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Post subject: Understanding a 66 Blackface Princeton Reverb
Posted: Mon Aug 28, 2017 1:50 pm
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Hey guys

Trying to wrap my head around this old 66 Princeton.

Tube chart AA764 (which I believe makes it AA1164) Chassis stamp: 10209 (which dates it as a 66)

Got it for a good price. The Speaker, Baffle, and grill cloth are not original, it has an old 12" Jensen.

She sounds good at very low volumes, but gets rough really early (Volume at 3) and then beyond that is just mud.

Internally, Its hard to make heads or tails.

To start with, it seems 2/3rds of the filter cap can is being employed and a Sprague 20uf 500vdc was 'added' on top? Is this normal? Im assuming a tech thought this would be easier.

From the PT (022722 - 8316809 Better coil and transformer Co, made in 68) Everything reads high. With the Rectifier tube removed, I get 470 VAC to pin 4. (125 VAC from the wall receptacle) With the 5AR4 in about 465 VAC. about 25-30 volts higher than the schem indicates.

So Im going through measuring and get more confused. But this last one may be normal. The scheme indicates no resistors on the output tubes, but they both have 470 Ohms. Also it indicates that 6 doesn't do anything. In my case the screen grid resistors terminate across 4-6, but the wiring all leaves off pin 6?

I decided to stop measuring at this point to see if this is all business as usual, or to ask if its all as out of whack as it seems.

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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Mon Aug 28, 2017 3:31 pm
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Hey there, ParksideDrive!

The amp seems to have undergone some non-original fixes. I'd prolly rewire the thing back to the OEM layout. Get rid of the lone Sprague Atom 20mfd filter cap and replace the can with a new CE unit. I've been using the 30/20/20/20 @ 525VDC can. As long as you aren't going to put a humungous magnet aftermarket speaker into the amp, you'll have plenty of room.

IMHO, the extra electrolyte fluid of this can will help keep the can from drying out as soon as the smaller 20/20/20/20 @ 475VDC can.

https://www.tubesandmore.com/products/c ... 30202020uf

Rewire all dropping resistors and bias supply back to OEM.

The higher voltages you are reading is prolly due to higher AC outlet voltages (compared to when the amp was built).

The PT has been replaced. Dating seems to indicate 9th week of 1968, as its birthdate.

The Princeton Reverb (both BF and SF) did NOT come with 470-ohm screen resistors. So, that was done by a tech, later. The screen supply voltage needs to be routed to unused pin 6. Then, the 470-ohm resistor is soldered between pins 4 & 6. Number 4 being the screen.

It's a good idea for each 6V6GT tube to have its own screen resistor, soldered to the socket --- like the Deluxe Reverbs.

Good luck with the fixes! :)


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Mon Aug 28, 2017 4:58 pm
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BMW2002Ti wrote:
The amp seems to have undergone some non-original fixes. I'd prolly rewire the thing back to the OEM layout. Get rid of the lone Sprague Atom 20mfd filter cap and replace the can with a new CE unit.


+1

You're better off starting from the original layout and working from there. Looks like someone prior to you must have had a new drill (and/or self tapping screws) and couldn't stand the temptation. Is the PT original? I wonder if someone added a cap board at some point trying to recreate a Deluxe Reverb from the amp.

That can is only 2 sections, they doubled up on one of them.

UN


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Tue Aug 29, 2017 7:04 pm
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Location: Coastal Bend, Tx.
You've come to the right place for help if you plan on doing the work yourself.
If you plan on sticking w/ the gz34, I'd recommend staying w/ the AA964 schematic as a reference or a previous date as long as it has a gz34 in the schematic.
Plan on replacing ALL the electrolytics including those used for specific tube operation.
Measure all the resistors and replace as necessary all those out of tolerance.
As BMW stated already your amp was designed around an input of 115-117vac, your higher voltage will cascade throughout the amplifier stages. Some resistor swaps may be required to get it tamed or better yet imho, build a box or buy a box that controls the input voltage! That way your amp circuit stays w/ original specs and sounds as close as possible to its intended design.
Definitely follow BMW's advise on the PSU capacitors and resistors as well as the other info.


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Thu Sep 14, 2017 12:00 pm
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Hey guys

Wanted to post a few updates.

So I replaced the Cap can with this Image

Which I bought here: https://www.tubesandmore.com/products/c ... ectrolytic

I also moved the wiring off pin 6 on the out put tubes, back to pin 4. And I left the screen resistors (terminating across pins 4 and 6) in place as Bmw advised.

So now I have rewired the cap can back into place as the layout explains

Image

Image

...and i get to this Letter D in the box thing.

Ive tried in the past to figure these letters out, but always get lost. I thought they were there to just say "follow letter d to letter d" to make the layout less cluttered....but there are often multiple instances of the same letter, which ruins that theory (i.e. follow point D to point D..and also point D, and also another point D...what?)

Image

Can you guys shed some light on this? I also see the box letters on the schematic near the cap can tab in question, but I still dont know what to do with it.

Thanks
Dave


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Sat Sep 16, 2017 10:18 am
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Anyone? :)


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Sat Sep 16, 2017 6:10 pm
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Parkside, "D" of the PSU charges the anode plates of the 12AX7 phase inverter. Best seen on the schematic:

http://ampwares.com/schematics/princeto ... aa1164.pdf

You should have a wire coming from an eyelet (red arrow) in the circuit board. This is where the PSU cap lead (D) should go. The two "D"s on the circuit board, are essentially the same electric point, in the circuit (0-ohms between them).

The wire between the cap can and the board, prolly goes under the board. I'd have to look at some of my photos.


Image


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Sat Sep 16, 2017 6:17 pm
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Nope, the wire is twisted on top of circuit, in my BF PR. Looks like it goes into one eyelet, then comes out the very next eyelet above it (from photo's perspective). You should do a continuity test with DVM, to be sure.



Image


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Mon Sep 18, 2017 2:50 pm
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Roadie
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Just jump every "D" together from right to left on the layout until you end up at the can cap. Don't "over think" it.

GW

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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Wed Sep 20, 2017 11:02 pm
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Hey guys

Thanks for the help. I think I figured it out before you responded. Checked out a few photos of bfprs

If you look at one of the previous photos, one of the 18k resistors had the wire soldered there. (Yellow cloth wire, near the pots). I don't know if it matters but I moved it back to the specified tab on the cap can.

Sounding good now, so I think that's enough surgery for the time being.

Someone should really break down 'how to read every symbol on a fender schematic' would probably solve most of these issues.

Thanks all


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Post subject: Re: Understanding a 66 Blackface Princeton Reverb
Posted: Thu Sep 21, 2017 6:15 pm
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If you compare the layout with the schematic, it makes the use of letters more understandable. These letters reduce clutter on the diagrams.

:mrgreen:


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