Princeton AA964

[Lowerleftcoast]

You missed the tail resistor on the bias pot. I show it going directly to the power side ground point.

An alternative for the ground scheme: Since the power for the tremolo triode comes from a different B+ node, the ground for that triode should associate with that node over at the power side ground point. There is no sense having the *less filtered* current flow through the sensitive input area. This requires the 47 Ohm resistor to be attached directly to the ground bus.

Are you using a 20-20-20-20 cap can? Just wondering if the first two caps are paralleled to 40uF sacrificing the second order filter of the original AA964 design.

Proposed changes:


EDIT: The B+ wires to the plate resistors are not shown on your layout so I did not add them to the above.

 

[King Fan]

For reference, a trip to Fullerton:

 

[mjcyates]

LLC, thanks much for the keen eye. Regarding the cap can I currently have a 20-20-20-20 but I may go ahead and get a 40-20-20. Here is the updated layout.

 

[mjcyates]

For reference, a trip to Fullerton:

View attachment 946237

View attachment 946236

Thanks KF. That is similar to the one that I am using but not exactly the same. This one is marked L-FD, the one I am using is marked I-FD. L-FD shows 420v on B+, I-FD shows 405v on B+. Not sure what other differences there might be that's just what stood out at first glance.

 

[King Fan]

Are you using a 20-20-20-20 cap can? Just wondering if the first two caps are paralleled to 40uF sacrificing the second order filter of the original AA964 design.

LLC, that's a good question, one I didn't think about when I built my PR. For those of us who flunked filter theory, could you tell us what the original second order scheme achieved or gave up?

This one is marked L-FD, the one I am using is marked I-FD. L-FD shows 420v on B+, I-FD shows 405v on B+. Not sure what other differences there might be that's just what stood out at first glance.

Yeah, multiple schematics were SOP at Fender. Sometimes there was a preproduction run and then an early (or sometimes much later) 'as built' version, but equally often CBS changed something in later years, or (famously) Fender changed the PT specs -- in many cases around 1970. On some amps, the original schematic and revised schematic show way different B+ with the same nominal PT -- many and varied reasons are given depending on the amp.

But here we have a bit of insight. For the I-FD vs. L-FD versions, this el34world thread has a great discussion -- see especially reply #1, which details the filtering and B+ differences, I suspect the ones LLC was mentioning. Reply #4 tells us I-FD was September '64 and L-FD was November.

https://el34world.com/Forum/index.php?topic=10252.0

Pending LLC's note on the filters, I'm also interested in their idea that the extra filter node may have helped drop the B+ -- and the discussion about how B+ affects trem function. And that gets back to my concern that losing the voltage drop of a tube rectifier might be a problem.

 

[Lowerleftcoast]

But here we have a bit of insight. For the I-FD vs. L-FD versions, this el34world thread has a great discussion -- see especially reply #1, which details the filtering and B+ differences

Learnt something today. I never even noticed the *Production Date Code* on the schematics. "Look at the production code on the schematic; AA964 L-FD. L=12 F=6 and D=4". This quote comes from a similar discussion on the Gear Page. https://www.thegearpage.net/board/index.php?threads/fender-princeton.1919982/page-2
Fender code talkers... L = 12.

 

[King Fan]

Learnt something today. I never even noticed the *Production Date Code* on the schematics. "Look at the production code on the schematic; AA964 L-FD. L=12 F=6 and D=4". This quote comes from a similar discussion on the Gear Page. https://www.thegearpage.net/board/index.php?threads/fender-princeton.1919982/page-2
Fender code talkers... L = 12.

Cool, I didn't recall that either, though I've read that EL34 thread in the past. We see what we're ready to see, I guess.

But now, my friend, can you 'splain more about the effects of the difference in filtering between those two schematics? I couldn't quite follow the explanation and speculation about the sonic effects in that EL34 "reply #1." Would the later scheme change the B+? what are your thoughts on B+ in this amp? The AA964, of course, is supposed to be 'cleaner' than the AA1164 -- but is B+ part of that?

 

[Lowerleftcoast]

For those of us who flunked filter theory, could you tell us what the original second order scheme achieved or gave up?

Resistor/Capacitors can make high pass, low pass, (and other) RC filters. The most common capacitors making up RC filters in audio amplifiers are A) the coupling caps, which form *High Pass Filters* and 2) the filter caps, which make *Low Pass Filters*. They perform similar but different functions.

RC filters have a *cutoff frequency* aka corner frequency. The cutoff frequency is 3dB below the the original signal strength. The easiest way to describe why this 3dB point is chosen is to consider a two way speaker crossover. The high range horn will have a high pass filter. The lower frequencies will be cut off from the passband reproduced by the horn driver. The low range speaker will have a low pass filter. The high frequencies will be cut off from the passband reproduced by the speaker. To maintain a flat response at the crossover frequency, the *knee* where the high frequency starts to be filtered must be a distance from the knee of the low frequency filter. The actual crossover frequency must be 3dB down so when the sound of the speaker and horn are added together there is not a change in volume at the crossover frequency. This 3dB is not too important when figuring the RC filters in an amplifier. It is an extra 3dB that can be added to the dB reduction the amp designer is trying to achieve.

