2K.

[fossilshark]

Thank you all for your replies! The goal for this amp is to get it sounding as good as possible while ideally leaving it as stock as possible.

So I found the bias was set perfectly and seems rock solid, all voltages seem fine until the plates on the 6v6s. The DC voltage is 370v wich seems a bit low BUT there is 76vac on the plates, leading me to believe one or all of the replacement filter caps have failed. All other voltages test normal.

In defense of the previous tech, the solder joints don't look too bad in person and I have found no cold/cracked solder joints. The new filter caps and bias supply components have all been siliconed in and are not just hanging by the solder connections.

I am a little bit unsure about how the AC wiring should be wired but as far as I can tell there is no death cap and all the connections seem solid and test fine, although I will probably re-dress some of them just to be sure.

Tested it using my test bench speaker and the hum is much much more pronounced, definitely going to replace the speaker thats currently installed.

I have a few questions before I move forward:
-The original Mallory filter cap is x4 20uF, the replacement linked above is x2 20uF and x2 40uF, would that be a suitable replacement?
-The heater lines appear to have no ground return, wich seems to be standard as per schematic. Would there be any benefit to adding a virtual ground (resistor coming off each heater line both tied together at a ground point)? 
-Does "stray DC" mean DC leaking over to a side of a capacitor where DC should not be (say, pin 5 of the 6V6s)? I have not found any DC where DC should not be so I think we are good on that front.

I am thinking after I replace the filter caps and (hopefully) that 76vac on the B+ line goes away the majority of the hum should be gone and I will do a signal trace with the scope just to make sure everything is good.

[fossilshark]

Here's a better shot of the AC wiring

[AlNewman]

Not burnt, like too much current, but burnt like too much heat from an iron.
There's several that look like the paint's been melted off.  Just something to take into consideration, and check tolerances if you haven't already.

[mresistor]

Sorry I linked the wrong one..   Here is 4x20uf CE cap can     https://www.tubesandmore.com/products/capacitor-ce-mfg-475v-20202020-f-electrolytic


You don't want DC on the pots or on any side of a coupling cap where it doesn't belong.


The heater winding has a center tap tied to the ground point where the filter caps and PT secondary HV center tap is grounded. It's a green wire with whitish stripe. There is no need for a faux center tap with resistors. We use one or the other not both. 

[HotBluePlates]

-The heater lines appear to have no ground return, wich seems to be standard as per schematic. Would there be any benefit to adding a virtual ground (resistor coming off each heater line both tied together at a ground point)? 

The heater center-tap (Green/Yellow) is grounded at a PT bolt with the high-voltage winding center-tap (Red/Yellow).

[tubeswell]

That could be HBP   maybe the OP would tell us if that is so.  That mo esistor on the other side is 8.2K.

Hard to see if the filter caps are properly grounded from the photo. If there is a wire connecting the 2 arrows (that we can’t see from that photo like HPB said), it might not be the source of the weird noise - but then again if there’s no connecting wire, those filter cap grounds look like they might be very poorly soldered to the chassis (like someone didn’t use a hot enough iron and didn’t use decent flux). Aside from that, I wouldn’t put IC caps in any amp

[scstill]

Why not use IC caps?
What are the preferred brands?

[tubeswell]

Why not use IC caps?

Generally unreliable. Every fender amp I’ve come across with those caps, one or more of the ICs are bulging. Even the crazy Zinky amps with multiple ICs stacked in parallel out of the factory!

[TitaniumValhalla]

Why not use IC caps?
What are the preferred brands?

Use F&T or MOD electrolytics.

[DummyLoad]

Why not use IC caps?
What are the preferred brands?

Because they f****** suck! Especially the 22uF 500V model, marked as TTA sometimes TTAA series.

#1 type/size I've replaced in all the repairs I've done over the years. The 47uF 500V are not far behind those. The 100uF seem to hold up a bit longer. Amps less than a few years old were coming in with the 22uF leaking, and most times if the 22uF were leaking, the 47uF and 100uF had bubbling vents, so not long to start bleeding.


--Pete

[tubeswell]

Which reminds me - me and Steve L comparing build notes on Zinky amps in this thread https://el34world.com/Forum/index.php?topic=29802.0