BF Princeton - aa964 / aa1164

[bluemonk]

I'm considering to build a BF Princeton aa964 clone.
I'm not really interested in Tremolo and think to use that triode for another switchable gain stage just like aa1164.
Does it need simply a bypass switch?
How should it be done exactly?

Thanks in advance

[bluemonk]

Higher voltage on preamp. Anybody?

My project is to build an AA964 clone adding a switchable gain stage (missing vibrato).
This stage should be cloned from AA1164, where it has 160V on plates.
In my project it replaces AA964 vibrato so should get 260V.

Is it safe?
Is there some tweaks?

Thanks in advance

[TheTinMan]

I have not built the attached circuit but do have built two Princeton Reverb clones. The 470K resistor to ground right before V3a serves two functions: grid return for V3a and, more importantly, second half of voltage divider in Princeton Reverb formed by 3.3 meg and 470k/Reverb pot. Learned to look for that kind of issue with a Super Reverb derivative...

The switchable cathode resistor bypass cap on V1b is the easiest way to implement a boost. There are others.

Also, I took the liberty of moving the PI plate supply up to node "C" for higher voltage. Lets you push the 6V6s a bit harder before the PI starts to distort.

Further liberties taken by adding a pi filter and adjustment pot to the bias supply. That arrangement worked very well in my last PR build.

I can add an editable ExpressSCH file if you want it.

Hope this helps,

Chip

[bluemonk]

Your suggestion are very helpful to me. Thanks.
I'm just wondering if the added gain stage (in place of aa964 vibrato) can be bypassed by a plain switch, to get stock Princeton aa964 circuit.
I wish that because I'd need a very clean amp... but with some optional growling voice.

I may think that running the added gain stage 220V in place of stock 160V (as is in aa1164) doesn't hurt. Is that correct?

TIA

[MWJB]

In reality you might not see as little as 160vdc on the preamp plates of any tolex Princeton, they will probably be somewhat higher. However, to tweak the preamp voltage you need to adjust the value of 18K resistor that feeds the 100Ks at pins 1 & 6 of the first 12AX7. Preamp tubes don't draw much current, so you might need to make substantial changes in value for noticeable results with voltage, you might not hear tangible changes in tone at increments of less than 20vdc. Build the amp first, see what you end up with and then select preferred preamp voltage by ear testing. Anything from 160-300vdc will 'work' safely (220v would be ideal for tolex Fender tone).

[TheTinMan]

OK - you want more clean headroom in the preamp. Higher plate voltages will do that. (Never noticed the big difference in voltages between the AA964 and the AA1164 - thanks for that.)

There are two 18K dropping/filter resistors in the schematic I posted. Change them both to 10K. That gets you close to the single 18K resistor in the AA964 and should give you similar B+ and plate voltages in the preamp. Also, the reverb drive and reverb transformer in the Princeton Reverb draw a lot of current relative to normal preamp gain stages so that will bring your B+ voltages up too.

The stock AA1164 is a clean preamp. Raising the voltages on the PI stage and the earlier preamp stages will make it cleaner. You're not going to get much "growl" out of it even with the cathode bypass cap I showed. Switching out that bypass cap should make it squeaky clean, relatively speaking ("Boost" switch). You might raise that cathode resistor (V1b) to 1.8K or even 2.2K if you want it cleaner, but that will minimize the chances of getting "growl" out of the 6V6s.

However, adding that 25K audio taper Mid pot will give you the ability to get some growl. It's a relatively tame version of the "Raw" control some guys talk about. It gradually reduces the insertion loss (and effectiveness) of the tone controls. I definitely would try that plus a 2.2meg resistor where the 3.3meg is shown. With the Mid control down in the 1-3 range and the Boost off (no cathode bypass cap on V1b), it should be quite clean. Turn on the Boost and crank the Mids up above 5 and you'll start to hear a lot more growl than a stock BF Princeton.

Hope this works out well for you.

Chip

[bluemonk]

Ok, it's time to go to build and then tweak&debug.
Thanks everybody

[spiky1987]

Hi, i'mn planning to build a AA964 clone. It's gonna be my first DIY amp. I bought almost all the big stuff and i'm now stalled with the capacitor.

I'd like to know what kind of capacitor is the 25/25 one ? are thay 25uF 25V ??

