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Post subject: Interesting discussion of the PR AA764
Posted: Fri Feb 28, 2014 11:10 pm
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Post subject: Re: Interesting discussion of the PR AA764
Posted: Fri Feb 28, 2014 11:57 pm
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I fully agree with the gentleman's assertions regarding the benefits of the GZ34 over the 5U4GB and why the latter should not be substituted for the former. However, he is mistaken about the existence of the AA1164 circuit revision (with GZ34 rectifier) -- I have an archived PDF copy of this schematic. And it seems to jive 100% with my own Princeton Reverb built in November of 1965. However, its tube chart is marked "AA764".

Arjay

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Post subject: Re: Interesting discussion of the PR AA764
Posted: Sat Mar 01, 2014 6:53 am
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I have both the AA764 and a AA1164 (GZ34). AFAIK, there is no difference between the two. The serial number indicates that the AA1164 was made before the AA764. I believe that the AA1164 is a 1965 and the AA764 is a 1966. I will need to dig the 1965 one out of storage, but if I remember correctly, it had no Fender logo on the front.

As for the GZ34 and the 5U4GB, they BOTH have cathodes. They need both cathode and anodes, in order to work. The GZ34 has an indirectly heated cathode and the 5U4GB has a directly heated one. That is one reason the 5U4GB needs more current. You heat the entire cathode in a 5U4GB. Whereas, the GZ34 --- you are only heating a separate coil. Since it takes time to indirectly heat the cathode, the GZ34 has the "slow-start" character.

I have measured the voltage across the heater line with the GZ34 and the 5U4GB, in the BF & SF PR and the Princeton. There is no appreciable drop in voltage with the 5U4GB compared to the GZ34. Either one is fine with the stock BF or SF OEM Princeton Reverb PT.

Hope this helps, a bit. :D


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Post subject: Re: Interesting discussion of the PR AA764
Posted: Sat Mar 01, 2014 10:10 am
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BMW2002Ti wrote:
I have measured the voltage across the heater line with the GZ34 and the 5U4GB, in the BF & SF PR and the Princeton. There is no appreciable drop in voltage with the 5U4GB compared to the GZ34. Either one is fine with the stock BF or SF OEM Princeton Reverb PT.


I think the operative parameter to be compared is current draw, not voltage drop.

Arjay

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Post subject: Re: Interesting discussion of the PR AA764
Posted: Sat Mar 01, 2014 12:35 pm
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Arjay,

That is the quick-&-dirty test for excessive current draw. You don't need to break the line connection to test the current draw. Read the AC voltage across two yellow leads off the PT --- leads to rectifier's cathode plate or heater. With no rectifier plugged in. Should be 5VAC (~10-15% tolerance).

Now, measure with the GZ34/5AR4 in. Measure with the 5U4GB in. If the voltage reading is more than 10% difference (drop) between the GZ34 and the 5U4GB --- the PT prolly can't handle the 5U4GB.

Since the PR used both the GZ34 and the 5U4GB in their AA1164 models (with same PT) --- think you'll be good to go, with either. Of course, the PT must be healthy.

BTW... since the rectifier's cathodes need to warm-up, before it begins to operate (either heated directly or indirectly) --- you should have a delay-on, with the 5U4GB. Not as long as the GZ34. But, definitely longer than an amp with solid-state rectification.

https://sp.yimg.com/ib/th?id=HN.6080329 ... 0&pid=15.1


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Post subject: Re: Interesting discussion of the PR AA764
Posted: Sat Mar 01, 2014 1:06 pm
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Before now I'd never noticed that there were variations in the AA1164 schematic. Are there other Fender designs that had changes without changing the schematic design number/code?

Related question: Other than the three-prong versus two-prong plug is the B1270 the same as the AA1164?

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Post subject: Re: Interesting discussion of the PR AA764
Posted: Sat Mar 01, 2014 1:39 pm
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So, my '73 SF PR is an AA1164?

I believe that it uses the 5U4GB, does this mean I can go with the GZ34?

My understanding is that the GZ34 is a better rectifier no..??

cheers!

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Post subject: Re: Interesting discussion of the PR AA764
Posted: Sat Mar 01, 2014 2:40 pm
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Lightnin MN wrote:
So, my '73 SF PR is an AA1164?


Your Princeton Reverb is the B1270 circuit.

Arjay

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Post subject: Re: Interesting discussion of the PR AA764
Posted: Sat Mar 01, 2014 6:28 pm
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Lightnin MN wrote:
So, my '73 SF PR is an AA1164?

I believe that it uses the 5U4GB, does this mean I can go with the GZ34?

My understanding is that the GZ34 is a better rectifier no..??

cheers!



Yes sir! Any amp that uses a 5U4GB as OEM, can easily go to the 5AR4/GZ34. It's a drop-in replacement. Now, which rectifier is "better," depends on amp, speaker(s), and what tone you are after. The GZ34/5AR4 has a higher B+ potential and less forward voltage drop (10-15VDC) = stiffer tone. Better bass, better transients.

The 5U4GB usually has 10-20VDC less final B+ in the same amp. Much more forward voltage drop (30-40VDC). Makes for more sag in the tone. A bit more "slow quality" to the transients. Some ppl like this in certain amps. Esp those with very efficient speakers, who are looking of a more Bluesy tone. I like the tone of the 5U4GB in my SFSR.

Experiment with both rectifiers. And find what suits your needs. Be sure to re-bias after changing any rectifier and output tube. Note B+ changes.


Good luck! :D


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