A *First Order* RC filter removes frequencies other than the passband at 6dB per octave.
A *Second Order* RC filter removes frequencies other than the passband at 12dB per octave.
A *Third Order* RC filter removes frequencies other than the passband at 18dB per octave.
A *Fourth Order* RC filter removes frequencies other than the passband at 24dB per octave.

In the Princeton (K-FD) example, the unused node creates a Second Order RC filter. Instead of a 6dB per octave slope it attenuates 12dB per octave. The cutoff frequency (18k resistors with 20uF capacitors) is 0.4Hz. The target frequency of ripple, we want to get rid of, in the B+ is 120Hz (or 100Hz in other countries). At 12dB per octave the attenuation is over 96dB of the 120Hz content in the B+. Not too shabby.

splain more about the effects of the difference in filtering between those two schematics?

Before we get into that, in other threads, there has been some observations of single ended amps and their B+ filtering. The observations seem to show the reservoir cap plays a significantly more important role in reducing the 120Hz (100Hz) content, than the RC filters that follow. Imo, doubling the reservoir cap to 40uF will have more effect reducing hum than having the Second Order Filter later in the B+ supply.

Would the later scheme change the B+? what are your thoughts on B+ in this amp?

When the Second Order Filter is not employed, one of the 18k resistors is not in the power rail. Without this resistor, the B+ will be higher in the preamp. Generally higher voltages on the preamp tubes will help to give the amp a cleaner character.

The Stokes Mod can be mentioned here, because it is similar to the difference in B+ we are talking about. The Stokes mod only increases the B+ voltage to the PI though. The Stokes mod moves the power source of the PI from after the two RC filters to between the two RC filters. The B+ supplied to the PI is higher. The object of the mod is to make the Princeton a little cleaner. (A byproduct of the Stokes mod is it changes the Second Order Filter to two First Order Filters.)

Imo, B+ plays a role in the character of an amp. KF, you have performed some tests with bucking transformers and commented on the sonic changes. You have also tried different PT secondary taps to attain browner sounds etc.

I agree with the guess that Chip made in the EL34 World thread. The difference in B+ will be heard.

Where Chip's observations may have gone wrong, is trusting the B+ voltages printed on the Fender schematics. I don't trust is the B+ voltages shown on the schematics. Sometimes we have seen Fender schematics that use the same transformers, different rectifiers, yet the schematics display the same voltages on the schematic. That doesn't happen in the real world without changes in other parts of the circuit.

 

[King Fan]

Wow, sir, thank you! I learned a ton right there. Extremely helpful and much appreciated. You write more clearly than a lot of published experts I've read.

I knew a bit about the Stokes mod in the PR, but not at that level. As for my tone explorations with varying B+, yeah, it's why I run my tweed amps on the bucking transformer but the Princeton Reverb on 120+.

FWIW, here's the earlier I-FD schematic (tho noter even the L-FD apparently came out before the amps were actually released, suggesting it was the one used on the shop floor).

And even tho schematics rule, it's also easy to see the node/filter difference on the two different layouts...

Sorry for the detour, @mjcyates . Now back to our regularly scheduled build thread....

 

[mjcyates]

Apologies not needed. I am all for learning and you guys are teaching .

 

[BenTobith]

Here is the board. Haven't soldered anything yet. View attachment 945742

Are those 1/2 watt or 1 watt PRPs?

 

[mjcyates]

They are 1 watt. Ordered from https://www.partsconnexion.com/prp-resistors.html.

 

[BenTobith]

They are 1 watt. Ordered from https://www.partsconnexion.com/prp-resistors.html.

Thanks! I was curious because I wanted to know how the size was, relative to a board and other components. They are beefy, but don’t seem overwhelming?

 

[mjcyates]

They are not. I like the size, plus they just look cool.

 

[BenTobith]

They are not. I like the size, plus they just look cool.

Ha! Agreed! Thinking of using some for a Princeton Reverb-Delete build.

 

[mjcyates]

Ok I think this is the final layout I am going with.

 

[supersonicobr]

Consider going with a 25uF or at most 33uF 100V capacitor on your bias circuitry, especially whereas you're using solid state rectification and no standby switch. I converted an 18W TMB kit into a Blackvibe 6V6 using the same bias circuit as on the Princeton (no 50V tap on the PT) and used what would have been its power amp cathode bypass capacitor at 470uF/63V as the bias reservoir - with the 100K ohm resistor upstream it takes it almost a full minute to reach stable negative bias voltage, I have to keep it on standby until it does. A smaller cap would charge up much more quickly, possibly quickly enough in your case to build up negative bias before the power tubes are warmed up and conducting. Just my 2 cents, have fun with the build!

 

[mjcyates]

Thanks for the input! Much appreciated.