I need to get more precise info before to buy my stock. Are 0.2 0.2uF or 0.2F ???

Thanks a lot guys !

[TheTinMan]

Spiky - I strongly recommend that you start with a kit instead of a scratch build.

You need to know which capacitors are electrolytics and which are not. On the schematic posted above, only the electrolytic caps have voltage specified in addition to capacitance. There also is a "+" sign showing the polarity of the electrolytics and that is very important. (note: filter caps in bias supply are "upside down" because they are filtering negative voltage) All other caps are 400 volt, and all capacitor values are "uf" except for the 250 picofarad cap in the tone stack. All resistors should be 1/2 watt unless marked otherwise.

Seriously, if you haven't built a tube amp before, starting on a never-been-tried prototype is a really bad idea. I said clearly above that I had NOT built that circuit. You do not have a layout for the circuit. A "known-good" layout is just as important as the circuit itself for building a workable amp.

My apologies if this comes off as preachy or condescending. Your questions left me worried.

Chip

[spiky1987]

Before finding this forum i was alone lol and i wanted to do the princeton because it looked simple and i had the schematic and this layout :
http://www.fenderholic.com/schem/pri...964_layout.gif

I didn't find any better but that looks enough for me. I'm not used with amps but i'm currently electricien and studying in electronics so i'm very interested to learn.

Thanks a lot for your tips, considering what you said, There's only 6 electrolytic cap and what kind of caps are the other ones ? Also i was wondering what kind of diode ther usualy put ? ( no infos on that )

[TheTinMan]

OK - starting with the Fender layout isn't a bad idea.

There are 8 electrolytics caps shown. However, the Fender used a cap can containing all four power rail filter caps in one package (at left end of board in your layout). You could use the JJ 40/20/20/20 cap can, just make sure you use the 40uf section for the first node (i.e. directly off rectifier).

The other caps are 400 volt polyester or polyethelene (sp?) film caps in axial configuration (leads on either end).

The diode is specified - 1N4007.

I just noticed that the schematic shows a 5U4GB rectifier tube. You may need to use a GZ34 (5AR4 equivalent) depending on the current rating of your power transformer. The 5U4GB requires 3 amps, but the GZ34 only needs 2 amps.

At the risk of REALLY confusing you, here's the layout I used for my second Princeton Reverb build: http://i222.photobucket.com/albums/d...g?t=1254972247 Please note that this is NOT the stock circuit. Here's the corresponding schematic: http://i222.photobucket.com/albums/d...g?t=1254972390

Chip

[bluemonk]

i'm very interested to learn.

what kind of caps are the other ones ?

Here is my experience: before I built my first amp (for instance it was a simple SE) I read a lot of Ampage's and Ax84's threads to get more knowledge. By searching in Ampage's archive you can find discussions and answers about many questions. AX84.com - The Cooperative Tube Guitar Amp Project is very helpful to begin.

[TheTinMan]

There are some other references that would be helpful. Kevin O'Connor's "The Ultimate Tone" series was very helpful to me, starting with Vol. 1. (there are other people who do not favor KOC).

Merlin, a contributor here, has just come out with a book that's getting very positive reviews. Here's his website:
How to design valve guitar amplifiers

Here's a thread with TONS of references:
Useful References to Check Before You Post Hoffman's forum is a wonderful "place" but it tends to be geared toward experienced builders and professional techs.

Chip

[Don Moose]

moving the PI plate supply up to node "C" for higher voltage. Lets you push the 6V6s a bit harder before the PI starts to distort.

Isn't this the 'Stokes' mod? YMMV, but I hear this kills Princeton PTs.

[TheTinMan]

Isn't this the 'Stokes' mod? YMMV, but I hear this kills Princeton PTs.

I wrote something else and then found this thread:
Paul C and Stokes mod - The Gear Page

I've built two BF Princeton Reverb clones. The first doesn't have the "Stokes" mod. The second one does and I like it better. It hasn't blown up yet. However, BOTH have adjustable bias. Maybe that serves as a partial offset?

I just don't see how raising the B+ a bit on the PI could generate so much more signal swing that it threatens the PT, but those guys on the Gearpage probably know a heck of a lot more than I do.

By the time you get to a LTPI, you've basically got a Deluxe. At that point, I can see potential danger.

Chip